1TCPDUMP(8) System Manager's Manual TCPDUMP(8)
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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|-P 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 -d Dump the compiled packet-matching code in a human readable form
102 to standard output and stop.
103 -dd Dump packet-matching code as a C program fragment.
105 -ddd Dump packet-matching code as decimal numbers (preceded with a
107 count).
108 -D
110 --list-interfaces
111 Print the list of the network interfaces available on the system
112 and on which tcpdump can capture packets. For each network
113 interface, a number and an interface name, possibly followed by
114 a text description of the interface, is printed. The interface
115 name or the number can be supplied to the -i flag to specify an
116 interface on which to capture.
117 This can be useful on systems that don't have a command to list
119 them (e.g., Windows systems, or UNIX systems lacking ifconfig
120 -a); the number can be useful on Windows 2000 and later systems,
121 where the interface name is a somewhat complex string.
122 The -D flag will not be supported if tcpdump was built with an
124 older version of libpcap that lacks the pcap_findalldevs() func‐
125 tion.
126 -e Print the link-level header on each dump line. This can be
128 used, for example, to print MAC layer addresses for protocols
129 such as Ethernet and IEEE 802.11.
130 -E Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that
132 are addressed to addr and contain Security Parameter Index value
133 spi. This combination may be repeated with comma or newline sep‐
134 aration.
135 Note that setting the secret for IPv4 ESP packets is supported
137 at this time.
138 Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc,
140 cast128-cbc, or none. The default is des-cbc. The ability to
141 decrypt packets is only present if tcpdump was compiled with
142 cryptography enabled.
143 secret is the ASCII text for ESP secret key. If preceded by 0x,
145 then a hex value will be read.
146 The option assumes RFC2406 ESP, not RFC1827 ESP. The option is
148 only for debugging purposes, and the use of this option with a
149 true `secret' key is discouraged. By presenting IPsec secret
150 key onto command line you make it visible to others, via ps(1)
151 and other occasions.
152 In addition to the above syntax, the syntax file name may be
154 used to have tcpdump read the provided file in. The file is
155 opened upon receiving the first ESP packet, so any special per‐
156 missions that tcpdump may have been given should already have
157 been given up.
158 -f Print `foreign' IPv4 addresses numerically rather than symboli‐
160 cally (this option is intended to get around serious brain dam‐
161 age in Sun's NIS server — usually it hangs forever translating
162 non-local internet numbers).
163 The test for `foreign' IPv4 addresses is done using the IPv4
165 address and netmask of the interface on which capture is being
166 done. If that address or netmask are not available, available,
167 either because the interface on which capture is being done has
168 no address or netmask or because the capture is being done on
169 the Linux "any" interface, which can capture on more than one
170 interface, this option will not work correctly.
171 -F file
173 Use file as input for the filter expression. An additional
174 expression given on the command line is ignored.
175 -G rotate_seconds
177 If specified, rotates the dump file specified with the -w option
178 every rotate_seconds seconds. Savefiles will have the name
179 specified by -w which should include a time format as defined by
180 strftime(3). If no time format is specified, each new file will
181 overwrite the previous.
182 If used in conjunction with the -C option, filenames will take
184 the form of `file<count>'.
185 -h
187 --help Print the tcpdump and libpcap version strings, print a usage
188 message, and exit.
189 --version
191 Print the tcpdump and libpcap version strings and exit.
192 -H Attempt to detect 802.11s draft mesh headers.
194 -i interface
196 --interface=interface
197 Listen on interface. If unspecified, tcpdump searches the sys‐
198 tem interface list for the lowest numbered, configured up inter‐
199 face (excluding loopback), which may turn out to be, for exam‐
200 ple, ``eth0''.
201 On Linux systems with 2.2 or later kernels, an interface argu‐
203 ment of ``any'' can be used to capture packets from all inter‐
204 faces. Note that captures on the ``any'' device will not be
205 done in promiscuous mode.
206 If the -D flag is supported, an interface number as printed by
208 that flag can be used as the interface argument, if no interface
209 on the system has that number as a name.
210 -I
212 --monitor-mode
213 Put the interface in "monitor mode"; this is supported only on
214 IEEE 802.11 Wi-Fi interfaces, and supported only on some operat‐
215 ing systems.
216 Note that in monitor mode the adapter might disassociate from
218 the network with which it's associated, so that you will not be
219 able to use any wireless networks with that adapter. This could
220 prevent accessing files on a network server, or resolving host
221 names or network addresses, if you are capturing in monitor mode
222 and are not connected to another network with another adapter.
223 This flag will affect the output of the -L flag. If -I isn't
225 specified, only those link-layer types available when not in
226 monitor mode will be shown; if -I is specified, only those link-
227 layer types available when in monitor mode will be shown.
228 --immediate-mode
230 Capture in "immediate mode". In this mode, packets are deliv‐
231 ered to tcpdump as soon as they arrive, rather than being
232 buffered for efficiency. This is the default when printing
233 packets rather than saving packets to a ``savefile'' if the
234 packets are being printed to a terminal rather than to a file or
235 pipe.
236 -j tstamp_type
238 --time-stamp-type=tstamp_type
239 Set the time stamp type for the capture to tstamp_type. The
240 names to use for the time stamp types are given in pcap-
241 tstamp(7); not all the types listed there will necessarily be
242 valid for any given interface.
243 -J
245 --list-time-stamp-types
246 List the supported time stamp types for the interface and exit.
247 If the time stamp type cannot be set for the interface, no time
248 stamp types are listed.
249 --time-stamp-precision=tstamp_precision
251 When capturing, set the time stamp precision for the capture to
252 tstamp_precision. Note that availability of high precision time
253 stamps (nanoseconds) and their actual accuracy is platform and
254 hardware dependent. Also note that when writing captures made
255 with nanosecond accuracy to a savefile, the time stamps are
256 written with nanosecond resolution, and the file is written with
257 a different magic number, to indicate that the time stamps are
258 in seconds and nanoseconds; not all programs that read pcap
259 savefiles will be able to read those captures.
260 When reading a savefile, convert time stamps to the precision specified
262 by timestamp_precision, and display them with that resolution. If the
263 precision specified is less than the precision of time stamps in the
264 file, the conversion will lose precision.
265 The supported values for timestamp_precision are micro for microsecond
267 resolution and nano for nanosecond resolution. The default is
268 microsecond resolution.
269 -K
271 --dont-verify-checksums
272 Don't attempt to verify IP, TCP, or UDP checksums. This is use‐
273 ful for interfaces that perform some or all of those checksum
274 calculation in hardware; otherwise, all outgoing TCP checksums
275 will be flagged as bad.
276 -l Make stdout line buffered. Useful if you want to see the data
278 while capturing it. E.g.,
279 tcpdump -l | tee dat
281 or
283 tcpdump -l > dat & tail -f dat
285 Note that on Windows,``line buffered'' means ``unbuffered'', so
287 that WinDump will write each character individually if -l is
288 specified.
289 -U is similar to -l in its behavior, but it will cause output to
291 be ``packet-buffered'', so that the output is written to stdout
292 at the end of each packet rather than at the end of each line;
293 this is buffered on all platforms, including Windows.
294 -L
296 --list-data-link-types
297 List the known data link types for the interface, in the speci‐
298 fied mode, and exit. The list of known data link types may be
299 dependent on the specified mode; for example, on some platforms,
300 a Wi-Fi interface might support one set of data link types when
301 not in monitor mode (for example, it might support only fake
302 Ethernet headers, or might support 802.11 headers but not sup‐
303 port 802.11 headers with radio information) and another set of
304 data link types when in monitor mode (for example, it might sup‐
305 port 802.11 headers, or 802.11 headers with radio information,
306 only in monitor mode).
307 -m module
309 Load SMI MIB module definitions from file module. This option
310 can be used several times to load several MIB modules into tcp‐
311 dump.
312 -M secret
314 Use secret as a shared secret for validating the digests found
315 in TCP segments with the TCP-MD5 option (RFC 2385), if present.
316 -n Don't convert host addresses to names. This can be used to
318 avoid DNS lookups.
319 -nn Don't convert protocol and port numbers etc. to names either.
321 -N Don't print domain name qualification of host names. E.g., if
323 you give this flag then tcpdump will print ``nic'' instead of
324 ``nic.ddn.mil''.
325 -#
327 --number
328 Print an optional packet number at the beginning of the line.
329 -O
331 --no-optimize
332 Do not run the packet-matching code optimizer. This is useful
333 only if you suspect a bug in the optimizer.
334 -p
336 --no-promiscuous-mode
337 Don't put the interface into promiscuous mode. Note that the
338 interface might be in promiscuous mode for some other reason;
339 hence, `-p' cannot be used as an abbreviation for `ether host
340 {local-hw-addr} or ether broadcast'.
341 -Q|-P direction
343 --direction=direction
344 Choose send/receive direction direction for which packets should
345 be captured. Possible values are `in', `out' and `inout'. Not
346 available on all platforms.
347 -q Quick (quiet?) output. Print less protocol information so out‐
349 put lines are shorter.
350 -r file
352 Read packets from file (which was created with the -w option or
353 by other tools that write pcap or pcap-ng files). Standard
354 input is used if file is ``-''.
355 -S
357 --absolute-tcp-sequence-numbers
358 Print absolute, rather than relative, TCP sequence numbers.
359 -s snaplen
361 --snapshot-length=snaplen
362 Snarf snaplen bytes of data from each packet rather than the
363 default of 262144 bytes. Packets truncated because of a limited
364 snapshot are indicated in the output with ``[|proto]'', where
365 proto is the name of the protocol level at which the truncation
366 has occurred. Note that taking larger snapshots both increases
367 the amount of time it takes to process packets and, effectively,
368 decreases the amount of packet buffering. This may cause pack‐
369 ets to be lost. You should limit snaplen to the smallest number
370 that will capture the protocol information you're interested in.
371 Setting snaplen to 0 sets it to the default of 262144, for back‐
372 wards compatibility with recent older versions of tcpdump.
373 -T type
375 Force packets selected by "expression" to be interpreted the
376 specified type. Currently known types are aodv (Ad-hoc On-
377 demand Distance Vector protocol), carp (Common Address Redun‐
378 dancy Protocol), cnfp (Cisco NetFlow protocol), lmp (Link Man‐
379 agement Protocol), pgm (Pragmatic General Multicast), pgm_zmtp1
380 (ZMTP/1.0 inside PGM/EPGM), resp (REdis Serialization Protocol),
381 radius (RADIUS), rpc (Remote Procedure Call), rtp (Real-Time
382 Applications protocol), rtcp (Real-Time Applications control
383 protocol), snmp (Simple Network Management Protocol), tftp
384 (Trivial File Transfer Protocol), vat (Visual Audio Tool), wb
385 (distributed White Board), zmtp1 (ZeroMQ Message Transport Pro‐
386 tocol 1.0) and vxlan (Virtual eXtensible Local Area Network).
387 Note that the pgm type above affects UDP interpretation only,
389 the native PGM is always recognised as IP protocol 113 regard‐
390 less. UDP-encapsulated PGM is often called "EPGM" or "PGM/UDP".
391 Note that the pgm_zmtp1 type above affects interpretation of
393 both native PGM and UDP at once. During the native PGM decoding
394 the application data of an ODATA/RDATA packet would be decoded
395 as a ZeroMQ datagram with ZMTP/1.0 frames. During the UDP
396 decoding in addition to that any UDP packet would be treated as
397 an encapsulated PGM packet.
398 -t Don't print a timestamp on each dump line.
400 -tt Print the timestamp, as seconds since January 1, 1970, 00:00:00,
402 UTC, and fractions of a second since that time, on each dump
403 line.
404 -ttt Print a delta (micro-second resolution) between current and pre‐
406 vious line on each dump line.
407 -tttt Print a timestamp, as hours, minutes, seconds, and fractions of
409 a second since midnight, preceded by the date, on each dump
410 line.
411 -ttttt Print a delta (micro-second resolution) between current and
413 first line on each dump line.
414 -u Print undecoded NFS handles.
416 -U
418 --packet-buffered
419 If the -w option is not specified, make the printed packet out‐
420 put ``packet-buffered''; i.e., as the description of the con‐
421 tents of each packet is printed, it will be written to the stan‐
422 dard output, rather than, when not writing to a terminal, being
423 written only when the output buffer fills.
424 If the -w option is specified, make the saved raw packet output
426 ``packet-buffered''; i.e., as each packet is saved, it will be
427 written to the output file, rather than being written only when
428 the output buffer fills.
429 The -U flag will not be supported if tcpdump was built with an
431 older version of libpcap that lacks the pcap_dump_flush() func‐
432 tion.
433 -v When parsing and printing, produce (slightly more) verbose out‐
435 put. For example, the time to live, identification, total
436 length and options in an IP packet are printed. Also enables
437 additional packet integrity checks such as verifying the IP and
438 ICMP header checksum.
439 When writing to a file with the -w option, report, every 10 sec‐
441 onds, the number of packets captured.
442 -vv Even more verbose output. For example, additional fields are
444 printed from NFS reply packets, and SMB packets are fully
445 decoded.
446 -vvv Even more verbose output. For example, telnet SB ... SE options
448 are printed in full. With -X Telnet options are printed in hex
449 as well.
450 -V file
452 Read a list of filenames from file. Standard input is used if
453 file is ``-''.
454 -w file
456 Write the raw packets to file rather than parsing and printing
457 them out. They can later be printed with the -r option. Stan‐
458 dard output is used if file is ``-''.
459 This output will be buffered if written to a file or pipe, so a
461 program reading from the file or pipe may not see packets for an
462 arbitrary amount of time after they are received. Use the -U
463 flag to cause packets to be written as soon as they are
464 received.
465 The MIME type application/vnd.tcpdump.pcap has been registered
467 with IANA for pcap files. The filename extension .pcap appears
468 to be the most commonly used along with .cap and .dmp. Tcpdump
469 itself doesn't check the extension when reading capture files
470 and doesn't add an extension when writing them (it uses magic
471 numbers in the file header instead). However, many operating
472 systems and applications will use the extension if it is present
473 and adding one (e.g. .pcap) is recommended.
474 See pcap-savefile(5) for a description of the file format.
476 -W Used in conjunction with the -C option, this will limit the num‐
478 ber of files created to the specified number, and begin over‐
479 writing files from the beginning, thus creating a 'rotating'
480 buffer. In addition, it will name the files with enough leading
481 0s to support the maximum number of files, allowing them to sort
482 correctly.
483 Used in conjunction with the -G option, this will limit the num‐
485 ber of rotated dump files that get created, exiting with status
486 0 when reaching the limit. If used with -C as well, the behavior
487 will result in cyclical files per timeslice.
488 -x When parsing and printing, in addition to printing the headers
490 of each packet, print the data of each packet (minus its link
491 level header) in hex. The smaller of the entire packet or
492 snaplen bytes will be printed. Note that this is the entire
493 link-layer packet, so for link layers that pad (e.g. Ethernet),
494 the padding bytes will also be printed when the higher layer
495 packet is shorter than the required padding.
496 -xx When parsing and printing, in addition to printing the headers
498 of each packet, print the data of each packet, including its
499 link level header, in hex.
500 -X When parsing and printing, in addition to printing the headers
502 of each packet, print the data of each packet (minus its link
503 level header) in hex and ASCII. This is very handy for
504 analysing new protocols.
505 -XX When parsing and printing, in addition to printing the headers
507 of each packet, print the data of each packet, including its
508 link level header, in hex and ASCII.
509 -y datalinktype
511 --linktype=datalinktype
512 Set the data link type to use while capturing packets to
513 datalinktype.
514 -z postrotate-command
516 Used in conjunction with the -C or -G options, this will make
517 tcpdump run " postrotate-command file " where file is the save‐
518 file being closed after each rotation. For example, specifying
519 -z gzip or -z bzip2 will compress each savefile using gzip or
520 bzip2.
521 Note that tcpdump will run the command in parallel to the cap‐
523 ture, using the lowest priority so that this doesn't disturb the
524 capture process.
525 And in case you would like to use a command that itself takes
527 flags or different arguments, you can always write a shell
528 script that will take the savefile name as the only argument,
529 make the flags & arguments arrangements and execute the command
530 that you want.
531 -Z user
533 --relinquish-privileges=user
534 If tcpdump is running as root, after opening the capture device
535 or input savefile, but before opening any savefiles for output,
536 change the user ID to user and the group ID to the primary group
537 of user.
538 This behavior can also be enabled by default at compile time.
540 expression
542 selects which packets will be dumped. If no expression is
543 given, all packets on the net will be dumped. Otherwise, only
544 packets for which expression is `true' will be dumped.
545 For the expression syntax, see pcap-filter(7).
547 The expression argument can be passed to tcpdump as either a
549 single Shell argument, or as multiple Shell arguments, whichever
550 is more convenient. Generally, if the expression contains Shell
551 metacharacters, such as backslashes used to escape protocol
552 names, it is easier to pass it as a single, quoted argument
553 rather than to escape the Shell metacharacters. Multiple argu‐
554 ments are concatenated with spaces before being parsed.
555
557 To print all packets arriving at or departing from sundown:
558 tcpdump host sundown
559 To print traffic between helios and either hot or ace:
561 tcpdump host helios and \( hot or ace \)
562 To print all IP packets between ace and any host except helios:
564 tcpdump ip host ace and not helios
565 To print all traffic between local hosts and hosts at Berkeley:
567 tcpdump net ucb-ether
568 To print all ftp traffic through internet gateway snup: (note that the
570 expression is quoted to prevent the shell from (mis-)interpreting the
571 parentheses):
572 tcpdump 'gateway snup and (port ftp or ftp-data)'
573 To print traffic neither sourced from nor destined for local hosts (if
575 you gateway to one other net, this stuff should never make it onto your
576 local net).
577 tcpdump ip and not net localnet
578 To print the start and end packets (the SYN and FIN packets) of each
580 TCP conversation that involves a non-local host.
581 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'
582 To print all IPv4 HTTP packets to and from port 80, i.e. print only
584 packets that contain data, not, for example, SYN and FIN packets and
585 ACK-only packets. (IPv6 is left as an exercise for the reader.)
586 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
587 To print IP packets longer than 576 bytes sent through gateway snup:
589 tcpdump 'gateway snup and ip[2:2] > 576'
590 To print IP broadcast or multicast packets that were not sent via Eth‐
592 ernet broadcast or multicast:
593 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
594 To print all ICMP packets that are not echo requests/replies (i.e., not
596 ping packets):
597 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
598
600 The output of tcpdump is protocol dependent. The following gives a
601 brief description and examples of most of the formats.
602 Timestamps
604 By default, all output lines are preceded by a timestamp. The time‐
606 stamp is the current clock time in the form
607 hh:mm:ss.frac
608 and is as accurate as the kernel's clock. The timestamp reflects the
609 time the kernel applied a time stamp to the packet. No attempt is made
610 to account for the time lag between when the network interface finished
611 receiving the packet from the network and when the kernel applied a
612 time stamp to the packet; that time lag could include a delay between
613 the time when the network interface finished receiving a packet from
614 the network and the time when an interrupt was delivered to the kernel
615 to get it to read the packet and a delay between the time when the ker‐
616 nel serviced the `new packet' interrupt and the time when it applied a
617 time stamp to the packet.
618 Link Level Headers
620 If the '-e' option is given, the link level header is printed out. On
622 Ethernets, the source and destination addresses, protocol, and packet
623 length are printed.
624 On FDDI networks, the '-e' option causes tcpdump to print the `frame
626 control' field, the source and destination addresses, and the packet
627 length. (The `frame control' field governs the interpretation of the
628 rest of the packet. Normal packets (such as those containing IP data‐
629 grams) are `async' packets, with a priority value between 0 and 7; for
630 example, `async4'. Such packets are assumed to contain an 802.2 Logi‐
631 cal Link Control (LLC) packet; the LLC header is printed if it is not
632 an ISO datagram or a so-called SNAP packet.
633 On Token Ring networks, the '-e' option causes tcpdump to print the
635 `access control' and `frame control' fields, the source and destination
636 addresses, and the packet length. As on FDDI networks, packets are
637 assumed to contain an LLC packet. Regardless of whether the '-e'
638 option is specified or not, the source routing information is printed
639 for source-routed packets.
640 On 802.11 networks, the '-e' option causes tcpdump to print the `frame
642 control' fields, all of the addresses in the 802.11 header, and the
643 packet length. As on FDDI networks, packets are assumed to contain an
644 LLC packet.
645 (N.B.: The following description assumes familiarity with the SLIP com‐
647 pression algorithm described in RFC-1144.)
648 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for out‐
650 bound), packet type, and compression information are printed out. The
651 packet type is printed first. The three types are ip, utcp, and ctcp.
652 No further link information is printed for ip packets. For TCP pack‐
653 ets, the connection identifier is printed following the type. If the
654 packet is compressed, its encoded header is printed out. The special
655 cases are printed out as *S+n and *SA+n, where n is the amount by which
656 the sequence number (or sequence number and ack) has changed. If it is
657 not a special case, zero or more changes are printed. A change is
658 indicated by U (urgent pointer), W (window), A (ack), S (sequence num‐
659 ber), and I (packet ID), followed by a delta (+n or -n), or a new value
660 (=n). Finally, the amount of data in the packet and compressed header
661 length are printed.
662 For example, the following line shows an outbound compressed TCP
664 packet, with an implicit connection identifier; the ack has changed by
665 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes
666 of data and 6 bytes of compressed header:
667 O ctcp * A+6 S+49 I+6 3 (6)
668 ARP/RARP Packets
670 Arp/rarp output shows the type of request and its arguments. The for‐
672 mat is intended to be self explanatory. Here is a short sample taken
673 from the start of an `rlogin' from host rtsg to host csam:
674 arp who-has csam tell rtsg
675 arp reply csam is-at CSAM
676 The first line says that rtsg sent an arp packet asking for the Ether‐
677 net address of internet host csam. Csam replies with its Ethernet
678 address (in this example, Ethernet addresses are in caps and internet
679 addresses in lower case).
680 This would look less redundant if we had done tcpdump -n:
682 arp who-has 128.3.254.6 tell 128.3.254.68
683 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4
684 If we had done tcpdump -e, the fact that the first packet is broadcast
686 and the second is point-to-point would be visible:
687 RTSG Broadcast 0806 64: arp who-has csam tell rtsg
688 CSAM RTSG 0806 64: arp reply csam is-at CSAM
689 For the first packet this says the Ethernet source address is RTSG, the
690 destination is the Ethernet broadcast address, the type field contained
691 hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
692 IPv4 Packets
694 If the link-layer header is not being printed, for IPv4 packets, IP is
696 printed after the time stamp.
697 If the -v flag is specified, information from the IPv4 header is shown
699 in parentheses after the IP or the link-layer header. The general for‐
700 mat of this information is:
701 tos tos, ttl ttl, id id, offset offset, flags [flags], proto proto, length length, options (options)
702 tos is the type of service field; if the ECN bits are non-zero, those
703 are reported as ECT(1), ECT(0), or CE. ttl is the time-to-live; it is
704 not reported if it is zero. id is the IP identification field. offset
705 is the fragment offset field; it is printed whether this is part of a
706 fragmented datagram or not. flags are the MF and DF flags; + is
707 reported if MF is set, and DFP is reported if F is set. If neither are
708 set, . is reported. proto is the protocol ID field. length is the
709 total length field. options are the IP options, if any.
710 Next, for TCP and UDP packets, the source and destination IP addresses
712 and TCP or UDP ports, with a dot between each IP address and its corre‐
713 sponding port, will be printed, with a > separating the source and des‐
714 tination. For other protocols, the addresses will be printed, with a >
715 separating the source and destination. Higher level protocol informa‐
716 tion, if any, will be printed after that.
717 For fragmented IP datagrams, the first fragment contains the higher
719 level protocol header; fragments after the first contain no higher
720 level protocol header. Fragmentation information will be printed only
721 with the -v flag, in the IP header information, as described above.
722 TCP Packets
724 (N.B.:The following description assumes familiarity with the TCP proto‐
726 col described in RFC-793. If you are not familiar with the protocol,
727 this description will not be of much use to you.)
728 The general format of a TCP protocol line is:
730 src > dst: Flags [tcpflags], seq data-seqno, ack ackno, win window, urg urgent, options [opts], length len
731 Src and dst are the source and destination IP addresses and ports.
732 Tcpflags are some combination of S (SYN), F (FIN), P (PUSH), R (RST), U
733 (URG), W (ECN CWR), E (ECN-Echo) or `.' (ACK), or `none' if no flags
734 are set. Data-seqno describes the portion of sequence space covered by
735 the data in this packet (see example below). Ackno is sequence number
736 of the next data expected the other direction on this connection. Win‐
737 dow is the number of bytes of receive buffer space available the other
738 direction on this connection. Urg indicates there is `urgent' data in
739 the packet. Opts are TCP options (e.g., mss 1024). Len is the length
740 of payload data.
741 Iptype, Src, dst, and flags are always present. The other fields
743 depend on the contents of the packet's TCP protocol header and are out‐
744 put only if appropriate.
745 Here is the opening portion of an rlogin from host rtsg to host csam.
747 IP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024]
748 IP csam.login > rtsg.1023: Flags [S.], seq, 947648:947648, ack 768513, win 4096, opts [mss 1024]
749 IP rtsg.1023 > csam.login: Flags [.], ack 1, win 4096
750 IP rtsg.1023 > csam.login: Flags [P.], seq 1:2, ack 1, win 4096, length 1
751 IP csam.login > rtsg.1023: Flags [.], ack 2, win 4096
752 IP rtsg.1023 > csam.login: Flags [P.], seq 2:21, ack 1, win 4096, length 19
753 IP csam.login > rtsg.1023: Flags [P.], seq 1:2, ack 21, win 4077, length 1
754 IP csam.login > rtsg.1023: Flags [P.], seq 2:3, ack 21, win 4077, urg 1, length 1
755 IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 1
756 The first line says that TCP port 1023 on rtsg sent a packet to port
757 login on csam. The S indicates that the SYN flag was set. The packet
758 sequence number was 768512 and it contained no data. (The notation is
759 `first:last' which means `sequence numbers first up to but not includ‐
760 ing last.) There was no piggy-backed ack, the available receive window
761 was 4096 bytes and there was a max-segment-size option requesting an
762 mss of 1024 bytes.
763 Csam replies with a similar packet except it includes a piggy-backed
765 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means the ACK
766 flag was set. The packet contained no data so there is no data
767 sequence number or length. Note that the ack sequence number is a
768 small integer (1). The first time tcpdump sees a TCP `conversation',
769 it prints the sequence number from the packet. On subsequent packets
770 of the conversation, the difference between the current packet's
771 sequence number and this initial sequence number is printed. This
772 means that sequence numbers after the first can be interpreted as rela‐
773 tive byte positions in the conversation's data stream (with the first
774 data byte each direction being `1'). `-S' will override this feature,
775 causing the original sequence numbers to be output.
776 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
778 in the rtsg → csam side of the conversation). The PUSH flag is set in
779 the packet. On the 7th line, csam says it's received data sent by rtsg
780 up to but not including byte 21. Most of this data is apparently sit‐
781 ting in the socket buffer since csam's receive window has gotten 19
782 bytes smaller. Csam also sends one byte of data to rtsg in this
783 packet. On the 8th and 9th lines, csam sends two bytes of urgent,
784 pushed data to rtsg.
785 If the snapshot was small enough that tcpdump didn't capture the full
787 TCP header, it interprets as much of the header as it can and then
788 reports ``[|tcp]'' to indicate the remainder could not be interpreted.
789 If the header contains a bogus option (one with a length that's either
790 too small or beyond the end of the header), tcpdump reports it as
791 ``[bad opt]'' and does not interpret any further options (since it's
792 impossible to tell where they start). If the header length indicates
793 options are present but the IP datagram length is not long enough for
794 the options to actually be there, tcpdump reports it as ``[bad hdr
795 length]''.
796 Capturing TCP packets with particular flag combinations (SYN-ACK, URG-
798 ACK, etc.)
799 There are 8 bits in the control bits section of the TCP header:
801 CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
803 Let's assume that we want to watch packets used in establishing a TCP
805 connection. Recall that TCP uses a 3-way handshake protocol when it
806 initializes a new connection; the connection sequence with regard to
807 the TCP control bits is
808 1) Caller sends SYN
810 2) Recipient responds with SYN, ACK
811 3) Caller sends ACK
812 Now we're interested in capturing packets that have only the SYN bit
814 set (Step 1). Note that we don't want packets from step 2 (SYN-ACK),
815 just a plain initial SYN. What we need is a correct filter expression
816 for tcpdump.
817 Recall the structure of a TCP header without options:
819 0 15 31
821 -----------------------------------------------------------------
822 | source port | destination port |
823 -----------------------------------------------------------------
824 | sequence number |
825 -----------------------------------------------------------------
826 | acknowledgment number |
827 -----------------------------------------------------------------
828 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
829 -----------------------------------------------------------------
830 | TCP checksum | urgent pointer |
831 -----------------------------------------------------------------
832 A TCP header usually holds 20 octets of data, unless options are
834 present. The first line of the graph contains octets 0 - 3, the second
835 line shows octets 4 - 7 etc.
836 Starting to count with 0, the relevant TCP control bits are contained
838 in octet 13:
839 0 7| 15| 23| 31
841 ----------------|---------------|---------------|----------------
842 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
843 ----------------|---------------|---------------|----------------
844 | | 13th octet | | |
845 Let's have a closer look at octet no. 13:
847 | |
849 |---------------|
850 |C|E|U|A|P|R|S|F|
851 |---------------|
852 |7 5 3 0|
853 These are the TCP control bits we are interested in. We have numbered
855 the bits in this octet from 0 to 7, right to left, so the PSH bit is
856 bit number 3, while the URG bit is number 5.
857 Recall that we want to capture packets with only SYN set. Let's see
859 what happens to octet 13 if a TCP datagram arrives with the SYN bit set
860 in its header:
861 |C|E|U|A|P|R|S|F|
863 |---------------|
864 |0 0 0 0 0 0 1 0|
865 |---------------|
866 |7 6 5 4 3 2 1 0|
867 Looking at the control bits section we see that only bit number 1 (SYN)
869 is set.
870 Assuming that octet number 13 is an 8-bit unsigned integer in network
872 byte order, the binary value of this octet is
873 00000010
875 and its decimal representation is
877 7 6 5 4 3 2 1 0
879 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
880 We're almost done, because now we know that if only SYN is set, the
882 value of the 13th octet in the TCP header, when interpreted as a 8-bit
883 unsigned integer in network byte order, must be exactly 2.
884 This relationship can be expressed as
886 tcp[13] == 2
887 We can use this expression as the filter for tcpdump in order to watch
889 packets which have only SYN set:
890 tcpdump -i xl0 tcp[13] == 2
891 The expression says "let the 13th octet of a TCP datagram have the dec‐
893 imal value 2", which is exactly what we want.
894 Now, let's assume that we need to capture SYN packets, but we don't
896 care if ACK or any other TCP control bit is set at the same time.
897 Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set
898 arrives:
899 |C|E|U|A|P|R|S|F|
901 |---------------|
902 |0 0 0 1 0 0 1 0|
903 |---------------|
904 |7 6 5 4 3 2 1 0|
905 Now bits 1 and 4 are set in the 13th octet. The binary value of octet
907 13 is
908 00010010
910 which translates to decimal
912 7 6 5 4 3 2 1 0
914 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
915 Now we can't just use 'tcp[13] == 18' in the tcpdump filter expression,
917 because that would select only those packets that have SYN-ACK set, but
918 not those with only SYN set. Remember that we don't care if ACK or any
919 other control bit is set as long as SYN is set.
920 In order to achieve our goal, we need to logically AND the binary value
922 of octet 13 with some other value to preserve the SYN bit. We know
923 that we want SYN to be set in any case, so we'll logically AND the
924 value in the 13th octet with the binary value of a SYN:
925 00010010 SYN-ACK 00000010 SYN
927 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
928 -------- --------
929 = 00000010 = 00000010
930 We see that this AND operation delivers the same result regardless
932 whether ACK or another TCP control bit is set. The decimal representa‐
933 tion of the AND value as well as the result of this operation is 2
934 (binary 00000010), so we know that for packets with SYN set the follow‐
935 ing relation must hold true:
936 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
938 This points us to the tcpdump filter expression
940 tcpdump -i xl0 'tcp[13] & 2 == 2'
941 Some offsets and field values may be expressed as names rather than as
943 numeric values. For example tcp[13] may be replaced with tcp[tcpflags].
944 The following TCP flag field values are also available: tcp-fin, tcp-
945 syn, tcp-rst, tcp-push, tcp-act, tcp-urg.
946 This can be demonstrated as:
948 tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
949 Note that you should use single quotes or a backslash in the expression
951 to hide the AND ('&') special character from the shell.
952 UDP Packets
954 UDP format is illustrated by this rwho packet:
956 actinide.who > broadcast.who: udp 84
957 This says that port who on host actinide sent a udp datagram to port
958 who on host broadcast, the Internet broadcast address. The packet con‐
959 tained 84 bytes of user data.
960 Some UDP services are recognized (from the source or destination port
962 number) and the higher level protocol information printed. In particu‐
963 lar, Domain Name service requests (RFC-1034/1035) and Sun RPC calls
964 (RFC-1050) to NFS.
965 UDP Name Server Requests
967 (N.B.:The following description assumes familiarity with the Domain
969 Service protocol described in RFC-1035. If you are not familiar with
970 the protocol, the following description will appear to be written in
971 greek.)
972 Name server requests are formatted as
974 src > dst: id op? flags qtype qclass name (len)
975 h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
976 Host h2opolo asked the domain server on helios for an address record
977 (qtype=A) associated with the name ucbvax.berkeley.edu. The query id
978 was `3'. The `+' indicates the recursion desired flag was set. The
979 query length was 37 bytes, not including the UDP and IP protocol head‐
980 ers. The query operation was the normal one, Query, so the op field
981 was omitted. If the op had been anything else, it would have been
982 printed between the `3' and the `+'. Similarly, the qclass was the
983 normal one, C_IN, and omitted. Any other qclass would have been
984 printed immediately after the `A'.
985 A few anomalies are checked and may result in extra fields enclosed in
987 square brackets: If a query contains an answer, authority records or
988 additional records section, ancount, nscount, or arcount are printed as
989 `[na]', `[nn]' or `[nau]' where n is the appropriate count. If any of
990 the response bits are set (AA, RA or rcode) or any of the `must be
991 zero' bits are set in bytes two and three, `[b2&3=x]' is printed, where
992 x is the hex value of header bytes two and three.
993 UDP Name Server Responses
995 Name server responses are formatted as
997 src > dst: id op rcode flags a/n/au type class data (len)
998 helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
999 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
1000 In the first example, helios responds to query id 3 from h2opolo with 3
1001 answer records, 3 name server records and 7 additional records. The
1002 first answer record is type A (address) and its data is internet
1003 address 128.32.137.3. The total size of the response was 273 bytes,
1004 excluding UDP and IP headers. The op (Query) and response code (NoEr‐
1005 ror) were omitted, as was the class (C_IN) of the A record.
1006 In the second example, helios responds to query 2 with a response code
1008 of non-existent domain (NXDomain) with no answers, one name server and
1009 no authority records. The `*' indicates that the authoritative answer
1010 bit was set. Since there were no answers, no type, class or data were
1011 printed.
1012 Other flag characters that might appear are `-' (recursion available,
1014 RA, not set) and `|' (truncated message, TC, set). If the `question'
1015 section doesn't contain exactly one entry, `[nq]' is printed.
1016 SMB/CIFS decoding
1018 tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on
1020 UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and Net‐
1021 BEUI SMB data is also done.
1022 By default a fairly minimal decode is done, with a much more detailed
1024 decode done if -v is used. Be warned that with -v a single SMB packet
1025 may take up a page or more, so only use -v if you really want all the
1026 gory details.
1027 For information on SMB packet formats and what all the fields mean see
1029 www.cifs.org or the pub/samba/specs/ directory on your favorite
1030 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1031 (tridge@samba.org).
1032 NFS Requests and Replies
1034 Sun NFS (Network File System) requests and replies are printed as:
1036 src.sport > dst.nfs: NFS request xid xid len op args
1037 src.nfs > dst.dport: NFS reply xid xid reply stat len op results
1038 sushi.1023 > wrl.nfs: NFS request xid 26377
1039 112 readlink fh 21,24/10.73165
1040 wrl.nfs > sushi.1023: NFS reply xid 26377
1041 reply ok 40 readlink "../var"
1042 sushi.1022 > wrl.nfs: NFS request xid 8219
1043 144 lookup fh 9,74/4096.6878 "xcolors"
1044 wrl.nfs > sushi.1022: NFS reply xid 8219
1045 reply ok 128 lookup fh 9,74/4134.3150
1046 In the first line, host sushi sends a transaction with id 26377 to wrl.
1047 The request was 112 bytes, excluding the UDP and IP headers. The oper‐
1048 ation was a readlink (read symbolic link) on file handle (fh)
1049 21,24/10.731657119. (If one is lucky, as in this case, the file handle
1050 can be interpreted as a major,minor device number pair, followed by the
1051 inode number and generation number.) In the second line, wrl replies
1052 `ok' with the same transaction id and the contents of the link.
1053 In the third line, sushi asks (using a new transaction id) wrl to
1055 lookup the name `xcolors' in directory file 9,74/4096.6878. In the
1056 fourth line, wrl sends a reply with the respective transaction id.
1057 Note that the data printed depends on the operation type. The format
1059 is intended to be self explanatory if read in conjunction with an NFS
1060 protocol spec. Also note that older versions of tcpdump printed NFS
1061 packets in a slightly different format: the transaction id (xid) would
1062 be printed instead of the non-NFS port number of the packet.
1063 If the -v (verbose) flag is given, additional information is printed.
1065 For example:
1066 sushi.1023 > wrl.nfs: NFS request xid 79658
1067 148 read fh 21,11/12.195 8192 bytes @ 24576
1068 wrl.nfs > sushi.1023: NFS reply xid 79658
1069 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1070 (-v also prints the IP header TTL, ID, length, and fragmentation
1071 fields, which have been omitted from this example.) In the first line,
1072 sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte off‐
1073 set 24576. Wrl replies `ok'; the packet shown on the second line is
1074 the first fragment of the reply, and hence is only 1472 bytes long (the
1075 other bytes will follow in subsequent fragments, but these fragments do
1076 not have NFS or even UDP headers and so might not be printed, depending
1077 on the filter expression used). Because the -v flag is given, some of
1078 the file attributes (which are returned in addition to the file data)
1079 are printed: the file type (``REG'', for regular file), the file mode
1080 (in octal), the uid and gid, and the file size.
1081 If the -v flag is given more than once, even more details are printed.
1083 Note that NFS requests are very large and much of the detail won't be
1085 printed unless snaplen is increased. Try using `-s 192' to watch NFS
1086 traffic.
1087 NFS reply packets do not explicitly identify the RPC operation.
1089 Instead, tcpdump keeps track of ``recent'' requests, and matches them
1090 to the replies using the transaction ID. If a reply does not closely
1091 follow the corresponding request, it might not be parsable.
1092 AFS Requests and Replies
1094 Transarc AFS (Andrew File System) requests and replies are printed as:
1096 src.sport > dst.dport: rx packet-type
1098 src.sport > dst.dport: rx packet-type service call call-name args
1099 src.sport > dst.dport: rx packet-type service reply call-name args
1100 elvis.7001 > pike.afsfs:
1101 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1102 new fid 536876964/1/1 ".newsrc"
1103 pike.afsfs > elvis.7001: rx data fs reply rename
1104 In the first line, host elvis sends a RX packet to pike. This was a RX
1105 data packet to the fs (fileserver) service, and is the start of an RPC
1106 call. The RPC call was a rename, with the old directory file id of
1107 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1108 file id of 536876964/1/1 and a new filename of `.newsrc'. The host
1109 pike responds with a RPC reply to the rename call (which was success‐
1110 ful, because it was a data packet and not an abort packet).
1111 In general, all AFS RPCs are decoded at least by RPC call name. Most
1113 AFS RPCs have at least some of the arguments decoded (generally only
1114 the `interesting' arguments, for some definition of interesting).
1115 The format is intended to be self-describing, but it will probably not
1117 be useful to people who are not familiar with the workings of AFS and
1118 RX.
1119 If the -v (verbose) flag is given twice, acknowledgement packets and
1121 additional header information is printed, such as the RX call ID, call
1122 number, sequence number, serial number, and the RX packet flags.
1123 If the -v flag is given twice, additional information is printed, such
1125 as the RX call ID, serial number, and the RX packet flags. The MTU
1126 negotiation information is also printed from RX ack packets.
1127 If the -v flag is given three times, the security index and service id
1129 are printed.
1130 Error codes are printed for abort packets, with the exception of Ubik
1132 beacon packets (because abort packets are used to signify a yes vote
1133 for the Ubik protocol).
1134 Note that AFS requests are very large and many of the arguments won't
1136 be printed unless snaplen is increased. Try using `-s 256' to watch
1137 AFS traffic.
1138 AFS reply packets do not explicitly identify the RPC operation.
1140 Instead, tcpdump keeps track of ``recent'' requests, and matches them
1141 to the replies using the call number and service ID. If a reply does
1142 not closely follow the corresponding request, it might not be parsable.
1143 KIP AppleTalk (DDP in UDP)
1146 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1148 and dumped as DDP packets (i.e., all the UDP header information is dis‐
1149 carded). The file /etc/atalk.names is used to translate AppleTalk net
1150 and node numbers to names. Lines in this file have the form
1151 number name
1152 1.254 ether
1154 16.1 icsd-net
1155 1.254.110 ace
1156 The first two lines give the names of AppleTalk networks. The third
1157 line gives the name of a particular host (a host is distinguished from
1158 a net by the 3rd octet in the number - a net number must have two
1159 octets and a host number must have three octets.) The number and name
1160 should be separated by whitespace (blanks or tabs). The
1161 /etc/atalk.names file may contain blank lines or comment lines (lines
1162 starting with a `#').
1163 AppleTalk addresses are printed in the form
1165 net.host.port
1166 144.1.209.2 > icsd-net.112.220
1168 office.2 > icsd-net.112.220
1169 jssmag.149.235 > icsd-net.2
1170 (If the /etc/atalk.names doesn't exist or doesn't contain an entry for
1171 some AppleTalk host/net number, addresses are printed in numeric form.)
1172 In the first example, NBP (DDP port 2) on net 144.1 node 209 is sending
1173 to whatever is listening on port 220 of net icsd node 112. The second
1174 line is the same except the full name of the source node is known
1175 (`office'). The third line is a send from port 235 on net jssmag node
1176 149 to broadcast on the icsd-net NBP port (note that the broadcast
1177 address (255) is indicated by a net name with no host number - for this
1178 reason it's a good idea to keep node names and net names distinct in
1179 /etc/atalk.names).
1180 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1182 packets have their contents interpreted. Other protocols just dump the
1183 protocol name (or number if no name is registered for the protocol) and
1184 packet size.
1185 NBP packets are formatted like the following examples:
1187 icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1188 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1189 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186
1190 The first line is a name lookup request for laserwriters sent by net
1191 icsd host 112 and broadcast on net jssmag. The nbp id for the lookup
1192 is 190. The second line shows a reply for this request (note that it
1193 has the same id) from host jssmag.209 saying that it has a laserwriter
1194 resource named "RM1140" registered on port 250. The third line is
1195 another reply to the same request saying host techpit has laserwriter
1196 "techpit" registered on port 186.
1197 ATP packet formatting is demonstrated by the following example:
1199 jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1200 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1201 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1202 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1203 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1204 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1205 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1206 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1207 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1208 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1209 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1210 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1211 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1212 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
1213 Jssmag.209 initiates transaction id 12266 with host helios by request‐
1214 ing up to 8 packets (the `<0-7>'). The hex number at the end of the
1215 line is the value of the `userdata' field in the request.
1216 Helios responds with 8 512-byte packets. The `:digit' following the
1218 transaction id gives the packet sequence number in the transaction and
1219 the number in parens is the amount of data in the packet, excluding the
1220 atp header. The `*' on packet 7 indicates that the EOM bit was set.
1221 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1223 resends them then jssmag.209 releases the transaction. Finally, jss‐
1224 mag.209 initiates the next request. The `*' on the request indicates
1225 that XO (`exactly once') was not set.
1226
1229 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5), pcap-fil‐
1230 ter(7), pcap-tstamp(7)
1231 http://www.iana.org/assignments/media-types/application/vnd.tcp‐
1233 dump.pcap
1234
1236 The original authors are:
1237 Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence
1239 Berkeley National Laboratory, University of California, Berkeley, CA.
1240 It is currently being maintained by tcpdump.org.
1242 The current version is available via http:
1244 http://www.tcpdump.org/
1246 The original distribution is available via anonymous ftp:
1248 ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z
1250 IPv6/IPsec support is added by WIDE/KAME project. This program uses
1252 Eric Young's SSLeay library, under specific configurations.
1253
1255 To report a security issue please send an e-mail to
1256 security@tcpdump.org.
1257 To report bugs and other problems, contribute patches, request a fea‐
1259 ture, provide generic feedback etc please see the file CONTRIBUTING in
1260 the tcpdump source tree root.
1261 NIT doesn't let you watch your own outbound traffic, BPF will. We rec‐
1263 ommend that you use the latter.
1264 On Linux systems with 2.0[.x] kernels:
1266 packets on the loopback device will be seen twice;
1268 packet filtering cannot be done in the kernel, so that all pack‐
1270 ets must be copied from the kernel in order to be filtered in
1271 user mode;
1272 all of a packet, not just the part that's within the snapshot
1274 length, will be copied from the kernel (the 2.0[.x] packet cap‐
1275 ture mechanism, if asked to copy only part of a packet to user‐
1276 land, will not report the true length of the packet; this would
1277 cause most IP packets to get an error from tcpdump);
1278 capturing on some PPP devices won't work correctly.
1280 We recommend that you upgrade to a 2.2 or later kernel.
1282 Some attempt should be made to reassemble IP fragments or, at least to
1284 compute the right length for the higher level protocol.
1285 Name server inverse queries are not dumped correctly: the (empty) ques‐
1287 tion section is printed rather than real query in the answer section.
1288 Some believe that inverse queries are themselves a bug and prefer to
1289 fix the program generating them rather than tcpdump.
1290 A packet trace that crosses a daylight savings time change will give
1292 skewed time stamps (the time change is ignored).
1293 Filter expressions on fields other than those in Token Ring headers
1295 will not correctly handle source-routed Token Ring packets.
1296 Filter expressions on fields other than those in 802.11 headers will
1298 not correctly handle 802.11 data packets with both To DS and From DS
1299 set.
1300 ip6 proto should chase header chain, but at this moment it does not.
1302 ip6 protochain is supplied for this behavior.
1303 Arithmetic expression against transport layer headers, like tcp[0],
1305 does not work against IPv6 packets. It only looks at IPv4 packets.
1306 2 February 2017 TCPDUMP(8)