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 \-AbdDefhIKlLnNOpqRStuUvxX
94 .I spi@ipaddr algo:secret,...
104 .I postrotate-command
118 \fITcpdump\fP prints out a description of the contents of packets on a
119 network interface that match the boolean \fIexpression\fP. It can also
122 flag, which causes it to save the packet data to a file for later
123 analysis, and/or with the
125 flag, which causes it to read from a saved packet file rather than to
126 read packets from a network interface. In all cases, only packets that
133 will, if not run with the
135 flag, continue capturing packets until it is interrupted by a SIGINT
136 signal (generated, for example, by typing your interrupt character,
137 typically control-C) or a SIGTERM signal (typically generated with the
139 command); if run with the
141 flag, it will capture packets until it is interrupted by a SIGINT or
142 SIGTERM signal or the specified number of packets have been processed.
146 finishes capturing packets, it will report counts of:
148 packets ``captured'' (this is the number of packets that
150 has received and processed);
152 packets ``received by filter'' (the meaning of this depends on the OS on
155 and possibly on the way the OS was configured - if a filter was
156 specified on the command line, on some OSes it counts packets regardless
157 of whether they were matched by the filter expression and, even if they
158 were matched by the filter expression, regardless of whether
160 has read and processed them yet, on other OSes it counts only packets that were
161 matched by the filter expression regardless of whether
163 has read and processed them yet, and on other OSes it counts only
164 packets that were matched by the filter expression and were processed by
167 packets ``dropped by kernel'' (this is the number of packets that were
168 dropped, due to a lack of buffer space, by the packet capture mechanism
171 is running, if the OS reports that information to applications; if not,
172 it will be reported as 0).
174 On platforms that support the SIGINFO signal, such as most BSDs
175 (including Mac OS X) and Digital/Tru64 UNIX, it will report those counts
176 when it receives a SIGINFO signal (generated, for example, by typing
177 your ``status'' character, typically control-T, although on some
178 platforms, such as Mac OS X, the ``status'' character is not set by
179 default, so you must set it with
181 in order to use it) and will continue capturing packets.
183 Reading packets from a network interface may require that you have
184 special privileges; see the
186 man page for details. Reading a saved packet file doesn't require
191 Print each packet (minus its link level header) in ASCII. Handy for
195 Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN
199 Set the operating system capture buffer size to \fIbuffer_size\fP.
202 Exit after receiving \fIcount\fP packets.
205 Before writing a raw packet to a savefile, check whether the file is
206 currently larger than \fIfile_size\fP and, if so, close the current
207 savefile and open a new one. Savefiles after the first savefile will
208 have the name specified with the
210 flag, with a number after it, starting at 1 and continuing upward.
211 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
212 not 1,048,576 bytes).
215 Dump the compiled packet-matching code in a human readable form to
216 standard output and stop.
219 Dump packet-matching code as a
224 Dump packet-matching code as decimal numbers (preceded with a count).
227 Print the list of the network interfaces available on the system and on
230 can capture packets. For each network interface, a number and an
231 interface name, possibly followed by a text description of the
232 interface, is printed. The interface name or the number can be supplied
235 flag to specify an interface on which to capture.
237 This can be useful on systems that don't have a command to list them
238 (e.g., Windows systems, or UNIX systems lacking
239 .BR "ifconfig \-a" );
240 the number can be useful on Windows 2000 and later systems, where the
241 interface name is a somewhat complex string.
245 flag will not be supported if
247 was built with an older version of
250 .B pcap_findalldevs()
254 Print the link-level header on each dump line.
257 Use \fIspi@ipaddr algo:secret\fP for decrypting IPsec ESP packets that
258 are addressed to \fIaddr\fP and contain Security Parameter Index value
259 \fIspi\fP. This combination may be repeated with comma or newline separation.
261 Note that setting the secret for IPv4 ESP packets is supported at this time.
268 \fBcast128-cbc\fP, or
270 The default is \fBdes-cbc\fP.
271 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
272 with cryptography enabled.
274 \fIsecret\fP is the ASCII text for ESP secret key.
275 If preceded by 0x, then a hex value will be read.
277 The option assumes RFC2406 ESP, not RFC1827 ESP.
278 The option is only for debugging purposes, and
279 the use of this option with a true `secret' key is discouraged.
280 By presenting IPsec secret key onto command line
281 you make it visible to others, via
285 In addition to the above syntax, the syntax \fIfile name\fP may be used
286 to have tcpdump read the provided file in. The file is opened upon
287 receiving the first ESP packet, so any special permissions that tcpdump
288 may have been given should already have been given up.
291 Print `foreign' IPv4 addresses numerically rather than symbolically
292 (this option is intended to get around serious brain damage in
293 Sun's NIS server \(em usually it hangs forever translating non-local
296 The test for `foreign' IPv4 addresses is done using the IPv4 address and
297 netmask of the interface on which capture is being done. If that
298 address or netmask are not available, available, either because the
299 interface on which capture is being done has no address or netmask or
300 because the capture is being done on the Linux "any" interface, which
301 can capture on more than one interface, this option will not work
305 Use \fIfile\fP as input for the filter expression.
306 An additional expression given on the command line is ignored.
309 If specified, rotates the dump file specified with the
311 option every \fIrotate_seconds\fP seconds.
312 Savefiles will have the name specified by
314 which should include a time format as defined by
316 If no time format is specified, each new file will overwrite the previous.
318 If used in conjunction with the
320 option, filenames will take the form of `\fIfile\fP<count>'.
323 Attempt to detect 802.11s draft mesh headers.
326 Listen on \fIinterface\fP.
327 If unspecified, \fItcpdump\fP searches the system interface list for the
328 lowest numbered, configured up interface (excluding loopback).
329 Ties are broken by choosing the earliest match.
331 On Linux systems with 2.2 or later kernels, an
333 argument of ``any'' can be used to capture packets from all interfaces.
334 Note that captures on the ``any'' device will not be done in promiscuous
339 flag is supported, an interface number as printed by that flag can be
345 Put the interface in "monitor mode"; this is supported only on IEEE
346 802.11 Wi-Fi interfaces, and supported only on some operating systems.
348 Note that in monitor mode the adapter might disassociate from the
349 network with which it's associated, so that you will not be able to use
350 any wireless networks with that adapter. This could prevent accessing
351 files on a network server, or resolving host names or network addresses,
352 if you are capturing in monitor mode and are not connected to another
353 network with another adapter.
355 This flag will affect the output of the
359 isn't specified, only those link-layer types available when not in
360 monitor mode will be shown; if
362 is specified, only those link-layer types available when in monitor mode
366 Don't attempt to verify IP, TCP, or UDP checksums. This is useful for
367 interfaces that perform some or all of those checksum calculation in
368 hardware; otherwise, all outgoing TCP checksums will be flagged as bad.
371 Make stdout line buffered.
372 Useful if you want to see the data
376 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
377 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
380 List the known data link types for the interface, in the specified mode,
381 and exit. The list of known data link types may be dependent on the
382 specified mode; for example, on some platforms, a Wi-Fi interface might
383 support one set of data link types when not in monitor mode (for
384 example, it might support only fake Ethernet headers, or might support
385 802.11 headers but not support 802.11 headers with radio information)
386 and another set of data link types when in monitor mode (for example, it
387 might support 802.11 headers, or 802.11 headers with radio information,
388 only in monitor mode).
391 Load SMI MIB module definitions from file \fImodule\fR.
393 can be used several times to load several MIB modules into \fItcpdump\fP.
396 Use \fIsecret\fP as a shared secret for validating the digests found in
397 TCP segments with the TCP-MD5 option (RFC 2385), if present.
400 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
403 Don't print domain name qualification of host names.
405 if you give this flag then \fItcpdump\fP will print ``nic''
406 instead of ``nic.ddn.mil''.
409 Do not run the packet-matching code optimizer.
411 if you suspect a bug in the optimizer.
414 \fIDon't\fP put the interface
415 into promiscuous mode.
416 Note that the interface might be in promiscuous
417 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
418 `ether host {local-hw-addr} or ether broadcast'.
421 Quick (quiet?) output.
422 Print less protocol information so output
426 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
427 If specified, \fItcpdump\fP will not print replay prevention field.
428 Since there is no protocol version field in ESP/AH specification,
429 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
432 Read packets from \fIfile\fR (which was created with the
435 Standard input is used if \fIfile\fR is ``-''.
438 Print absolute, rather than relative, TCP sequence numbers.
441 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
442 default of 65535 bytes.
443 Packets truncated because of a limited snapshot
444 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
445 is the name of the protocol level at which the truncation has occurred.
446 Note that taking larger snapshots both increases
447 the amount of time it takes to process packets and, effectively,
448 decreases the amount of packet buffering.
449 This may cause packets to be
451 You should limit \fIsnaplen\fP to the smallest number that will
452 capture the protocol information you're interested in.
454 \fIsnaplen\fP to 0 sets it to the default of 65535,
455 for backwards compatibility with recent older versions of
459 Force packets selected by "\fIexpression\fP" to be interpreted the
460 specified \fItype\fR.
461 Currently known types are
462 \fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
463 \fBcnfp\fR (Cisco NetFlow protocol),
464 \fBrpc\fR (Remote Procedure Call),
465 \fBrtp\fR (Real-Time Applications protocol),
466 \fBrtcp\fR (Real-Time Applications control protocol),
467 \fBsnmp\fR (Simple Network Management Protocol),
468 \fBtftp\fR (Trivial File Transfer Protocol),
469 \fBvat\fR (Visual Audio Tool),
471 \fBwb\fR (distributed White Board).
474 \fIDon't\fP print a timestamp on each dump line.
477 Print an unformatted timestamp on each dump line.
480 Print a delta (micro-second resolution) between current and previous line
484 Print a timestamp in default format proceeded by date on each dump line.
487 Print a delta (micro-second resolution) between current and first line
491 Print undecoded NFS handles.
494 Make output saved via the
496 option ``packet-buffered''; i.e., as each packet is saved, it will be
497 written to the output file, rather than being written only when the
502 flag will not be supported if
504 was built with an older version of
511 When parsing and printing, produce (slightly more) verbose output.
512 For example, the time to live,
513 identification, total length and options in an IP packet are printed.
514 Also enables additional packet integrity checks such as verifying the
515 IP and ICMP header checksum.
517 When writing to a file with the
519 option, report, every 10 seconds, the number of packets captured.
522 Even more verbose output.
523 For example, additional fields are
524 printed from NFS reply packets, and SMB packets are fully decoded.
527 Even more verbose output.
529 telnet \fBSB\fP ... \fBSE\fP options
533 Telnet options are printed in hex as well.
536 Write the raw packets to \fIfile\fR rather than parsing and printing
538 They can later be printed with the \-r option.
539 Standard output is used if \fIfile\fR is ``-''.
541 .BR pcap-savefile (@MAN_FILE_FORMATS@)
542 for a description of the file format.
545 Used in conjunction with the
547 option, this will limit the number
548 of files created to the specified number, and begin overwriting files
549 from the beginning, thus creating a 'rotating' buffer.
550 In addition, it will name
551 the files with enough leading 0s to support the maximum number of
552 files, allowing them to sort correctly.
554 Used in conjunction with the
556 option, this will limit the number of rotated dump files that get
557 created, exiting with status 0 when reaching the limit. If used with
559 as well, the behavior will result in cyclical files per timeslice.
562 When parsing and printing,
563 in addition to printing the headers of each packet, print the data of
564 each packet (minus its link level header) in hex.
565 The smaller of the entire packet or
567 bytes will be printed. Note that this is the entire link-layer
568 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
569 will also be printed when the higher layer packet is shorter than the
573 When parsing and printing,
574 in addition to printing the headers of each packet, print the data of
577 its link level header, in hex.
580 When parsing and printing,
581 in addition to printing the headers of each packet, print the data of
582 each packet (minus its link level header) in hex and ASCII.
583 This is very handy for analysing new protocols.
586 When parsing and printing,
587 in addition to printing the headers of each packet, print the data of
590 its link level header, in hex and ASCII.
593 Set the data link type to use while capturing packets to \fIdatalinktype\fP.
596 Used in conjunction with the
600 options, this will make
606 is the savefile being closed after each rotation. For example, specifying
610 will compress each savefile using gzip or bzip2.
612 Note that tcpdump will run the command in parallel to the capture, using
613 the lowest priority so that this doesn't disturb the capture process.
615 And in case you would like to use a command that itself takes flags or
616 different arguments, you can always write a shell script that will take the
617 savefile name as the only argument, make the flags & arguments arrangements
618 and execute the command that you want.
621 Drops privileges (if root) and changes user ID to
623 and the group ID to the primary group of
626 This behavior can also be enabled by default at compile time.
627 .IP "\fI expression\fP"
629 selects which packets will be dumped.
630 If no \fIexpression\fP
631 is given, all packets on the net will be dumped.
633 only packets for which \fIexpression\fP is `true' will be dumped.
635 For the \fIexpression\fP syntax, see
636 .BR pcap-filter (@MAN_MISC_INFO@).
638 Expression arguments can be passed to \fItcpdump\fP as either a single
639 argument or as multiple arguments, whichever is more convenient.
640 Generally, if the expression contains Shell metacharacters, it is
641 easier to pass it as a single, quoted argument.
642 Multiple arguments are concatenated with spaces before being parsed.
645 To print all packets arriving at or departing from \fIsundown\fP:
648 \fBtcpdump host sundown\fP
652 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
655 \fBtcpdump host helios and \\( hot or ace \\)\fP
659 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
662 \fBtcpdump ip host ace and not helios\fP
666 To print all traffic between local hosts and hosts at Berkeley:
670 tcpdump net ucb-ether
674 To print all ftp traffic through internet gateway \fIsnup\fP:
675 (note that the expression is quoted to prevent the shell from
676 (mis-)interpreting the parentheses):
680 tcpdump 'gateway snup and (port ftp or ftp-data)'
684 To print traffic neither sourced from nor destined for local hosts
685 (if you gateway to one other net, this stuff should never make it
686 onto your local net).
690 tcpdump ip and not net \fIlocalnet\fP
694 To print the start and end packets (the SYN and FIN packets) of each
695 TCP conversation that involves a non-local host.
699 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
703 To print all IPv4 HTTP packets to and from port 80, i.e. print only
704 packets that contain data, not, for example, SYN and FIN packets and
705 ACK-only packets. (IPv6 is left as an exercise for the reader.)
709 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
713 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
717 tcpdump 'gateway snup and ip[2:2] > 576'
721 To print IP broadcast or multicast packets that were
723 sent via Ethernet broadcast or multicast:
727 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
731 To print all ICMP packets that are not echo requests/replies (i.e., not
736 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
741 The output of \fItcpdump\fP is protocol dependent.
743 gives a brief description and examples of most of the formats.
751 If the '-e' option is given, the link level header is printed out.
752 On Ethernets, the source and destination addresses, protocol,
753 and packet length are printed.
755 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
756 the `frame control' field, the source and destination addresses,
757 and the packet length.
758 (The `frame control' field governs the
759 interpretation of the rest of the packet.
761 as those containing IP datagrams) are `async' packets, with a priority
762 value between 0 and 7; for example, `\fBasync4\fR'.
764 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
765 the LLC header is printed if it is \fInot\fR an ISO datagram or a
766 so-called SNAP packet.
768 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
769 the `access control' and `frame control' fields, the source and
770 destination addresses, and the packet length.
772 packets are assumed to contain an LLC packet.
773 Regardless of whether
774 the '-e' option is specified or not, the source routing information is
775 printed for source-routed packets.
777 On 802.11 networks, the '-e' option causes \fItcpdump\fP to print
778 the `frame control' fields, all of the addresses in the 802.11 header,
779 and the packet length.
781 packets are assumed to contain an LLC packet.
783 \fI(N.B.: The following description assumes familiarity with
784 the SLIP compression algorithm described in RFC-1144.)\fP
786 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
787 packet type, and compression information are printed out.
788 The packet type is printed first.
789 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
790 No further link information is printed for \fIip\fR packets.
791 For TCP packets, the connection identifier is printed following the type.
792 If the packet is compressed, its encoded header is printed out.
793 The special cases are printed out as
794 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
795 the sequence number (or sequence number and ack) has changed.
796 If it is not a special case,
797 zero or more changes are printed.
798 A change is indicated by U (urgent pointer), W (window), A (ack),
799 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
801 Finally, the amount of data in the packet and compressed header length
804 For example, the following line shows an outbound compressed TCP packet,
805 with an implicit connection identifier; the ack has changed by 6,
806 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
807 data and 6 bytes of compressed header:
810 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
816 Arp/rarp output shows the type of request and its arguments.
818 format is intended to be self explanatory.
819 Here is a short sample taken from the start of an `rlogin' from
820 host \fIrtsg\fP to host \fIcsam\fP:
824 \f(CWarp who-has csam tell rtsg
825 arp reply csam is-at CSAM\fR
829 The first line says that rtsg sent an arp packet asking
830 for the Ethernet address of internet host csam.
832 replies with its Ethernet address (in this example, Ethernet addresses
833 are in caps and internet addresses in lower case).
835 This would look less redundant if we had done \fItcpdump \-n\fP:
839 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
840 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
844 If we had done \fItcpdump \-e\fP, the fact that the first packet is
845 broadcast and the second is point-to-point would be visible:
849 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
850 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
854 For the first packet this says the Ethernet source address is RTSG, the
855 destination is the Ethernet broadcast address, the type field
856 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
860 \fI(N.B.:The following description assumes familiarity with
861 the TCP protocol described in RFC-793.
862 If you are not familiar
863 with the protocol, neither this description nor \fItcpdump\fP will
864 be of much use to you.)\fP
866 The general format of a tcp protocol line is:
870 \fIsrc > dst: flags data-seqno ack window urgent options\fP
874 \fISrc\fP and \fIdst\fP are the source and destination IP
876 \fIFlags\fP are some combination of S (SYN),
877 F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or
878 `.' (ACK), or `none' if no flags are set.
879 \fIData-seqno\fP describes the portion of sequence space covered
880 by the data in this packet (see example below).
881 \fIAck\fP is sequence number of the next data expected the other
882 direction on this connection.
883 \fIWindow\fP is the number of bytes of receive buffer space available
884 the other direction on this connection.
885 \fIUrg\fP indicates there is `urgent' data in the packet.
886 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
888 \fISrc, dst\fP and \fIflags\fP are always present.
890 depend on the contents of the packet's tcp protocol header and
891 are output only if appropriate.
893 Here is the opening portion of an rlogin from host \fIrtsg\fP to
898 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
899 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
900 rtsg.1023 > csam.login: . ack 1 win 4096
901 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
902 csam.login > rtsg.1023: . ack 2 win 4096
903 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
904 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
905 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
906 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
910 The first line says that tcp port 1023 on rtsg sent a packet
913 The \fBS\fP indicates that the \fISYN\fP flag was set.
914 The packet sequence number was 768512 and it contained no data.
915 (The notation is `first:last(nbytes)' which means `sequence
917 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
918 There was no piggy-backed ack, the available receive window was 4096
919 bytes and there was a max-segment-size option requesting an mss of
922 Csam replies with a similar packet except it includes a piggy-backed
924 Rtsg then acks csam's SYN.
925 The `.' means the ACK flag was set.
926 The packet contained no data so there is no data sequence number.
927 Note that the ack sequence
928 number is a small integer (1).
929 The first time \fItcpdump\fP sees a
930 tcp `conversation', it prints the sequence number from the packet.
931 On subsequent packets of the conversation, the difference between
932 the current packet's sequence number and this initial sequence number
934 This means that sequence numbers after the
935 first can be interpreted
936 as relative byte positions in the conversation's data stream (with the
937 first data byte each direction being `1').
938 `-S' will override this
939 feature, causing the original sequence numbers to be output.
941 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
942 in the rtsg \(-> csam side of the conversation).
943 The PUSH flag is set in the packet.
944 On the 7th line, csam says it's received data sent by rtsg up to
945 but not including byte 21.
946 Most of this data is apparently sitting in the
947 socket buffer since csam's receive window has gotten 19 bytes smaller.
948 Csam also sends one byte of data to rtsg in this packet.
949 On the 8th and 9th lines,
950 csam sends two bytes of urgent, pushed data to rtsg.
952 If the snapshot was small enough that \fItcpdump\fP didn't capture
953 the full TCP header, it interprets as much of the header as it can
954 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
956 If the header contains a bogus option (one with a length
957 that's either too small or beyond the end of the header), \fItcpdump\fP
958 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
959 options (since it's impossible to tell where they start).
961 length indicates options are present but the IP datagram length is not
962 long enough for the options to actually be there, \fItcpdump\fP reports
963 it as ``[\fIbad hdr length\fP]''.
965 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
967 There are 8 bits in the control bits section of the TCP header:
969 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
971 Let's assume that we want to watch packets used in establishing
973 Recall that TCP uses a 3-way handshake protocol
974 when it initializes a new connection; the connection sequence with
975 regard to the TCP control bits is
981 2) Recipient responds with SYN, ACK
987 Now we're interested in capturing packets that have only the
988 SYN bit set (Step 1).
989 Note that we don't want packets from step 2
990 (SYN-ACK), just a plain initial SYN.
991 What we need is a correct filter
992 expression for \fItcpdump\fP.
994 Recall the structure of a TCP header without options:
998 -----------------------------------------------------------------
999 | source port | destination port |
1000 -----------------------------------------------------------------
1002 -----------------------------------------------------------------
1003 | acknowledgment number |
1004 -----------------------------------------------------------------
1005 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1006 -----------------------------------------------------------------
1007 | TCP checksum | urgent pointer |
1008 -----------------------------------------------------------------
1011 A TCP header usually holds 20 octets of data, unless options are
1013 The first line of the graph contains octets 0 - 3, the
1014 second line shows octets 4 - 7 etc.
1016 Starting to count with 0, the relevant TCP control bits are contained
1021 ----------------|---------------|---------------|----------------
1022 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1023 ----------------|---------------|---------------|----------------
1024 | | 13th octet | | |
1027 Let's have a closer look at octet no. 13:
1037 These are the TCP control bits we are interested
1039 We have numbered the bits in this octet from 0 to 7, right to
1040 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1042 Recall that we want to capture packets with only SYN set.
1043 Let's see what happens to octet 13 if a TCP datagram arrives
1044 with the SYN bit set in its header:
1055 control bits section we see that only bit number 1 (SYN) is set.
1057 Assuming that octet number 13 is an 8-bit unsigned integer in
1058 network byte order, the binary value of this octet is
1062 and its decimal representation is
1066 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1069 We're almost done, because now we know that if only SYN is set,
1070 the value of the 13th octet in the TCP header, when interpreted
1071 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1073 This relationship can be expressed as
1079 We can use this expression as the filter for \fItcpdump\fP in order
1080 to watch packets which have only SYN set:
1083 tcpdump -i xl0 tcp[13] == 2
1086 The expression says "let the 13th octet of a TCP datagram have
1087 the decimal value 2", which is exactly what we want.
1089 Now, let's assume that we need to capture SYN packets, but we
1090 don't care if ACK or any other TCP control bit is set at the
1092 Let's see what happens to octet 13 when a TCP datagram
1093 with SYN-ACK set arrives:
1103 Now bits 1 and 4 are set in the 13th octet.
1109 which translates to decimal
1113 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1116 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1117 expression, because that would select only those packets that have
1118 SYN-ACK set, but not those with only SYN set.
1119 Remember that we don't care
1120 if ACK or any other control bit is set as long as SYN is set.
1122 In order to achieve our goal, we need to logically AND the
1123 binary value of octet 13 with some other value to preserve
1125 We know that we want SYN to be set in any case,
1126 so we'll logically AND the value in the 13th octet with
1127 the binary value of a SYN:
1131 00010010 SYN-ACK 00000010 SYN
1132 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1134 = 00000010 = 00000010
1137 We see that this AND operation delivers the same result
1138 regardless whether ACK or another TCP control bit is set.
1139 The decimal representation of the AND value as well as
1140 the result of this operation is 2 (binary 00000010),
1141 so we know that for packets with SYN set the following
1142 relation must hold true:
1144 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1146 This points us to the \fItcpdump\fP filter expression
1149 tcpdump -i xl0 'tcp[13] & 2 == 2'
1152 Some offsets and field values may be expressed as names
1153 rather than as numeric values. For example tcp[13] may
1154 be replaced with tcp[tcpflags]. The following TCP flag
1155 field values are also available: tcp-fin, tcp-syn, tcp-rst,
1156 tcp-push, tcp-act, tcp-urg.
1158 This can be demonstrated as:
1161 tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
1164 Note that you should use single quotes or a backslash
1165 in the expression to hide the AND ('&') special character
1171 UDP format is illustrated by this rwho packet:
1175 \f(CWactinide.who > broadcast.who: udp 84\fP
1179 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1180 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1182 The packet contained 84 bytes of user data.
1184 Some UDP services are recognized (from the source or destination
1185 port number) and the higher level protocol information printed.
1186 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1187 RPC calls (RFC-1050) to NFS.
1189 UDP Name Server Requests
1191 \fI(N.B.:The following description assumes familiarity with
1192 the Domain Service protocol described in RFC-1035.
1193 If you are not familiar
1194 with the protocol, the following description will appear to be written
1197 Name server requests are formatted as
1201 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1203 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1207 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1208 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1209 The query id was `3'.
1210 The `+' indicates the \fIrecursion desired\fP flag
1212 The query length was 37 bytes, not including the UDP and
1213 IP protocol headers.
1214 The query operation was the normal one, \fIQuery\fP,
1215 so the op field was omitted.
1216 If the op had been anything else, it would
1217 have been printed between the `3' and the `+'.
1218 Similarly, the qclass was the normal one,
1219 \fIC_IN\fP, and omitted.
1220 Any other qclass would have been printed
1221 immediately after the `A'.
1223 A few anomalies are checked and may result in extra fields enclosed in
1224 square brackets: If a query contains an answer, authority records or
1225 additional records section,
1230 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1231 is the appropriate count.
1232 If any of the response bits are set (AA, RA or rcode) or any of the
1233 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1234 is printed, where \fIx\fP is the hex value of header bytes two and three.
1236 UDP Name Server Responses
1238 Name server responses are formatted as
1242 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1244 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1245 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1249 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1250 with 3 answer records, 3 name server records and 7 additional records.
1251 The first answer record is type A (address) and its data is internet
1252 address 128.32.137.3.
1253 The total size of the response was 273 bytes,
1254 excluding UDP and IP headers.
1255 The op (Query) and response code
1256 (NoError) were omitted, as was the class (C_IN) of the A record.
1258 In the second example, \fIhelios\fP responds to query 2 with a
1259 response code of non-existent domain (NXDomain) with no answers,
1260 one name server and no authority records.
1261 The `*' indicates that
1262 the \fIauthoritative answer\fP bit was set.
1264 answers, no type, class or data were printed.
1266 Other flag characters that might appear are `\-' (recursion available,
1267 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1269 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1275 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1276 on UDP/137, UDP/138 and TCP/139.
1277 Some primitive decoding of IPX and
1278 NetBEUI SMB data is also done.
1280 By default a fairly minimal decode is done, with a much more detailed
1281 decode done if -v is used.
1282 Be warned that with -v a single SMB packet
1283 may take up a page or more, so only use -v if you really want all the
1286 For information on SMB packet formats and what all te fields mean see
1287 www.cifs.org or the pub/samba/specs/ directory on your favorite
1288 samba.org mirror site.
1289 The SMB patches were written by Andrew Tridgell
1293 NFS Requests and Replies
1295 Sun NFS (Network File System) requests and replies are printed as:
1299 \fIsrc.xid > dst.nfs: len op args\fP
1300 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1303 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1304 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1305 sushi.201b > wrl.nfs:
1306 144 lookup fh 9,74/4096.6878 "xcolors"
1307 wrl.nfs > sushi.201b:
1308 reply ok 128 lookup fh 9,74/4134.3150
1313 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1314 to \fIwrl\fP (note that the number following the src host is a
1315 transaction id, \fInot\fP the source port).
1316 The request was 112 bytes,
1317 excluding the UDP and IP headers.
1318 The operation was a \fIreadlink\fP
1319 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1320 (If one is lucky, as in this case, the file handle can be interpreted
1321 as a major,minor device number pair, followed by the inode number and
1323 \fIWrl\fP replies `ok' with the contents of the link.
1325 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1326 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1327 Note that the data printed
1328 depends on the operation type.
1329 The format is intended to be self
1330 explanatory if read in conjunction with
1331 an NFS protocol spec.
1333 If the \-v (verbose) flag is given, additional information is printed.
1339 sushi.1372a > wrl.nfs:
1340 148 read fh 21,11/12.195 8192 bytes @ 24576
1341 wrl.nfs > sushi.1372a:
1342 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1347 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1348 which have been omitted from this example.) In the first line,
1349 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1350 at byte offset 24576.
1351 \fIWrl\fP replies `ok'; the packet shown on the
1352 second line is the first fragment of the reply, and hence is only 1472
1353 bytes long (the other bytes will follow in subsequent fragments, but
1354 these fragments do not have NFS or even UDP headers and so might not be
1355 printed, depending on the filter expression used).
1356 Because the \-v flag
1357 is given, some of the file attributes (which are returned in addition
1358 to the file data) are printed: the file type (``REG'', for regular file),
1359 the file mode (in octal), the uid and gid, and the file size.
1361 If the \-v flag is given more than once, even more details are printed.
1363 Note that NFS requests are very large and much of the detail won't be printed
1364 unless \fIsnaplen\fP is increased.
1365 Try using `\fB\-s 192\fP' to watch
1368 NFS reply packets do not explicitly identify the RPC operation.
1370 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1371 replies using the transaction ID.
1372 If a reply does not closely follow the
1373 corresponding request, it might not be parsable.
1375 AFS Requests and Replies
1377 Transarc AFS (Andrew File System) requests and replies are printed
1383 \fIsrc.sport > dst.dport: rx packet-type\fP
1384 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1385 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1388 elvis.7001 > pike.afsfs:
1389 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1390 new fid 536876964/1/1 ".newsrc"
1391 pike.afsfs > elvis.7001: rx data fs reply rename
1396 In the first line, host elvis sends a RX packet to pike.
1398 a RX data packet to the fs (fileserver) service, and is the start of
1400 The RPC call was a rename, with the old directory file id
1401 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1402 file id of 536876964/1/1 and a new filename of `.newsrc'.
1404 responds with a RPC reply to the rename call (which was successful, because
1405 it was a data packet and not an abort packet).
1407 In general, all AFS RPCs are decoded at least by RPC call name.
1409 AFS RPCs have at least some of the arguments decoded (generally only
1410 the `interesting' arguments, for some definition of interesting).
1412 The format is intended to be self-describing, but it will probably
1413 not be useful to people who are not familiar with the workings of
1416 If the -v (verbose) flag is given twice, acknowledgement packets and
1417 additional header information is printed, such as the the RX call ID,
1418 call number, sequence number, serial number, and the RX packet flags.
1420 If the -v flag is given twice, additional information is printed,
1421 such as the the RX call ID, serial number, and the RX packet flags.
1422 The MTU negotiation information is also printed from RX ack packets.
1424 If the -v flag is given three times, the security index and service id
1427 Error codes are printed for abort packets, with the exception of Ubik
1428 beacon packets (because abort packets are used to signify a yes vote
1429 for the Ubik protocol).
1431 Note that AFS requests are very large and many of the arguments won't
1432 be printed unless \fIsnaplen\fP is increased.
1433 Try using `\fB-s 256\fP'
1434 to watch AFS traffic.
1436 AFS reply packets do not explicitly identify the RPC operation.
1438 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1439 replies using the call number and service ID.
1440 If a reply does not closely
1442 corresponding request, it might not be parsable.
1445 KIP AppleTalk (DDP in UDP)
1447 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1448 and dumped as DDP packets (i.e., all the UDP header information is
1452 is used to translate AppleTalk net and node numbers to names.
1453 Lines in this file have the form
1465 The first two lines give the names of AppleTalk networks.
1467 line gives the name of a particular host (a host is distinguished
1468 from a net by the 3rd octet in the number \-
1469 a net number \fImust\fP have two octets and a host number \fImust\fP
1470 have three octets.) The number and name should be separated by
1471 whitespace (blanks or tabs).
1474 file may contain blank lines or comment lines (lines starting with
1477 AppleTalk addresses are printed in the form
1483 \f(CW144.1.209.2 > icsd-net.112.220
1484 office.2 > icsd-net.112.220
1485 jssmag.149.235 > icsd-net.2\fR
1491 doesn't exist or doesn't contain an entry for some AppleTalk
1492 host/net number, addresses are printed in numeric form.)
1493 In the first example, NBP (DDP port 2) on net 144.1 node 209
1494 is sending to whatever is listening on port 220 of net icsd node 112.
1495 The second line is the same except the full name of the source node
1496 is known (`office').
1497 The third line is a send from port 235 on
1498 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1499 the broadcast address (255) is indicated by a net name with no host
1500 number \- for this reason it's a good idea to keep node names and
1501 net names distinct in /etc/atalk.names).
1503 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1504 packets have their contents interpreted.
1505 Other protocols just dump
1506 the protocol name (or number if no name is registered for the
1507 protocol) and packet size.
1509 \fBNBP packets\fP are formatted like the following examples:
1513 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1514 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1515 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1519 The first line is a name lookup request for laserwriters sent by net icsd host
1520 112 and broadcast on net jssmag.
1521 The nbp id for the lookup is 190.
1522 The second line shows a reply for this request (note that it has the
1523 same id) from host jssmag.209 saying that it has a laserwriter
1524 resource named "RM1140" registered on port 250.
1526 another reply to the same request saying host techpit has laserwriter
1527 "techpit" registered on port 186.
1529 \fBATP packet\fP formatting is demonstrated by the following example:
1533 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1534 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1535 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1536 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1537 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1538 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1539 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1540 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1541 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1542 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1543 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1544 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1545 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1546 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1550 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1551 up to 8 packets (the `<0-7>').
1552 The hex number at the end of the line
1553 is the value of the `userdata' field in the request.
1555 Helios responds with 8 512-byte packets.
1556 The `:digit' following the
1557 transaction id gives the packet sequence number in the transaction
1558 and the number in parens is the amount of data in the packet,
1559 excluding the atp header.
1560 The `*' on packet 7 indicates that the
1563 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1565 resends them then jssmag.209 releases the transaction.
1567 jssmag.209 initiates the next request.
1568 The `*' on the request
1569 indicates that XO (`exactly once') was \fInot\fP set.
1574 Fragmented Internet datagrams are printed as
1578 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1579 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1583 (The first form indicates there are more fragments.
1585 indicates this is the last fragment.)
1587 \fIId\fP is the fragment id.
1588 \fISize\fP is the fragment
1589 size (in bytes) excluding the IP header.
1590 \fIOffset\fP is this
1591 fragment's offset (in bytes) in the original datagram.
1593 The fragment information is output for each fragment.
1595 fragment contains the higher level protocol header and the frag
1596 info is printed after the protocol info.
1598 after the first contain no higher level protocol header and the
1599 frag info is printed after the source and destination addresses.
1600 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1601 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1605 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1606 arizona > rtsg: (frag 595a:204@328)
1607 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1611 There are a couple of things to note here: First, addresses in the
1612 2nd line don't include port numbers.
1613 This is because the TCP
1614 protocol information is all in the first fragment and we have no idea
1615 what the port or sequence numbers are when we print the later fragments.
1616 Second, the tcp sequence information in the first line is printed as if there
1617 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1618 the first frag and 204 in the second).
1619 If you are looking for holes
1620 in the sequence space or trying to match up acks
1621 with packets, this can fool you.
1623 A packet with the IP \fIdon't fragment\fP flag is marked with a
1624 trailing \fB(DF)\fP.
1628 By default, all output lines are preceded by a timestamp.
1630 is the current clock time in the form
1636 and is as accurate as the kernel's clock.
1637 The timestamp reflects the time the kernel first saw the packet.
1639 is made to account for the time lag between when the
1640 Ethernet interface removed the packet from the wire and when the kernel
1641 serviced the `new packet' interrupt.
1643 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@),
1644 pcap-filter(@MAN_MISC_INFO@)
1646 The original authors are:
1650 Steven McCanne, all of the
1651 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1653 It is currently being maintained by tcpdump.org.
1655 The current version is available via http:
1658 .I http://www.tcpdump.org/
1661 The original distribution is available via anonymous ftp:
1664 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1667 IPv6/IPsec support is added by WIDE/KAME project.
1668 This program uses Eric Young's SSLeay library, under specific configurations.
1670 Please send problems, bugs, questions, desirable enhancements, patches
1674 tcpdump-workers@lists.tcpdump.org
1677 NIT doesn't let you watch your own outbound traffic, BPF will.
1678 We recommend that you use the latter.
1680 On Linux systems with 2.0[.x] kernels:
1682 packets on the loopback device will be seen twice;
1684 packet filtering cannot be done in the kernel, so that all packets must
1685 be copied from the kernel in order to be filtered in user mode;
1687 all of a packet, not just the part that's within the snapshot length,
1688 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1689 asked to copy only part of a packet to userland, will not report the
1690 true length of the packet; this would cause most IP packets to get an
1694 capturing on some PPP devices won't work correctly.
1696 We recommend that you upgrade to a 2.2 or later kernel.
1698 Some attempt should be made to reassemble IP fragments or, at least
1699 to compute the right length for the higher level protocol.
1701 Name server inverse queries are not dumped correctly: the (empty)
1702 question section is printed rather than real query in the answer
1704 Some believe that inverse queries are themselves a bug and
1705 prefer to fix the program generating them rather than \fItcpdump\fP.
1707 A packet trace that crosses a daylight savings time change will give
1708 skewed time stamps (the time change is ignored).
1710 Filter expressions on fields other than those in Token Ring headers will
1711 not correctly handle source-routed Token Ring packets.
1713 Filter expressions on fields other than those in 802.11 headers will not
1714 correctly handle 802.11 data packets with both To DS and From DS set.
1717 should chase header chain, but at this moment it does not.
1718 .BR "ip6 protochain"
1719 is supplied for this behavior.
1721 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1722 does not work against IPv6 packets.
1723 It only looks at IPv4 packets.
projects / tcpdump / blob