1NMAP(1) Nmap Reference Guide NMAP(1)
2
6 nmap - Network exploration tool and security / port scanner
7
9 nmap [Scan Type...] [Options] {target specification}
10
12 Nmap (“Network Mapper”) is an open source tool for network exploration
13 and security auditing. It was designed to rapidly scan large networks,
14 although it works fine against single hosts. Nmap uses raw IP packets
15 in novel ways to determine what hosts are available on the network,
16 what services (application name and version) those hosts are offering,
17 what operating systems (and OS versions) they are running, what type of
18 packet filters/firewalls are in use, and dozens of other
19 characteristics. While Nmap is commonly used for security audits, many
20 systems and network administrators find it useful for routine tasks
21 such as network inventory, managing service upgrade schedules, and
22 monitoring host or service uptime.
23 The output from Nmap is a list of scanned targets, with supplemental
25 information on each depending on the options used. Key among that
26 information is the “interesting ports table”. That table lists the port
27 number and protocol, service name, and state. The state is either open,
28 filtered, closed, or unfiltered. Open means that an application on the
29 target machine is listening for connections/packets on that port.
30 Filtered means that a firewall, filter, or other network obstacle is
31 blocking the port so that Nmap cannot tell whether it is open or
32 closed. Closed ports have no application listening on them, though
33 they could open up at any time. Ports are classified as unfiltered when
34 they are responsive to Nmap's probes, but Nmap cannot determine whether
35 they are open or closed. Nmap reports the state combinations
36 open|filtered and closed|filtered when it cannot determine which of the
37 two states describe a port. The port table may also include software
38 version details when version detection has been requested. When an IP
39 protocol scan is requested (-sO), Nmap provides information on
40 supported IP protocols rather than listening ports.
41 In addition to the interesting ports table, Nmap can provide further
43 information on targets, including reverse DNS names, operating system
44 guesses, device types, and MAC addresses.
45 A typical Nmap scan is shown in Example 14.1, “A representative Nmap
47 scan”. The only Nmap arguments used in this example are -A, to enable
48 OS and version detection, -T4 for faster execution, and then the two
49 target hostnames. Example 14.1. A representative Nmap scan.sp
50 # nmap -A -T4 scanme.nmap.org playground
51 Starting nmap ( http://insecure.org/nmap/ )
53 Interesting ports on scanme.nmap.org (205.217.153.62):
54 (The 1663 ports scanned but not shown below are in state: filtered)
55 PORT STATE SERVICE VERSION
56 22/tcp open ssh OpenSSH 3.9p1 (protocol 1.99)
57 53/tcp open domain
58 70/tcp closed gopher
59 80/tcp open http Apache httpd 2.0.52 ((Fedora))
60 113/tcp closed auth
61 Device type: general purpose
62 Running: Linux 2.4.X|2.5.X|2.6.X
63 OS details: Linux 2.4.7 - 2.6.11, Linux 2.6.0 - 2.6.11
64 Uptime 33.908 days (since Thu Jul 21 03:38:03 2005)
65 Interesting ports on playground.nmap.org (192.168.0.40):
67 (The 1659 ports scanned but not shown below are in state: closed)
68 PORT STATE SERVICE VERSION
69 135/tcp open msrpc Microsoft Windows RPC
70 139/tcp open netbios-ssn
71 389/tcp open ldap?
72 445/tcp open microsoft-ds Microsoft Windows XP microsoft-ds
73 1002/tcp open windows-icfw?
74 1025/tcp open msrpc Microsoft Windows RPC
75 1720/tcp open H.323/Q.931 CompTek AquaGateKeeper
76 5800/tcp open vnc-http RealVNC 4.0 (Resolution 400x250; VNC TCP port: 5900)
77 5900/tcp open vnc VNC (protocol 3.8)
78 MAC Address: 00:A0:CC:63:85:4B (Lite-on Communications)
79 Device type: general purpose
80 Running: Microsoft Windows NT/2K/XP
81 OS details: Microsoft Windows XP Pro RC1+ through final release
82 Service Info: OSs: Windows, Windows XP
83 Nmap finished: 2 IP addresses (2 hosts up) scanned in 88.392 seconds
85 The newest version of Nmap can be obtained from
87 http://insecure.org/nmap/. The newest version of the man page is
88 available from http://insecure.org/nmap/man/.
89
91 This options summary is printed when Nmap is run with no arguments, and
92 the latest version is always available at
93 http://insecure.org/nmap/data/nmap.usage.txt. It helps people remember
94 the most common options, but is no substitute for the in-depth
95 documentation in the rest of this manual. Some obscure options aren't
96 even included here.
97 Nmap 4.20RC1 ( http://insecure.org )
99 Usage: nmap [Scan Type(s)] [Options] {target specification}
100 TARGET SPECIFICATION:
101 Can pass hostnames, IP addresses, networks, etc.
102 Ex: scanme.nmap.org, 192.168.0.1; 10.0.0-255.1-254
103 -iL <inputfilename>: Input from list of hosts/networks
104 -iR <num hosts>: Choose random targets
105 --exclude <host1[,host2][,host3],...>: Exclude hosts/networks
106 --excludefile <exclude_file>: Exclude list from file
107 HOST DISCOVERY:
108 -sL: List Scan - simply list targets to scan
109 -sP: Ping Scan - go no further than determining if host is online
110 -P0: Treat all hosts as online -- skip host discovery
111 -PS/PA/PU [portlist]: TCP SYN/ACK or UDP discovery to given ports
112 -PE/PP/PM: ICMP echo, timestamp, and netmask request discovery probes
113 -n/-R: Never do DNS resolution/Always resolve [default: sometimes]
114 --dns-servers <serv1[,serv2],...>: Specify custom DNS servers
115 --system-dns: Use OS's DNS resolver
116 SCAN TECHNIQUES:
117 -sS/sT/sA/sW/sM: TCP SYN/Connect()/ACK/Window/Maimon scans
118 -sU: UDP Scan
119 -sN/sF/sX: TCP Null, FIN, and Xmas scans
120 --scanflags <flags>: Customize TCP scan flags
121 -sI <zombie host[:probeport]>: Idlescan
122 -sO: IP protocol scan
123 -b <ftp relay host>: FTP bounce scan
124 PORT SPECIFICATION AND SCAN ORDER:
125 -p <port ranges>: Only scan specified ports
126 Ex: -p22; -p1-65535; -p U:53,111,137,T:21-25,80,139,8080
127 -F: Fast - Scan only the ports listed in the nmap-services file)
128 -r: Scan ports consecutively - don't randomize
129 SERVICE/VERSION DETECTION:
130 -sV: Probe open ports to determine service/version info
131 --version-intensity <level>: Set from 0 (light) to 9 (try all probes)
132 --version-light: Limit to most likely probes (intensity 2)
133 --version-all: Try every single probe (intensity 9)
134 --version-trace: Show detailed version scan activity (for debugging)
135 OS DETECTION:
136 -O: Enable OS detection (try 2nd generation w/fallback to 1st)
137 -O2: Only use the new OS detection system (no fallback)
138 -O1: Only use the old (1st generation) OS detection system
139 --osscan-limit: Limit OS detection to promising targets
140 --osscan-guess: Guess OS more aggressively
141 TIMING AND PERFORMANCE:
142 Options which take <time> are in milliseconds, unless you append 's'
143 (seconds), 'm' (minutes), or 'h' (hours) to the value (e.g. 30m).
144 -T[0-5]: Set timing template (higher is faster)
145 --min-hostgroup/max-hostgroup <size>: Parallel host scan group sizes
146 --min-parallelism/max-parallelism <time>: Probe parallelization
147 --min-rtt-timeout/max-rtt-timeout/initial-rtt-timeout <time>: Specifies
148 probe round trip time.
149 --max-retries <tries>: Caps number of port scan probe retransmissions.
150 --host-timeout <time>: Give up on target after this long
151 --scan-delay/--max-scan-delay <time>: Adjust delay between probes
152 FIREWALL/IDS EVASION AND SPOOFING:
153 -f; --mtu <val>: fragment packets (optionally w/given MTU)
154 -D <decoy1,decoy2[,ME],...>: Cloak a scan with decoys
155 -S <IP_Address>: Spoof source address
156 -e <iface>: Use specified interface
157 -g/--source-port <portnum>: Use given port number
158 --data-length <num>: Append random data to sent packets
159 --ip-options <options>: Send packets with specified ip options
160 --ttl <val>: Set IP time-to-live field
161 --spoof-mac <mac address/prefix/vendor name>: Spoof your MAC address
162 --badsum: Send packets with a bogus TCP/UDP checksum
163 OUTPUT:
164 -oN/-oX/-oS/-oG <file>: Output scan in normal, XML, s|<rIpt kIddi3,
165 and Grepable format, respectively, to the given filename.
166 -oA <basename>: Output in the three major formats at once
167 -v: Increase verbosity level (use twice for more effect)
168 -d[level]: Set or increase debugging level (Up to 9 is meaningful)
169 --open: Only show open (or possibly open) ports
170 --packet-trace: Show all packets sent and received
171 --iflist: Print host interfaces and routes (for debugging)
172 --log-errors: Log errors/warnings to the normal-format output file
173 --append-output: Append to rather than clobber specified output files
174 --resume <filename>: Resume an aborted scan
175 --stylesheet <path/URL>: XSL stylesheet to transform XML output to HTML
176 --webxml: Reference stylesheet from Insecure.Org for more portable XML
177 --no-stylesheet: Prevent associating of XSL stylesheet w/XML output
178 MISC:
179 -6: Enable IPv6 scanning
180 -A: Enables OS detection and Version detection
181 --datadir <dirname>: Specify custom Nmap data file location
182 --send-eth/--send-ip: Send using raw ethernet frames or IP packets
183 --privileged: Assume that the user is fully privileged
184 --unprivileged: Assume the user lacks raw socket privileges
185 -V: Print version number
186 -h: Print this help summary page.
187 EXAMPLES:
188 nmap -v -A scanme.nmap.org
189 nmap -v -sP 192.168.0.0/16 10.0.0.0/8
190 nmap -v -iR 10000 -P0 -p 80
191
194 Everything on the Nmap command-line that isn't an option (or option
195 argument) is treated as a target host specification. The simplest case
196 is to specify a target IP address or hostname for scanning.
197 Sometimes you wish to scan a whole network of adjacent hosts. For this,
199 Nmap supports CIDR-style addressing. You can append
200 /numbits to an IP address or hostname and Nmap will scan every IP
202 address for which the first numbits are the same as for the reference
203 IP or hostname given. For example, 192.168.10.0/24 would scan the 256
204 hosts between 192.168.10.0 (binary: 11000000 10101000 00001010
205 00000000) and 192.168.10.255 (binary: 11000000 10101000 00001010
206 11111111), inclusive. 192.168.10.40/24 would do exactly the same thing.
207 Given that the host scanme.nmap.org is at the IP address
208 205.217.153.62, the specification scanme.nmap.org/16 would scan the
209 65,536 IP addresses between 205.217.0.0 and 205.217.255.255. The
210 smallest allowed value is /1, which scans half the Internet. The
211 largest value is 32, which scans just the named host or IP address
212 because all address bits are fixed.
213 CIDR notation is short but not always flexible enough. For example, you
215 might want to scan 192.168.0.0/16 but skip any IPs ending with .0 or
216 .255 because they are commonly broadcast addresses. Nmap supports this
217 through octet range addressing. Rather than specify a normal IP
218 address, you can specify a comma separated list of numbers or ranges
219 for each octet. For example, 192.168.0-255.1-254 will skip all
220 addresses in the range that end in .0 and or .255. Ranges need not be
221 limited to the final octets: the specifier 0-255.0-255.13.37 will
222 perform an Internet-wide scan for all IP addresses ending in 13.37.
223 This sort of broad sampling can be useful for Internet surveys and
224 research.
225 IPv6 addresses can only be specified by their fully qualified IPv6
227 address or hostname. CIDR and octet ranges aren't supported for IPv6
228 because they are rarely useful.
229 Nmap accepts multiple host specifications on the command line, and they
231 don't need to be the same type. The command nmap scanme.nmap.org
232 192.168.0.0/16 10.0.0,1,3-7.0-255 does what you would expect.
233 While targets are usually specified on the command lines, the following
235 options are also available to control target selection:
236 -iL <inputfilename> (Input from list)
238 Reads target specifications from inputfilename. Passing a huge
239 list of hosts is often awkward on the command line, yet it is a
240 common desire. For example, your DHCP server might export a list
241 of 10,000 current leases that you wish to scan. Or maybe you
242 want to scan all IP addresses except for those to locate hosts
243 using unauthorized static IP addresses. Simply generate the list
244 of hosts to scan and pass that filename to Nmap as an argument
245 to the -iL option. Entries can be in any of the formats accepted
246 by Nmap on the command line (IP address, hostname, CIDR, IPv6,
247 or octet ranges). Each entry must be separated by one or more
248 spaces, tabs, or newlines. You can specify a hyphen (-) as the
249 filename if you want Nmap to read hosts from standard input
250 rather than an actual file.
251 -iR <num hosts> (Choose random targets)
253 For Internet-wide surveys and other research, you may want to
254 choose targets at random. The num hosts argument tells Nmap how
255 many IPs to generate. Undesirable IPs such as those in certain
256 private, multicast, or unallocated address ranges are
257 automatically skipped. The argument 0 can be specified for a
258 never-ending scan. Keep in mind that some network administrators
259 bristle at unauthorized scans of their networks and may
260 complain. Use this option at your own risk! If you find yourself
261 really bored one rainy afternoon, try the command nmap -sS -PS80
262 -iR 0 -p 80 to locate random web servers for browsing.
263 --exclude <host1[,host2][,host3],...> (Exclude hosts/networks)
265 Specifies a comma-separated list of targets to be excluded from
266 the scan even if they are part of the overall network range you
267 specify. The list you pass in uses normal Nmap syntax, so it can
268 include hostnames, CIDR netblocks, octet ranges, etc. This can
269 be useful when the network you wish to scan includes untouchable
270 mission-critical servers, systems that are known to react
271 adversely to port scans, or subnetworks administered by other
272 people.
273 --excludefile <exclude_file> (Exclude list from file)
275 This offers the same functionality as the --exclude option,
276 except that the excluded targets are provided in a newline,
277 space, or tab delimited exclude_file rather than on the command
278 line.
279
281 One of the very first steps in any network reconnaissance mission is to
282 reduce a (sometimes huge) set of IP ranges into a list of active or
283 interesting hosts. Scanning every port of every single IP address is
284 slow and usually unnecessary. Of course what makes a host interesting
285 depends greatly on the scan purposes. Network administrators may only
286 be interested in hosts running a certain service, while security
287 auditors may care about every single device with an IP address. An
288 administrator may be comfortable using just an ICMP ping to locate
289 hosts on his internal network, while an external penetration tester may
290 use a diverse set of dozens of probes in an attempt to evade firewall
291 restrictions.
292 Because host discovery needs are so diverse, Nmap offers a wide variety
294 of options for customizing the techniques used. Host discovery is
295 sometimes called ping scan, but it goes well beyond the simple ICMP
296 echo request packets associated with the ubiquitous ping tool. Users
297 can skip the ping step entirely with a list scan (-sL) or by disabling
298 ping (-P0), or engage the network with arbitrary combinations of
299 multi-port TCP SYN/ACK, UDP, and ICMP probes. The goal of these probes
300 is to solicit responses which demonstrate that an IP address is
301 actually active (is being used by a host or network device). On many
302 networks, only a small percentage of IP addresses are active at any
303 given time. This is particularly common with RFC1918-blessed private
304 address space such as 10.0.0.0/8. That network has 16 million IPs, but
305 I have seen it used by companies with less than a thousand machines.
306 Host discovery can find those machines in a sparsely allocated sea of
307 IP addresses.
308 If no host discovery options are given, Nmap sends a TCP ACK packet
310 destined for port 80 and an ICMP Echo Request query to each target
311 machine. An exception to this is that an ARP scan is used for any
312 targets which are on a local ethernet network. For unprivileged UNIX
313 shell users, a SYN packet is sent instead of the ack using the
314 connect() system call. These defaults are equivalent to the -PA -PE
315 options. This host discovery is often sufficient when scanning local
316 networks, but a more comprehensive set of discovery probes is
317 recommended for security auditing.
318 The -P* options (which select ping types) can be combined. You can
320 increase your odds of penetrating strict firewalls by sending many
321 probe types using different TCP ports/flags and ICMP codes. Also note
322 that ARP discovery (-PR) is done by default against targets on a local
323 ethernet network even if you specify other -P* options, because it is
324 almost always faster and more effective.
325 By default, Nmap does host discovery and then performs a port scan
327 against each host it determines is online. This is true even if you
328 specify non-default host discovery types such as UDP probes (-PU). Read
329 about the -sP option to learn how to perform only host discovery, or
330 use -P0 to skip host discovery and port scan all target hosts. The
331 following options control host discovery:
332 -sL (List Scan)
334 The list scan is a degenerate form of host discovery that simply
335 lists each host of the network(s) specified, without sending any
336 packets to the target hosts. By default, Nmap still does
337 reverse-DNS resolution on the hosts to learn their names. It is
338 often surprising how much useful information simple hostnames
339 give out. For example, fw.chi is the name of one company's
340 Chicago firewall. Nmap also reports the total number of IP
341 addresses at the end. The list scan is a good sanity check to
342 ensure that you have proper IP addresses for your targets. If
343 the hosts sport domain names you do not recognize, it is worth
344 investigating further to prevent scanning the wrong company's
345 network.
346 Since the idea is to simply print a list of target hosts,
348 options for higher level functionality such as port scanning, OS
349 detection, or ping scanning cannot be combined with this. If you
350 wish to disable ping scanning while still performing such higher
351 level functionality, read up on the -P0 option.
352 -sP (Ping Scan)
354 This option tells Nmap to only perform a ping scan (host
355 discovery), then print out the available hosts that responded to
356 the scan. No further testing (such as port scanning or OS
357 detection) is performed. This is one step more intrusive than
358 the list scan, and can often be used for the same purposes. It
359 allows light reconnaissance of a target network without
360 attracting much attention. Knowing how many hosts are up is more
361 valuable to attackers than the list provided by list scan of
362 every single IP and host name.
363 Systems administrators often find this option valuable as well.
365 It can easily be used to count available machines on a network
366 or monitor server availability. This is often called a ping
367 sweep, and is more reliable than pinging the broadcast address
368 because many hosts do not reply to broadcast queries.
369 The -sP option sends an ICMP echo request and a TCP packet to
371 port 80 by default. When executed by an unprivileged user, a SYN
372 packet is sent (using a connect() call) to port 80 on the
373 target. When a privileged user tries to scan targets on a local
374 ethernet network, ARP requests (-PR) are used unless --send-ip
375 was specified. The -sP option can be combined with any of the
376 discovery probe types (the -P* options, excluding -P0) for
377 greater flexibility. If any of those probe type and port number
378 options are used, the default probes (ACK and echo request) are
379 overridden. When strict firewalls are in place between the
380 source host running Nmap and the target network, using those
381 advanced techniques is recommended. Otherwise hosts could be
382 missed when the firewall drops probes or their responses.
383 -P0 (No ping)
385 This option skips the Nmap discovery stage altogether. Normally,
386 Nmap uses this stage to determine active machines for heavier
387 scanning. By default, Nmap only performs heavy probing such as
388 port scans, version detection, or OS detection against hosts
389 that are found to be up. Disabling host discovery with -P0
390 causes Nmap to attempt the requested scanning functions against
391 every target IP address specified. So if a class B sized target
392 address space (/16) is specified on the command line, all 65,536
393 IP addresses are scanned. That second option character in -P0 is
394 a zero and not the letter O. Proper host discovery is skipped as
395 with the list scan, but instead of stopping and printing the
396 target list, Nmap continues to perform requested functions as if
397 each target IP is active.
398 -PS [portlist] (TCP SYN Ping)
400 This option sends an empty TCP packet with the SYN flag set. The
401 default destination port is 80 (configurable at compile time by
402 changing DEFAULT_TCP_PROBE_PORT in nmap.h), but an alternate
403 port can be specified as a parameter. A comma separated list of
404 ports can even be specified (e.g.
405 -PS22,23,25,80,113,1050,35000), in which case probes will be
406 attempted against each port in parallel.
407 The SYN flag suggests to the remote system that you are
409 attempting to establish a connection. Normally the destination
410 port will be closed, and a RST (reset) packet sent back. If the
411 port happens to be open, the target will take the second step of
412 a TCP 3-way-handshake by responding with a SYN/ACK TCP packet.
413 The machine running Nmap then tears down the nascent connection
414 by responding with a RST rather than sending an ACK packet which
415 would complete the 3-way-handshake and establish a full
416 connection. The RST packet is sent by the kernel of the machine
417 running Nmap in response to the unexpected SYN/ACK, not by Nmap
418 itself.
419 Nmap does not care whether the port is open or closed. Either
421 the RST or SYN/ACK response discussed previously tell Nmap that
422 the host is available and responsive.
423 On UNIX boxes, only the privileged user root is generally able
425 to send and receive raw TCP packets. For unprivileged users, a
426 workaround is automatically employed whereby the connect()
427 system call is initiated against each target port. This has the
428 effect of sending a SYN packet to the target host, in an attempt
429 to establish a connection. If connect() returns with a quick
430 success or an ECONNREFUSED failure, the underlying TCP stack
431 must have received a SYN/ACK or RST and the host is marked
432 available. If the connection attempt is left hanging until a
433 timeout is reached, the host is marked as down. This workaround
434 is also used for IPv6 connections, as raw IPv6 packet building
435 support is not yet available in Nmap.
436 -PA [portlist] (TCP ACK Ping)
438 The TCP ACK ping is quite similar to the just-discussed SYN
439 ping. The difference, as you could likely guess, is that the TCP
440 ACK flag is set instead of the SYN flag. Such an ACK packet
441 purports to be acknowledging data over an established TCP
442 connection, but no such connection exists. So remote hosts
443 should always respond with a RST packet, disclosing their
444 existence in the process.
445 The -PA option uses the same default port as the SYN probe (80)
447 and can also take a list of destination ports in the same
448 format. If an unprivileged user tries this, or an IPv6 target is
449 specified, the connect() workaround discussed previously is
450 used. This workaround is imperfect because connect() is actually
451 sending a SYN packet rather than an ACK.
452 The reason for offering both SYN and ACK ping probes is to
454 maximize the chances of bypassing firewalls. Many administrators
455 configure routers and other simple firewalls to block incoming
456 SYN packets except for those destined for public services like
457 the company web site or mail server. This prevents other
458 incoming connections to the organization, while allowing users
459 to make unobstructed outgoing connections to the Internet. This
460 non-stateful approach takes up few resources on the
461 firewall/router and is widely supported by hardware and software
462 filters. The Linux Netfilter/iptables firewall software offers
463 the --syn convenience option to implement this stateless
464 approach. When stateless firewall rules such as this are in
465 place, SYN ping probes (-PS) are likely to be blocked when sent
466 to closed target ports. In such cases, the ACK probe shines as
467 it cuts right through these rules.
468 Another common type of firewall uses stateful rules that drop
470 unexpected packets. This feature was initially found mostly on
471 high-end firewalls, though it has become much more common over
472 the years. The Linux Netfilter/iptables system supports this
473 through the --state option, which categorizes packets based on
474 connection state. A SYN probe is more likely to work against
475 such a system, as unexpected ACK packets are generally
476 recognized as bogus and dropped. A solution to this quandary is
477 to send both SYN and ACK probes by specifying -PS and -PA.
478 -PU [portlist] (UDP Ping)
480 Another host discovery option is the UDP ping, which sends an
481 empty (unless --data-length is specified) UDP packet to the
482 given ports. The portlist takes the same format as with the
483 previously discussed -PS and -PA options. If no ports are
484 specified, the default is 31338. This default can be configured
485 at compile-time by changing DEFAULT_UDP_PROBE_PORT in nmap.h. A
486 highly uncommon port is used by default because sending to open
487 ports is often undesirable for this particular scan type.
488 Upon hitting a closed port on the target machine, the UDP probe
490 should elicit an ICMP port unreachable packet in return. This
491 signifies to Nmap that the machine is up and available. Many
492 other types of ICMP errors, such as host/network unreachables or
493 TTL exceeded are indicative of a down or unreachable host. A
494 lack of response is also interpreted this way. If an open port
495 is reached, most services simply ignore the empty packet and
496 fail to return any response. This is why the default probe port
497 is 31338, which is highly unlikely to be in use. A few services,
498 such as chargen, will respond to an empty UDP packet, and thus
499 disclose to Nmap that the machine is available.
500 The primary advantage of this scan type is that it bypasses
502 firewalls and filters that only screen TCP. For example, I once
503 owned a Linksys BEFW11S4 wireless broadband router. The external
504 interface of this device filtered all TCP ports by default, but
505 UDP probes would still elicit port unreachable messages and thus
506 give away the device.
507 -PE; -PP; -PM (ICMP Ping Types)
509 In addition to the unusual TCP and UDP host discovery types
510 discussed previously, Nmap can send the standard packets sent by
511 the ubiquitous ping program. Nmap sends an ICMP type 8 (echo
512 request) packet to the target IP addresses, expecting a type 0
513 (Echo Reply) in return from available hosts. Unfortunately for
514 network explorers, many hosts and firewalls now block these
515 packets, rather than responding as required by [1]RFC 1122. For
516 this reason, ICMP-only scans are rarely reliable enough against
517 unknown targets over the Internet. But for system administrators
518 monitoring an internal network, they can be a practical and
519 efficient approach. Use the -PE option to enable this echo
520 request behavior.
521 While echo request is the standard ICMP ping query, Nmap does
523 not stop there. The ICMP standard ([2]RFC 792) also specifies
524 timestamp request, information request, and address mask request
525 packets as codes 13, 15, and 17, respectively. While the
526 ostensible purpose for these queries is to learn information
527 such as address masks and current times, they can easily be used
528 for host discovery. A system that replies is up and available.
529 Nmap does not currently implement information request packets,
530 as they are not widely supported. RFC 1122 insists that “a host
531 SHOULD NOT implement these messages”. Timestamp and address mask
532 queries can be sent with the -PP and -PM options, respectively.
533 A timestamp reply (ICMP code 14) or address mask reply (code 18)
534 discloses that the host is available. These two queries can be
535 valuable when admins specifically block echo request packets
536 while forgetting that other ICMP queries can be used for the
537 same purpose.
538 -PR (ARP Ping)
540 One of the most common Nmap usage scenarios is to scan an
541 ethernet LAN. On most LANs, especially those using
542 RFC1918-blessed private address ranges, the vast majority of IP
543 addresses are unused at any given time. When Nmap tries to send
544 a raw IP packet such as an ICMP echo request, the operating
545 system must determine the destination hardware (ARP) address
546 corresponding to the target IP so that it can properly address
547 the ethernet frame. This is often slow and problematic, since
548 operating systems weren't written with the expectation that they
549 would need to do millions of ARP requests against unavailable
550 hosts in a short time period.
551 ARP scan puts Nmap and its optimized algorithms in charge of ARP
553 requests. And if it gets a response back, Nmap doesn't even need
554 to worry about the IP-based ping packets since it already knows
555 the host is up. This makes ARP scan much faster and more
556 reliable than IP-based scans. So it is done by default when
557 scanning ethernet hosts that Nmap detects are on a local
558 ethernet network. Even if different ping types (such as -PE or
559 -PS) are specified, Nmap uses ARP instead for any of the targets
560 which are on the same LAN. If you absolutely don't want to do an
561 ARP scan, specify --send-ip.
562 -n (No DNS resolution)
564 Tells Nmap to never do reverse DNS resolution on the active IP
565 addresses it finds. Since DNS can be slow even with Nmap's
566 built-in parallel stub resolver, this option can slash scanning
567 times.
568 -R (DNS resolution for all targets)
570 Tells Nmap to always do reverse DNS resolution on the target IP
571 addresses. Normally reverse DNS is only performed against
572 responsive (online) hosts.
573 --system-dns (Use system DNS resolver)
575 By default, Nmap resolves IP addresses by sending queries
576 directly to the name servers configured on your host and then
577 listening for responses. Many requests (often dozens) are
578 performed in parallel to improve performance. Specify this
579 option to use your system resolver instead (one IP at a time via
580 the getnameinfo() call). This is slower and rarely useful unless
581 you find a bug in the Nmap parallel resolver (please let us know
582 if you do). The system resolver is always used for IPv6 scans.
583 --dns-servers <server1[,server2],...> (Servers to use for reverse DNS
585 queries)
586 By default Nmap will try to determine your DNS servers (for rDNS
587 resolution) from your resolv.conf file (UNIX) or the registry
588 (Win32). Alternatively, you may use this option to specify
589 alternate servers. This option is not honored if you are using
590 --system-dns or an IPv6 scan. Using multiple DNS servers is
591 often faster, especially if you choose authoritative servers for
592 your target IP space. This option can also improve stealth, as
593 your requests can be bounced off just about any recursive DNS
594 server on the internet.
595 This option also comes in handy when scanning private networks.
597 Sometimes only a few name servers provide proper rDNS
598 information, and you may not even know where they are. You can
599 scan the network for port 53 (perhaps with version detection),
600 then try Nmap list scans (-sL) specifying each name server one
601 at a time with --dns-servers until you find one which works.
602
604 While Nmap has grown in functionality over the years, it began as an
605 efficient port scanner, and that remains its core function. The simple
606 command nmap target scans more than 1660 TCP ports on the host target.
607 While many port scanners have traditionally lumped all ports into the
608 open or closed states, Nmap is much more granular. It divides ports
609 into six states: open, closed, filtered, unfiltered, open|filtered, or
610 closed|filtered.
611 These states are not intrinsic properties of the port itself, but
613 describe how Nmap sees them. For example, an Nmap scan from the same
614 network as the target may show port 135/tcp as open, while a scan at
615 the same time with the same options from across the Internet might show
616 that port as filtered.
617 The six port states recognized by Nmap
619 open An application is actively accepting TCP connections or UDP
621 packets on this port. Finding these is often the primary goal of
622 port scanning. Security-minded people know that each open port
623 is an avenue for attack. Attackers and pen-testers want to
624 exploit the open ports, while administrators try to close or
625 protect them with firewalls without thwarting legitimate users.
626 Open ports are also interesting for non-security scans because
627 they show services available for use on the network.
628 closed A closed port is accessible (it receives and responds to Nmap
630 probe packets), but there is no application listening on it.
631 They can be helpful in showing that a host is up on an IP
632 address (host discovery, or ping scanning), and as part of OS
633 detection. Because closed ports are reachable, it may be worth
634 scanning later in case some open up. Administrators may want to
635 consider blocking such ports with a firewall. Then they would
636 appear in the filtered state, discussed next.
637 filtered
639 Nmap cannot determine whether the port is open because packet
640 filtering prevents its probes from reaching the port. The
641 filtering could be from a dedicated firewall device, router
642 rules, or host-based firewall software. These ports frustrate
643 attackers because they provide so little information. Sometimes
644 they respond with ICMP error messages such as type 3 code 13
645 (destination unreachable: communication administratively
646 prohibited), but filters that simply drop probes without
647 responding are far more common. This forces Nmap to retry
648 several times just in case the probe was dropped due to network
649 congestion rather than filtering. This slows down the scan
650 dramatically.
651 unfiltered
653 The unfiltered state means that a port is accessible, but Nmap
654 is unable to determine whether it is open or closed. Only the
655 ACK scan, which is used to map firewall rulesets, classifies
656 ports into this state. Scanning unfiltered ports with other scan
657 types such as Window scan, SYN scan, or FIN scan, may help
658 resolve whether the port is open.
659 open|filtered
661 Nmap places ports in this state when it is unable to determine
662 whether a port is open or filtered. This occurs for scan types
663 in which open ports give no response. The lack of response could
664 also mean that a packet filter dropped the probe or any response
665 it elicited. So Nmap does not know for sure whether the port is
666 open or being filtered. The UDP, IP Protocol, FIN, Null, and
667 Xmas scans classify ports this way.
668 closed|filtered
670 This state is used when Nmap is unable to determine whether a
671 port is closed or filtered. It is only used for the IPID Idle
672 scan.
673
675 As a novice performing automotive repair, I can struggle for hours
676 trying to fit my rudimentary tools (hammer, duct tape, wrench, etc.) to
677 the task at hand. When I fail miserably and tow my jalopy to a real
678 mechanic, he invariably fishes around in a huge tool chest until
679 pulling out the perfect gizmo which makes the job seem effortless. The
680 art of port scanning is similar. Experts understand the dozens of scan
681 techniques and choose the appropriate one (or combination) for a given
682 task. Inexperienced users and script kiddies, on the other hand, try to
683 solve every problem with the default SYN scan. Since Nmap is free, the
684 only barrier to port scanning mastery is knowledge. That certainly
685 beats the automotive world, where it may take great skill to determine
686 that you need a strut spring compressor, then you still have to pay
687 thousands of dollars for it.
688 Most of the scan types are only available to privileged users. This is
690 because they send and receive raw packets, which requires root access
691 on UNIX systems. Using an administrator account on Windows is
692 recommended, though Nmap sometimes works for unprivileged users on that
693 platform when WinPcap has already been loaded into the OS. Requiring
694 root privileges was a serious limitation when Nmap was released in
695 1997, as many users only had access to shared shell accounts. Now, the
696 world is different. Computers are cheaper, far more people have
697 always-on direct Internet access, and desktop UNIX systems (including
698 Linux and MAC OS X) are prevalent. A Windows version of Nmap is now
699 available, allowing it to run on even more desktops. For all these
700 reasons, users have less need to run Nmap from limited shared shell
701 accounts. This is fortunate, as the privileged options make Nmap far
702 more powerful and flexible.
703 While Nmap attempts to produce accurate results, keep in mind that all
705 of its insights are based on packets returned by the target machines
706 (or firewalls in front of them). Such hosts may be untrustworthy and
707 send responses intended to confuse or mislead Nmap. Much more common
708 are non-RFC-compliant hosts that do not respond as they should to Nmap
709 probes. FIN, Null, and Xmas scans are particularly susceptible to this
710 problem. Such issues are specific to certain scan types and so are
711 discussed in the individual scan type entries.
712 This section documents the dozen or so port scan techniques supported
714 by Nmap. Only one method may be used at a time, except that UDP scan
715 (-sU) may be combined with any one of the TCP scan types. As a memory
716 aid, port scan type options are of the form -sC, where C is a prominent
717 character in the scan name, usually the first. The one exception to
718 this is the deprecated FTP bounce scan (-b). By default, Nmap performs
719 a SYN Scan, though it substitutes a connect scan if the user does not
720 have proper privileges to send raw packets (requires root access on
721 UNIX) or if IPv6 targets were specified. Of the scans listed in this
722 section, unprivileged users can only execute connect and ftp bounce
723 scans.
724 -sS (TCP SYN scan)
726 SYN scan is the default and most popular scan option for good
727 reasons. It can be performed quickly, scanning thousands of
728 ports per second on a fast network not hampered by intrusive
729 firewalls. SYN scan is relatively unobtrusive and stealthy,
730 since it never completes TCP connections. It also works against
731 any compliant TCP stack rather than depending on idiosyncrasies
732 of specific platforms as Nmap's Fin/Null/Xmas, Maimon and Idle
733 scans do. It also allows clear, reliable differentiation between
734 the open, closed, and filtered states.
735 This technique is often referred to as half-open scanning,
737 because you don't open a full TCP connection. You send a SYN
738 packet, as if you are going to open a real connection and then
739 wait for a response. A SYN/ACK indicates the port is listening
740 (open), while a RST (reset) is indicative of a non-listener. If
741 no response is received after several retransmissions, the port
742 is marked as filtered. The port is also marked filtered if an
743 ICMP unreachable error (type 3, code 1,2, 3, 9, 10, or 13) is
744 received.
745 -sT (TCP connect scan)
747 TCP connect scan is the default TCP scan type when SYN scan is
748 not an option. This is the case when a user does not have raw
749 packet privileges or is scanning IPv6 networks. Instead of
750 writing raw packets as most other scan types do, Nmap asks the
751 underlying operating system to establish a connection with the
752 target machine and port by issuing the connect() system call.
753 This is the same high-level system call that web browsers, P2P
754 clients, and most other network-enabled applications use to
755 establish a connection. It is part of a programming interface
756 known as the Berkeley Sockets API. Rather than read raw packet
757 responses off the wire, Nmap uses this API to obtain status
758 information on each connection attempt.
759 When SYN scan is available, it is usually a better choice. Nmap
761 has less control over the high level connect() call than with
762 raw packets, making it less efficient. The system call completes
763 connections to open target ports rather than performing the
764 half-open reset that SYN scan does. Not only does this take
765 longer and require more packets to obtain the same information,
766 but target machines are more likely to log the connection. A
767 decent IDS will catch either, but most machines have no such
768 alarm system. Many services on your average UNIX system will add
769 a note to syslog, and sometimes a cryptic error message, when
770 Nmap connects and then closes the connection without sending
771 data. Truly pathetic services crash when this happens, though
772 that is uncommon. An administrator who sees a bunch of
773 connection attempts in her logs from a single system should know
774 that she has been connect scanned.
775 -sU (UDP scans)
777 While most popular services on the Internet run over the TCP
778 protocol, [3]UDP services are widely deployed. DNS, SNMP, and
779 DHCP (registered ports 53, 161/162, and 67/68) are three of the
780 most common. Because UDP scanning is generally slower and more
781 difficult than TCP, some security auditors ignore these ports.
782 This is a mistake, as exploitable UDP services are quite common
783 and attackers certainly don't ignore the whole protocol.
784 Fortunately, Nmap can help inventory UDP ports.
785 UDP scan is activated with the -sU option. It can be combined
787 with a TCP scan type such as SYN scan (-sS) to check both
788 protocols during the same run.
789 UDP scan works by sending an empty (no data) UDP header to every
791 targeted port. If an ICMP port unreachable error (type 3, code
792 3) is returned, the port is closed. Other ICMP unreachable
793 errors (type 3, codes 1, 2, 9, 10, or 13) mark the port as
794 filtered. Occasionally, a service will respond with a UDP
795 packet, proving that it is open. If no response is received
796 after retransmissions, the port is classified as open|filtered.
797 This means that the port could be open, or perhaps packet
798 filters are blocking the communication. Versions scan (-sV) can
799 be used to help differentiate the truly open ports from the
800 filtered ones.
801 A big challenge with UDP scanning is doing it quickly. Open and
803 filtered ports rarely send any response, leaving Nmap to time
804 out and then conduct retransmissions just in case the probe or
805 response were lost. Closed ports are often an even bigger
806 problem. They usually send back an ICMP port unreachable error.
807 But unlike the RST packets sent by closed TCP ports in response
808 to a SYN or connect scan, many hosts rate limit ICMP port
809 unreachable messages by default. Linux and Solaris are
810 particularly strict about this. For example, the Linux 2.4.20
811 kernel limits destination unreachable messages to one per second
812 (in net/ipv4/icmp.c).
813 Nmap detects rate limiting and slows down accordingly to avoid
815 flooding the network with useless packets that the target
816 machine will drop. Unfortunately, a Linux-style limit of one
817 packet per second makes a 65,536-port scan take more than 18
818 hours. Ideas for speeding your UDP scans up include scanning
819 more hosts in parallel, doing a quick scan of just the popular
820 ports first, scanning from behind the firewall, and using
821 --host-timeout to skip slow hosts.
822 -sN; -sF; -sX (TCP Null, FIN, and Xmas scans)
824 These three scan types (even more are possible with the
825 --scanflags option described in the next section) exploit a
826 subtle loophole in the [4]TCP RFC to differentiate between open
827 and closed ports. Page 65 says that “if the [destination] port
828 state is CLOSED .... an incoming segment not containing a RST
829 causes a RST to be sent in response.” Then the next page
830 discusses packets sent to open ports without the SYN, RST, or
831 ACK bits set, stating that: “you are unlikely to get here, but
832 if you do, drop the segment, and return.”
833 When scanning systems compliant with this RFC text, any packet
835 not containing SYN, RST, or ACK bits will result in a returned
836 RST if the port is closed and no response at all if the port is
837 open. As long as none of those three bits are included, any
838 combination of the other three (FIN, PSH, and URG) are OK. Nmap
839 exploits this with three scan types:
840 Null scan (-sN)
842 Does not set any bits (tcp flag header is 0)
843 FIN scan (-sF)
845 Sets just the TCP FIN bit.
846 Xmas scan (-sX)
848 Sets the FIN, PSH, and URG flags, lighting the packet up
849 like a Christmas tree.
850 These three scan types are exactly the same in behavior except
852 for the TCP flags set in probe packets. If a RST packet is
853 received, the port is considered closed, while no response means
854 it is open|filtered. The port is marked filtered if an ICMP
855 unreachable error (type 3, code 1, 2, 3, 9, 10, or 13) is
856 received.
857 The key advantage to these scan types is that they can sneak
859 through certain non-stateful firewalls and packet filtering
860 routers. Another advantage is that these scan types are a little
861 more stealthy than even a SYN scan. Don't count on this though
862 -- most modern IDS products can be configured to detect them.
863 The big downside is that not all systems follow RFC 793 to the
864 letter. A number of systems send RST responses to the probes
865 regardless of whether the port is open or not. This causes all
866 of the ports to be labeled closed. Major operating systems that
867 do this are Microsoft Windows, many Cisco devices, BSDI, and IBM
868 OS/400. This scan does work against most UNIX-based systems
869 though. Another downside of these scans is that they can't
870 distinguish open ports from certain filtered ones, leaving you
871 with the response open|filtered.
872 -sA (TCP ACK scan)
874 This scan is different than the others discussed so far in that
875 it never determines open (or even open|filtered) ports. It is
876 used to map out firewall rulesets, determining whether they are
877 stateful or not and which ports are filtered.
878 The ACK scan probe packet has only the ACK flag set (unless you
880 use --scanflags). When scanning unfiltered systems, open and
881 closed ports will both return a RST packet. Nmap then labels
882 them as unfiltered, meaning that they are reachable by the ACK
883 packet, but whether they are open or closed is undetermined.
884 Ports that don't respond, or send certain ICMP error messages
885 back (type 3, code 1, 2, 3, 9, 10, or 13), are labeled filtered.
886 -sW (TCP Window scan)
888 Window scan is exactly the same as ACK scan except that it
889 exploits an implementation detail of certain systems to
890 differentiate open ports from closed ones, rather than always
891 printing unfiltered when a RST is returned. It does this by
892 examining the TCP Window field of the RST packets returned. On
893 some systems, open ports use a positive window size (even for
894 RST packets) while closed ones have a zero window. So instead of
895 always listing a port as unfiltered when it receives a RST back,
896 Window scan lists the port as open or closed if the TCP Window
897 value in that reset is positive or zero, respectively.
898 This scan relies on an implementation detail of a minority of
900 systems out on the Internet, so you can't always trust it.
901 Systems that don't support it will usually return all ports
902 closed. Of course, it is possible that the machine really has no
903 open ports. If most scanned ports are closed but a few common
904 port numbers (such as 22, 25, 53) are filtered, the system is
905 most likely susceptible. Occasionally, systems will even show
906 the exact opposite behavior. If your scan shows 1000 open ports
907 and 3 closed or filtered ports, then those three may very well
908 be the truly open ones.
909 -sM (TCP Maimon scan)
911 The Maimon scan is named after its discoverer, Uriel Maimon. He
912 described the technique in Phrack Magazine issue #49 (November
913 1996). Nmap, which included this technique, was released two
914 issues later. This technique is exactly the same as Null, FIN,
915 and Xmas scans, except that the probe is FIN/ACK. According to
916 RFC 793 (TCP), a RST packet should be generated in response to
917 such a probe whether the port is open or closed. However, Uriel
918 noticed that many BSD-derived systems simply drop the packet if
919 the port is open.
920 --scanflags (Custom TCP scan)
922 Truly advanced Nmap users need not limit themselves to the
923 canned scan types offered. The --scanflags option allows you to
924 design your own scan by specifying arbitrary TCP flags. Let your
925 creative juices flow, while evading intrusion detection systems
926 whose vendors simply paged through the Nmap man page adding
927 specific rules!
928 The --scanflags argument can be a numerical flag value such as 9
930 (PSH and FIN), but using symbolic names is easier. Just mash
931 together any combination of URG, ACK, PSH, RST, SYN, and FIN.
932 For example, --scanflags URGACKPSHRSTSYNFIN sets everything,
933 though it's not very useful for scanning. The order these are
934 specified in is irrelevant.
935 In addition to specifying the desired flags, you can specify a
937 TCP scan type (such as -sA or -sF). That base type tells Nmap
938 how to interpret responses. For example, a SYN scan considers
939 no-response to indicate a filtered port, while a FIN scan treats
940 the same as open|filtered. Nmap will behave the same way it does
941 for the base scan type, except that it will use the TCP flags
942 you specify instead. If you don't specify a base type, SYN scan
943 is used.
944 -sI <zombie host[:probeport]> (Idlescan)
946 This advanced scan method allows for a truly blind TCP port scan
947 of the target (meaning no packets are sent to the target from
948 your real IP address). Instead, a unique side-channel attack
949 exploits predictable IP fragmentation ID sequence generation on
950 the zombie host to glean information about the open ports on the
951 target. IDS systems will display the scan as coming from the
952 zombie machine you specify (which must be up and meet certain
953 criteria). This fascinating scan type is too complex to fully
954 describe in this reference guide, so I wrote and posted an
955 informal paper with full details at
956 http://insecure.org/nmap/idlescan.html.
957 Besides being extraordinarily stealthy (due to its blind
959 nature), this scan type permits mapping out IP-based trust
960 relationships between machines. The port listing shows open
961 ports from the perspective of the zombie host. So you can try
962 scanning a target using various zombies that you think might be
963 trusted (via router/packet filter rules).
964 You can add a colon followed by a port number to the zombie host
966 if you wish to probe a particular port on the zombie for IPID
967 changes. Otherwise Nmap will use the port it uses by default for
968 tcp pings (80).
969 -sO (IP protocol scan)
971 IP Protocol scan allows you to determine which IP protocols
972 (TCP, ICMP, IGMP, etc.) are supported by target machines. This
973 isn't technically a port scan, since it cycles through IP
974 protocol numbers rather than TCP or UDP port numbers. Yet it
975 still uses the -p option to select scanned protocol numbers,
976 reports its results within the normal port table format, and
977 even uses the same underlying scan engine as the true port
978 scanning methods. So it is close enough to a port scan that it
979 belongs here.
980 Besides being useful in its own right, protocol scan
982 demonstrates the power of open source software. While the
983 fundamental idea is pretty simple, I had not thought to add it
984 nor received any requests for such functionality. Then in the
985 summer of 2000, Gerhard Rieger conceived the idea, wrote an
986 excellent patch implementing it, and sent it to the nmap-hackers
987 mailing list. I incorporated that patch into the Nmap tree and
988 released a new version the next day. Few pieces of commercial
989 software have users enthusiastic enough to design and contribute
990 their own improvements!
991 Protocol scan works in a similar fashion to UDP scan. Instead of
993 iterating through the port number field of a UDP packet, it
994 sends IP packet headers and iterates through the 8-bit IP
995 protocol field. The headers are usually empty, containing no
996 data and not even the proper header for the claimed protocol.
997 The three exceptions are TCP, UDP, and ICMP. A proper protocol
998 header for those is included since some systems won't send them
999 otherwise and because Nmap already has functions to create them.
1000 Instead of watching for ICMP port unreachable messages, protocol
1001 scan is on the lookout for ICMP protocol unreachable messages.
1002 If Nmap receives any response in any protocol from the target
1003 host, Nmap marks that protocol as open. An ICMP protocol
1004 unreachable error (type 3, code 2) causes the protocol to be
1005 marked as closed Other ICMP unreachable errors (type 3, code 1,
1006 3, 9, 10, or 13) cause the protocol to be marked filtered
1007 (though they prove that ICMP is open at the same time). If no
1008 response is received after retransmissions, the protocol is
1009 marked open|filtered
1010 -b <ftp relay host> (FTP bounce scan)
1012 An interesting feature of the FTP protocol ([5]RFC 959) is
1013 support for so-called proxy ftp connections. This allows a user
1014 to connect to one FTP server, then ask that files be sent to a
1015 third-party server. Such a feature is ripe for abuse on many
1016 levels, so most servers have ceased supporting it. One of the
1017 abuses this feature allows is causing the FTP server to port
1018 scan other hosts. Simply ask the FTP server to send a file to
1019 each interesting port of a target host in turn. The error
1020 message will describe whether the port is open or not. This is a
1021 good way to bypass firewalls because organizational FTP servers
1022 are often placed where they have more access to other internal
1023 hosts than any old Internet host would. Nmap supports ftp bounce
1024 scan with the -b option. It takes an argument of the form
1025 username:password@server:port. Server is the name or IP address
1026 of a vulnerable FTP server. As with a normal URL, you may omit
1027 username:password, in which case anonymous login credentials
1028 (user: anonymous password:-wwwuser@) are used. The port number
1029 (and preceding colon) may be omitted as well, in which case the
1030 default FTP port (21) on server is used.
1031 This vulnerability was widespread in 1997 when Nmap was
1033 released, but has largely been fixed. Vulnerable servers are
1034 still around, so it is worth trying when all else fails. If
1035 bypassing a firewall is your goal, scan the target network for
1036 open port 21 (or even for any ftp services if you scan all ports
1037 with version detection), then try a bounce scan using each. Nmap
1038 will tell you whether the host is vulnerable or not. If you are
1039 just trying to cover your tracks, you don't need to (and, in
1040 fact, shouldn't) limit yourself to hosts on the target network.
1041 Before you go scanning random Internet addresses for vulnerable
1042 FTP servers, consider that sysadmins may not appreciate you
1043 abusing their servers in this way.
1044
1046 In addition to all of the scan methods discussed previously, Nmap
1047 offers options for specifying which ports are scanned and whether the
1048 scan order is randomized or sequential. By default, Nmap scans all
1049 ports up to and including 1024 as well as higher numbered ports listed
1050 in the nmap-services file for the protocol(s) being scanned.
1051 -p <port ranges> (Only scan specified ports)
1053 This option specifies which ports you want to scan and overrides
1054 the default. Individual port numbers are OK, as are ranges
1055 separated by a hyphen (e.g. 1-1023). The beginning and/or end
1056 values of a range may be omitted, causing Nmap to use 1 and
1057 65535, respectively. So you can specify -p- to scan ports from 1
1058 through 65535. Scanning port zero is allowed if you specify it
1059 explicitly. For IP protocol scanning (-sO), this option
1060 specifies the protocol numbers you wish to scan for (0-255).
1061 When scanning both TCP and UDP ports, you can specify a
1063 particular protocol by preceding the port numbers by T: or U:.
1064 The qualifier lasts until you specify another qualifier. For
1065 example, the argument -p U:53,111,137,T:21-25,80,139,8080 would
1066 scan UDP ports 53,111,and 137, as well as the listed TCP ports.
1067 Note that to scan both UDP & TCP, you have to specify -sU and at
1068 least one TCP scan type (such as -sS, -sF, or -sT). If no
1069 protocol qualifier is given, the port numbers are added to all
1070 protocol lists.
1071 -F (Fast (limited port) scan)
1073 Specifies that you only wish to scan for ports listed in the
1074 nmap-services file which comes with nmap (or the protocols file
1075 for -sO). This is much faster than scanning all 65535 ports on a
1076 host. Because this list contains so many TCP ports (more than
1077 1200), the speed difference from a default TCP scan (about 1650
1078 ports) isn't dramatic. The difference can be enormous if you
1079 specify your own tiny nmap-services file using the --datadir
1080 option.
1081 -r (Don't randomize ports)
1083 By default, Nmap randomizes the scanned port order (except that
1084 certain commonly accessible ports are moved near the beginning
1085 for efficiency reasons). This randomization is normally
1086 desirable, but you can specify -r for sequential port scanning
1087 instead.
1088
1090 Point Nmap at a remote machine and it might tell you that ports 25/tcp,
1091 80/tcp, and 53/udp are open. Using its nmap-services database of about
1092 2,200 well-known services, Nmap would report that those ports probably
1093 correspond to a mail server (SMTP), web server (HTTP), and name server
1094 (DNS) respectively. This lookup is usually accurate -- the vast
1095 majority of daemons listening on TCP port 25 are, in fact, mail
1096 servers. However, you should not bet your security on this! People can
1097 and do run services on strange ports.
1098 Even if Nmap is right, and the hypothetical server above is running
1100 SMTP, HTTP, and DNS servers, that is not a lot of information. When
1101 doing vulnerability assessments (or even simple network inventories) of
1102 your companies or clients, you really want to know which mail and DNS
1103 servers and versions are running. Having an accurate version number
1104 helps dramatically in determining which exploits a server is vulnerable
1105 to. Version detection helps you obtain this information.
1106 After TCP and/or UDP ports are discovered using one of the other scan
1108 methods, version detection interrogates those ports to determine more
1109 about what is actually running. The nmap-service-probes database
1110 contains probes for querying various services and match expressions to
1111 recognize and parse responses. Nmap tries to determine the service
1112 protocol (e.g. ftp, ssh, telnet, http), the application name (e.g. ISC
1113 Bind, Apache httpd, Solaris telnetd), the version number, hostname,
1114 device type (e.g. printer, router), the OS family (e.g. Windows, Linux)
1115 and sometimes miscellaneous details like whether an X server is open to
1116 connections, the SSH protocol version, or the KaZaA user name). Of
1117 course, most services don't provide all of this information. If Nmap
1118 was compiled with OpenSSL support, it will connect to SSL servers to
1119 deduce the service listening behind that encryption layer. When RPC
1120 services are discovered, the Nmap RPC grinder (-sR) is automatically
1121 used to determine the RPC program and version numbers. Some UDP ports
1122 are left in the open|filtered state after a UDP port scan is unable to
1123 determine whether the port is open or filtered. Version detection will
1124 try to elicit a response from these ports (just as it does with open
1125 ports), and change the state to open if it succeeds. open|filtered TCP
1126 ports are treated the same way. Note that the Nmap -A option enables
1127 version detection among other things. A paper documenting the workings,
1128 usage, and customization of version detection is available at
1129 http://insecure.org/nmap/vscan/.
1130 When Nmap receives responses from a service but cannot match them to
1132 its database, it prints out a special fingerprint and a URL for you to
1133 submit if to if you know for sure what is running on the port. Please
1134 take a couple minutes to make the submission so that your find can
1135 benefit everyone. Thanks to these submissions, Nmap has about 3,000
1136 pattern matches for more than 350 protocols such as smtp, ftp, http,
1137 etc.
1138 Version detection is enabled and controlled with the following options:
1140 -sV (Version detection)
1142 Enables version detection, as discussed above. Alternatively,
1143 you can use -A to enable both OS detection and version
1144 detection.
1145 --allports (Don't exclude any ports from version detection)
1147 By default, Nmap version detection skips TCP port 9100 because
1148 some printers simply print anything sent to that port, leading
1149 to dozens of pages of HTTP get requests, binary SSL session
1150 requests, etc. This behavior can be changed by modifying or
1151 removing the Exclude directive in nmap-service-probes, or you
1152 can specify --allports to scan all ports regardless of any
1153 Exclude directive.
1154 --version-intensity <intensity> (Set version scan intensity)
1156 When performing a version scan (-sV), nmap sends a series of
1157 probes, each of which is assigned a rarity value between 1 and
1158 9. The lower-numbered probes are effective against a wide
1159 variety of common services, while the higher numbered ones are
1160 rarely useful. The intensity level specifies which probes should
1161 be applied. The higher the number, the more likely it is the
1162 service will be correctly identified. However, high intensity
1163 scans take longer. The intensity must be between 0 and 9. The
1164 default is 7. When a probe is registered to the target port via
1165 the nmap-service-probesports directive, that probe is tried
1166 regardless of intensity level. This ensures that the DNS probes
1167 will always be attempted against any open port 53, the SSL probe
1168 will be done against 443, etc.
1169 --version-light (Enable light mode)
1171 This is a convenience alias for --version-intensity 2. This
1172 light mode makes version scanning much faster, but it is
1173 slightly less likely to identify services.
1174 --version-all (Try every single probe)
1176 An alias for --version-intensity 9, ensuring that every single
1177 probe is attempted against each port.
1178 --version-trace (Trace version scan activity)
1180 This causes Nmap to print out extensive debugging info about
1181 what version scanning is doing. It is a subset of what you get
1182 with --packet-trace.
1183 -sR (RPC scan)
1185 This method works in conjunction with the various port scan
1186 methods of Nmap. It takes all the TCP/UDP ports found open and
1187 floods them with SunRPC program NULL commands in an attempt to
1188 determine whether they are RPC ports, and if so, what program
1189 and version number they serve up. Thus you can effectively
1190 obtain the same info as rpcinfo -p even if the target's
1191 portmapper is behind a firewall (or protected by TCP wrappers).
1192 Decoys do not currently work with RPC scan. This is
1193 automatically enabled as part of version scan (-sV) if you
1194 request that. As version detection includes this and is much
1195 more comprehensive, -sR is rarely needed.
1196
1198 One of Nmap's best-known features is remote OS detection using TCP/IP
1199 stack fingerprinting. Nmap sends a series of TCP and UDP packets to the
1200 remote host and examines practically every bit in the responses. After
1201 performing dozens of tests such as TCP ISN sampling, TCP options
1202 support and ordering, IPID sampling, and the initial window size check,
1203 Nmap compares the results to its nmap-os-fingerprints database of more
1204 than 1500 known OS fingerprints and prints out the OS details if there
1205 is a match. Each fingerprint includes a freeform textual description of
1206 the OS, and a classification which provides the vendor name (e.g. Sun),
1207 underlying OS (e.g. Solaris), OS generation (e.g. 10), and device type
1208 (general purpose, router, switch, game console, etc).
1209 If Nmap is unable to guess the OS of a machine, and conditions are good
1211 (e.g. at least one open port and one closed port were found), Nmap will
1212 provide a URL you can use to submit the fingerprint if you know (for
1213 sure) the OS running on the machine. By doing this you contribute to
1214 the pool of operating systems known to Nmap and thus it will be more
1215 accurate for everyone.
1216 OS detection enables several other tests which make use of information
1218 that is gathered during the process anyway. One of these is uptime
1219 measurement, which uses the TCP timestamp option (RFC 1323) to guess
1220 when a machine was last rebooted. This is only reported for machines
1221 which provide this information. Another is TCP Sequence Predictability
1222 Classification. This measures approximately how hard it is to establish
1223 a forged TCP connection against the remote host. It is useful for
1224 exploiting source-IP based trust relationships (rlogin, firewall
1225 filters, etc) or for hiding the source of an attack. This sort of
1226 spoofing is rarely performed any more, but many machines are still
1227 vulnerable to it. The actual difficulty number is based on statistical
1228 sampling and may fluctuate. It is generally better to use the English
1229 classification such as “worthy challenge” or “trivial joke”. This is
1230 only reported in normal output in verbose (-v) mode. When verbose mode
1231 is enabled along with -O, IPID Sequence Generation is also reported.
1232 Most machines are in the “incremental” class, which means that they
1233 increment the ID field in the IP header for each packet they send. This
1234 makes them vulnerable to several advanced information gathering and
1235 spoofing attacks.
1236 A paper documenting the workings, usage, and customization of OS
1238 detection is available at http://insecure.org/nmap/osdetect/.
1239 OS detection is enabled and controlled with the following options:
1241 -O (Enable OS detection)
1243 Enables OS detection, as discussed above. Alternatively, you can
1244 use -A to enable both OS detection and version detection. 2nd
1245 generation OS detection is tried first. If that fails, Nmap will
1246 either print out the host fingerprint and ask you to submit it
1247 (if you are certain about what the target host is running), or
1248 Nmap will fall back to the 1st generation OS detection system in
1249 case its larger database has a match.
1250 -O2 (2nd Generation OS Detection Only)
1252 Enables 2nd generation OS detection, but never falls back to the
1253 old (1st generation) system, even if it fails to find any match.
1254 This saves time and can reduce the number of packets sent to
1255 each target.
1256 -O1 (1nd Generation OS Detection Only)
1258 Tells Nmap to only use the old OS detection system. If -O2 just
1259 gives you a fingerprint to submit, but you don't know what OS
1260 the target is running, try -O1. But in that case, don't submit
1261 the fingerprint as you don't know for sure whether -O1 guess
1262 correctly. If it was perfect, we wouldn't have bothered to
1263 create -O2.
1264 This option, and all other vestiges of the old OS detection
1266 system, will likely be removed in late 2006 or in 2007.
1267 --osscan-limit (Limit OS detection to promising targets)
1269 OS detection is far more effective if at least one open and one
1270 closed TCP port are found. Set this option and Nmap will not
1271 even try OS detection against hosts that do not meet this
1272 criteria. This can save substantial time, particularly on -P0
1273 scans against many hosts. It only matters when OS detection is
1274 requested with -O or -A.
1275 --osscan-guess; --fuzzy (Guess OS detection results)
1277 When Nmap is unable to detect a perfect OS match, it sometimes
1278 offers up near-matches as possibilities. The match has to be
1279 very close for Nmap to do this by default. Either of these
1280 (equivalent) options make Nmap guess more aggressively. Nmap
1281 will still tell you when an imperfect match is printed and
1282 display its confidence level (percentage) for each guess.
1283 --max-os-tries (Set the maximum number of OS detection tries against a
1285 target)
1286 When Nmap performs OS detection against a target and fails to
1287 find a perfect match, it usually repeats the attempt. By
1288 default, Nmap tries five times if conditions are favorable for
1289 OS fingerprint submission, and twice when conditions aren't so
1290 good. Specifying a lower --max-os-tries value (such as 1) speeds
1291 Nmap up, though you miss out on retries which could potentially
1292 identify the OS. Alternatively, a high value may be set to allow
1293 even more retries when conditions are favorable. This is rarely
1294 done, except to generate better fingerprints for submission and
1295 integration into the Nmap OS database. This option only affects
1296 second generation OS detection (-O2, the default) and not the
1297 old system (-O1).
1298
1300 One of my highest Nmap development priorities has always been
1301 performance. A default scan (nmap hostname) of a host on my local
1302 network takes a fifth of a second. That is barely enough time to blink,
1303 but adds up when you are scanning tens or hundreds of thousands of
1304 hosts. Moreover, certain scan options such as UDP scanning and version
1305 detection can increase scan times substantially. So can certain
1306 firewall configurations, particularly response rate limiting. While
1307 Nmap utilizes parallelism and many advanced algorithms to accelerate
1308 these scans, the user has ultimate control over how Nmap runs. Expert
1309 users carefully craft Nmap commands to obtain only the information they
1310 care about while meeting their time constraints.
1311 Techniques for improving scan times include omitting non-critical
1313 tests, and upgrading to the latest version of Nmap (performance
1314 enhancements are made frequently). Optimizing timing parameters can
1315 also make a substantial difference. Those options are listed below.
1316 Some options accept a time parameter. This is specified in milliseconds
1318 by default, though you can append ‘s’, ‘m’, or ‘h’ to the value to
1319 specify seconds, minutes, or hours. So the --host-timeout arguments
1320 900000, 900s, and 15m all do the same thing.
1321 --min-hostgroup <numhosts>; --max-hostgroup <numhosts> (Adjust parallel
1323 scan group sizes)
1324 Nmap has the ability to port scan or version scan multiple hosts
1325 in parallel. Nmap does this by dividing the target IP space into
1326 groups and then scanning one group at a time. In general, larger
1327 groups are more efficient. The downside is that host results
1328 can't be provided until the whole group is finished. So if Nmap
1329 started out with a group size of 50, the user would not receive
1330 any reports (except for the updates offered in verbose mode)
1331 until the first 50 hosts are completed.
1332 By default, Nmap takes a compromise approach to this conflict.
1334 It starts out with a group size as low as five so the first
1335 results come quickly and then increases the groupsize to as high
1336 as 1024. The exact default numbers depend on the options given.
1337 For efficiency reasons, Nmap uses larger group sizes for UDP or
1338 few-port TCP scans.
1339 When a maximum group size is specified with --max-hostgroup,
1341 Nmap will never exceed that size. Specify a minimum size with
1342 --min-hostgroup and Nmap will try to keep group sizes above that
1343 level. Nmap may have to use smaller groups than you specify if
1344 there are not enough target hosts left on a given interface to
1345 fulfill the specified minimum. Both may be set to keep the group
1346 size within a specific range, though this is rarely desired.
1347 The primary use of these options is to specify a large minimum
1349 group size so that the full scan runs more quickly. A common
1350 choice is 256 to scan a network in Class C sized chunks. For a
1351 scan with many ports, exceeding that number is unlikely to help
1352 much. For scans of just a few port numbers, host group sizes of
1353 2048 or more may be helpful.
1354 --min-parallelism <numprobes>; --max-parallelism <numprobes> (Adjust
1356 probe parallelization)
1357 These options control the total number of probes that may be
1358 outstanding for a host group. They are used for port scanning
1359 and host discovery. By default, Nmap calculates an ever-changing
1360 ideal parallelism based on network performance. If packets are
1361 being dropped, Nmap slows down and allows fewer outstanding
1362 probes. The ideal probe number slowly rises as the network
1363 proves itself worthy. These options place minimum or maximum
1364 bounds on that variable. By default, the ideal parallelism can
1365 drop to 1 if the network proves unreliable and rise to several
1366 hundred in perfect conditions.
1367 The most common usage is to set --min-parallelism to a number
1369 higher than one to speed up scans of poorly performing hosts or
1370 networks. This is a risky option to play with, as setting it too
1371 high may affect accuracy. Setting this also reduces Nmap's
1372 ability to control parallelism dynamically based on network
1373 conditions. A value of ten might be reasonable, though I only
1374 adjust this value as a last resort.
1375 The --max-parallelism option is sometimes set to one to prevent
1377 Nmap from sending more than one probe at a time to hosts. This
1378 can be useful in combination with --scan-delay (discussed
1379 later), although the latter usually serves the purpose well
1380 enough by itself.
1381 --min-rtt-timeout <time>, --max-rtt-timeout <time>,
1383 --initial-rtt-timeout <time> (Adjust probe timeouts)
1384 Nmap maintains a running timeout value for determining how long
1385 it will wait for a probe response before giving up or
1386 retransmitting the probe. This is calculated based on the
1387 response times of previous probes. If the network latency shows
1388 itself to be significant and variable, this timeout can grow to
1389 several seconds. It also starts at a conservative (high) level
1390 and may stay that way for a while when Nmap scans unresponsive
1391 hosts.
1392 Specifying a lower --max-rtt-timeout and --initial-rtt-timeout
1394 than the defaults can cut scan times significantly. This is
1395 particularly true for pingless (-P0) scans, and those against
1396 heavily filtered networks. Don't get too aggressive though. The
1397 scan can end up taking longer if you specify such a low value
1398 that many probes are timing out and retransmitting while the
1399 response is in transit.
1400 If all the hosts are on a local network, 100 milliseconds is a
1402 reasonable aggressive --max-rtt-timeout value. If routing is
1403 involved, ping a host on the network first with the ICMP ping
1404 utility, or with a custom packet crafter such as hping2 that is
1405 more likely to get through a firewall. Look at the maximum round
1406 trip time out of ten packets or so. You might want to double
1407 that for the --initial-rtt-timeout and triple or quadruple it
1408 for the --max-rtt-timeout. I generally do not set the maximum
1409 rtt below 100ms, no matter what the ping times are. Nor do I
1410 exceed 1000ms.
1411 --min-rtt-timeout is a rarely used option that could be useful
1413 when a network is so unreliable that even Nmap's default is too
1414 aggressive. Since Nmap only reduces the timeout down to the
1415 minimum when the network seems to be reliable, this need is
1416 unusual and should be reported as a bug to the nmap-dev mailing
1417 list.
1418 --max-retries <numtries> (Specify the maximum number of port scan probe
1420 retransmissions)
1421 When Nmap receives no response to a port scan probe, it could
1422 mean the port is filtered. Or maybe the probe or response was
1423 simply lost on the network. It is also possible that the target
1424 host has rate limiting enabled that temporarily blocked the
1425 response. So Nmap tries again by retransmitting the initial
1426 probe. If Nmap detects poor network reliability, it may try many
1427 more times before giving up on a port. While this benefits
1428 accuracy, it also lengthen scan times. When performance is
1429 critical, scans may be sped up by limiting the number of
1430 retransmissions allowed. You can even specify --max-retries 0 to
1431 prevent any retransmissions, though that is rarely recommended.
1432 The default (with no -T template) is to allow ten
1434 retransmissions. If a network seems reliable and the target
1435 hosts aren't rate limiting, Nmap usually only does one
1436 retransmission. So most target scans aren't even affected by
1437 dropping --max-retries to a low value such as three. Such values
1438 can substantially speed scans of slow (rate limited) hosts. You
1439 usually lose some information when Nmap gives up on ports early,
1440 though that may be preferable to letting the --host-timeout
1441 expire and losing all information about the target.
1442 --host-timeout <time> (Give up on slow target hosts)
1444 Some hosts simply take a long time to scan. This may be due to
1445 poorly performing or unreliable networking hardware or software,
1446 packet rate limiting, or a restrictive firewall. The slowest few
1447 percent of the scanned hosts can eat up a majority of the scan
1448 time. Sometimes it is best to cut your losses and skip those
1449 hosts initially. Specify --host-timeout with the maximum amount
1450 of time you are willing to wait. I often specify 30m to ensure
1451 that Nmap doesn't waste more than half an hour on a single host.
1452 Note that Nmap may be scanning other hosts at the same time
1453 during that half an hour as well, so it isn't a complete loss. A
1454 host that times out is skipped. No port table, OS detection, or
1455 version detection results are printed for that host.
1456 --scan-delay <time>; --max-scan-delay <time> (Adjust delay between
1458 probes)
1459 This option causes Nmap to wait at least the given amount of
1460 time between each probe it sends to a given host. This is
1461 particularly useful in the case of rate limiting. Solaris
1462 machines (among many others) will usually respond to UDP scan
1463 probe packets with only one ICMP message per second. Any more
1464 than that sent by Nmap will be wasteful. A --scan-delay of 1s
1465 will keep Nmap at that slow rate. Nmap tries to detect rate
1466 limiting and adjust the scan delay accordingly, but it doesn't
1467 hurt to specify it explicitly if you already know what rate
1468 works best.
1469 When Nmap adjusts the scan delay upward to cope with rate
1471 limiting, the scan slows down dramatically. The --max-scan-delay
1472 option specifies the largest delay that Nmap will allow. Setting
1473 this value too low can lead to wasteful packet retransmissions
1474 and possible missed ports when the target implements strict rate
1475 limiting.
1476 Another use of --scan-delay is to evade threshold based
1478 intrusion detection and prevention systems (IDS/IPS).
1479 --defeat-rst-ratelimit
1481 Many hosts have long used rate limiting to reduce the number of
1482 ICMP error messages (such as port-unreachable errors) they send.
1483 Some systems now apply similar rate limits to the RST (reset)
1484 packets they generate. This can slow Nmap down dramatically as
1485 it adjusts its timing to reflect those rate limits. You can tell
1486 Nmap to ignore those rate limits (for port scans such as SYN
1487 scan which don't treat nonresponsive ports as open) by
1488 specifying --defeat-rst-ratelimit.
1489 Using this option can reduce accuracy, as some ports will appear
1491 nonresponse because Nmap didn't wait long enough for a
1492 rate-limited RST response. With a SYN scan, the non-response
1493 results in the port being labeled filtered rather than the
1494 closed state we see when RST packets are received. This optional
1495 is useful when you only care about open ports, and
1496 distinguishing between closed and filtered ports isn't worth the
1497 extra time.
1498 -T <Paranoid|Sneaky|Polite|Normal|Aggressive|Insane> (Set a timing
1500 template)
1501 While the fine grained timing controls discussed in the previous
1502 section are powerful and effective, some people find them
1503 confusing. Moreover, choosing the appropriate values can
1504 sometimes take more time than the scan you are trying to
1505 optimize. So Nmap offers a simpler approach, with six timing
1506 templates. You can specify them with the -T option and their
1507 number (0 - 5) or their name. The template names are paranoid
1508 (0), sneaky (1), polite (2), normal (3), aggressive (4), and
1509 insane (5). The first two are for IDS evasion. Polite mode slows
1510 down the scan to use less bandwidth and target machine
1511 resources. Normal mode is the default and so -T3 does nothing.
1512 Aggressive mode speeds scans up by making the assumption that
1513 you are on a reasonably fast and reliable network. Finally
1514 Insane mode assumes that you are on an extraordinarily fast
1515 network or are willing to sacrifice some accuracy for speed.
1516 These templates allow the user to specify how aggressive they
1518 wish to be, while leaving Nmap to pick the exact timing values.
1519 The templates also make some minor speed adjustments for which
1520 fine grained control options do not currently exist. For
1521 example, -T4 prohibits the dynamic scan delay from exceeding
1522 10ms for TCP ports and -T5 caps that value at 5 milliseconds.
1523 Templates can be used in combination with fine grained controls,
1524 and the fine-grained controls will you specify will take
1525 precedence over the timing template default for that parameter.
1526 I recommend using -T4 when scanning reasonably modern and
1527 reliable networks. Keep that option even when you add fine
1528 grained controls so that you benefit from those extra minor
1529 optimizations that it enables.
1530 If you are on a decent broadband or ethernet connection, I would
1532 recommend always using -T4. Some people love -T5 though it is
1533 too aggressive for my taste. People sometimes specify -T2
1534 because they think it is less likely to crash hosts or because
1535 they consider themselves to be polite in general. They often
1536 don't realize just how slow -T Polite really is. Their scan may
1537 take ten times longer than a default scan. Machine crashes and
1538 bandwidth problems are rare with the default timing options
1539 (-T3) and so I normally recommend that for cautious scanners.
1540 Omitting version detection is far more effective than playing
1541 with timing values at reducing these problems.
1542 While -T0 and -T1 may be useful for avoiding IDS alerts, they
1544 will take an extraordinarily long time to scan thousands of
1545 machines or ports. For such a long scan, you may prefer to set
1546 the exact timing values you need rather than rely on the canned
1547 -T0 and -T1 values.
1548 The main effects of T0 are serializing the scan so only one port
1550 is scanned at a time, and waiting five minutes between sending
1551 each probe. T1 and T2 are similar but they only wait 15 seconds
1552 and 0.4 seconds, respectively, between probes. T3 is Nmap's
1553 default behavior, which includes parallelization. T4 does the
1554 equivalent of --max-rtt-timeout 1250 --initial-rtt-timeout 500
1555 --max-retries 6 and sets the maximum TCP scan delay to 10
1556 milliseconds. T5 does the equivalent of --max-rtt-timeout 300
1557 --min-rtt-timeout 50 --initial-rtt-timeout 250 --max-retries 2
1558 --host-timeout 15m as well as setting the maximum TCP scan delay
1559 to 5ms.
1560
1562 Many Internet pioneers envisioned a global open network with a
1563 universal IP address space allowing virtual connections between any two
1564 nodes. This allows hosts to act as true peers, serving and retrieving
1565 information from each other. People could access all of their home
1566 systems from work, changing the climate control settings or unlocking
1567 the doors for early guests. This vision of universal connectivity has
1568 been stifled by address space shortages and security concerns. In the
1569 early 1990s, organizations began deploying firewalls for the express
1570 purpose of reducing connectivity. Huge networks were cordoned off from
1571 the unfiltered Internet by application proxies, network address
1572 translation, and packet filters. The unrestricted flow of information
1573 gave way to tight regulation of approved communication channels and the
1574 content that passes over them.
1575 Network obstructions such as firewalls can make mapping a network
1577 exceedingly difficult. It will not get any easier, as stifling casual
1578 reconnaissance is often a key goal of implementing the devices.
1579 Nevertheless, Nmap offers many features to help understand these
1580 complex networks, and to verify that filters are working as intended.
1581 It even supports mechanisms for bypassing poorly implemented defenses.
1582 One of the best methods of understanding your network security posture
1583 is to try to defeat it. Place yourself in the mindset of an attacker,
1584 and deploy techniques from this section against your networks. Launch
1585 an FTP bounce scan, Idle scan, fragmentation attack, or try to tunnel
1586 through one of your own proxies.
1587 In addition to restricting network activity, companies are increasingly
1589 monitoring traffic with intrusion detection systems (IDS). All of the
1590 major IDSs ship with rules designed to detect Nmap scans because scans
1591 are sometimes a precursor to attacks. Many of these products have
1592 recently morphed into intrusion prevention systems (IPS) that actively
1593 block traffic deemed malicious. Unfortunately for network
1594 administrators and IDS vendors, reliably detecting bad intentions by
1595 analyzing packet data is a tough problem. Attackers with patience,
1596 skill, and the help of certain Nmap options can usually pass by IDSs
1597 undetected. Meanwhile, administrators must cope with large numbers of
1598 false positive results where innocent activity is misdiagnosed and
1599 alerted on or blocked.
1600 Occasionally people suggest that Nmap should not offer features for
1602 evading firewall rules or sneaking past IDSs. They argue that these
1603 features are just as likely to be misused by attackers as used by
1604 administrators to enhance security. The problem with this logic is that
1605 these methods would still be used by attackers, who would just find
1606 other tools or patch the functionality into Nmap. Meanwhile,
1607 administrators would find it that much harder to do their jobs.
1608 Deploying only modern, patched FTP servers is a far more powerful
1609 defense than trying to prevent the distribution of tools implementing
1610 the FTP bounce attack.
1611 There is no magic bullet (or Nmap option) for detecting and subverting
1613 firewalls and IDS systems. It takes skill and experience. A tutorial is
1614 beyond the scope of this reference guide, which only lists the relevant
1615 options and describes what they do.
1616 -f (fragment packets); --mtu (using the specified MTU)
1618 The -f option causes the requested scan (including ping scans)
1619 to use tiny fragmented IP packets. The idea is to split up the
1620 TCP header over several packets to make it harder for packet
1621 filters, intrusion detection systems, and other annoyances to
1622 detect what you are doing. Be careful with this! Some programs
1623 have trouble handling these tiny packets. The old-school sniffer
1624 named Sniffit segmentation faulted immediately upon receiving
1625 the first fragment. Specify this option once, and Nmap splits
1626 the packets into 8 bytes or less after the IP header. So a
1627 20-byte TCP header would be split into 3 packets. Two with eight
1628 bytes of the TCP header, and one with the final four. Of course
1629 each fragment also has an IP header. Specify -f again to use 16
1630 bytes per fragment (reducing the number of fragments). Or you
1631 can specify your own offset size with the --mtu option. Don't
1632 also specify -f if you use --mtu. The offset must be a multiple
1633 of 8. While fragmented packets won't get by packet filters and
1634 firewalls that queue all IP fragments, such as the
1635 CONFIG_IP_ALWAYS_DEFRAG option in the Linux kernel, some
1636 networks can't afford the performance hit this causes and thus
1637 leave it disabled. Others can't enable this because fragments
1638 may take different routes into their networks. Some source
1639 systems defragment outgoing packets in the kernel. Linux with
1640 the iptables connection tracking module is one such example. Do
1641 a scan while a sniffer such as Ethereal is running to ensure
1642 that sent packets are fragmented. If your host OS is causing
1643 problems, try the --send-eth option to bypass the IP layer and
1644 send raw ethernet frames.
1645 -D <decoy1 [,decoy2][,ME],...> (Cloak a scan with decoys)
1647 Causes a decoy scan to be performed, which makes it appear to
1648 the remote host that the host(s) you specify as decoys are
1649 scanning the target network too. Thus their IDS might report
1650 5-10 port scans from unique IP addresses, but they won't know
1651 which IP was scanning them and which were innocent decoys. While
1652 this can be defeated through router path tracing,
1653 response-dropping, and other active mechanisms, it is generally
1654 an effective technique for hiding your IP address.
1655 Separate each decoy host with commas, and you can optionally use
1657 ME as one of the decoys to represent the position for your real
1658 IP address. If you put ME in the 6th position or later, some
1659 common port scan detectors (such as Solar Designer's excellent
1660 scanlogd) are unlikely to show your IP address at all. If you
1661 don't use ME, nmap will put you in a random position.
1662 Note that the hosts you use as decoys should be up or you might
1664 accidentally SYN flood your targets. Also it will be pretty easy
1665 to determine which host is scanning if only one is actually up
1666 on the network. You might want to use IP addresses instead of
1667 names (so the decoy networks don't see you in their nameserver
1668 logs).
1669 Decoys are used both in the initial ping scan (using ICMP, SYN,
1671 ACK, or whatever) and during the actual port scanning phase.
1672 Decoys are also used during remote OS detection (-O). Decoys do
1673 not work with version detection or TCP connect scan.
1674 It is worth noting that using too many decoys may slow your scan
1676 and potentially even make it less accurate. Also, some ISPs will
1677 filter out your spoofed packets, but many do not restrict
1678 spoofed IP packets at all.
1679 -S <IP_Address> (Spoof source address)
1681 In some circumstances, Nmap may not be able to determine your
1682 source address ( Nmap will tell you if this is the case). In
1683 this situation, use -S with the IP address of the interface you
1684 wish to send packets through.
1685 Another possible use of this flag is to spoof the scan to make
1687 the targets think that someone else is scanning them. Imagine a
1688 company being repeatedly port scanned by a competitor! The -e
1689 option and -P0 are generally required for this sort of usage.
1690 Note that you usually won't receive reply packets back (they
1691 will be addressed to the IP you are spoofing), so Nmap won't
1692 produce useful reports.
1693 -e <interface> (Use specified interface)
1695 Tells Nmap what interface to send and receive packets on. Nmap
1696 should be able to detect this automatically, but it will tell
1697 you if it cannot.
1698 --source-port <portnumber>; -g <portnumber> (Spoof source port number)
1700 One surprisingly common misconfiguration is to trust traffic
1701 based only on the source port number. It is easy to understand
1702 how this comes about. An administrator will set up a shiny new
1703 firewall, only to be flooded with complains from ungrateful
1704 users whose applications stopped working. In particular, DNS may
1705 be broken because the UDP DNS replies from external servers can
1706 no longer enter the network. FTP is another common example. In
1707 active FTP transfers, the remote server tries to establish a
1708 connection back to the client to transfer the requested file.
1709 Secure solutions to these problems exist, often in the form of
1711 application-level proxies or protocol-parsing firewall modules.
1712 Unfortunately there are also easier, insecure solutions. Noting
1713 that DNS replies come from port 53 and active ftp from port 20,
1714 many admins have fallen into the trap of simply allowing
1715 incoming traffic from those ports. They often assume that no
1716 attacker would notice and exploit such firewall holes. In other
1717 cases, admins consider this a short-term stop-gap measure until
1718 they can implement a more secure solution. Then they forget the
1719 security upgrade.
1720 Overworked network administrators are not the only ones to fall
1722 into this trap. Numerous products have shipped with these
1723 insecure rules. Even Microsoft has been guilty. The IPsec
1724 filters that shipped with Windows 2000 and Windows XP contain an
1725 implicit rule that allows all TCP or UDP traffic from port 88
1726 (Kerberos). In another well-known case, versions of the Zone
1727 Alarm personal firewall up to 2.1.25 allowed any incoming UDP
1728 packets with the source port 53 (DNS) or 67 (DHCP).
1729 Nmap offers the -g and --source-port options (they are
1731 equivalent) to exploit these weaknesses. Simply provide a port
1732 number and Nmap will send packets from that port where possible.
1733 Nmap must use different port numbers for certain OS detection
1734 tests to work properly, and DNS requests ignore the
1735 --source-port flag because Nmap relies on system libraries to
1736 handle those. Most TCP scans, including SYN scan, support the
1737 option completely, as does UDP scan.
1738 --data-length <number> (Append random data to sent packets)
1740 Normally Nmap sends minimalist packets containing only a header.
1741 So its TCP packets are generally 40 bytes and ICMP echo requests
1742 are just 28. This option tells Nmap to append the given number
1743 of random bytes to most of the packets it sends. OS detection
1744 (-O) packets are not affected because accuracy there requires
1745 probe consistency, but most pinging and portscan packets support
1746 this. It slows things down a little, but can make a scan
1747 slightly less conspicuous.
1748 --ip-options <S|R [route]|L [route]|T|U ... >; --ip-options <hex
1750 string> (Send packets with specified ip options)
1751 The [6]IP protocol offers several options which may be placed in
1752 packet headers. Unlike the ubiquitous TCP options, IP options
1753 are rarely seen due to practicality and security concerns. In
1754 fact, many Internet routers block the most dangerous options
1755 such as source routing. Yet options can still be useful in some
1756 cases for determining and manipulating the network route to
1757 target machines. For example, you may be able to use the record
1758 route option to determine a path to a target even when more
1759 traditional traceroute-style approaches fail. Or if your packets
1760 are being dropped by a certain firewall, you may be able to
1761 specify a different route with the strict or loose source
1762 routing options.
1763 The most powerful way to specify IP options is to simply pass in
1765 values as the argument to --ip-options. Precede each hex number
1766 with \x then the two digits. You may repeat certain characters
1767 by following them with an asterisk and then the number of times
1768 you wish them to repeat. For example, \x01\x07\x04\x00*36\x01 is
1769 a hex string containing 36 NUL bytes.
1770 Nmap also offers a shortcut mechanism for specifying options.
1772 Simply pass the letter R, T, or U to request record-route,
1773 record-timestamp, or both options together, respectively. Loose
1774 or strict source routing may be specified with an L or S
1775 followed by a space and then a space-separated list of IP
1776 addresses.
1777 If you wish to see the options in packets sent and received,
1779 specify --packet-trace. For more information and examples of
1780 using IP options with Nmap, see
1781 http://seclists.org/nmap-dev/2006/q3/0052.html.
1782 --ttl <value> (Set IP time-to-live field)
1784 Sets the IPv4 time-to-live field in sent packets to the given
1785 value.
1786 --randomize-hosts (Randomize target host order)
1788 Tells Nmap to shuffle each group of up to 8096 hosts before it
1789 scans them. This can make the scans less obvious to various
1790 network monitoring systems, especially when you combine it with
1791 slow timing options. If you want to randomize over larger group
1792 sizes, increase PING_GROUP_SZ in nmap.h and recompile. An
1793 alternative solution is to generate the target IP list with a
1794 list scan (-sL -n -oN filename), randomize it with a Perl
1795 script, then provide the whole list to Nmap with -iL.
1796 --spoof-mac <mac address, prefix, or vendor name> (Spoof MAC address)
1798 Asks Nmap to use the given MAC address for all of the raw
1799 ethernet frames it sends. This option implies --send-eth to
1800 ensure that Nmap actually sends ethernet-level packets. The MAC
1801 given can take several formats. If it is simply the string “0”,
1802 Nmap chooses a completely random MAC for the session. If the
1803 given string is an even number of hex digits (with the pairs
1804 optionally separated by a colon), Nmap will use those as the
1805 MAC. If less than 12 hex digits are provided, Nmap fills in the
1806 remainder of the 6 bytes with random values. If the argument
1807 isn't a 0 or hex string, Nmap looks through nmap-mac-prefixes to
1808 find a vendor name containing the given string (it is case
1809 insensitive). If a match is found, Nmap uses the vendor's OUI
1810 (3-byte prefix) and fills out the remaining 3 bytes randomly.
1811 Valid --spoof-mac argument examples are Apple, 0,
1812 01:02:03:04:05:06, deadbeefcafe, 0020F2, and Cisco.
1813 --badsum (Send packets with bogus TCP/UDP checksums)
1815 Asks Nmap to use an invalid TCP or UDP checksum for packets sent
1816 to target hosts. Since virtually all host IP stacks properly
1817 drop these packets, any responses received are likely coming
1818 from a firewall or IDS that didn't bother to verify the
1819 checksum. For more details on this technique, see
1820 http://www.phrack.org/phrack/60/p60-0x0c.txt
1821
1823 Any security tools is only as useful as the output it generates.
1824 Complex tests and algorithms are of little value if they aren't
1825 presented in an organized and comprehensible fashion. Given the number
1826 of ways Nmap is used by people and other software, no single format can
1827 please everyone. So Nmap offers several formats, including the
1828 interactive mode for humans to read directly and XML for easy parsing
1829 by software.
1830 In addition to offering different output formats, Nmap provides options
1832 for controlling the verbosity of output as well as debugging messages.
1833 Output types may be sent to standard output or to named files, which
1834 Nmap can append to or clobber. Output files may also be used to resume
1835 aborted scans.
1836 Nmap makes output available in five different formats. The default is
1838 called interactive output, and it is sent to standard output (stdout).
1839 There is also normal output, which is similar to interactive except
1840 that it displays less runtime information and warnings since it is
1841 expected to be analyzed after the scan completes rather than
1842 interactively.
1843 XML output is one of the most important output types, as it can be
1845 converted to HTML, easily parsed by programs such as Nmap graphical
1846 user interfaces, or imported into databases.
1847 The two remaining output types are the simple grepable output which
1849 includes most information for a target host on a single line, and
1850 sCRiPt KiDDi3 0utPUt for users who consider themselves |<-r4d.
1851 While interactive output is the default and has no associated
1853 command-line options, the other four format options use the same
1854 syntax. They take one argument, which is the filename that results
1855 should be stored in. Multiple formats may be specified, but each format
1856 may only be specified once. For example, you may wish to save normal
1857 output for your own review while saving XML of the same scan for
1858 programmatic analysis. You might do this with the options -oX
1859 myscan.xml -oN myscan.nmap. While this chapter uses the simple names
1860 like myscan.xml for brevity, more descriptive names are generally
1861 recommended. The names chosen are a matter of personal preference,
1862 though I use long ones that incorporate the scan date and a word or two
1863 describing the scan, placed in a directory named after the company I'm
1864 scanning.
1865 While these options save results to files, Nmap still prints
1867 interactive output to stdout as usual. For example, the command nmap
1868 -oX myscan.xml target prints XML to myscan.xml and fills standard
1869 output with the same interactive results it would have printed if -oX
1870 wasn't specified at all. You can change this by passing a hyphen
1871 character as the argument to one of the format types. This causes Nmap
1872 to deactivate interactive output, and instead print results in the
1873 format you specified to the standard output stream. So the command nmap
1874 -oX - target will send only XML output to stdout. Serious errors may
1875 still be printed to the normal error stream, stderr.
1876 Unlike some Nmap arguments, the space between the logfile option flag
1878 (such as -oX) and the filename or hyphen is mandatory. If you omit the
1879 flags and give arguments such as -oG- or -oXscan.xml, a backwards
1880 compatibility feature of Nmap will cause the creation of normal format
1881 output files named G- and Xscan.xml respectively.
1882 Nmap also offers options to control scan verbosity and to append to
1884 output files rather than clobbering them. All of these options are
1885 described below.
1886 Nmap Output Formats
1888 -oN <filespec> (Normal output)
1890 Requests that normal output be directed to the given filename.
1891 As discussed above, this differs slightly from interactive
1892 output.
1893 -oX <filespec> (XML output)
1895 Requests that XML output be directed to the given filename. Nmap
1896 includes a document type definition (DTD) which allows XML
1897 parsers to validate Nmap XML output. While it is primarily
1898 intended for programmatic use, it can also help humans interpret
1899 Nmap XML output. The DTD defines the legal elements of the
1900 format, and often enumerates the attributes and values they can
1901 take on. The latest version is always available from
1902 http://insecure.org/nmap/data/nmap.dtd.
1903 XML offers a stable format that is easily parsed by software.
1905 Free XML parsers are available for all major computer languages,
1906 including C/C++, Perl, Python, and Java. People have even
1907 written bindings for most of these languages to handle Nmap
1908 output and execution specifically. Examples are [7]Nmap::Scanner
1909 and [8]Nmap::Parser in Perl CPAN. In almost all cases that a
1910 non-trivial application interfaces with Nmap, XML is the
1911 preferred format.
1912 The XML output references an XSL stylesheet which can be used to
1914 format the results as HTML. The easiest way to use this is
1915 simply to load the XML output in a web browser such as Firefox
1916 or IE. By default, this will only work on the machine you ran
1917 Nmap on (or a similarly configured one) due to the hard-coded
1918 nmap.xsl filesystem path. Use the --webxml or --stylesheet
1919 options to create portable XML files that render as HTML on any
1920 web-connected machine.
1921 -oS <filespec> (ScRipT KIdd|3 oUTpuT)
1923 Script kiddie output is like interactive output, except that it
1924 is post-processed to better suit the l33t HaXXorZ who previously
1925 looked down on Nmap due to its consistent capitalization and
1926 spelling. Humor impaired people should note that this option is
1927 making fun of the script kiddies before flaming me for
1928 supposedly “helping them”.
1929 -oG <filespec> (Grepable output)
1931 This output format is covered last because it is deprecated. The
1932 XML output format is far more powerful, and is nearly as
1933 convenient for experienced users. XML is a standard for which
1934 dozens of excellent parsers are available, while grepable output
1935 is my own simple hack. XML is extensible to support new Nmap
1936 features as they are released, while I often must omit those
1937 features from grepable output for lack of a place to put them.
1938 Nevertheless, grepable output is still quite popular. It is a
1940 simple format that lists each host on one line and can be
1941 trivially searched and parsed with standard UNIX tools such as
1942 grep, awk, cut, sed, diff, and Perl. Even I usually use it for
1943 one-off tests done at the command line. Finding all the hosts
1944 with the ssh port open or that are running Solaris takes only a
1945 simple grep to identify the hosts, piped to an awk or cut
1946 command to print the desired fields.
1947 Grepable output consists of comments (lines starting with a
1949 pound (#)) and target lines. A target line includes a
1950 combination of 6 labeled fields, separated by tabs and followed
1951 with a colon. The fields are Host, Ports, Protocols, Ignored
1952 State, OS, Seq Index, IPID, and Status.
1953 The most important of these fields is generally Ports, which
1955 gives details on each interesting port. It is a comma separated
1956 list of port entries. Each port entry represents one interesting
1957 port, and takes the form of seven slash (/) separated subfields.
1958 Those subfields are: Port number, State, Protocol, Owner,
1959 Service, SunRPC info, and Version info.
1960 As with XML output, this man page does not allow for documenting
1962 the entire format. A more detailed look at the Nmap grepable
1963 output format is available from
1964 http://www.unspecific.com/nmap-oG-output.
1965 -oA <basename> (Output to all formats)
1967 As a convenience, you may specify -oA basename to store scan
1969 results in normal, XML, and grepable formats at once. They are
1970 stored in basename.nmap, basename.xml, and basename.gnmap,
1971 respectively. As with most programs, you can prefix the
1972 filenames with a directory path, such as ~/nmaplogs/foocorp/ on
1973 UNIX or c:\hacking\sco on Windows.
1974 Verbosity and debugging options
1976 -v (Increase verbosity level)
1978 Increases the verbosity level, causing Nmap to print more
1979 information about the scan in progress. Open ports are shown as
1980 they are found and completion time estimates are provided when
1981 Nmap thinks a scan will take more than a few minutes. Use it
1982 twice for even greater verbosity. Using it more than twice has
1983 no effect.
1984 Most changes only affect interactive output, and some also
1986 affect normal and script kiddie output. The other output types
1987 are meant to be processed by machines, so Nmap can give
1988 substantial detail by default in those formats without fatiguing
1989 a human user. However, there are a few changes in other modes
1990 where output size can be reduced substantially by omitting some
1991 detail. For example, a comment line in the grepable output that
1992 provides a list of all ports scanned is only printed in verbose
1993 mode because it can be quite long.
1994 -d [level] (Increase or set debugging level)
1996 When even verbose mode doesn't provide sufficient data for you,
1997 debugging is available to flood you with much more! As with the
1998 verbosity option (-v), debugging is enabled with a command-line
1999 flag (-d) and the debug level can be increased by specifying it
2000 multiple times. Alternatively, you can set a debug level by
2001 giving an argument to -d. For example, -d9 sets level nine. That
2002 is the highest effective level and will produce thousands of
2003 lines unless you run a very simple scan with very few ports and
2004 targets.
2005 Debugging output is useful when a bug is suspected in Nmap, or
2007 if you are simply confused as to what Nmap is doing and why. As
2008 this feature is mostly intended for developers, debug lines
2009 aren't always self-explanatory. You may get something like:
2010 Timeout vals: srtt: -1 rttvar: -1 to: 1000000 delta 14987 ==>
2011 srtt: 14987 rttvar: 14987 to: 100000. If you don't understand a
2012 line, your only recourses are to ignore it, look it up in the
2013 source code, or request help from the development list
2014 (nmap-dev). Some lines are self explanatory, but the messages
2015 become more obscure as the debug level is increased.
2016 --packet-trace (Trace packets and data sent and received)
2018 Causes Nmap to print a summary of every packet sent or received.
2019 This is often used for debugging, but is also a valuable way for
2020 new users to understand exactly what Nmap is doing under the
2021 covers. To avoid printing thousands of lines, you may want to
2022 specify a limited number of ports to scan, such as -p20-30. If
2023 you only care about the goings on of the version detection
2024 subsystem, use --version-trace instead.
2025 --open (Show only open (or possibly open) ports)
2027 Sometimes you only care about ports you can actually connect to
2028 (open ones), and don't want results cluttered with closed,
2029 filtered, and closed|filtered ports. Output customization is
2030 normally done after the scan using tools such as grep, awk, and
2031 Perl, but this feature was added due to overwhelming requests.
2032 Specify --open to only see open, open|filtered, and unfiltered
2033 ports. These three ports are treated just as they normally are,
2034 which means that open|filtered and unfiltered may be condensed
2035 into counts if there are an overwhelming number of them.
2036 --iflist (List interfaces and routes)
2038 Prints the interface list and system routes as detected by Nmap.
2039 This is useful for debugging routing problems or device
2040 mischaracterization (such as Nmap treating a PPP connection as
2041 Ethernet).
2042 --log-errors (Log errors/warnings to normal mode output file)
2044 Warnings and errors printed by Nmap usually go only to the
2045 screen (interactive output), leaving any specified normal-fomat
2046 output files uncluttered. But when you do want to see those
2047 messages in the normal output file you specified, add this
2048 option. It is useful when you aren't watching the interactive
2049 output or are trying to debug a problem. The messages will also
2050 still appear in interactive mode. This will not work for most
2051 errors related to bad command-line arguments, as Nmap may not
2052 have initialized its output files yet. In addition, some Nmap
2053 error/warning messages use a different system that does not yet
2054 support this option. An alternative to using this option is
2055 redirecting interactive output (including the standard error
2056 stream) to a file. While most UNIX shells make that approach
2057 easy, it can be difficult on Windows.
2058 Miscellaneous output options
2060 --append-output (Append to rather than clobber output files)
2062 When you specify a filename to an output format flag such as -oX
2063 or -oN, that file is overwritten by default. If you prefer to
2064 keep the existing content of the file and append the new
2065 results, specify the --append-output option. All output
2066 filenames specified in that Nmap execution will then be appended
2067 to rather than clobbered. This doesn't work well for XML (-oX)
2068 scan data as the resultant file generally won't parse properly
2069 until you fix it up by hand.
2070 --resume <filename> (Resume aborted scan)
2072 Some extensive Nmap runs take a very long time -- on the order
2073 of days. Such scans don't always run to completion. Restrictions
2074 may prevent Nmap from being run during working hours, the
2075 network could go down, the machine Nmap is running on might
2076 suffer a planned or unplanned reboot, or Nmap itself could
2077 crash. The admin running Nmap could cancel it for any other
2078 reason as well, by pressing ctrl-C. Restarting the whole scan
2079 from the beginning may be undesirable. Fortunately, if normal
2080 (-oN) or grepable (-oG) logs were kept, the user can ask Nmap to
2081 resume scanning with the target it was working on when execution
2082 ceased. Simply specify the --resume option and pass the
2083 normal/grepable output file as its argument. No other arguments
2084 are permitted, as Nmap parses the output file to use the same
2085 ones specified previously. Simply call Nmap as nmap --resume
2086 logfilename. Nmap will append new results to the data files
2087 specified in the previous execution. Resumption does not support
2088 the XML output format because combining the two runs into one
2089 valid XML file would be difficult.
2090 --stylesheet <path or URL> (Set XSL stylesheet to transform XML output)
2092 Nmap ships with an XSL stylesheet named nmap.xsl for viewing or
2093 translating XML output to HTML. The XML output includes an
2094 xml-stylesheet directive which points to nmap.xml where it was
2095 initially installed by Nmap (or in the current working directory
2096 on Windows). Simply load Nmap's XML output in a modern web
2097 browser and it should retrieve nmap.xsl from the filesystem and
2098 use it to render results. If you wish to use a different
2099 stylesheet, specify it as the argument to --stylesheet. You must
2100 pass the full pathname or URL. One common invocation is
2101 --stylesheet http://insecure.org/nmap/data/nmap.xsl. This tells
2102 a browser to load the latest version of the stylesheet from
2103 Insecure.Org. The --webxml option does the same thing with less
2104 typing and memorization. Loading the XSL from Insecure.Org makes
2105 it easier to view results on a machine that doesn't have Nmap
2106 (and thus nmap.xsl) installed. So the URL is often more useful,
2107 but the local filesystem location of nmap.xsl is used by default
2108 for privacy reasons.
2109 --webxml (Load stylesheet from Insecure.Org)
2111 This convenience option is simply an alias for --stylesheet
2112 http://insecure.org/nmap/data/nmap.xsl.
2113 --no_stylesheet (Omit XSL stylesheet declaration from XML)
2115 Specify this option to prevent Nmap from associating any XSL
2116 stylesheet with its XML output. The xml-stylesheet directive is
2117 omitted.
2118
2120 This section describes some important (and not-so-important) options
2121 that don't really fit anywhere else.
2122 -6 (Enable IPv6 scanning)
2124 Since 2002, Nmap has offered IPv6 support for its most popular
2125 features. In particular, ping scanning (TCP-only), connect
2126 scanning, and version detection all support IPv6. The command
2127 syntax is the same as usual except that you also add the -6
2128 option. Of course, you must use IPv6 syntax if you specify an
2129 address rather than a hostname. An address might look like
2130 3ffe:7501:4819:2000:210:f3ff:fe03:14d0, so hostnames are
2131 recommended. The output looks the same as usual, with the IPv6
2132 address on the “interesting ports” line being the only IPv6 give
2133 away.
2134 While IPv6 hasn't exactly taken the world by storm, it gets
2136 significant use in some (usually Asian) countries and most
2137 modern operating systems support it. To use Nmap with IPv6, both
2138 the source and target of your scan must be configured for IPv6.
2139 If your ISP (like most of them) does not allocate IPv6 addresses
2140 to you, free tunnel brokers are widely available and work fine
2141 with Nmap. One of the better ones is run by BT Exact at
2142 https://tb.ipv6.btexact.com/. I have also used one that
2143 Hurricane Electric provides at http://ipv6tb.he.net/. 6to4
2144 tunnels are another popular, free approach.
2145 -A (Aggressive scan options)
2147 This option enables additional advanced and aggressive options.
2148 I haven't decided exactly which it stands for yet. Presently
2149 this enables OS Detection (-O) and version scanning (-sV). More
2150 features may be added in the future. The point is to enable a
2151 comprehensive set of scan options without people having to
2152 remember a large set of flags. This option only enables
2153 features, and not timing options (such as -T4) or verbosity
2154 options (-v) that you might want as well.
2155 --datadir <directoryname> (Specify custom Nmap data file location)
2157 Nmap obtains some special data at runtime in files named
2158 nmap-service-probes, nmap-services, nmap-protocols, nmap-rpc,
2159 nmap-mac-prefixes, and nmap-os-fingerprints. Nmap first searches
2160 these files in the directory specified with the --datadir option
2161 (if any). Any files not found there, are searched for in the
2162 directory specified by the NMAPDIR environmental variable. Next
2163 comes ~/.nmap for real and effective UIDs (POSIX systems only)
2164 or location of the Nmap executable (Win32 only), and then a
2165 compiled-in location such as /usr/local/share/nmap or
2166 /usr/share/nmap
2167 --send-eth (Use raw ethernet sending)
2169 Asks Nmap to send packets at the raw ethernet (data link) layer
2170 rather than the higher IP (network) layer. By default, Nmap
2171 chooses the one which is generally best for the platform it is
2172 running on. Raw sockets (IP layer) are generally most efficient
2173 for UNIX machines, while ethernet frames are required for
2174 Windows operation since Microsoft disabled raw socket support.
2175 Nmap still uses raw IP packets on UNIX despite this option when
2176 there is no other choice (such as non-ethernet connections).
2177 --send-ip (Send at raw IP level)
2179 Asks Nmap to send packets via raw IP sockets rather than sending
2180 lower level ethernet frames. It is the complement to the
2181 --send-eth option discussed previously.
2182 --privileged (Assume that the user is fully privileged)
2184 Tells Nmap to simply assume that it is privileged enough to
2185 perform raw socket sends, packet sniffing, and similar
2186 operations that usually require root privileges on UNIX systems.
2187 By default Nmap quits if such operations are requested but
2188 geteuid() is not zero. --privileged is useful with Linux kernel
2189 capabilities and similar systems that may be configured to allow
2190 unprivileged users to perform raw-packet scans. Be sure to
2191 provide this option flag before any flags for options that
2192 require privileges (SYN scan, OS detection, etc.). The
2193 NMAP_PRIVILEGED variable may be set as an equivalent alternative
2194 to --privileged.
2195 --unprivileged (Assume that the user lacks raw socket privileges)
2197 This option is the opposite of --privileged. It tells Nmap to
2198 treat the user as lacking network raw socket and sniffing
2199 privileges. This is useful for testing, debugging, or when the
2200 raw network functionality of your operating system is somehow
2201 broken.
2202 --release-memory (Release memory before quitting)
2204 This option is only useful for memory-leak debugging. It causes
2205 Nmap to release allocated memory just before it quits so that
2206 actual memory leaks are easier to spot. Normally Nmap skips this
2207 as the OS does this anyway upon process termination.
2208 --interactive (Start in interactive mode)
2210 Starts Nmap in interactive mode, which offers an interactive
2211 Nmap prompt allowing easy launching of multiple scans (either
2212 synchronously or in the background). This is useful for people
2213 who scan from multi-user systems as they often want to test
2214 their security without letting everyone else on the system know
2215 exactly which systems they are scanning. Use --interactive to
2216 activate this mode and then type h for help. This option is
2217 rarely used because proper shells are usually more familiar and
2218 feature-complete. This option includes a bang (!) operator for
2219 executing shell commands, which is one of many reasons not to
2220 install Nmap setuid root.
2221 -V; --version (Print version number)
2223 Prints the Nmap version number and exits.
2224 -h; --help (Print help summary page)
2226 Prints a short help screen with the most common command flags.
2227 Running Nmap without any arguments does the same thing.
2228
2230 During the execution of nmap, all key presses are captured. This allows
2231 you to interact with the program without aborting and restarting it.
2232 Certain special keys will change options, while any other keys will
2233 print out a status message telling you about the scan. The convention
2234 is that lowercase letters increase the amount of printing, and
2235 uppercase letters decrease the printing. You may also press ‘?’ for
2236 help.
2237 v / V Increase / Decrease the Verbosity
2239 d / D Increase / Decrease the Debugging Level
2241 p / P Turn on / off Packet Tracing
2243 ? Print a runtime interaction help screen
2245 Anything else
2247 Print out a status message like this:
2248 Stats: 0:00:08 elapsed; 111 hosts completed (5 up), 5 undergoing
2250 Service Scan
2251 Service scan Timing: About 28.00% done; ETC: 16:18 (0:00:15
2253 remaining)
2254
2256 Here are some Nmap usage examples, from the simple and routine to a
2257 little more complex and esoteric. Some actual IP addresses and domain
2258 names are used to make things more concrete. In their place you should
2259 substitute addresses/names from your own network.. While I don't think
2260 port scanning other networks is or should be illegal, some network
2261 administrators don't appreciate unsolicited scanning of their networks
2262 and may complain. Getting permission first is the best approach.
2263 For testing purposes, you have permission to scan the host
2265 scanme.nmap.org. This permission only includes scanning via Nmap and
2266 not testing exploits or denial of service attacks. To conserve
2267 bandwidth, please do not initiate more than a dozen scans against that
2268 host per day. If this free scanning target service is abused, it will
2269 be taken down and Nmap will report Failed to resolve given hostname/IP:
2270 scanme.nmap.org. These permissions also apply to the hosts
2271 scanme2.nmap.org, scanme3.nmap.org, and so on, though those hosts do
2272 not currently exist.
2273 nmap -v scanme.nmap.org
2275 This option scans all reserved TCP ports on the machine scanme.nmap.org
2277 -v option enables verbose mode.
2278 nmap -sS -O scanme.nmap.org/24
2280 Launches a stealth SYN scan against each machine that is up out of the
2282 255 machines on “class C” network where Scanme resides. It also tries
2283 to determine what operating system is running on each host that is up
2284 and running. This requires root privileges because of the SYN scan and
2285 OS detection.
2286 nmap -sV -p 22,53,110,143,4564 198.116.0-255.1-127
2288 Launches host enumeration and a TCP scan at the first half of each of
2290 the 255 possible 8 bit subnets in the 198.116 class B address space.
2291 This tests whether the systems run sshd, DNS, pop3d, imapd, or port
2292 4564. For any of these ports found open, version detection is used to
2293 determine what application is running.
2294 nmap -v -iR 100000 -P0 -p 80
2296 Asks Nmap to choose 100,000 hosts at random and scan them for web
2298 servers (port 80). Host enumeration is disabled with -P0 since first
2299 sending a couple probes to determine whether a host is up is wasteful
2300 when you are only probing one port on each target host anyway.
2301 nmap -P0 -p80 -oX logs/pb-port80scan.xml -oG logs/pb-port80scan.gnmap
2303 216.163.128.20/20
2304 This scans 4096 IPs for any webservers (without pinging them) and saves
2306 the output in grepable and XML formats.
2307
2309 Like its author, Nmap isn't perfect. But you can help make it better by
2310 sending bug reports or even writing patches. If Nmap doesn't behave the
2311 way you expect, first upgrade to the latest version available from
2312 http://insecure.org/nmap/. If the problem persists, do some research to
2313 determine whether it has already been discovered and addressed. Try
2314 Googling the error message or browsing the Nmap-dev archives at
2315 http://seclists.org/. Read this full munual page as well. If nothing
2316 comes of this, mail a bug report to <nmap-dev@insecure.org>. Please
2317 include everything you have learned about the problem, as well as what
2318 version of Nmap you are running and what operating system version it is
2319 running on. Problem reports and Nmap usage questions sent to
2320 nmap-dev@insecure.org are far more likely to be answered than those
2321 sent to Fyodor directly.
2322 Code patches to fix bugs are even better than bug reports. Basic
2324 instructions for creating patch files with your changes are available
2325 at http://insecure.org/nmap/data/HACKING. Patches may be sent to
2326 nmap-dev (recommended) or to Fyodor directly.
2327
2329 Fyodor <fyodor@insecure.org> (http://insecure.org)
2330 Hundreds of people have made valuable contributions to Nmap over the
2332 years. These are detailed in the CHANGELOG file which is distributed
2333 with Nmap and also available from
2334 http://insecure.org/nmap/changelog.html.
2335
2337 Nmap Copyright and Licensing
2338 The Nmap Security Scanner is (C) 1996-2005 Insecure.Com LLC. Nmap is
2339 also a registered trademark of Insecure.Com LLC. This program is free
2340 software; you may redistribute and/or modify it under the terms of the
2341 GNU General Public License as published by the Free Software
2342 Foundation; Version 2. This guarantees your right to use, modify, and
2343 redistribute this software under certain conditions. If you wish to
2344 embed Nmap technology into proprietary software, we may be willing to
2345 sell alternative licenses (contact <sales@insecure.com>). Many security
2346 scanner vendors already license Nmap technology such as host discovery,
2347 port scanning, OS detection, and service/version detection.
2348 Note that the GPL places important restrictions on “derived works”, yet
2350 it does not provide a detailed definition of that term. To avoid
2351 misunderstandings, we consider an application to constitute a
2352 “derivative work” for the purpose of this license if it does any of the
2353 following:
2354 · Integrates source code from Nmap
2356 · Reads or includes Nmap copyrighted data files, such as
2358 nmap-os-fingerprints or nmap-service-probes.
2359 · Executes Nmap and parses the results (as opposed to typical shell or
2361 execution-menu apps, which simply display raw Nmap output and so are
2362 not derivative works.)
2363 · Integrates/includes/aggregates Nmap into a proprietary executable
2365 installer, such as those produced by InstallShield.
2366 · Links to a library or executes a program that does any of the above.
2368 The term “Nmap” should be taken to also include any portions or derived
2370 works of Nmap. This list is not exclusive, but is just meant to clarify
2371 our interpretation of derived works with some common examples. These
2372 restrictions only apply when you actually redistribute Nmap. For
2373 example, nothing stops you from writing and selling a proprietary
2374 front-end to Nmap. Just distribute it by itself, and point people to
2375 http://insecure.org/nmap/ to download Nmap.
2376 We don't consider these to be added restrictions on top of the GPL, but
2378 just a clarification of how we interpret “derived works” as it applies
2379 to our GPL-licensed Nmap product. This is similar to the way Linus
2380 Torvalds has announced his interpretation of how “derived works”
2381 applies to Linux kernel modules. Our interpretation refers only to Nmap
2382 - we don't speak for any other GPL products.
2383 If you have any questions about the GPL licensing restrictions on using
2385 Nmap in non-GPL works, we would be happy to help. As mentioned above,
2386 we also offer alternative license to integrate Nmap into proprietary
2387 applications and appliances. These contracts have been sold to many
2388 security vendors, and generally include a perpetual license as well as
2389 providing for priority support and updates as well as helping to fund
2390 the continued development of Nmap technology. Please email
2391 <sales@insecure.com> for further information.
2392 As a special exception to the GPL terms, Insecure.Com LLC grants
2394 permission to link the code of this program with any version of the
2395 OpenSSL library which is distributed under a license identical to that
2396 listed in the included Copying.OpenSSL file, and distribute linked
2397 combinations including the two. You must obey the GNU GPL in all
2398 respects for all of the code used other than OpenSSL. If you modify
2399 this file, you may extend this exception to your version of the file,
2400 but you are not obligated to do so.
2401 If you received these files with a written license agreement or
2403 contract stating terms other than the terms above, then that
2404 alternative license agreement takes precedence over these comments.
2405 Creative Commons license for this Nmap guide
2407 This Nmap Reference Guide is (C) 2005 Insecure.Com LLC. It is hereby
2408 placed under version 2.5 of the [9]Creative Commons Attribution
2409 License. This allows you redistribute and modify the work as you
2410 desire, as long as you credit the original source. Alternatively, you
2411 may choose to treat this document as falling under the same license as
2412 Nmap itself (discussed previously).
2413 Source code availability and community contributions
2415 Source is provided to this software because we believe users have a
2416 right to know exactly what a program is going to do before they run it.
2417 This also allows you to audit the software for security holes (none
2418 have been found so far).
2419 Source code also allows you to port Nmap to new platforms, fix bugs,
2421 and add new features. You are highly encouraged to send your changes to
2422 <fyodor@insecure.org> for possible incorporation into the main
2423 distribution. By sending these changes to Fyodor or one of the
2424 Insecure.Org development mailing lists, it is assumed that you are
2425 offering Fyodor and Insecure.Com LLC the unlimited, non-exclusive right
2426 to reuse, modify, and relicense the code. Nmap will always be available
2427 Open Source, but this is important because the inability to relicense
2428 code has caused devastating problems for other Free Software projects
2429 (such as KDE and NASM). We also occasionally relicense the code to
2430 third parties as discussed above. If you wish to specify special
2431 license conditions of your contributions, just say so when you send
2432 them.
2433 No Warranty
2435 This program is distributed in the hope that it will be useful, but
2436 WITHOUT ANY WARRANTY; without even the implied warranty of
2437 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
2438 General Public License for more details at
2439 http://www.gnu.org/copyleft/gpl.html, or in the COPYING file included
2440 with Nmap.
2441 It should also be noted that Nmap has occasionally been known to crash
2443 poorly written applications, TCP/IP stacks, and even operating systems.
2444 While this is extremely rare, it is important to keep in mind. Nmap
2445 should never be run against mission critical systems unless you are
2446 prepared to suffer downtime. We acknowledge here that Nmap may crash
2447 your systems or networks and we disclaim all liability for any damage
2448 or problems Nmap could cause.
2449 Inappropriate Usage
2451 Because of the slight risk of crashes and because a few black hats like
2452 to use Nmap for reconnaissance prior to attacking systems, there are
2453 administrators who become upset and may complain when their system is
2454 scanned. Thus, it is often advisable to request permission before doing
2455 even a light scan of a network.
2456 Nmap should never be installed with special privileges (e.g. suid root)
2458 for security reasons.
2459 Third-Party Software
2461 This product includes software developed by the [10]Apache Software
2462 Foundation. A modified version of the [11]Libpcap portable packet
2463 capture library is distributed along with nmap. The Windows version of
2464 Nmap utilized the libpcap-derived [12]WinPcap library instead. Regular
2465 expression support is provided by the [13]PCRE library, which is open
2466 source software, written by Philip Hazel. Certain raw networking
2467 functions use the [14]Libdnet networking library, which was written by
2468 Dug Song. A modified version is distributed with Nmap. Nmap can
2469 optionally link with the [15]OpenSSL cryptography toolkit for SSL
2470 version detection support. All of the third-party software described in
2471 this paragraph is freely redistributable under BSD-style software
2472 licenses.
2473 US Export Control Classification
2475 US Export Control: Insecure.Com LLC believes that Nmap falls under US
2476 ECCN (export control classification number) 5D992. This category is
2477 called “Information Security software not controlled by 5D002”. The
2478 only restriction of this classification is AT (anti-terrorism), which
2479 applies to almost all goods and denies export to a handful of rogue
2480 nations such as Iran and North Korea. Thus exporting Nmap does not
2481 require any special license, permit, or other governmental
2482 authorization.
2483
2485 1. RFC 1122
2486 http://www.rfc-editor.org/rfc/rfc1122.txt
2487 2. RFC 792
2489 http://www.rfc-editor.org/rfc/rfc792.txt
2490 3. UDP
2492 http://www.rfc-editor.org/rfc/rfc768.txt
2493 4. TCP RFC
2495 http://www.rfc-editor.org/rfc/rfc793.txt
2496 5. RFC 959
2498 http://www.rfc-editor.org/rfc/rfc959.txt
2499 6. IP protocol
2501 http://www.ietf.org/rfc/rfc0791.txt
2502 7. Nmap::Scanner
2504 http://sourceforge.net/projects/nmap-scanner/
2505 8. Nmap::Parser
2507 http://www.nmapparser.com
2508 9. Creative Commons Attribution License
2510 http://creativecommons.org/licenses/by/2.5/
2511 10. Apache Software Foundation
2513 http://www.apache.org
2514 11. Libpcap portable packet capture library
2516 http://www.tcpdump.org
2517 12. WinPcap library
2519 http://www.winpcap.org
2520 13. PCRE library
2522 http://www.pcre.org
2523 14. Libdnet
2525 http://libdnet.sourceforge.net
2526 15. OpenSSL cryptography toolkit
2528 http://www.openssl.org
2529 12/07/2006 NMAP(1)