1TCPDUMP(8) System Manager's Manual TCPDUMP(8)
2
6 tcpdump - dump traffic on a network
7
9 tcpdump [ -AbdDefhHIJKlLnNOpqStuUvxX# ] [ -B buffer_size ]
10 [ -c count ]
11 [ -C file_size ] [ -G rotate_seconds ] [ -F file ]
12 [ -i interface ] [ -j tstamp_type ] [ -m module ] [ -M secret ]
13 [ --number ] [ -Q in|out|inout ]
14 [ -r file ] [ -V file ] [ -s snaplen ] [ -T type ] [ -w file ]
15 [ -W filecount ]
16 [ -E spi@ipaddr algo:secret,... ]
17 [ -y datalinktype ] [ -z postrotate-command ] [ -Z user ]
18 [ --time-stamp-precision=tstamp_precision ]
19 [ --immediate-mode ] [ --version ]
20 [ expression ]
21
23 Tcpdump prints out a description of the contents of packets on a net‐
24 work interface that match the boolean expression; the description is
25 preceded by a time stamp, printed, by default, as hours, minutes, sec‐
26 onds, and fractions of a second since midnight. It can also be run
27 with the -w flag, which causes it to save the packet data to a file for
28 later analysis, and/or with the -r flag, which causes it to read from a
29 saved packet file rather than to read packets from a network interface.
30 It can also be run with the -V flag, which causes it to read a list of
31 saved packet files. In all cases, only packets that match expression
32 will be processed by tcpdump.
33 Tcpdump will, if not run with the -c flag, continue capturing packets
35 until it is interrupted by a SIGINT signal (generated, for example, by
36 typing your interrupt character, typically control-C) or a SIGTERM sig‐
37 nal (typically generated with the kill(1) command); if run with the -c
38 flag, it will capture packets until it is interrupted by a SIGINT or
39 SIGTERM signal or the specified number of packets have been processed.
40 When tcpdump finishes capturing packets, it will report counts of:
42 packets ``captured'' (this is the number of packets that tcpdump
44 has received and processed);
45 packets ``received by filter'' (the meaning of this depends on
47 the OS on which you're running tcpdump, and possibly on the way
48 the OS was configured - if a filter was specified on the command
49 line, on some OSes it counts packets regardless of whether they
50 were matched by the filter expression and, even if they were
51 matched by the filter expression, regardless of whether tcpdump
52 has read and processed them yet, on other OSes it counts only
53 packets that were matched by the filter expression regardless of
54 whether tcpdump has read and processed them yet, and on other
55 OSes it counts only packets that were matched by the filter
56 expression and were processed by tcpdump);
57 packets ``dropped by kernel'' (this is the number of packets
59 that were dropped, due to a lack of buffer space, by the packet
60 capture mechanism in the OS on which tcpdump is running, if the
61 OS reports that information to applications; if not, it will be
62 reported as 0).
63 On platforms that support the SIGINFO signal, such as most BSDs
65 (including Mac OS X) and Digital/Tru64 UNIX, it will report those
66 counts when it receives a SIGINFO signal (generated, for example, by
67 typing your ``status'' character, typically control-T, although on some
68 platforms, such as Mac OS X, the ``status'' character is not set by
69 default, so you must set it with stty(1) in order to use it) and will
70 continue capturing packets. On platforms that do not support the SIG‐
71 INFO signal, the same can be achieved by using the SIGUSR1 signal.
72 Reading packets from a network interface may require that you have spe‐
74 cial privileges; see the pcap (3PCAP) man page for details. Reading a
75 saved packet file doesn't require special privileges.
76
78 -A Print each packet (minus its link level header) in ASCII. Handy
79 for capturing web pages.
80 -b Print the AS number in BGP packets in ASDOT notation rather than
82 ASPLAIN notation.
83 -B buffer_size
85 --buffer-size=buffer_size
86 Set the operating system capture buffer size to buffer_size, in
87 units of KiB (1024 bytes).
88 -c count
90 Exit after receiving count packets.
91 -C file_size
93 Before writing a raw packet to a savefile, check whether the
94 file is currently larger than file_size and, if so, close the
95 current savefile and open a new one. Savefiles after the first
96 savefile will have the name specified with the -w flag, with a
97 number after it, starting at 1 and continuing upward. The units
98 of file_size are millions of bytes (1,000,000 bytes, not
99 1,048,576 bytes).
100 Note that when used with -Z option (enabled by default), privi‐
102 leges are dropped before opening first savefile.
103 -d Dump the compiled packet-matching code in a human readable form
105 to standard output and stop.
106 -dd Dump packet-matching code as a C program fragment.
108 -ddd Dump packet-matching code as decimal numbers (preceded with a
110 count).
111 -D
113 --list-interfaces
114 Print the list of the network interfaces available on the system
115 and on which tcpdump can capture packets. For each network
116 interface, a number and an interface name, possibly followed by
117 a text description of the interface, is printed. The interface
118 name or the number can be supplied to the -i flag to specify an
119 interface on which to capture.
120 This can be useful on systems that don't have a command to list
122 them (e.g., Windows systems, or UNIX systems lacking ifconfig
123 -a); the number can be useful on Windows 2000 and later systems,
124 where the interface name is a somewhat complex string.
125 The -D flag will not be supported if tcpdump was built with an
127 older version of libpcap that lacks the pcap_findalldevs() func‐
128 tion.
129 -e Print the link-level header on each dump line. This can be
131 used, for example, to print MAC layer addresses for protocols
132 such as Ethernet and IEEE 802.11.
133 -E Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that
135 are addressed to addr and contain Security Parameter Index value
136 spi. This combination may be repeated with comma or newline sep‐
137 aration.
138 Note that setting the secret for IPv4 ESP packets is supported
140 at this time.
141 Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc,
143 cast128-cbc, or none. The default is des-cbc. The ability to
144 decrypt packets is only present if tcpdump was compiled with
145 cryptography enabled.
146 secret is the ASCII text for ESP secret key. If preceded by 0x,
148 then a hex value will be read.
149 The option assumes RFC2406 ESP, not RFC1827 ESP. The option is
151 only for debugging purposes, and the use of this option with a
152 true `secret' key is discouraged. By presenting IPsec secret
153 key onto command line you make it visible to others, via ps(1)
154 and other occasions.
155 In addition to the above syntax, the syntax file name may be
157 used to have tcpdump read the provided file in. The file is
158 opened upon receiving the first ESP packet, so any special per‐
159 missions that tcpdump may have been given should already have
160 been given up.
161 -f Print `foreign' IPv4 addresses numerically rather than symboli‐
163 cally (this option is intended to get around serious brain dam‐
164 age in Sun's NIS server — usually it hangs forever translating
165 non-local internet numbers).
166 The test for `foreign' IPv4 addresses is done using the IPv4
168 address and netmask of the interface on which capture is being
169 done. If that address or netmask are not available, available,
170 either because the interface on which capture is being done has
171 no address or netmask or because the capture is being done on
172 the Linux "any" interface, which can capture on more than one
173 interface, this option will not work correctly.
174 -F file
176 Use file as input for the filter expression. An additional
177 expression given on the command line is ignored.
178 -G rotate_seconds
180 If specified, rotates the dump file specified with the -w option
181 every rotate_seconds seconds. Savefiles will have the name
182 specified by -w which should include a time format as defined by
183 strftime(3). If no time format is specified, each new file will
184 overwrite the previous.
185 If used in conjunction with the -C option, filenames will take
187 the form of `file<count>'.
188 -h
190 --help Print the tcpdump and libpcap version strings, print a usage
191 message, and exit.
192 --version
194 Print the tcpdump and libpcap version strings and exit.
195 -H Attempt to detect 802.11s draft mesh headers.
197 -i interface
199 --interface=interface
200 Listen on interface. If unspecified, tcpdump searches the sys‐
201 tem interface list for the lowest numbered, configured up inter‐
202 face (excluding loopback), which may turn out to be, for exam‐
203 ple, ``eth0''.
204 On Linux systems with 2.2 or later kernels, an interface argu‐
206 ment of ``any'' can be used to capture packets from all inter‐
207 faces. Note that captures on the ``any'' device will not be
208 done in promiscuous mode.
209 If the -D flag is supported, an interface number as printed by
211 that flag can be used as the interface argument, if no interface
212 on the system has that number as a name.
213 -I
215 --monitor-mode
216 Put the interface in "monitor mode"; this is supported only on
217 IEEE 802.11 Wi-Fi interfaces, and supported only on some operat‐
218 ing systems.
219 Note that in monitor mode the adapter might disassociate from
221 the network with which it's associated, so that you will not be
222 able to use any wireless networks with that adapter. This could
223 prevent accessing files on a network server, or resolving host
224 names or network addresses, if you are capturing in monitor mode
225 and are not connected to another network with another adapter.
226 This flag will affect the output of the -L flag. If -I isn't
228 specified, only those link-layer types available when not in
229 monitor mode will be shown; if -I is specified, only those link-
230 layer types available when in monitor mode will be shown.
231 --immediate-mode
233 Capture in "immediate mode". In this mode, packets are deliv‐
234 ered to tcpdump as soon as they arrive, rather than being
235 buffered for efficiency. This is the default when printing
236 packets rather than saving packets to a ``savefile'' if the
237 packets are being printed to a terminal rather than to a file or
238 pipe.
239 -j tstamp_type
241 --time-stamp-type=tstamp_type
242 Set the time stamp type for the capture to tstamp_type. The
243 names to use for the time stamp types are given in pcap-
244 tstamp(7); not all the types listed there will necessarily be
245 valid for any given interface.
246 -J
248 --list-time-stamp-types
249 List the supported time stamp types for the interface and exit.
250 If the time stamp type cannot be set for the interface, no time
251 stamp types are listed.
252 --time-stamp-precision=tstamp_precision
254 When capturing, set the time stamp precision for the capture to
255 tstamp_precision. Note that availability of high precision time
256 stamps (nanoseconds) and their actual accuracy is platform and
257 hardware dependent. Also note that when writing captures made
258 with nanosecond accuracy to a savefile, the time stamps are
259 written with nanosecond resolution, and the file is written with
260 a different magic number, to indicate that the time stamps are
261 in seconds and nanoseconds; not all programs that read pcap
262 savefiles will be able to read those captures.
263 When reading a savefile, convert time stamps to the precision specified
265 by timestamp_precision, and display them with that resolution. If the
266 precision specified is less than the precision of time stamps in the
267 file, the conversion will lose precision.
268 The supported values for timestamp_precision are micro for microsecond
270 resolution and nano for nanosecond resolution. The default is
271 microsecond resolution.
272 -K
274 --dont-verify-checksums
275 Don't attempt to verify IP, TCP, or UDP checksums. This is use‐
276 ful for interfaces that perform some or all of those checksum
277 calculation in hardware; otherwise, all outgoing TCP checksums
278 will be flagged as bad.
279 -l Make stdout line buffered. Useful if you want to see the data
281 while capturing it. E.g.,
282 tcpdump -l | tee dat
284 or
286 tcpdump -l > dat & tail -f dat
288 Note that on Windows,``line buffered'' means ``unbuffered'', so
290 that WinDump will write each character individually if -l is
291 specified.
292 -U is similar to -l in its behavior, but it will cause output to
294 be ``packet-buffered'', so that the output is written to stdout
295 at the end of each packet rather than at the end of each line;
296 this is buffered on all platforms, including Windows.
297 -L
299 --list-data-link-types
300 List the known data link types for the interface, in the speci‐
301 fied mode, and exit. The list of known data link types may be
302 dependent on the specified mode; for example, on some platforms,
303 a Wi-Fi interface might support one set of data link types when
304 not in monitor mode (for example, it might support only fake
305 Ethernet headers, or might support 802.11 headers but not sup‐
306 port 802.11 headers with radio information) and another set of
307 data link types when in monitor mode (for example, it might sup‐
308 port 802.11 headers, or 802.11 headers with radio information,
309 only in monitor mode).
310 -m module
312 Load SMI MIB module definitions from file module. This option
313 can be used several times to load several MIB modules into tcp‐
314 dump.
315 -M secret
317 Use secret as a shared secret for validating the digests found
318 in TCP segments with the TCP-MD5 option (RFC 2385), if present.
319 -n Don't convert host addresses to names. This can be used to
321 avoid DNS lookups.
322 -nn Don't convert protocol and port numbers etc. to names either.
324 -N Don't print domain name qualification of host names. E.g., if
326 you give this flag then tcpdump will print ``nic'' instead of
327 ``nic.ddn.mil''.
328 -#
330 --number
331 Print an optional packet number at the beginning of the line.
332 -O
334 --no-optimize
335 Do not run the packet-matching code optimizer. This is useful
336 only if you suspect a bug in the optimizer.
337 -p
339 --no-promiscuous-mode
340 Don't put the interface into promiscuous mode. Note that the
341 interface might be in promiscuous mode for some other reason;
342 hence, `-p' cannot be used as an abbreviation for `ether host
343 {local-hw-addr} or ether broadcast'.
344 -Q direction
346 --direction=direction
347 Choose send/receive direction direction for which packets should
348 be captured. Possible values are `in', `out' and `inout'. Not
349 available on all platforms.
350 -q Quick (quiet?) output. Print less protocol information so out‐
352 put lines are shorter.
353 -r file
355 Read packets from file (which was created with the -w option or
356 by other tools that write pcap or pcap-ng files). Standard
357 input is used if file is ``-''.
358 -S
360 --absolute-tcp-sequence-numbers
361 Print absolute, rather than relative, TCP sequence numbers.
362 -s snaplen
364 --snapshot-length=snaplen
365 Snarf snaplen bytes of data from each packet rather than the
366 default of 262144 bytes. Packets truncated because of a limited
367 snapshot are indicated in the output with ``[|proto]'', where
368 proto is the name of the protocol level at which the truncation
369 has occurred. Note that taking larger snapshots both increases
370 the amount of time it takes to process packets and, effectively,
371 decreases the amount of packet buffering. This may cause pack‐
372 ets to be lost. You should limit snaplen to the smallest number
373 that will capture the protocol information you're interested in.
374 Setting snaplen to 0 sets it to the default of 262144, for back‐
375 wards compatibility with recent older versions of tcpdump.
376 -T type
378 Force packets selected by "expression" to be interpreted the
379 specified type. Currently known types are aodv (Ad-hoc On-
380 demand Distance Vector protocol), carp (Common Address Redun‐
381 dancy Protocol), cnfp (Cisco NetFlow protocol), lmp (Link Man‐
382 agement Protocol), pgm (Pragmatic General Multicast), pgm_zmtp1
383 (ZMTP/1.0 inside PGM/EPGM), resp (REdis Serialization Protocol),
384 radius (RADIUS), rpc (Remote Procedure Call), rtp (Real-Time
385 Applications protocol), rtcp (Real-Time Applications control
386 protocol), snmp (Simple Network Management Protocol), tftp
387 (Trivial File Transfer Protocol), vat (Visual Audio Tool), wb
388 (distributed White Board), zmtp1 (ZeroMQ Message Transport Pro‐
389 tocol 1.0) and vxlan (Virtual eXtensible Local Area Network).
390 Note that the pgm type above affects UDP interpretation only,
392 the native PGM is always recognised as IP protocol 113 regard‐
393 less. UDP-encapsulated PGM is often called "EPGM" or "PGM/UDP".
394 Note that the pgm_zmtp1 type above affects interpretation of
396 both native PGM and UDP at once. During the native PGM decoding
397 the application data of an ODATA/RDATA packet would be decoded
398 as a ZeroMQ datagram with ZMTP/1.0 frames. During the UDP
399 decoding in addition to that any UDP packet would be treated as
400 an encapsulated PGM packet.
401 -t Don't print a timestamp on each dump line.
403 -tt Print the timestamp, as seconds since January 1, 1970, 00:00:00,
405 UTC, and fractions of a second since that time, on each dump
406 line.
407 -ttt Print a delta (micro-second resolution) between current and pre‐
409 vious line on each dump line.
410 -tttt Print a timestamp, as hours, minutes, seconds, and fractions of
412 a second since midnight, preceded by the date, on each dump
413 line.
414 -ttttt Print a delta (micro-second resolution) between current and
416 first line on each dump line.
417 -u Print undecoded NFS handles.
419 -U
421 --packet-buffered
422 If the -w option is not specified, make the printed packet out‐
423 put ``packet-buffered''; i.e., as the description of the con‐
424 tents of each packet is printed, it will be written to the stan‐
425 dard output, rather than, when not writing to a terminal, being
426 written only when the output buffer fills.
427 If the -w option is specified, make the saved raw packet output
429 ``packet-buffered''; i.e., as each packet is saved, it will be
430 written to the output file, rather than being written only when
431 the output buffer fills.
432 The -U flag will not be supported if tcpdump was built with an
434 older version of libpcap that lacks the pcap_dump_flush() func‐
435 tion.
436 -v When parsing and printing, produce (slightly more) verbose out‐
438 put. For example, the time to live, identification, total
439 length and options in an IP packet are printed. Also enables
440 additional packet integrity checks such as verifying the IP and
441 ICMP header checksum.
442 When writing to a file with the -w option, report, every 10 sec‐
444 onds, the number of packets captured.
445 -vv Even more verbose output. For example, additional fields are
447 printed from NFS reply packets, and SMB packets are fully
448 decoded.
449 -vvv Even more verbose output. For example, telnet SB ... SE options
451 are printed in full. With -X Telnet options are printed in hex
452 as well.
453 -V file
455 Read a list of filenames from file. Standard input is used if
456 file is ``-''.
457 -w file
459 Write the raw packets to file rather than parsing and printing
460 them out. They can later be printed with the -r option. Stan‐
461 dard output is used if file is ``-''.
462 This output will be buffered if written to a file or pipe, so a
464 program reading from the file or pipe may not see packets for an
465 arbitrary amount of time after they are received. Use the -U
466 flag to cause packets to be written as soon as they are
467 received.
468 The MIME type application/vnd.tcpdump.pcap has been registered
470 with IANA for pcap files. The filename extension .pcap appears
471 to be the most commonly used along with .cap and .dmp. Tcpdump
472 itself doesn't check the extension when reading capture files
473 and doesn't add an extension when writing them (it uses magic
474 numbers in the file header instead). However, many operating
475 systems and applications will use the extension if it is present
476 and adding one (e.g. .pcap) is recommended.
477 See pcap-savefile(5) for a description of the file format.
479 -W Used in conjunction with the -C option, this will limit the num‐
481 ber of files created to the specified number, and begin over‐
482 writing files from the beginning, thus creating a 'rotating'
483 buffer. In addition, it will name the files with enough leading
484 0s to support the maximum number of files, allowing them to sort
485 correctly.
486 Used in conjunction with the -G option, this will limit the num‐
488 ber of rotated dump files that get created, exiting with status
489 0 when reaching the limit. If used with -C as well, the behavior
490 will result in cyclical files per timeslice.
491 -x When parsing and printing, in addition to printing the headers
493 of each packet, print the data of each packet (minus its link
494 level header) in hex. The smaller of the entire packet or
495 snaplen bytes will be printed. Note that this is the entire
496 link-layer packet, so for link layers that pad (e.g. Ethernet),
497 the padding bytes will also be printed when the higher layer
498 packet is shorter than the required padding.
499 -xx When parsing and printing, in addition to printing the headers
501 of each packet, print the data of each packet, including its
502 link level header, in hex.
503 -X When parsing and printing, in addition to printing the headers
505 of each packet, print the data of each packet (minus its link
506 level header) in hex and ASCII. This is very handy for
507 analysing new protocols.
508 -XX When parsing and printing, in addition to printing the headers
510 of each packet, print the data of each packet, including its
511 link level header, in hex and ASCII.
512 -y datalinktype
514 --linktype=datalinktype
515 Set the data link type to use while capturing packets to
516 datalinktype.
517 -z postrotate-command
519 Used in conjunction with the -C or -G options, this will make
520 tcpdump run " postrotate-command file " where file is the save‐
521 file being closed after each rotation. For example, specifying
522 -z gzip or -z bzip2 will compress each savefile using gzip or
523 bzip2.
524 Note that tcpdump will run the command in parallel to the cap‐
526 ture, using the lowest priority so that this doesn't disturb the
527 capture process.
528 And in case you would like to use a command that itself takes
530 flags or different arguments, you can always write a shell
531 script that will take the savefile name as the only argument,
532 make the flags & arguments arrangements and execute the command
533 that you want.
534 -Z user
536 --relinquish-privileges=user
537 If tcpdump is running as root, after opening the capture device
538 or input savefile, but before opening any savefiles for output,
539 change the user ID to user and the group ID to the primary group
540 of user.
541 This behavior is enabled by default (-Z tcpdump), and can be
543 disabled by -Z root.
544 expression
547 selects which packets will be dumped. If no expression is
548 given, all packets on the net will be dumped. Otherwise, only
549 packets for which expression is `true' will be dumped.
550 For the expression syntax, see pcap-filter(7).
552 The expression argument can be passed to tcpdump as either a
554 single Shell argument, or as multiple Shell arguments, whichever
555 is more convenient. Generally, if the expression contains Shell
556 metacharacters, such as backslashes used to escape protocol
557 names, it is easier to pass it as a single, quoted argument
558 rather than to escape the Shell metacharacters. Multiple argu‐
559 ments are concatenated with spaces before being parsed.
560
562 To print all packets arriving at or departing from sundown:
563 tcpdump host sundown
564 To print traffic between helios and either hot or ace:
566 tcpdump host helios and \( hot or ace \)
567 To print all IP packets between ace and any host except helios:
569 tcpdump ip host ace and not helios
570 To print all traffic between local hosts and hosts at Berkeley:
572 tcpdump net ucb-ether
573 To print all ftp traffic through internet gateway snup: (note that the
575 expression is quoted to prevent the shell from (mis-)interpreting the
576 parentheses):
577 tcpdump 'gateway snup and (port ftp or ftp-data)'
578 To print traffic neither sourced from nor destined for local hosts (if
580 you gateway to one other net, this stuff should never make it onto your
581 local net).
582 tcpdump ip and not net localnet
583 To print the start and end packets (the SYN and FIN packets) of each
585 TCP conversation that involves a non-local host.
586 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'
587 To print all IPv4 HTTP packets to and from port 80, i.e. print only
589 packets that contain data, not, for example, SYN and FIN packets and
590 ACK-only packets. (IPv6 is left as an exercise for the reader.)
591 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
592 To print IP packets longer than 576 bytes sent through gateway snup:
594 tcpdump 'gateway snup and ip[2:2] > 576'
595 To print IP broadcast or multicast packets that were not sent via Eth‐
597 ernet broadcast or multicast:
598 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
599 To print all ICMP packets that are not echo requests/replies (i.e., not
601 ping packets):
602 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
603
605 The output of tcpdump is protocol dependent. The following gives a
606 brief description and examples of most of the formats.
607 Timestamps
609 By default, all output lines are preceded by a timestamp. The time‐
611 stamp is the current clock time in the form
612 hh:mm:ss.frac
613 and is as accurate as the kernel's clock. The timestamp reflects the
614 time the kernel applied a time stamp to the packet. No attempt is made
615 to account for the time lag between when the network interface finished
616 receiving the packet from the network and when the kernel applied a
617 time stamp to the packet; that time lag could include a delay between
618 the time when the network interface finished receiving a packet from
619 the network and the time when an interrupt was delivered to the kernel
620 to get it to read the packet and a delay between the time when the ker‐
621 nel serviced the `new packet' interrupt and the time when it applied a
622 time stamp to the packet.
623 Link Level Headers
625 If the '-e' option is given, the link level header is printed out. On
627 Ethernets, the source and destination addresses, protocol, and packet
628 length are printed.
629 On FDDI networks, the '-e' option causes tcpdump to print the `frame
631 control' field, the source and destination addresses, and the packet
632 length. (The `frame control' field governs the interpretation of the
633 rest of the packet. Normal packets (such as those containing IP data‐
634 grams) are `async' packets, with a priority value between 0 and 7; for
635 example, `async4'. Such packets are assumed to contain an 802.2 Logi‐
636 cal Link Control (LLC) packet; the LLC header is printed if it is not
637 an ISO datagram or a so-called SNAP packet.
638 On Token Ring networks, the '-e' option causes tcpdump to print the
640 `access control' and `frame control' fields, the source and destination
641 addresses, and the packet length. As on FDDI networks, packets are
642 assumed to contain an LLC packet. Regardless of whether the '-e'
643 option is specified or not, the source routing information is printed
644 for source-routed packets.
645 On 802.11 networks, the '-e' option causes tcpdump to print the `frame
647 control' fields, all of the addresses in the 802.11 header, and the
648 packet length. As on FDDI networks, packets are assumed to contain an
649 LLC packet.
650 (N.B.: The following description assumes familiarity with the SLIP com‐
652 pression algorithm described in RFC-1144.)
653 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for out‐
655 bound), packet type, and compression information are printed out. The
656 packet type is printed first. The three types are ip, utcp, and ctcp.
657 No further link information is printed for ip packets. For TCP pack‐
658 ets, the connection identifier is printed following the type. If the
659 packet is compressed, its encoded header is printed out. The special
660 cases are printed out as *S+n and *SA+n, where n is the amount by which
661 the sequence number (or sequence number and ack) has changed. If it is
662 not a special case, zero or more changes are printed. A change is
663 indicated by U (urgent pointer), W (window), A (ack), S (sequence num‐
664 ber), and I (packet ID), followed by a delta (+n or -n), or a new value
665 (=n). Finally, the amount of data in the packet and compressed header
666 length are printed.
667 For example, the following line shows an outbound compressed TCP
669 packet, with an implicit connection identifier; the ack has changed by
670 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes
671 of data and 6 bytes of compressed header:
672 O ctcp * A+6 S+49 I+6 3 (6)
673 ARP/RARP Packets
675 Arp/rarp output shows the type of request and its arguments. The for‐
677 mat is intended to be self explanatory. Here is a short sample taken
678 from the start of an `rlogin' from host rtsg to host csam:
679 arp who-has csam tell rtsg
680 arp reply csam is-at CSAM
681 The first line says that rtsg sent an arp packet asking for the Ether‐
682 net address of internet host csam. Csam replies with its Ethernet
683 address (in this example, Ethernet addresses are in caps and internet
684 addresses in lower case).
685 This would look less redundant if we had done tcpdump -n:
687 arp who-has 128.3.254.6 tell 128.3.254.68
688 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4
689 If we had done tcpdump -e, the fact that the first packet is broadcast
691 and the second is point-to-point would be visible:
692 RTSG Broadcast 0806 64: arp who-has csam tell rtsg
693 CSAM RTSG 0806 64: arp reply csam is-at CSAM
694 For the first packet this says the Ethernet source address is RTSG, the
695 destination is the Ethernet broadcast address, the type field contained
696 hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
697 IPv4 Packets
699 If the link-layer header is not being printed, for IPv4 packets, IP is
701 printed after the time stamp.
702 If the -v flag is specified, information from the IPv4 header is shown
704 in parentheses after the IP or the link-layer header. The general for‐
705 mat of this information is:
706 tos tos, ttl ttl, id id, offset offset, flags [flags], proto proto, length length, options (options)
707 tos is the type of service field; if the ECN bits are non-zero, those
708 are reported as ECT(1), ECT(0), or CE. ttl is the time-to-live; it is
709 not reported if it is zero. id is the IP identification field. offset
710 is the fragment offset field; it is printed whether this is part of a
711 fragmented datagram or not. flags are the MF and DF flags; + is
712 reported if MF is set, and DFP is reported if F is set. If neither are
713 set, . is reported. proto is the protocol ID field. length is the
714 total length field. options are the IP options, if any.
715 Next, for TCP and UDP packets, the source and destination IP addresses
717 and TCP or UDP ports, with a dot between each IP address and its corre‐
718 sponding port, will be printed, with a > separating the source and des‐
719 tination. For other protocols, the addresses will be printed, with a >
720 separating the source and destination. Higher level protocol informa‐
721 tion, if any, will be printed after that.
722 For fragmented IP datagrams, the first fragment contains the higher
724 level protocol header; fragments after the first contain no higher
725 level protocol header. Fragmentation information will be printed only
726 with the -v flag, in the IP header information, as described above.
727 TCP Packets
729 (N.B.:The following description assumes familiarity with the TCP proto‐
731 col described in RFC-793. If you are not familiar with the protocol,
732 this description will not be of much use to you.)
733 The general format of a TCP protocol line is:
735 src > dst: Flags [tcpflags], seq data-seqno, ack ackno, win window, urg urgent, options [opts], length len
736 Src and dst are the source and destination IP addresses and ports.
737 Tcpflags are some combination of S (SYN), F (FIN), P (PUSH), R (RST), U
738 (URG), W (ECN CWR), E (ECN-Echo) or `.' (ACK), or `none' if no flags
739 are set. Data-seqno describes the portion of sequence space covered by
740 the data in this packet (see example below). Ackno is sequence number
741 of the next data expected the other direction on this connection. Win‐
742 dow is the number of bytes of receive buffer space available the other
743 direction on this connection. Urg indicates there is `urgent' data in
744 the packet. Opts are TCP options (e.g., mss 1024). Len is the length
745 of payload data.
746 Iptype, Src, dst, and flags are always present. The other fields
748 depend on the contents of the packet's TCP protocol header and are out‐
749 put only if appropriate.
750 Here is the opening portion of an rlogin from host rtsg to host csam.
752 IP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024]
753 IP csam.login > rtsg.1023: Flags [S.], seq, 947648:947648, ack 768513, win 4096, opts [mss 1024]
754 IP rtsg.1023 > csam.login: Flags [.], ack 1, win 4096
755 IP rtsg.1023 > csam.login: Flags [P.], seq 1:2, ack 1, win 4096, length 1
756 IP csam.login > rtsg.1023: Flags [.], ack 2, win 4096
757 IP rtsg.1023 > csam.login: Flags [P.], seq 2:21, ack 1, win 4096, length 19
758 IP csam.login > rtsg.1023: Flags [P.], seq 1:2, ack 21, win 4077, length 1
759 IP csam.login > rtsg.1023: Flags [P.], seq 2:3, ack 21, win 4077, urg 1, length 1
760 IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 1
761 The first line says that TCP port 1023 on rtsg sent a packet to port
762 login on csam. The S indicates that the SYN flag was set. The packet
763 sequence number was 768512 and it contained no data. (The notation is
764 `first:last' which means `sequence numbers first up to but not includ‐
765 ing last.) There was no piggy-backed ack, the available receive window
766 was 4096 bytes and there was a max-segment-size option requesting an
767 mss of 1024 bytes.
768 Csam replies with a similar packet except it includes a piggy-backed
770 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means the ACK
771 flag was set. The packet contained no data so there is no data
772 sequence number or length. Note that the ack sequence number is a
773 small integer (1). The first time tcpdump sees a TCP `conversation',
774 it prints the sequence number from the packet. On subsequent packets
775 of the conversation, the difference between the current packet's
776 sequence number and this initial sequence number is printed. This
777 means that sequence numbers after the first can be interpreted as rela‐
778 tive byte positions in the conversation's data stream (with the first
779 data byte each direction being `1'). `-S' will override this feature,
780 causing the original sequence numbers to be output.
781 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
783 in the rtsg → csam side of the conversation). The PUSH flag is set in
784 the packet. On the 7th line, csam says it's received data sent by rtsg
785 up to but not including byte 21. Most of this data is apparently sit‐
786 ting in the socket buffer since csam's receive window has gotten 19
787 bytes smaller. Csam also sends one byte of data to rtsg in this
788 packet. On the 8th and 9th lines, csam sends two bytes of urgent,
789 pushed data to rtsg.
790 If the snapshot was small enough that tcpdump didn't capture the full
792 TCP header, it interprets as much of the header as it can and then
793 reports ``[|tcp]'' to indicate the remainder could not be interpreted.
794 If the header contains a bogus option (one with a length that's either
795 too small or beyond the end of the header), tcpdump reports it as
796 ``[bad opt]'' and does not interpret any further options (since it's
797 impossible to tell where they start). If the header length indicates
798 options are present but the IP datagram length is not long enough for
799 the options to actually be there, tcpdump reports it as ``[bad hdr
800 length]''.
801 Capturing TCP packets with particular flag combinations (SYN-ACK, URG-
803 ACK, etc.)
804 There are 8 bits in the control bits section of the TCP header:
806 CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
808 Let's assume that we want to watch packets used in establishing a TCP
810 connection. Recall that TCP uses a 3-way handshake protocol when it
811 initializes a new connection; the connection sequence with regard to
812 the TCP control bits is
813 1) Caller sends SYN
815 2) Recipient responds with SYN, ACK
816 3) Caller sends ACK
817 Now we're interested in capturing packets that have only the SYN bit
819 set (Step 1). Note that we don't want packets from step 2 (SYN-ACK),
820 just a plain initial SYN. What we need is a correct filter expression
821 for tcpdump.
822 Recall the structure of a TCP header without options:
824 0 15 31
826 -----------------------------------------------------------------
827 | source port | destination port |
828 -----------------------------------------------------------------
829 | sequence number |
830 -----------------------------------------------------------------
831 | acknowledgment number |
832 -----------------------------------------------------------------
833 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
834 -----------------------------------------------------------------
835 | TCP checksum | urgent pointer |
836 -----------------------------------------------------------------
837 A TCP header usually holds 20 octets of data, unless options are
839 present. The first line of the graph contains octets 0 - 3, the second
840 line shows octets 4 - 7 etc.
841 Starting to count with 0, the relevant TCP control bits are contained
843 in octet 13:
844 0 7| 15| 23| 31
846 ----------------|---------------|---------------|----------------
847 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
848 ----------------|---------------|---------------|----------------
849 | | 13th octet | | |
850 Let's have a closer look at octet no. 13:
852 | |
854 |---------------|
855 |C|E|U|A|P|R|S|F|
856 |---------------|
857 |7 5 3 0|
858 These are the TCP control bits we are interested in. We have numbered
860 the bits in this octet from 0 to 7, right to left, so the PSH bit is
861 bit number 3, while the URG bit is number 5.
862 Recall that we want to capture packets with only SYN set. Let's see
864 what happens to octet 13 if a TCP datagram arrives with the SYN bit set
865 in its header:
866 |C|E|U|A|P|R|S|F|
868 |---------------|
869 |0 0 0 0 0 0 1 0|
870 |---------------|
871 |7 6 5 4 3 2 1 0|
872 Looking at the control bits section we see that only bit number 1 (SYN)
874 is set.
875 Assuming that octet number 13 is an 8-bit unsigned integer in network
877 byte order, the binary value of this octet is
878 00000010
880 and its decimal representation is
882 7 6 5 4 3 2 1 0
884 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
885 We're almost done, because now we know that if only SYN is set, the
887 value of the 13th octet in the TCP header, when interpreted as a 8-bit
888 unsigned integer in network byte order, must be exactly 2.
889 This relationship can be expressed as
891 tcp[13] == 2
892 We can use this expression as the filter for tcpdump in order to watch
894 packets which have only SYN set:
895 tcpdump -i xl0 tcp[13] == 2
896 The expression says "let the 13th octet of a TCP datagram have the dec‐
898 imal value 2", which is exactly what we want.
899 Now, let's assume that we need to capture SYN packets, but we don't
901 care if ACK or any other TCP control bit is set at the same time.
902 Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set
903 arrives:
904 |C|E|U|A|P|R|S|F|
906 |---------------|
907 |0 0 0 1 0 0 1 0|
908 |---------------|
909 |7 6 5 4 3 2 1 0|
910 Now bits 1 and 4 are set in the 13th octet. The binary value of octet
912 13 is
913 00010010
915 which translates to decimal
917 7 6 5 4 3 2 1 0
919 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
920 Now we can't just use 'tcp[13] == 18' in the tcpdump filter expression,
922 because that would select only those packets that have SYN-ACK set, but
923 not those with only SYN set. Remember that we don't care if ACK or any
924 other control bit is set as long as SYN is set.
925 In order to achieve our goal, we need to logically AND the binary value
927 of octet 13 with some other value to preserve the SYN bit. We know
928 that we want SYN to be set in any case, so we'll logically AND the
929 value in the 13th octet with the binary value of a SYN:
930 00010010 SYN-ACK 00000010 SYN
932 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
933 -------- --------
934 = 00000010 = 00000010
935 We see that this AND operation delivers the same result regardless
937 whether ACK or another TCP control bit is set. The decimal representa‐
938 tion of the AND value as well as the result of this operation is 2
939 (binary 00000010), so we know that for packets with SYN set the follow‐
940 ing relation must hold true:
941 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
943 This points us to the tcpdump filter expression
945 tcpdump -i xl0 'tcp[13] & 2 == 2'
946 Some offsets and field values may be expressed as names rather than as
948 numeric values. For example tcp[13] may be replaced with tcp[tcpflags].
949 The following TCP flag field values are also available: tcp-fin, tcp-
950 syn, tcp-rst, tcp-push, tcp-act, tcp-urg.
951 This can be demonstrated as:
953 tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
954 Note that you should use single quotes or a backslash in the expression
956 to hide the AND ('&') special character from the shell.
957 UDP Packets
959 UDP format is illustrated by this rwho packet:
961 actinide.who > broadcast.who: udp 84
962 This says that port who on host actinide sent a udp datagram to port
963 who on host broadcast, the Internet broadcast address. The packet con‐
964 tained 84 bytes of user data.
965 Some UDP services are recognized (from the source or destination port
967 number) and the higher level protocol information printed. In particu‐
968 lar, Domain Name service requests (RFC-1034/1035) and Sun RPC calls
969 (RFC-1050) to NFS.
970 UDP Name Server Requests
972 (N.B.:The following description assumes familiarity with the Domain
974 Service protocol described in RFC-1035. If you are not familiar with
975 the protocol, the following description will appear to be written in
976 greek.)
977 Name server requests are formatted as
979 src > dst: id op? flags qtype qclass name (len)
980 h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
981 Host h2opolo asked the domain server on helios for an address record
982 (qtype=A) associated with the name ucbvax.berkeley.edu. The query id
983 was `3'. The `+' indicates the recursion desired flag was set. The
984 query length was 37 bytes, not including the UDP and IP protocol head‐
985 ers. The query operation was the normal one, Query, so the op field
986 was omitted. If the op had been anything else, it would have been
987 printed between the `3' and the `+'. Similarly, the qclass was the
988 normal one, C_IN, and omitted. Any other qclass would have been
989 printed immediately after the `A'.
990 A few anomalies are checked and may result in extra fields enclosed in
992 square brackets: If a query contains an answer, authority records or
993 additional records section, ancount, nscount, or arcount are printed as
994 `[na]', `[nn]' or `[nau]' where n is the appropriate count. If any of
995 the response bits are set (AA, RA or rcode) or any of the `must be
996 zero' bits are set in bytes two and three, `[b2&3=x]' is printed, where
997 x is the hex value of header bytes two and three.
998 UDP Name Server Responses
1000 Name server responses are formatted as
1002 src > dst: id op rcode flags a/n/au type class data (len)
1003 helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1004 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
1005 In the first example, helios responds to query id 3 from h2opolo with 3
1006 answer records, 3 name server records and 7 additional records. The
1007 first answer record is type A (address) and its data is internet
1008 address 128.32.137.3. The total size of the response was 273 bytes,
1009 excluding UDP and IP headers. The op (Query) and response code (NoEr‐
1010 ror) were omitted, as was the class (C_IN) of the A record.
1011 In the second example, helios responds to query 2 with a response code
1013 of non-existent domain (NXDomain) with no answers, one name server and
1014 no authority records. The `*' indicates that the authoritative answer
1015 bit was set. Since there were no answers, no type, class or data were
1016 printed.
1017 Other flag characters that might appear are `-' (recursion available,
1019 RA, not set) and `|' (truncated message, TC, set). If the `question'
1020 section doesn't contain exactly one entry, `[nq]' is printed.
1021 SMB/CIFS decoding
1023 tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on
1025 UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and Net‐
1026 BEUI SMB data is also done.
1027 By default a fairly minimal decode is done, with a much more detailed
1029 decode done if -v is used. Be warned that with -v a single SMB packet
1030 may take up a page or more, so only use -v if you really want all the
1031 gory details.
1032 For information on SMB packet formats and what all the fields mean see
1034 www.cifs.org or the pub/samba/specs/ directory on your favorite
1035 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1036 (tridge@samba.org).
1037 NFS Requests and Replies
1039 Sun NFS (Network File System) requests and replies are printed as:
1041 src.sport > dst.nfs: NFS request xid xid len op args
1042 src.nfs > dst.dport: NFS reply xid xid reply stat len op results
1043 sushi.1023 > wrl.nfs: NFS request xid 26377
1044 112 readlink fh 21,24/10.73165
1045 wrl.nfs > sushi.1023: NFS reply xid 26377
1046 reply ok 40 readlink "../var"
1047 sushi.1022 > wrl.nfs: NFS request xid 8219
1048 144 lookup fh 9,74/4096.6878 "xcolors"
1049 wrl.nfs > sushi.1022: NFS reply xid 8219
1050 reply ok 128 lookup fh 9,74/4134.3150
1051 In the first line, host sushi sends a transaction with id 26377 to wrl.
1052 The request was 112 bytes, excluding the UDP and IP headers. The oper‐
1053 ation was a readlink (read symbolic link) on file handle (fh)
1054 21,24/10.731657119. (If one is lucky, as in this case, the file handle
1055 can be interpreted as a major,minor device number pair, followed by the
1056 inode number and generation number.) In the second line, wrl replies
1057 `ok' with the same transaction id and the contents of the link.
1058 In the third line, sushi asks (using a new transaction id) wrl to
1060 lookup the name `xcolors' in directory file 9,74/4096.6878. In the
1061 fourth line, wrl sends a reply with the respective transaction id.
1062 Note that the data printed depends on the operation type. The format
1064 is intended to be self explanatory if read in conjunction with an NFS
1065 protocol spec. Also note that older versions of tcpdump printed NFS
1066 packets in a slightly different format: the transaction id (xid) would
1067 be printed instead of the non-NFS port number of the packet.
1068 If the -v (verbose) flag is given, additional information is printed.
1070 For example:
1071 sushi.1023 > wrl.nfs: NFS request xid 79658
1072 148 read fh 21,11/12.195 8192 bytes @ 24576
1073 wrl.nfs > sushi.1023: NFS reply xid 79658
1074 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1075 (-v also prints the IP header TTL, ID, length, and fragmentation
1076 fields, which have been omitted from this example.) In the first line,
1077 sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte off‐
1078 set 24576. Wrl replies `ok'; the packet shown on the second line is
1079 the first fragment of the reply, and hence is only 1472 bytes long (the
1080 other bytes will follow in subsequent fragments, but these fragments do
1081 not have NFS or even UDP headers and so might not be printed, depending
1082 on the filter expression used). Because the -v flag is given, some of
1083 the file attributes (which are returned in addition to the file data)
1084 are printed: the file type (``REG'', for regular file), the file mode
1085 (in octal), the uid and gid, and the file size.
1086 If the -v flag is given more than once, even more details are printed.
1088 Note that NFS requests are very large and much of the detail won't be
1090 printed unless snaplen is increased. Try using `-s 192' to watch NFS
1091 traffic.
1092 NFS reply packets do not explicitly identify the RPC operation.
1094 Instead, tcpdump keeps track of ``recent'' requests, and matches them
1095 to the replies using the transaction ID. If a reply does not closely
1096 follow the corresponding request, it might not be parsable.
1097 AFS Requests and Replies
1099 Transarc AFS (Andrew File System) requests and replies are printed as:
1101 src.sport > dst.dport: rx packet-type
1103 src.sport > dst.dport: rx packet-type service call call-name args
1104 src.sport > dst.dport: rx packet-type service reply call-name args
1105 elvis.7001 > pike.afsfs:
1106 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1107 new fid 536876964/1/1 ".newsrc"
1108 pike.afsfs > elvis.7001: rx data fs reply rename
1109 In the first line, host elvis sends a RX packet to pike. This was a RX
1110 data packet to the fs (fileserver) service, and is the start of an RPC
1111 call. The RPC call was a rename, with the old directory file id of
1112 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1113 file id of 536876964/1/1 and a new filename of `.newsrc'. The host
1114 pike responds with a RPC reply to the rename call (which was success‐
1115 ful, because it was a data packet and not an abort packet).
1116 In general, all AFS RPCs are decoded at least by RPC call name. Most
1118 AFS RPCs have at least some of the arguments decoded (generally only
1119 the `interesting' arguments, for some definition of interesting).
1120 The format is intended to be self-describing, but it will probably not
1122 be useful to people who are not familiar with the workings of AFS and
1123 RX.
1124 If the -v (verbose) flag is given twice, acknowledgement packets and
1126 additional header information is printed, such as the RX call ID, call
1127 number, sequence number, serial number, and the RX packet flags.
1128 If the -v flag is given twice, additional information is printed, such
1130 as the RX call ID, serial number, and the RX packet flags. The MTU
1131 negotiation information is also printed from RX ack packets.
1132 If the -v flag is given three times, the security index and service id
1134 are printed.
1135 Error codes are printed for abort packets, with the exception of Ubik
1137 beacon packets (because abort packets are used to signify a yes vote
1138 for the Ubik protocol).
1139 Note that AFS requests are very large and many of the arguments won't
1141 be printed unless snaplen is increased. Try using `-s 256' to watch
1142 AFS traffic.
1143 AFS reply packets do not explicitly identify the RPC operation.
1145 Instead, tcpdump keeps track of ``recent'' requests, and matches them
1146 to the replies using the call number and service ID. If a reply does
1147 not closely follow the corresponding request, it might not be parsable.
1148 KIP AppleTalk (DDP in UDP)
1151 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1153 and dumped as DDP packets (i.e., all the UDP header information is dis‐
1154 carded). The file /etc/atalk.names is used to translate AppleTalk net
1155 and node numbers to names. Lines in this file have the form
1156 number name
1157 1.254 ether
1159 16.1 icsd-net
1160 1.254.110 ace
1161 The first two lines give the names of AppleTalk networks. The third
1162 line gives the name of a particular host (a host is distinguished from
1163 a net by the 3rd octet in the number - a net number must have two
1164 octets and a host number must have three octets.) The number and name
1165 should be separated by whitespace (blanks or tabs). The
1166 /etc/atalk.names file may contain blank lines or comment lines (lines
1167 starting with a `#').
1168 AppleTalk addresses are printed in the form
1170 net.host.port
1171 144.1.209.2 > icsd-net.112.220
1173 office.2 > icsd-net.112.220
1174 jssmag.149.235 > icsd-net.2
1175 (If the /etc/atalk.names doesn't exist or doesn't contain an entry for
1176 some AppleTalk host/net number, addresses are printed in numeric form.)
1177 In the first example, NBP (DDP port 2) on net 144.1 node 209 is sending
1178 to whatever is listening on port 220 of net icsd node 112. The second
1179 line is the same except the full name of the source node is known
1180 (`office'). The third line is a send from port 235 on net jssmag node
1181 149 to broadcast on the icsd-net NBP port (note that the broadcast
1182 address (255) is indicated by a net name with no host number - for this
1183 reason it's a good idea to keep node names and net names distinct in
1184 /etc/atalk.names).
1185 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1187 packets have their contents interpreted. Other protocols just dump the
1188 protocol name (or number if no name is registered for the protocol) and
1189 packet size.
1190 NBP packets are formatted like the following examples:
1192 icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1193 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1194 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
1195 The first line is a name lookup request for laserwriters sent by net
1196 icsd host 112 and broadcast on net jssmag. The nbp id for the lookup
1197 is 190. The second line shows a reply for this request (note that it
1198 has the same id) from host jssmag.209 saying that it has a laserwriter
1199 resource named "RM1140" registered on port 250. The third line is
1200 another reply to the same request saying host techpit has laserwriter
1201 "techpit" registered on port 186.
1202 ATP packet formatting is demonstrated by the following example:
1204 jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1205 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1206 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1207 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1208 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1209 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1210 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1211 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1212 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1213 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1214 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1215 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1216 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1217 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
1218 Jssmag.209 initiates transaction id 12266 with host helios by request‐
1219 ing up to 8 packets (the `<0-7>'). The hex number at the end of the
1220 line is the value of the `userdata' field in the request.
1221 Helios responds with 8 512-byte packets. The `:digit' following the
1223 transaction id gives the packet sequence number in the transaction and
1224 the number in parens is the amount of data in the packet, excluding the
1225 atp header. The `*' on packet 7 indicates that the EOM bit was set.
1226 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1228 resends them then jssmag.209 releases the transaction. Finally, jss‐
1229 mag.209 initiates the next request. The `*' on the request indicates
1230 that XO (`exactly once') was not set.
1231
1234 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5), pcap-fil‐
1235 ter(7), pcap-tstamp(7)
1236 http://www.iana.org/assignments/media-types/application/vnd.tcp‐
1238 dump.pcap
1239
1241 The original authors are:
1242 Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence
1244 Berkeley National Laboratory, University of California, Berkeley, CA.
1245 It is currently being maintained by tcpdump.org.
1247 The current version is available via http:
1249 http://www.tcpdump.org/
1251 The original distribution is available via anonymous ftp:
1253 ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z
1255 IPv6/IPsec support is added by WIDE/KAME project. This program uses
1257 Eric Young's SSLeay library, under specific configurations.
1258
1260 To report a security issue please send an e-mail to
1261 security@tcpdump.org.
1262 To report bugs and other problems, contribute patches, request a fea‐
1264 ture, provide generic feedback etc please see the file CONTRIBUTING in
1265 the tcpdump source tree root.
1266 NIT doesn't let you watch your own outbound traffic, BPF will. We rec‐
1268 ommend that you use the latter.
1269 On Linux systems with 2.0[.x] kernels:
1271 packets on the loopback device will be seen twice;
1273 packet filtering cannot be done in the kernel, so that all pack‐
1275 ets must be copied from the kernel in order to be filtered in
1276 user mode;
1277 all of a packet, not just the part that's within the snapshot
1279 length, will be copied from the kernel (the 2.0[.x] packet cap‐
1280 ture mechanism, if asked to copy only part of a packet to user‐
1281 land, will not report the true length of the packet; this would
1282 cause most IP packets to get an error from tcpdump);
1283 capturing on some PPP devices won't work correctly.
1285 We recommend that you upgrade to a 2.2 or later kernel.
1287 Some attempt should be made to reassemble IP fragments or, at least to
1289 compute the right length for the higher level protocol.
1290 Name server inverse queries are not dumped correctly: the (empty) ques‐
1292 tion section is printed rather than real query in the answer section.
1293 Some believe that inverse queries are themselves a bug and prefer to
1294 fix the program generating them rather than tcpdump.
1295 A packet trace that crosses a daylight savings time change will give
1297 skewed time stamps (the time change is ignored).
1298 Filter expressions on fields other than those in Token Ring headers
1300 will not correctly handle source-routed Token Ring packets.
1301 Filter expressions on fields other than those in 802.11 headers will
1303 not correctly handle 802.11 data packets with both To DS and From DS
1304 set.
1305 ip6 proto should chase header chain, but at this moment it does not.
1307 ip6 protochain is supplied for this behavior.
1308 Arithmetic expression against transport layer headers, like tcp[0],
1310 does not work against IPv6 packets. It only looks at IPv4 packets.
1311 2 February 2017 TCPDUMP(8)