1 .\" @(#) $Header: /tcpdump/master/tcpdump/tcpdump.1.in,v 1.2 2008-11-09 23:35:03 mcr Exp $ (LBL)
3 .\" $NetBSD: tcpdump.8,v 1.9 2003/03/31 00:18:17 perry Exp $
5 .\" Copyright (c) 1987, 1988, 1989, 1990, 1991, 1992, 1994, 1995, 1996, 1997
6 .\" The Regents of the University of California. All rights reserved.
7 .\" All rights reserved.
9 .\" Redistribution and use in source and binary forms, with or without
10 .\" modification, are permitted provided that: (1) source code distributions
11 .\" retain the above copyright notice and this paragraph in its entirety, (2)
12 .\" distributions including binary code include the above copyright notice and
13 .\" this paragraph in its entirety in the documentation or other materials
14 .\" provided with the distribution, and (3) all advertising materials mentioning
15 .\" features or use of this software display the following acknowledgement:
16 .\" ``This product includes software developed by the University of California,
17 .\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
18 .\" the University nor the names of its contributors may be used to endorse
19 .\" or promote products derived from this software without specific prior
20 .\" written permission.
21 .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
22 .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
23 .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
25 .TH TCPDUMP 1 "05 March 2009"
27 tcpdump \- dump traffic on a network
32 .B \-AbdDefhIJKlLnNOpqRStuUvxX
98 .I spi@ipaddr algo:secret,...
108 .I postrotate-command
122 \fITcpdump\fP prints out a description of the contents of packets on a
123 network interface that match the boolean \fIexpression\fP. It can also
126 flag, which causes it to save the packet data to a file for later
127 analysis, and/or with the
129 flag, which causes it to read from a saved packet file rather than to
130 read packets from a network interface. In all cases, only packets that
137 will, if not run with the
139 flag, continue capturing packets until it is interrupted by a SIGINT
140 signal (generated, for example, by typing your interrupt character,
141 typically control-C) or a SIGTERM signal (typically generated with the
143 command); if run with the
145 flag, it will capture packets until it is interrupted by a SIGINT or
146 SIGTERM signal or the specified number of packets have been processed.
150 finishes capturing packets, it will report counts of:
152 packets ``captured'' (this is the number of packets that
154 has received and processed);
156 packets ``received by filter'' (the meaning of this depends on the OS on
159 and possibly on the way the OS was configured - if a filter was
160 specified on the command line, on some OSes it counts packets regardless
161 of whether they were matched by the filter expression and, even if they
162 were matched by the filter expression, regardless of whether
164 has read and processed them yet, on other OSes it counts only packets that were
165 matched by the filter expression regardless of whether
167 has read and processed them yet, and on other OSes it counts only
168 packets that were matched by the filter expression and were processed by
171 packets ``dropped by kernel'' (this is the number of packets that were
172 dropped, due to a lack of buffer space, by the packet capture mechanism
175 is running, if the OS reports that information to applications; if not,
176 it will be reported as 0).
178 On platforms that support the SIGINFO signal, such as most BSDs
179 (including Mac OS X) and Digital/Tru64 UNIX, it will report those counts
180 when it receives a SIGINFO signal (generated, for example, by typing
181 your ``status'' character, typically control-T, although on some
182 platforms, such as Mac OS X, the ``status'' character is not set by
183 default, so you must set it with
185 in order to use it) and will continue capturing packets.
187 Reading packets from a network interface may require that you have
188 special privileges; see the
190 man page for details. Reading a saved packet file doesn't require
195 Print each packet (minus its link level header) in ASCII. Handy for
199 Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN
203 Set the operating system capture buffer size to \fIbuffer_size\fP.
206 Exit after receiving \fIcount\fP packets.
209 Before writing a raw packet to a savefile, check whether the file is
210 currently larger than \fIfile_size\fP and, if so, close the current
211 savefile and open a new one. Savefiles after the first savefile will
212 have the name specified with the
214 flag, with a number after it, starting at 1 and continuing upward.
215 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
216 not 1,048,576 bytes).
219 Dump the compiled packet-matching code in a human readable form to
220 standard output and stop.
223 Dump packet-matching code as a
228 Dump packet-matching code as decimal numbers (preceded with a count).
231 Print the list of the network interfaces available on the system and on
234 can capture packets. For each network interface, a number and an
235 interface name, possibly followed by a text description of the
236 interface, is printed. The interface name or the number can be supplied
239 flag to specify an interface on which to capture.
241 This can be useful on systems that don't have a command to list them
242 (e.g., Windows systems, or UNIX systems lacking
243 .BR "ifconfig \-a" );
244 the number can be useful on Windows 2000 and later systems, where the
245 interface name is a somewhat complex string.
249 flag will not be supported if
251 was built with an older version of
254 .B pcap_findalldevs()
258 Print the link-level header on each dump line.
261 Use \fIspi@ipaddr algo:secret\fP for decrypting IPsec ESP packets that
262 are addressed to \fIaddr\fP and contain Security Parameter Index value
263 \fIspi\fP. This combination may be repeated with comma or newline separation.
265 Note that setting the secret for IPv4 ESP packets is supported at this time.
272 \fBcast128-cbc\fP, or
274 The default is \fBdes-cbc\fP.
275 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
276 with cryptography enabled.
278 \fIsecret\fP is the ASCII text for ESP secret key.
279 If preceded by 0x, then a hex value will be read.
281 The option assumes RFC2406 ESP, not RFC1827 ESP.
282 The option is only for debugging purposes, and
283 the use of this option with a true `secret' key is discouraged.
284 By presenting IPsec secret key onto command line
285 you make it visible to others, via
289 In addition to the above syntax, the syntax \fIfile name\fP may be used
290 to have tcpdump read the provided file in. The file is opened upon
291 receiving the first ESP packet, so any special permissions that tcpdump
292 may have been given should already have been given up.
295 Print `foreign' IPv4 addresses numerically rather than symbolically
296 (this option is intended to get around serious brain damage in
297 Sun's NIS server \(em usually it hangs forever translating non-local
300 The test for `foreign' IPv4 addresses is done using the IPv4 address and
301 netmask of the interface on which capture is being done. If that
302 address or netmask are not available, available, either because the
303 interface on which capture is being done has no address or netmask or
304 because the capture is being done on the Linux "any" interface, which
305 can capture on more than one interface, this option will not work
309 Use \fIfile\fP as input for the filter expression.
310 An additional expression given on the command line is ignored.
313 If specified, rotates the dump file specified with the
315 option every \fIrotate_seconds\fP seconds.
316 Savefiles will have the name specified by
318 which should include a time format as defined by
320 If no time format is specified, each new file will overwrite the previous.
322 If used in conjunction with the
324 option, filenames will take the form of `\fIfile\fP<count>'.
327 Attempt to detect 802.11s draft mesh headers.
330 Listen on \fIinterface\fP.
331 If unspecified, \fItcpdump\fP searches the system interface list for the
332 lowest numbered, configured up interface (excluding loopback).
333 Ties are broken by choosing the earliest match.
335 On Linux systems with 2.2 or later kernels, an
337 argument of ``any'' can be used to capture packets from all interfaces.
338 Note that captures on the ``any'' device will not be done in promiscuous
343 flag is supported, an interface number as printed by that flag can be
349 Put the interface in "monitor mode"; this is supported only on IEEE
350 802.11 Wi-Fi interfaces, and supported only on some operating systems.
352 Note that in monitor mode the adapter might disassociate from the
353 network with which it's associated, so that you will not be able to use
354 any wireless networks with that adapter. This could prevent accessing
355 files on a network server, or resolving host names or network addresses,
356 if you are capturing in monitor mode and are not connected to another
357 network with another adapter.
359 This flag will affect the output of the
363 isn't specified, only those link-layer types available when not in
364 monitor mode will be shown; if
366 is specified, only those link-layer types available when in monitor mode
370 Set the time stamp type for the capture to \fItstamp_type\fP. The names
371 to use for the time stamp types are given in
372 .BR pcap-tstamp-type (@MAN_MISC_INFO@);
373 not all the types listed there will necessarily be valid for any given
377 List the supported time stamp types for the interface and exit. If the
378 time stamp type cannot be set for the interface, no time stamp types are
382 Don't attempt to verify IP, TCP, or UDP checksums. This is useful for
383 interfaces that perform some or all of those checksum calculation in
384 hardware; otherwise, all outgoing TCP checksums will be flagged as bad.
387 Make stdout line buffered.
388 Useful if you want to see the data
392 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
393 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
396 List the known data link types for the interface, in the specified mode,
397 and exit. The list of known data link types may be dependent on the
398 specified mode; for example, on some platforms, a Wi-Fi interface might
399 support one set of data link types when not in monitor mode (for
400 example, it might support only fake Ethernet headers, or might support
401 802.11 headers but not support 802.11 headers with radio information)
402 and another set of data link types when in monitor mode (for example, it
403 might support 802.11 headers, or 802.11 headers with radio information,
404 only in monitor mode).
407 Load SMI MIB module definitions from file \fImodule\fR.
409 can be used several times to load several MIB modules into \fItcpdump\fP.
412 Use \fIsecret\fP as a shared secret for validating the digests found in
413 TCP segments with the TCP-MD5 option (RFC 2385), if present.
416 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
419 Don't print domain name qualification of host names.
421 if you give this flag then \fItcpdump\fP will print ``nic''
422 instead of ``nic.ddn.mil''.
425 Do not run the packet-matching code optimizer.
427 if you suspect a bug in the optimizer.
430 \fIDon't\fP put the interface
431 into promiscuous mode.
432 Note that the interface might be in promiscuous
433 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
434 `ether host {local-hw-addr} or ether broadcast'.
437 Quick (quiet?) output.
438 Print less protocol information so output
442 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
443 If specified, \fItcpdump\fP will not print replay prevention field.
444 Since there is no protocol version field in ESP/AH specification,
445 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
448 Read packets from \fIfile\fR (which was created with the
451 Standard input is used if \fIfile\fR is ``-''.
454 Print absolute, rather than relative, TCP sequence numbers.
457 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
458 default of 65535 bytes.
459 Packets truncated because of a limited snapshot
460 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
461 is the name of the protocol level at which the truncation has occurred.
462 Note that taking larger snapshots both increases
463 the amount of time it takes to process packets and, effectively,
464 decreases the amount of packet buffering.
465 This may cause packets to be
467 You should limit \fIsnaplen\fP to the smallest number that will
468 capture the protocol information you're interested in.
470 \fIsnaplen\fP to 0 sets it to the default of 65535,
471 for backwards compatibility with recent older versions of
475 Force packets selected by "\fIexpression\fP" to be interpreted the
476 specified \fItype\fR.
477 Currently known types are
478 \fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
479 \fBcnfp\fR (Cisco NetFlow protocol),
480 \fBrpc\fR (Remote Procedure Call),
481 \fBrtp\fR (Real-Time Applications protocol),
482 \fBrtcp\fR (Real-Time Applications control protocol),
483 \fBsnmp\fR (Simple Network Management Protocol),
484 \fBtftp\fR (Trivial File Transfer Protocol),
485 \fBvat\fR (Visual Audio Tool),
487 \fBwb\fR (distributed White Board).
490 \fIDon't\fP print a timestamp on each dump line.
493 Print an unformatted timestamp on each dump line.
496 Print a delta (micro-second resolution) between current and previous line
500 Print a timestamp in default format proceeded by date on each dump line.
503 Print a delta (micro-second resolution) between current and first line
507 Print undecoded NFS handles.
510 Make output saved via the
512 option ``packet-buffered''; i.e., as each packet is saved, it will be
513 written to the output file, rather than being written only when the
518 flag will not be supported if
520 was built with an older version of
527 When parsing and printing, produce (slightly more) verbose output.
528 For example, the time to live,
529 identification, total length and options in an IP packet are printed.
530 Also enables additional packet integrity checks such as verifying the
531 IP and ICMP header checksum.
533 When writing to a file with the
535 option, report, every 10 seconds, the number of packets captured.
538 Even more verbose output.
539 For example, additional fields are
540 printed from NFS reply packets, and SMB packets are fully decoded.
543 Even more verbose output.
545 telnet \fBSB\fP ... \fBSE\fP options
549 Telnet options are printed in hex as well.
552 Write the raw packets to \fIfile\fR rather than parsing and printing
554 They can later be printed with the \-r option.
555 Standard output is used if \fIfile\fR is ``-''.
557 .BR pcap-savefile (@MAN_FILE_FORMATS@)
558 for a description of the file format.
561 Used in conjunction with the
563 option, this will limit the number
564 of files created to the specified number, and begin overwriting files
565 from the beginning, thus creating a 'rotating' buffer.
566 In addition, it will name
567 the files with enough leading 0s to support the maximum number of
568 files, allowing them to sort correctly.
570 Used in conjunction with the
572 option, this will limit the number of rotated dump files that get
573 created, exiting with status 0 when reaching the limit. If used with
575 as well, the behavior will result in cyclical files per timeslice.
578 When parsing and printing,
579 in addition to printing the headers of each packet, print the data of
580 each packet (minus its link level header) in hex.
581 The smaller of the entire packet or
583 bytes will be printed. Note that this is the entire link-layer
584 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
585 will also be printed when the higher layer packet is shorter than the
589 When parsing and printing,
590 in addition to printing the headers of each packet, print the data of
593 its link level header, in hex.
596 When parsing and printing,
597 in addition to printing the headers of each packet, print the data of
598 each packet (minus its link level header) in hex and ASCII.
599 This is very handy for analysing new protocols.
602 When parsing and printing,
603 in addition to printing the headers of each packet, print the data of
606 its link level header, in hex and ASCII.
609 Set the data link type to use while capturing packets to \fIdatalinktype\fP.
612 Used in conjunction with the
616 options, this will make
622 is the savefile being closed after each rotation. For example, specifying
626 will compress each savefile using gzip or bzip2.
628 Note that tcpdump will run the command in parallel to the capture, using
629 the lowest priority so that this doesn't disturb the capture process.
631 And in case you would like to use a command that itself takes flags or
632 different arguments, you can always write a shell script that will take the
633 savefile name as the only argument, make the flags & arguments arrangements
634 and execute the command that you want.
639 is running as root, after opening the capture device or input savefile,
640 but before opening any savefiles for output, change the user ID to
642 and the group ID to the primary group of
645 This behavior can also be enabled by default at compile time.
646 .IP "\fI expression\fP"
648 selects which packets will be dumped.
649 If no \fIexpression\fP
650 is given, all packets on the net will be dumped.
652 only packets for which \fIexpression\fP is `true' will be dumped.
654 For the \fIexpression\fP syntax, see
655 .BR pcap-filter (@MAN_MISC_INFO@).
657 Expression arguments can be passed to \fItcpdump\fP as either a single
658 argument or as multiple arguments, whichever is more convenient.
659 Generally, if the expression contains Shell metacharacters, it is
660 easier to pass it as a single, quoted argument.
661 Multiple arguments are concatenated with spaces before being parsed.
664 To print all packets arriving at or departing from \fIsundown\fP:
667 \fBtcpdump host sundown\fP
671 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
674 \fBtcpdump host helios and \\( hot or ace \\)\fP
678 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
681 \fBtcpdump ip host ace and not helios\fP
685 To print all traffic between local hosts and hosts at Berkeley:
689 tcpdump net ucb-ether
693 To print all ftp traffic through internet gateway \fIsnup\fP:
694 (note that the expression is quoted to prevent the shell from
695 (mis-)interpreting the parentheses):
699 tcpdump 'gateway snup and (port ftp or ftp-data)'
703 To print traffic neither sourced from nor destined for local hosts
704 (if you gateway to one other net, this stuff should never make it
705 onto your local net).
709 tcpdump ip and not net \fIlocalnet\fP
713 To print the start and end packets (the SYN and FIN packets) of each
714 TCP conversation that involves a non-local host.
718 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
722 To print all IPv4 HTTP packets to and from port 80, i.e. print only
723 packets that contain data, not, for example, SYN and FIN packets and
724 ACK-only packets. (IPv6 is left as an exercise for the reader.)
728 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
732 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
736 tcpdump 'gateway snup and ip[2:2] > 576'
740 To print IP broadcast or multicast packets that were
742 sent via Ethernet broadcast or multicast:
746 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
750 To print all ICMP packets that are not echo requests/replies (i.e., not
755 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
760 The output of \fItcpdump\fP is protocol dependent.
762 gives a brief description and examples of most of the formats.
770 If the '-e' option is given, the link level header is printed out.
771 On Ethernets, the source and destination addresses, protocol,
772 and packet length are printed.
774 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
775 the `frame control' field, the source and destination addresses,
776 and the packet length.
777 (The `frame control' field governs the
778 interpretation of the rest of the packet.
780 as those containing IP datagrams) are `async' packets, with a priority
781 value between 0 and 7; for example, `\fBasync4\fR'.
783 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
784 the LLC header is printed if it is \fInot\fR an ISO datagram or a
785 so-called SNAP packet.
787 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
788 the `access control' and `frame control' fields, the source and
789 destination addresses, and the packet length.
791 packets are assumed to contain an LLC packet.
792 Regardless of whether
793 the '-e' option is specified or not, the source routing information is
794 printed for source-routed packets.
796 On 802.11 networks, the '-e' option causes \fItcpdump\fP to print
797 the `frame control' fields, all of the addresses in the 802.11 header,
798 and the packet length.
800 packets are assumed to contain an LLC packet.
802 \fI(N.B.: The following description assumes familiarity with
803 the SLIP compression algorithm described in RFC-1144.)\fP
805 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
806 packet type, and compression information are printed out.
807 The packet type is printed first.
808 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
809 No further link information is printed for \fIip\fR packets.
810 For TCP packets, the connection identifier is printed following the type.
811 If the packet is compressed, its encoded header is printed out.
812 The special cases are printed out as
813 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
814 the sequence number (or sequence number and ack) has changed.
815 If it is not a special case,
816 zero or more changes are printed.
817 A change is indicated by U (urgent pointer), W (window), A (ack),
818 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
820 Finally, the amount of data in the packet and compressed header length
823 For example, the following line shows an outbound compressed TCP packet,
824 with an implicit connection identifier; the ack has changed by 6,
825 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
826 data and 6 bytes of compressed header:
829 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
835 Arp/rarp output shows the type of request and its arguments.
837 format is intended to be self explanatory.
838 Here is a short sample taken from the start of an `rlogin' from
839 host \fIrtsg\fP to host \fIcsam\fP:
843 \f(CWarp who-has csam tell rtsg
844 arp reply csam is-at CSAM\fR
848 The first line says that rtsg sent an arp packet asking
849 for the Ethernet address of internet host csam.
851 replies with its Ethernet address (in this example, Ethernet addresses
852 are in caps and internet addresses in lower case).
854 This would look less redundant if we had done \fItcpdump \-n\fP:
858 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
859 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
863 If we had done \fItcpdump \-e\fP, the fact that the first packet is
864 broadcast and the second is point-to-point would be visible:
868 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
869 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
873 For the first packet this says the Ethernet source address is RTSG, the
874 destination is the Ethernet broadcast address, the type field
875 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
879 \fI(N.B.:The following description assumes familiarity with
880 the TCP protocol described in RFC-793.
881 If you are not familiar
882 with the protocol, neither this description nor \fItcpdump\fP will
883 be of much use to you.)\fP
885 The general format of a tcp protocol line is:
889 \fIsrc > dst: flags data-seqno ack window urgent options\fP
893 \fISrc\fP and \fIdst\fP are the source and destination IP
895 \fIFlags\fP are some combination of S (SYN),
896 F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or
897 `.' (ACK), or `none' if no flags are set.
898 \fIData-seqno\fP describes the portion of sequence space covered
899 by the data in this packet (see example below).
900 \fIAck\fP is sequence number of the next data expected the other
901 direction on this connection.
902 \fIWindow\fP is the number of bytes of receive buffer space available
903 the other direction on this connection.
904 \fIUrg\fP indicates there is `urgent' data in the packet.
905 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
907 \fISrc, dst\fP and \fIflags\fP are always present.
909 depend on the contents of the packet's tcp protocol header and
910 are output only if appropriate.
912 Here is the opening portion of an rlogin from host \fIrtsg\fP to
917 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
918 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
919 rtsg.1023 > csam.login: . ack 1 win 4096
920 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
921 csam.login > rtsg.1023: . ack 2 win 4096
922 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
923 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
924 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
925 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
929 The first line says that tcp port 1023 on rtsg sent a packet
932 The \fBS\fP indicates that the \fISYN\fP flag was set.
933 The packet sequence number was 768512 and it contained no data.
934 (The notation is `first:last(nbytes)' which means `sequence
936 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
937 There was no piggy-backed ack, the available receive window was 4096
938 bytes and there was a max-segment-size option requesting an mss of
941 Csam replies with a similar packet except it includes a piggy-backed
943 Rtsg then acks csam's SYN.
944 The `.' means the ACK flag was set.
945 The packet contained no data so there is no data sequence number.
946 Note that the ack sequence
947 number is a small integer (1).
948 The first time \fItcpdump\fP sees a
949 tcp `conversation', it prints the sequence number from the packet.
950 On subsequent packets of the conversation, the difference between
951 the current packet's sequence number and this initial sequence number
953 This means that sequence numbers after the
954 first can be interpreted
955 as relative byte positions in the conversation's data stream (with the
956 first data byte each direction being `1').
957 `-S' will override this
958 feature, causing the original sequence numbers to be output.
960 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
961 in the rtsg \(-> csam side of the conversation).
962 The PUSH flag is set in the packet.
963 On the 7th line, csam says it's received data sent by rtsg up to
964 but not including byte 21.
965 Most of this data is apparently sitting in the
966 socket buffer since csam's receive window has gotten 19 bytes smaller.
967 Csam also sends one byte of data to rtsg in this packet.
968 On the 8th and 9th lines,
969 csam sends two bytes of urgent, pushed data to rtsg.
971 If the snapshot was small enough that \fItcpdump\fP didn't capture
972 the full TCP header, it interprets as much of the header as it can
973 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
975 If the header contains a bogus option (one with a length
976 that's either too small or beyond the end of the header), \fItcpdump\fP
977 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
978 options (since it's impossible to tell where they start).
980 length indicates options are present but the IP datagram length is not
981 long enough for the options to actually be there, \fItcpdump\fP reports
982 it as ``[\fIbad hdr length\fP]''.
984 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
986 There are 8 bits in the control bits section of the TCP header:
988 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
990 Let's assume that we want to watch packets used in establishing
992 Recall that TCP uses a 3-way handshake protocol
993 when it initializes a new connection; the connection sequence with
994 regard to the TCP control bits is
1000 2) Recipient responds with SYN, ACK
1006 Now we're interested in capturing packets that have only the
1007 SYN bit set (Step 1).
1008 Note that we don't want packets from step 2
1009 (SYN-ACK), just a plain initial SYN.
1010 What we need is a correct filter
1011 expression for \fItcpdump\fP.
1013 Recall the structure of a TCP header without options:
1017 -----------------------------------------------------------------
1018 | source port | destination port |
1019 -----------------------------------------------------------------
1021 -----------------------------------------------------------------
1022 | acknowledgment number |
1023 -----------------------------------------------------------------
1024 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1025 -----------------------------------------------------------------
1026 | TCP checksum | urgent pointer |
1027 -----------------------------------------------------------------
1030 A TCP header usually holds 20 octets of data, unless options are
1032 The first line of the graph contains octets 0 - 3, the
1033 second line shows octets 4 - 7 etc.
1035 Starting to count with 0, the relevant TCP control bits are contained
1040 ----------------|---------------|---------------|----------------
1041 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1042 ----------------|---------------|---------------|----------------
1043 | | 13th octet | | |
1046 Let's have a closer look at octet no. 13:
1056 These are the TCP control bits we are interested
1058 We have numbered the bits in this octet from 0 to 7, right to
1059 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1061 Recall that we want to capture packets with only SYN set.
1062 Let's see what happens to octet 13 if a TCP datagram arrives
1063 with the SYN bit set in its header:
1074 control bits section we see that only bit number 1 (SYN) is set.
1076 Assuming that octet number 13 is an 8-bit unsigned integer in
1077 network byte order, the binary value of this octet is
1081 and its decimal representation is
1085 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1088 We're almost done, because now we know that if only SYN is set,
1089 the value of the 13th octet in the TCP header, when interpreted
1090 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1092 This relationship can be expressed as
1098 We can use this expression as the filter for \fItcpdump\fP in order
1099 to watch packets which have only SYN set:
1102 tcpdump -i xl0 tcp[13] == 2
1105 The expression says "let the 13th octet of a TCP datagram have
1106 the decimal value 2", which is exactly what we want.
1108 Now, let's assume that we need to capture SYN packets, but we
1109 don't care if ACK or any other TCP control bit is set at the
1111 Let's see what happens to octet 13 when a TCP datagram
1112 with SYN-ACK set arrives:
1122 Now bits 1 and 4 are set in the 13th octet.
1128 which translates to decimal
1132 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1135 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1136 expression, because that would select only those packets that have
1137 SYN-ACK set, but not those with only SYN set.
1138 Remember that we don't care
1139 if ACK or any other control bit is set as long as SYN is set.
1141 In order to achieve our goal, we need to logically AND the
1142 binary value of octet 13 with some other value to preserve
1144 We know that we want SYN to be set in any case,
1145 so we'll logically AND the value in the 13th octet with
1146 the binary value of a SYN:
1150 00010010 SYN-ACK 00000010 SYN
1151 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1153 = 00000010 = 00000010
1156 We see that this AND operation delivers the same result
1157 regardless whether ACK or another TCP control bit is set.
1158 The decimal representation of the AND value as well as
1159 the result of this operation is 2 (binary 00000010),
1160 so we know that for packets with SYN set the following
1161 relation must hold true:
1163 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1165 This points us to the \fItcpdump\fP filter expression
1168 tcpdump -i xl0 'tcp[13] & 2 == 2'
1171 Some offsets and field values may be expressed as names
1172 rather than as numeric values. For example tcp[13] may
1173 be replaced with tcp[tcpflags]. The following TCP flag
1174 field values are also available: tcp-fin, tcp-syn, tcp-rst,
1175 tcp-push, tcp-act, tcp-urg.
1177 This can be demonstrated as:
1180 tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
1183 Note that you should use single quotes or a backslash
1184 in the expression to hide the AND ('&') special character
1190 UDP format is illustrated by this rwho packet:
1194 \f(CWactinide.who > broadcast.who: udp 84\fP
1198 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1199 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1201 The packet contained 84 bytes of user data.
1203 Some UDP services are recognized (from the source or destination
1204 port number) and the higher level protocol information printed.
1205 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1206 RPC calls (RFC-1050) to NFS.
1208 UDP Name Server Requests
1210 \fI(N.B.:The following description assumes familiarity with
1211 the Domain Service protocol described in RFC-1035.
1212 If you are not familiar
1213 with the protocol, the following description will appear to be written
1216 Name server requests are formatted as
1220 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1222 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1226 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1227 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1228 The query id was `3'.
1229 The `+' indicates the \fIrecursion desired\fP flag
1231 The query length was 37 bytes, not including the UDP and
1232 IP protocol headers.
1233 The query operation was the normal one, \fIQuery\fP,
1234 so the op field was omitted.
1235 If the op had been anything else, it would
1236 have been printed between the `3' and the `+'.
1237 Similarly, the qclass was the normal one,
1238 \fIC_IN\fP, and omitted.
1239 Any other qclass would have been printed
1240 immediately after the `A'.
1242 A few anomalies are checked and may result in extra fields enclosed in
1243 square brackets: If a query contains an answer, authority records or
1244 additional records section,
1249 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1250 is the appropriate count.
1251 If any of the response bits are set (AA, RA or rcode) or any of the
1252 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1253 is printed, where \fIx\fP is the hex value of header bytes two and three.
1255 UDP Name Server Responses
1257 Name server responses are formatted as
1261 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1263 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1264 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1268 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1269 with 3 answer records, 3 name server records and 7 additional records.
1270 The first answer record is type A (address) and its data is internet
1271 address 128.32.137.3.
1272 The total size of the response was 273 bytes,
1273 excluding UDP and IP headers.
1274 The op (Query) and response code
1275 (NoError) were omitted, as was the class (C_IN) of the A record.
1277 In the second example, \fIhelios\fP responds to query 2 with a
1278 response code of non-existent domain (NXDomain) with no answers,
1279 one name server and no authority records.
1280 The `*' indicates that
1281 the \fIauthoritative answer\fP bit was set.
1283 answers, no type, class or data were printed.
1285 Other flag characters that might appear are `\-' (recursion available,
1286 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1288 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1294 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1295 on UDP/137, UDP/138 and TCP/139.
1296 Some primitive decoding of IPX and
1297 NetBEUI SMB data is also done.
1299 By default a fairly minimal decode is done, with a much more detailed
1300 decode done if -v is used.
1301 Be warned that with -v a single SMB packet
1302 may take up a page or more, so only use -v if you really want all the
1305 For information on SMB packet formats and what all te fields mean see
1306 www.cifs.org or the pub/samba/specs/ directory on your favorite
1307 samba.org mirror site.
1308 The SMB patches were written by Andrew Tridgell
1312 NFS Requests and Replies
1314 Sun NFS (Network File System) requests and replies are printed as:
1318 \fIsrc.xid > dst.nfs: len op args\fP
1319 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1322 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1323 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1324 sushi.201b > wrl.nfs:
1325 144 lookup fh 9,74/4096.6878 "xcolors"
1326 wrl.nfs > sushi.201b:
1327 reply ok 128 lookup fh 9,74/4134.3150
1332 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1333 to \fIwrl\fP (note that the number following the src host is a
1334 transaction id, \fInot\fP the source port).
1335 The request was 112 bytes,
1336 excluding the UDP and IP headers.
1337 The operation was a \fIreadlink\fP
1338 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1339 (If one is lucky, as in this case, the file handle can be interpreted
1340 as a major,minor device number pair, followed by the inode number and
1342 \fIWrl\fP replies `ok' with the contents of the link.
1344 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1345 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1346 Note that the data printed
1347 depends on the operation type.
1348 The format is intended to be self
1349 explanatory if read in conjunction with
1350 an NFS protocol spec.
1352 If the \-v (verbose) flag is given, additional information is printed.
1358 sushi.1372a > wrl.nfs:
1359 148 read fh 21,11/12.195 8192 bytes @ 24576
1360 wrl.nfs > sushi.1372a:
1361 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1366 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1367 which have been omitted from this example.) In the first line,
1368 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1369 at byte offset 24576.
1370 \fIWrl\fP replies `ok'; the packet shown on the
1371 second line is the first fragment of the reply, and hence is only 1472
1372 bytes long (the other bytes will follow in subsequent fragments, but
1373 these fragments do not have NFS or even UDP headers and so might not be
1374 printed, depending on the filter expression used).
1375 Because the \-v flag
1376 is given, some of the file attributes (which are returned in addition
1377 to the file data) are printed: the file type (``REG'', for regular file),
1378 the file mode (in octal), the uid and gid, and the file size.
1380 If the \-v flag is given more than once, even more details are printed.
1382 Note that NFS requests are very large and much of the detail won't be printed
1383 unless \fIsnaplen\fP is increased.
1384 Try using `\fB\-s 192\fP' to watch
1387 NFS reply packets do not explicitly identify the RPC operation.
1389 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1390 replies using the transaction ID.
1391 If a reply does not closely follow the
1392 corresponding request, it might not be parsable.
1394 AFS Requests and Replies
1396 Transarc AFS (Andrew File System) requests and replies are printed
1402 \fIsrc.sport > dst.dport: rx packet-type\fP
1403 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1404 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1407 elvis.7001 > pike.afsfs:
1408 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1409 new fid 536876964/1/1 ".newsrc"
1410 pike.afsfs > elvis.7001: rx data fs reply rename
1415 In the first line, host elvis sends a RX packet to pike.
1417 a RX data packet to the fs (fileserver) service, and is the start of
1419 The RPC call was a rename, with the old directory file id
1420 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1421 file id of 536876964/1/1 and a new filename of `.newsrc'.
1423 responds with a RPC reply to the rename call (which was successful, because
1424 it was a data packet and not an abort packet).
1426 In general, all AFS RPCs are decoded at least by RPC call name.
1428 AFS RPCs have at least some of the arguments decoded (generally only
1429 the `interesting' arguments, for some definition of interesting).
1431 The format is intended to be self-describing, but it will probably
1432 not be useful to people who are not familiar with the workings of
1435 If the -v (verbose) flag is given twice, acknowledgement packets and
1436 additional header information is printed, such as the the RX call ID,
1437 call number, sequence number, serial number, and the RX packet flags.
1439 If the -v flag is given twice, additional information is printed,
1440 such as the the RX call ID, serial number, and the RX packet flags.
1441 The MTU negotiation information is also printed from RX ack packets.
1443 If the -v flag is given three times, the security index and service id
1446 Error codes are printed for abort packets, with the exception of Ubik
1447 beacon packets (because abort packets are used to signify a yes vote
1448 for the Ubik protocol).
1450 Note that AFS requests are very large and many of the arguments won't
1451 be printed unless \fIsnaplen\fP is increased.
1452 Try using `\fB-s 256\fP'
1453 to watch AFS traffic.
1455 AFS reply packets do not explicitly identify the RPC operation.
1457 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1458 replies using the call number and service ID.
1459 If a reply does not closely
1461 corresponding request, it might not be parsable.
1464 KIP AppleTalk (DDP in UDP)
1466 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1467 and dumped as DDP packets (i.e., all the UDP header information is
1471 is used to translate AppleTalk net and node numbers to names.
1472 Lines in this file have the form
1484 The first two lines give the names of AppleTalk networks.
1486 line gives the name of a particular host (a host is distinguished
1487 from a net by the 3rd octet in the number \-
1488 a net number \fImust\fP have two octets and a host number \fImust\fP
1489 have three octets.) The number and name should be separated by
1490 whitespace (blanks or tabs).
1493 file may contain blank lines or comment lines (lines starting with
1496 AppleTalk addresses are printed in the form
1502 \f(CW144.1.209.2 > icsd-net.112.220
1503 office.2 > icsd-net.112.220
1504 jssmag.149.235 > icsd-net.2\fR
1510 doesn't exist or doesn't contain an entry for some AppleTalk
1511 host/net number, addresses are printed in numeric form.)
1512 In the first example, NBP (DDP port 2) on net 144.1 node 209
1513 is sending to whatever is listening on port 220 of net icsd node 112.
1514 The second line is the same except the full name of the source node
1515 is known (`office').
1516 The third line is a send from port 235 on
1517 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1518 the broadcast address (255) is indicated by a net name with no host
1519 number \- for this reason it's a good idea to keep node names and
1520 net names distinct in /etc/atalk.names).
1522 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1523 packets have their contents interpreted.
1524 Other protocols just dump
1525 the protocol name (or number if no name is registered for the
1526 protocol) and packet size.
1528 \fBNBP packets\fP are formatted like the following examples:
1532 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1533 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1534 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1538 The first line is a name lookup request for laserwriters sent by net icsd host
1539 112 and broadcast on net jssmag.
1540 The nbp id for the lookup is 190.
1541 The second line shows a reply for this request (note that it has the
1542 same id) from host jssmag.209 saying that it has a laserwriter
1543 resource named "RM1140" registered on port 250.
1545 another reply to the same request saying host techpit has laserwriter
1546 "techpit" registered on port 186.
1548 \fBATP packet\fP formatting is demonstrated by the following example:
1552 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1553 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1554 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1555 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1556 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1557 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1558 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1559 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1560 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1561 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1562 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1563 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1564 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1565 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1569 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1570 up to 8 packets (the `<0-7>').
1571 The hex number at the end of the line
1572 is the value of the `userdata' field in the request.
1574 Helios responds with 8 512-byte packets.
1575 The `:digit' following the
1576 transaction id gives the packet sequence number in the transaction
1577 and the number in parens is the amount of data in the packet,
1578 excluding the atp header.
1579 The `*' on packet 7 indicates that the
1582 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1584 resends them then jssmag.209 releases the transaction.
1586 jssmag.209 initiates the next request.
1587 The `*' on the request
1588 indicates that XO (`exactly once') was \fInot\fP set.
1593 Fragmented Internet datagrams are printed as
1597 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1598 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1602 (The first form indicates there are more fragments.
1604 indicates this is the last fragment.)
1606 \fIId\fP is the fragment id.
1607 \fISize\fP is the fragment
1608 size (in bytes) excluding the IP header.
1609 \fIOffset\fP is this
1610 fragment's offset (in bytes) in the original datagram.
1612 The fragment information is output for each fragment.
1614 fragment contains the higher level protocol header and the frag
1615 info is printed after the protocol info.
1617 after the first contain no higher level protocol header and the
1618 frag info is printed after the source and destination addresses.
1619 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1620 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1624 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1625 arizona > rtsg: (frag 595a:204@328)
1626 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1630 There are a couple of things to note here: First, addresses in the
1631 2nd line don't include port numbers.
1632 This is because the TCP
1633 protocol information is all in the first fragment and we have no idea
1634 what the port or sequence numbers are when we print the later fragments.
1635 Second, the tcp sequence information in the first line is printed as if there
1636 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1637 the first frag and 204 in the second).
1638 If you are looking for holes
1639 in the sequence space or trying to match up acks
1640 with packets, this can fool you.
1642 A packet with the IP \fIdon't fragment\fP flag is marked with a
1643 trailing \fB(DF)\fP.
1647 By default, all output lines are preceded by a timestamp.
1649 is the current clock time in the form
1655 and is as accurate as the kernel's clock.
1656 The timestamp reflects the time the kernel first saw the packet.
1658 is made to account for the time lag between when the
1659 Ethernet interface removed the packet from the wire and when the kernel
1660 serviced the `new packet' interrupt.
1662 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@),
1663 pcap-filter(@MAN_MISC_INFO@), pcap-tstamp-type(@MAN_MISC_INFO@)
1665 The original authors are:
1669 Steven McCanne, all of the
1670 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1672 It is currently being maintained by tcpdump.org.
1674 The current version is available via http:
1677 .I http://www.tcpdump.org/
1680 The original distribution is available via anonymous ftp:
1683 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1686 IPv6/IPsec support is added by WIDE/KAME project.
1687 This program uses Eric Young's SSLeay library, under specific configurations.
1689 Please send problems, bugs, questions, desirable enhancements, patches
1693 tcpdump-workers@lists.tcpdump.org
1696 NIT doesn't let you watch your own outbound traffic, BPF will.
1697 We recommend that you use the latter.
1699 On Linux systems with 2.0[.x] kernels:
1701 packets on the loopback device will be seen twice;
1703 packet filtering cannot be done in the kernel, so that all packets must
1704 be copied from the kernel in order to be filtered in user mode;
1706 all of a packet, not just the part that's within the snapshot length,
1707 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1708 asked to copy only part of a packet to userland, will not report the
1709 true length of the packet; this would cause most IP packets to get an
1713 capturing on some PPP devices won't work correctly.
1715 We recommend that you upgrade to a 2.2 or later kernel.
1717 Some attempt should be made to reassemble IP fragments or, at least
1718 to compute the right length for the higher level protocol.
1720 Name server inverse queries are not dumped correctly: the (empty)
1721 question section is printed rather than real query in the answer
1723 Some believe that inverse queries are themselves a bug and
1724 prefer to fix the program generating them rather than \fItcpdump\fP.
1726 A packet trace that crosses a daylight savings time change will give
1727 skewed time stamps (the time change is ignored).
1729 Filter expressions on fields other than those in Token Ring headers will
1730 not correctly handle source-routed Token Ring packets.
1732 Filter expressions on fields other than those in 802.11 headers will not
1733 correctly handle 802.11 data packets with both To DS and From DS set.
1736 should chase header chain, but at this moment it does not.
1737 .BR "ip6 protochain"
1738 is supplied for this behavior.
1740 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1741 does not work against IPv6 packets.
1742 It only looks at IPv4 packets.
projects / tcpdump / blob