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 $
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25 .TH TCPDUMP 1 "12 July 2012"
27 tcpdump \- dump traffic on a network
32 .B \-AbdDefhHIJKlLnNOpqRStuUvxX
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, in
204 units of KiB (1024 bytes).
207 Exit after receiving \fIcount\fP packets.
210 Before writing a raw packet to a savefile, check whether the file is
211 currently larger than \fIfile_size\fP and, if so, close the current
212 savefile and open a new one. Savefiles after the first savefile will
213 have the name specified with the
215 flag, with a number after it, starting at 1 and continuing upward.
216 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
217 not 1,048,576 bytes).
220 Dump the compiled packet-matching code in a human readable form to
221 standard output and stop.
224 Dump packet-matching code as a
229 Dump packet-matching code as decimal numbers (preceded with a count).
232 Print the list of the network interfaces available on the system and on
235 can capture packets. For each network interface, a number and an
236 interface name, possibly followed by a text description of the
237 interface, is printed. The interface name or the number can be supplied
240 flag to specify an interface on which to capture.
242 This can be useful on systems that don't have a command to list them
243 (e.g., Windows systems, or UNIX systems lacking
244 .BR "ifconfig \-a" );
245 the number can be useful on Windows 2000 and later systems, where the
246 interface name is a somewhat complex string.
250 flag will not be supported if
252 was built with an older version of
255 .B pcap_findalldevs()
259 Print the link-level header on each dump line. This can be used, for
260 example, to print MAC layer addresses for protocols such as Ethernet and
264 Use \fIspi@ipaddr algo:secret\fP for decrypting IPsec ESP packets that
265 are addressed to \fIaddr\fP and contain Security Parameter Index value
266 \fIspi\fP. This combination may be repeated with comma or newline separation.
268 Note that setting the secret for IPv4 ESP packets is supported at this time.
275 \fBcast128-cbc\fP, or
277 The default is \fBdes-cbc\fP.
278 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
279 with cryptography enabled.
281 \fIsecret\fP is the ASCII text for ESP secret key.
282 If preceded by 0x, then a hex value will be read.
284 The option assumes RFC2406 ESP, not RFC1827 ESP.
285 The option is only for debugging purposes, and
286 the use of this option with a true `secret' key is discouraged.
287 By presenting IPsec secret key onto command line
288 you make it visible to others, via
292 In addition to the above syntax, the syntax \fIfile name\fP may be used
293 to have tcpdump read the provided file in. The file is opened upon
294 receiving the first ESP packet, so any special permissions that tcpdump
295 may have been given should already have been given up.
298 Print `foreign' IPv4 addresses numerically rather than symbolically
299 (this option is intended to get around serious brain damage in
300 Sun's NIS server \(em usually it hangs forever translating non-local
303 The test for `foreign' IPv4 addresses is done using the IPv4 address and
304 netmask of the interface on which capture is being done. If that
305 address or netmask are not available, available, either because the
306 interface on which capture is being done has no address or netmask or
307 because the capture is being done on the Linux "any" interface, which
308 can capture on more than one interface, this option will not work
312 Use \fIfile\fP as input for the filter expression.
313 An additional expression given on the command line is ignored.
316 If specified, rotates the dump file specified with the
318 option every \fIrotate_seconds\fP seconds.
319 Savefiles will have the name specified by
321 which should include a time format as defined by
323 If no time format is specified, each new file will overwrite the previous.
325 If used in conjunction with the
327 option, filenames will take the form of `\fIfile\fP<count>'.
330 Print the tcpdump and libpcap version strings, print a usage message,
334 Attempt to detect 802.11s draft mesh headers.
337 Listen on \fIinterface\fP.
338 If unspecified, \fItcpdump\fP searches the system interface list for the
339 lowest numbered, configured up interface (excluding loopback).
340 Ties are broken by choosing the earliest match.
342 On Linux systems with 2.2 or later kernels, an
344 argument of ``any'' can be used to capture packets from all interfaces.
345 Note that captures on the ``any'' device will not be done in promiscuous
350 flag is supported, an interface number as printed by that flag can be
356 Put the interface in "monitor mode"; this is supported only on IEEE
357 802.11 Wi-Fi interfaces, and supported only on some operating systems.
359 Note that in monitor mode the adapter might disassociate from the
360 network with which it's associated, so that you will not be able to use
361 any wireless networks with that adapter. This could prevent accessing
362 files on a network server, or resolving host names or network addresses,
363 if you are capturing in monitor mode and are not connected to another
364 network with another adapter.
366 This flag will affect the output of the
370 isn't specified, only those link-layer types available when not in
371 monitor mode will be shown; if
373 is specified, only those link-layer types available when in monitor mode
377 Set the time stamp type for the capture to \fItstamp_type\fP. The names
378 to use for the time stamp types are given in
379 .BR pcap-tstamp-type (@MAN_MISC_INFO@);
380 not all the types listed there will necessarily be valid for any given
384 List the supported time stamp types for the interface and exit. If the
385 time stamp type cannot be set for the interface, no time stamp types are
389 Don't attempt to verify IP, TCP, or UDP checksums. This is useful for
390 interfaces that perform some or all of those checksum calculation in
391 hardware; otherwise, all outgoing TCP checksums will be flagged as bad.
394 Make stdout line buffered.
395 Useful if you want to see the data
402 \fBtcpdump \-l | tee dat\fP
412 \fBtcpdump \-l > dat & tail \-f dat\fP
417 Note that on Windows,``line buffered'' means ``unbuffered'', so that
418 WinDump will write each character individually if
425 in its behavior, but it will cause output to be ``packet-buffered'', so
426 that the output is written to stdout at the end of each packet rather
427 than at the end of each line; this is buffered on all platforms,
431 List the known data link types for the interface, in the specified mode,
432 and exit. The list of known data link types may be dependent on the
433 specified mode; for example, on some platforms, a Wi-Fi interface might
434 support one set of data link types when not in monitor mode (for
435 example, it might support only fake Ethernet headers, or might support
436 802.11 headers but not support 802.11 headers with radio information)
437 and another set of data link types when in monitor mode (for example, it
438 might support 802.11 headers, or 802.11 headers with radio information,
439 only in monitor mode).
442 Load SMI MIB module definitions from file \fImodule\fR.
444 can be used several times to load several MIB modules into \fItcpdump\fP.
447 Use \fIsecret\fP as a shared secret for validating the digests found in
448 TCP segments with the TCP-MD5 option (RFC 2385), if present.
451 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
454 Don't print domain name qualification of host names.
456 if you give this flag then \fItcpdump\fP will print ``nic''
457 instead of ``nic.ddn.mil''.
460 Do not run the packet-matching code optimizer.
462 if you suspect a bug in the optimizer.
465 \fIDon't\fP put the interface
466 into promiscuous mode.
467 Note that the interface might be in promiscuous
468 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
469 `ether host {local-hw-addr} or ether broadcast'.
472 Quick (quiet?) output.
473 Print less protocol information so output
477 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
478 If specified, \fItcpdump\fP will not print replay prevention field.
479 Since there is no protocol version field in ESP/AH specification,
480 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
483 Read packets from \fIfile\fR (which was created with the
486 Standard input is used if \fIfile\fR is ``-''.
489 Print absolute, rather than relative, TCP sequence numbers.
492 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
493 default of 65535 bytes.
494 Packets truncated because of a limited snapshot
495 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
496 is the name of the protocol level at which the truncation has occurred.
497 Note that taking larger snapshots both increases
498 the amount of time it takes to process packets and, effectively,
499 decreases the amount of packet buffering.
500 This may cause packets to be
502 You should limit \fIsnaplen\fP to the smallest number that will
503 capture the protocol information you're interested in.
505 \fIsnaplen\fP to 0 sets it to the default of 65535,
506 for backwards compatibility with recent older versions of
510 Force packets selected by "\fIexpression\fP" to be interpreted the
511 specified \fItype\fR.
512 Currently known types are
513 \fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
514 \fBcarp\fR (Common Address Redundancy Protocol),
515 \fBcnfp\fR (Cisco NetFlow protocol),
516 \fBradius\fR (RADIUS),
517 \fBrpc\fR (Remote Procedure Call),
518 \fBrtp\fR (Real-Time Applications protocol),
519 \fBrtcp\fR (Real-Time Applications control protocol),
520 \fBsnmp\fR (Simple Network Management Protocol),
521 \fBtftp\fR (Trivial File Transfer Protocol),
522 \fBvat\fR (Visual Audio Tool),
524 \fBwb\fR (distributed White Board).
527 \fIDon't\fP print a timestamp on each dump line.
530 Print an unformatted timestamp on each dump line.
533 Print a delta (micro-second resolution) between current and previous line
537 Print a timestamp in default format proceeded by date on each dump line.
540 Print a delta (micro-second resolution) between current and first line
544 Print undecoded NFS handles.
549 option is not specified, make the printed packet output
550 ``packet-buffered''; i.e., as the description of the contents of each
551 packet is printed, it will be written to the standard output, rather
552 than, when not writing to a terminal, being written only when the output
557 option is specified, make the saved raw packet output
558 ``packet-buffered''; i.e., as each packet is saved, it will be written
559 to the output file, rather than being written only when the output
564 flag will not be supported if
566 was built with an older version of
573 When parsing and printing, produce (slightly more) verbose output.
574 For example, the time to live,
575 identification, total length and options in an IP packet are printed.
576 Also enables additional packet integrity checks such as verifying the
577 IP and ICMP header checksum.
579 When writing to a file with the
581 option, report, every 10 seconds, the number of packets captured.
584 Even more verbose output.
585 For example, additional fields are
586 printed from NFS reply packets, and SMB packets are fully decoded.
589 Even more verbose output.
591 telnet \fBSB\fP ... \fBSE\fP options
595 Telnet options are printed in hex as well.
598 Write the raw packets to \fIfile\fR rather than parsing and printing
600 They can later be printed with the \-r option.
601 Standard output is used if \fIfile\fR is ``-''.
603 This output will be buffered if written to a file or pipe, so a program
604 reading from the file or pipe may not see packets for an arbitrary
605 amount of time after they are received. Use the
607 flag to cause packets to be written as soon as they are received.
609 The MIME type \fIapplication/vnd.tcpdump.pcap\fP has been registered
610 with IANA for \fIpcap\fP files. The filename extension \fI.pcap\fP
611 appears to be the most commonly used along with \fI.cap\fP and
612 \fI.dmp\fP. \fITcpdump\fP itself doesn't check the extension when
613 reading capture files and doesn't add an extension when writing them
614 (it uses magic numbers in the file header instead). However, many
615 operating systems and applications will use the extension if it is
616 present and adding one (e.g. .pcap) is recommended.
619 .BR pcap-savefile (@MAN_FILE_FORMATS@)
620 for a description of the file format.
623 Used in conjunction with the
625 option, this will limit the number
626 of files created to the specified number, and begin overwriting files
627 from the beginning, thus creating a 'rotating' buffer.
628 In addition, it will name
629 the files with enough leading 0s to support the maximum number of
630 files, allowing them to sort correctly.
632 Used in conjunction with the
634 option, this will limit the number of rotated dump files that get
635 created, exiting with status 0 when reaching the limit. If used with
637 as well, the behavior will result in cyclical files per timeslice.
640 When parsing and printing,
641 in addition to printing the headers of each packet, print the data of
642 each packet (minus its link level header) in hex.
643 The smaller of the entire packet or
645 bytes will be printed. Note that this is the entire link-layer
646 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
647 will also be printed when the higher layer packet is shorter than the
651 When parsing and printing,
652 in addition to printing the headers of each packet, print the data of
655 its link level header, in hex.
658 When parsing and printing,
659 in addition to printing the headers of each packet, print the data of
660 each packet (minus its link level header) in hex and ASCII.
661 This is very handy for analysing new protocols.
664 When parsing and printing,
665 in addition to printing the headers of each packet, print the data of
668 its link level header, in hex and ASCII.
671 Set the data link type to use while capturing packets to \fIdatalinktype\fP.
674 Used in conjunction with the
678 options, this will make
684 is the savefile being closed after each rotation. For example, specifying
688 will compress each savefile using gzip or bzip2.
690 Note that tcpdump will run the command in parallel to the capture, using
691 the lowest priority so that this doesn't disturb the capture process.
693 And in case you would like to use a command that itself takes flags or
694 different arguments, you can always write a shell script that will take the
695 savefile name as the only argument, make the flags & arguments arrangements
696 and execute the command that you want.
701 is running as root, after opening the capture device or input savefile,
702 but before opening any savefiles for output, change the user ID to
704 and the group ID to the primary group of
707 This behavior can also be enabled by default at compile time.
708 .IP "\fI expression\fP"
710 selects which packets will be dumped.
711 If no \fIexpression\fP
712 is given, all packets on the net will be dumped.
714 only packets for which \fIexpression\fP is `true' will be dumped.
716 For the \fIexpression\fP syntax, see
717 .BR pcap-filter (@MAN_MISC_INFO@).
719 Expression arguments can be passed to \fItcpdump\fP as either a single
720 argument or as multiple arguments, whichever is more convenient.
721 Generally, if the expression contains Shell metacharacters, such as
722 backslashes used to escape protocol names, it is easier to pass it as
723 a single, quoted argument rather than to escape the Shell
725 Multiple arguments are concatenated with spaces before being parsed.
728 To print all packets arriving at or departing from \fIsundown\fP:
731 \fBtcpdump host sundown\fP
735 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
738 \fBtcpdump host helios and \\( hot or ace \\)\fP
742 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
745 \fBtcpdump ip host ace and not helios\fP
749 To print all traffic between local hosts and hosts at Berkeley:
753 tcpdump net ucb-ether
757 To print all ftp traffic through internet gateway \fIsnup\fP:
758 (note that the expression is quoted to prevent the shell from
759 (mis-)interpreting the parentheses):
763 tcpdump 'gateway snup and (port ftp or ftp-data)'
767 To print traffic neither sourced from nor destined for local hosts
768 (if you gateway to one other net, this stuff should never make it
769 onto your local net).
773 tcpdump ip and not net \fIlocalnet\fP
777 To print the start and end packets (the SYN and FIN packets) of each
778 TCP conversation that involves a non-local host.
782 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
786 To print all IPv4 HTTP packets to and from port 80, i.e. print only
787 packets that contain data, not, for example, SYN and FIN packets and
788 ACK-only packets. (IPv6 is left as an exercise for the reader.)
792 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
796 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
800 tcpdump 'gateway snup and ip[2:2] > 576'
804 To print IP broadcast or multicast packets that were
806 sent via Ethernet broadcast or multicast:
810 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
814 To print all ICMP packets that are not echo requests/replies (i.e., not
819 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
824 The output of \fItcpdump\fP is protocol dependent.
826 gives a brief description and examples of most of the formats.
834 If the '-e' option is given, the link level header is printed out.
835 On Ethernets, the source and destination addresses, protocol,
836 and packet length are printed.
838 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
839 the `frame control' field, the source and destination addresses,
840 and the packet length.
841 (The `frame control' field governs the
842 interpretation of the rest of the packet.
844 as those containing IP datagrams) are `async' packets, with a priority
845 value between 0 and 7; for example, `\fBasync4\fR'.
847 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
848 the LLC header is printed if it is \fInot\fR an ISO datagram or a
849 so-called SNAP packet.
851 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
852 the `access control' and `frame control' fields, the source and
853 destination addresses, and the packet length.
855 packets are assumed to contain an LLC packet.
856 Regardless of whether
857 the '-e' option is specified or not, the source routing information is
858 printed for source-routed packets.
860 On 802.11 networks, the '-e' option causes \fItcpdump\fP to print
861 the `frame control' fields, all of the addresses in the 802.11 header,
862 and the packet length.
864 packets are assumed to contain an LLC packet.
866 \fI(N.B.: The following description assumes familiarity with
867 the SLIP compression algorithm described in RFC-1144.)\fP
869 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
870 packet type, and compression information are printed out.
871 The packet type is printed first.
872 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
873 No further link information is printed for \fIip\fR packets.
874 For TCP packets, the connection identifier is printed following the type.
875 If the packet is compressed, its encoded header is printed out.
876 The special cases are printed out as
877 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
878 the sequence number (or sequence number and ack) has changed.
879 If it is not a special case,
880 zero or more changes are printed.
881 A change is indicated by U (urgent pointer), W (window), A (ack),
882 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
884 Finally, the amount of data in the packet and compressed header length
887 For example, the following line shows an outbound compressed TCP packet,
888 with an implicit connection identifier; the ack has changed by 6,
889 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
890 data and 6 bytes of compressed header:
893 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
899 Arp/rarp output shows the type of request and its arguments.
901 format is intended to be self explanatory.
902 Here is a short sample taken from the start of an `rlogin' from
903 host \fIrtsg\fP to host \fIcsam\fP:
907 \f(CWarp who-has csam tell rtsg
908 arp reply csam is-at CSAM\fR
912 The first line says that rtsg sent an arp packet asking
913 for the Ethernet address of internet host csam.
915 replies with its Ethernet address (in this example, Ethernet addresses
916 are in caps and internet addresses in lower case).
918 This would look less redundant if we had done \fItcpdump \-n\fP:
922 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
923 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
927 If we had done \fItcpdump \-e\fP, the fact that the first packet is
928 broadcast and the second is point-to-point would be visible:
932 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
933 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
937 For the first packet this says the Ethernet source address is RTSG, the
938 destination is the Ethernet broadcast address, the type field
939 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
943 \fI(N.B.:The following description assumes familiarity with
944 the TCP protocol described in RFC-793.
945 If you are not familiar
946 with the protocol, neither this description nor \fItcpdump\fP will
947 be of much use to you.)\fP
949 The general format of a tcp protocol line is:
953 \fIsrc > dst: flags data-seqno ack window urgent options\fP
957 \fISrc\fP and \fIdst\fP are the source and destination IP
959 \fIFlags\fP are some combination of S (SYN),
960 F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or
961 `.' (ACK), or `none' if no flags are set.
962 \fIData-seqno\fP describes the portion of sequence space covered
963 by the data in this packet (see example below).
964 \fIAck\fP is sequence number of the next data expected the other
965 direction on this connection.
966 \fIWindow\fP is the number of bytes of receive buffer space available
967 the other direction on this connection.
968 \fIUrg\fP indicates there is `urgent' data in the packet.
969 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
971 \fISrc, dst\fP and \fIflags\fP are always present.
973 depend on the contents of the packet's tcp protocol header and
974 are output only if appropriate.
976 Here is the opening portion of an rlogin from host \fIrtsg\fP to
981 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
982 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
983 rtsg.1023 > csam.login: . ack 1 win 4096
984 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
985 csam.login > rtsg.1023: . ack 2 win 4096
986 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
987 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
988 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
989 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
993 The first line says that tcp port 1023 on rtsg sent a packet
996 The \fBS\fP indicates that the \fISYN\fP flag was set.
997 The packet sequence number was 768512 and it contained no data.
998 (The notation is `first:last(nbytes)' which means `sequence
1000 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
1001 There was no piggy-backed ack, the available receive window was 4096
1002 bytes and there was a max-segment-size option requesting an mss of
1005 Csam replies with a similar packet except it includes a piggy-backed
1007 Rtsg then acks csam's SYN.
1008 The `.' means the ACK flag was set.
1009 The packet contained no data so there is no data sequence number.
1010 Note that the ack sequence
1011 number is a small integer (1).
1012 The first time \fItcpdump\fP sees a
1013 tcp `conversation', it prints the sequence number from the packet.
1014 On subsequent packets of the conversation, the difference between
1015 the current packet's sequence number and this initial sequence number
1017 This means that sequence numbers after the
1018 first can be interpreted
1019 as relative byte positions in the conversation's data stream (with the
1020 first data byte each direction being `1').
1021 `-S' will override this
1022 feature, causing the original sequence numbers to be output.
1024 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
1025 in the rtsg \(-> csam side of the conversation).
1026 The PUSH flag is set in the packet.
1027 On the 7th line, csam says it's received data sent by rtsg up to
1028 but not including byte 21.
1029 Most of this data is apparently sitting in the
1030 socket buffer since csam's receive window has gotten 19 bytes smaller.
1031 Csam also sends one byte of data to rtsg in this packet.
1032 On the 8th and 9th lines,
1033 csam sends two bytes of urgent, pushed data to rtsg.
1035 If the snapshot was small enough that \fItcpdump\fP didn't capture
1036 the full TCP header, it interprets as much of the header as it can
1037 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
1039 If the header contains a bogus option (one with a length
1040 that's either too small or beyond the end of the header), \fItcpdump\fP
1041 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
1042 options (since it's impossible to tell where they start).
1044 length indicates options are present but the IP datagram length is not
1045 long enough for the options to actually be there, \fItcpdump\fP reports
1046 it as ``[\fIbad hdr length\fP]''.
1048 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
1050 There are 8 bits in the control bits section of the TCP header:
1052 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
1054 Let's assume that we want to watch packets used in establishing
1056 Recall that TCP uses a 3-way handshake protocol
1057 when it initializes a new connection; the connection sequence with
1058 regard to the TCP control bits is
1064 2) Recipient responds with SYN, ACK
1070 Now we're interested in capturing packets that have only the
1071 SYN bit set (Step 1).
1072 Note that we don't want packets from step 2
1073 (SYN-ACK), just a plain initial SYN.
1074 What we need is a correct filter
1075 expression for \fItcpdump\fP.
1077 Recall the structure of a TCP header without options:
1081 -----------------------------------------------------------------
1082 | source port | destination port |
1083 -----------------------------------------------------------------
1085 -----------------------------------------------------------------
1086 | acknowledgment number |
1087 -----------------------------------------------------------------
1088 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1089 -----------------------------------------------------------------
1090 | TCP checksum | urgent pointer |
1091 -----------------------------------------------------------------
1094 A TCP header usually holds 20 octets of data, unless options are
1096 The first line of the graph contains octets 0 - 3, the
1097 second line shows octets 4 - 7 etc.
1099 Starting to count with 0, the relevant TCP control bits are contained
1104 ----------------|---------------|---------------|----------------
1105 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1106 ----------------|---------------|---------------|----------------
1107 | | 13th octet | | |
1110 Let's have a closer look at octet no. 13:
1120 These are the TCP control bits we are interested
1122 We have numbered the bits in this octet from 0 to 7, right to
1123 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1125 Recall that we want to capture packets with only SYN set.
1126 Let's see what happens to octet 13 if a TCP datagram arrives
1127 with the SYN bit set in its header:
1138 control bits section we see that only bit number 1 (SYN) is set.
1140 Assuming that octet number 13 is an 8-bit unsigned integer in
1141 network byte order, the binary value of this octet is
1145 and its decimal representation is
1149 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1152 We're almost done, because now we know that if only SYN is set,
1153 the value of the 13th octet in the TCP header, when interpreted
1154 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1156 This relationship can be expressed as
1162 We can use this expression as the filter for \fItcpdump\fP in order
1163 to watch packets which have only SYN set:
1166 tcpdump -i xl0 tcp[13] == 2
1169 The expression says "let the 13th octet of a TCP datagram have
1170 the decimal value 2", which is exactly what we want.
1172 Now, let's assume that we need to capture SYN packets, but we
1173 don't care if ACK or any other TCP control bit is set at the
1175 Let's see what happens to octet 13 when a TCP datagram
1176 with SYN-ACK set arrives:
1186 Now bits 1 and 4 are set in the 13th octet.
1192 which translates to decimal
1196 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1199 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1200 expression, because that would select only those packets that have
1201 SYN-ACK set, but not those with only SYN set.
1202 Remember that we don't care
1203 if ACK or any other control bit is set as long as SYN is set.
1205 In order to achieve our goal, we need to logically AND the
1206 binary value of octet 13 with some other value to preserve
1208 We know that we want SYN to be set in any case,
1209 so we'll logically AND the value in the 13th octet with
1210 the binary value of a SYN:
1214 00010010 SYN-ACK 00000010 SYN
1215 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1217 = 00000010 = 00000010
1220 We see that this AND operation delivers the same result
1221 regardless whether ACK or another TCP control bit is set.
1222 The decimal representation of the AND value as well as
1223 the result of this operation is 2 (binary 00000010),
1224 so we know that for packets with SYN set the following
1225 relation must hold true:
1227 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1229 This points us to the \fItcpdump\fP filter expression
1232 tcpdump -i xl0 'tcp[13] & 2 == 2'
1235 Some offsets and field values may be expressed as names
1236 rather than as numeric values. For example tcp[13] may
1237 be replaced with tcp[tcpflags]. The following TCP flag
1238 field values are also available: tcp-fin, tcp-syn, tcp-rst,
1239 tcp-push, tcp-act, tcp-urg.
1241 This can be demonstrated as:
1244 tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
1247 Note that you should use single quotes or a backslash
1248 in the expression to hide the AND ('&') special character
1254 UDP format is illustrated by this rwho packet:
1258 \f(CWactinide.who > broadcast.who: udp 84\fP
1262 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1263 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1265 The packet contained 84 bytes of user data.
1267 Some UDP services are recognized (from the source or destination
1268 port number) and the higher level protocol information printed.
1269 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1270 RPC calls (RFC-1050) to NFS.
1272 UDP Name Server Requests
1274 \fI(N.B.:The following description assumes familiarity with
1275 the Domain Service protocol described in RFC-1035.
1276 If you are not familiar
1277 with the protocol, the following description will appear to be written
1280 Name server requests are formatted as
1284 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1286 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1290 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1291 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1292 The query id was `3'.
1293 The `+' indicates the \fIrecursion desired\fP flag
1295 The query length was 37 bytes, not including the UDP and
1296 IP protocol headers.
1297 The query operation was the normal one, \fIQuery\fP,
1298 so the op field was omitted.
1299 If the op had been anything else, it would
1300 have been printed between the `3' and the `+'.
1301 Similarly, the qclass was the normal one,
1302 \fIC_IN\fP, and omitted.
1303 Any other qclass would have been printed
1304 immediately after the `A'.
1306 A few anomalies are checked and may result in extra fields enclosed in
1307 square brackets: If a query contains an answer, authority records or
1308 additional records section,
1313 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1314 is the appropriate count.
1315 If any of the response bits are set (AA, RA or rcode) or any of the
1316 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1317 is printed, where \fIx\fP is the hex value of header bytes two and three.
1319 UDP Name Server Responses
1321 Name server responses are formatted as
1325 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1327 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1328 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1332 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1333 with 3 answer records, 3 name server records and 7 additional records.
1334 The first answer record is type A (address) and its data is internet
1335 address 128.32.137.3.
1336 The total size of the response was 273 bytes,
1337 excluding UDP and IP headers.
1338 The op (Query) and response code
1339 (NoError) were omitted, as was the class (C_IN) of the A record.
1341 In the second example, \fIhelios\fP responds to query 2 with a
1342 response code of non-existent domain (NXDomain) with no answers,
1343 one name server and no authority records.
1344 The `*' indicates that
1345 the \fIauthoritative answer\fP bit was set.
1347 answers, no type, class or data were printed.
1349 Other flag characters that might appear are `\-' (recursion available,
1350 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1352 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1357 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1358 on UDP/137, UDP/138 and TCP/139.
1359 Some primitive decoding of IPX and
1360 NetBEUI SMB data is also done.
1362 By default a fairly minimal decode is done, with a much more detailed
1363 decode done if -v is used.
1364 Be warned that with -v a single SMB packet
1365 may take up a page or more, so only use -v if you really want all the
1368 For information on SMB packet formats and what all the fields mean see
1369 www.cifs.org or the pub/samba/specs/ directory on your favorite
1370 samba.org mirror site.
1371 The SMB patches were written by Andrew Tridgell
1374 NFS Requests and Replies
1376 Sun NFS (Network File System) requests and replies are printed as:
1380 \fIsrc.xid > dst.nfs: len op args\fP
1381 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1384 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1385 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1386 sushi.201b > wrl.nfs:
1387 144 lookup fh 9,74/4096.6878 "xcolors"
1388 wrl.nfs > sushi.201b:
1389 reply ok 128 lookup fh 9,74/4134.3150
1394 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1395 to \fIwrl\fP (note that the number following the src host is a
1396 transaction id, \fInot\fP the source port).
1397 The request was 112 bytes,
1398 excluding the UDP and IP headers.
1399 The operation was a \fIreadlink\fP
1400 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1401 (If one is lucky, as in this case, the file handle can be interpreted
1402 as a major,minor device number pair, followed by the inode number and
1404 \fIWrl\fP replies `ok' with the contents of the link.
1406 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1407 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1408 Note that the data printed
1409 depends on the operation type.
1410 The format is intended to be self
1411 explanatory if read in conjunction with
1412 an NFS protocol spec.
1414 If the \-v (verbose) flag is given, additional information is printed.
1420 sushi.1372a > wrl.nfs:
1421 148 read fh 21,11/12.195 8192 bytes @ 24576
1422 wrl.nfs > sushi.1372a:
1423 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1428 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1429 which have been omitted from this example.) In the first line,
1430 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1431 at byte offset 24576.
1432 \fIWrl\fP replies `ok'; the packet shown on the
1433 second line is the first fragment of the reply, and hence is only 1472
1434 bytes long (the other bytes will follow in subsequent fragments, but
1435 these fragments do not have NFS or even UDP headers and so might not be
1436 printed, depending on the filter expression used).
1437 Because the \-v flag
1438 is given, some of the file attributes (which are returned in addition
1439 to the file data) are printed: the file type (``REG'', for regular file),
1440 the file mode (in octal), the uid and gid, and the file size.
1442 If the \-v flag is given more than once, even more details are printed.
1444 Note that NFS requests are very large and much of the detail won't be printed
1445 unless \fIsnaplen\fP is increased.
1446 Try using `\fB\-s 192\fP' to watch
1449 NFS reply packets do not explicitly identify the RPC operation.
1451 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1452 replies using the transaction ID.
1453 If a reply does not closely follow the
1454 corresponding request, it might not be parsable.
1456 AFS Requests and Replies
1458 Transarc AFS (Andrew File System) requests and replies are printed
1464 \fIsrc.sport > dst.dport: rx packet-type\fP
1465 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1466 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1469 elvis.7001 > pike.afsfs:
1470 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1471 new fid 536876964/1/1 ".newsrc"
1472 pike.afsfs > elvis.7001: rx data fs reply rename
1477 In the first line, host elvis sends a RX packet to pike.
1479 a RX data packet to the fs (fileserver) service, and is the start of
1481 The RPC call was a rename, with the old directory file id
1482 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1483 file id of 536876964/1/1 and a new filename of `.newsrc'.
1485 responds with a RPC reply to the rename call (which was successful, because
1486 it was a data packet and not an abort packet).
1488 In general, all AFS RPCs are decoded at least by RPC call name.
1490 AFS RPCs have at least some of the arguments decoded (generally only
1491 the `interesting' arguments, for some definition of interesting).
1493 The format is intended to be self-describing, but it will probably
1494 not be useful to people who are not familiar with the workings of
1497 If the -v (verbose) flag is given twice, acknowledgement packets and
1498 additional header information is printed, such as the RX call ID,
1499 call number, sequence number, serial number, and the RX packet flags.
1501 If the -v flag is given twice, additional information is printed,
1502 such as the RX call ID, serial number, and the RX packet flags.
1503 The MTU negotiation information is also printed from RX ack packets.
1505 If the -v flag is given three times, the security index and service id
1508 Error codes are printed for abort packets, with the exception of Ubik
1509 beacon packets (because abort packets are used to signify a yes vote
1510 for the Ubik protocol).
1512 Note that AFS requests are very large and many of the arguments won't
1513 be printed unless \fIsnaplen\fP is increased.
1514 Try using `\fB-s 256\fP'
1515 to watch AFS traffic.
1517 AFS reply packets do not explicitly identify the RPC operation.
1519 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1520 replies using the call number and service ID.
1521 If a reply does not closely
1523 corresponding request, it might not be parsable.
1526 KIP AppleTalk (DDP in UDP)
1528 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1529 and dumped as DDP packets (i.e., all the UDP header information is
1533 is used to translate AppleTalk net and node numbers to names.
1534 Lines in this file have the form
1546 The first two lines give the names of AppleTalk networks.
1548 line gives the name of a particular host (a host is distinguished
1549 from a net by the 3rd octet in the number \-
1550 a net number \fImust\fP have two octets and a host number \fImust\fP
1551 have three octets.) The number and name should be separated by
1552 whitespace (blanks or tabs).
1555 file may contain blank lines or comment lines (lines starting with
1558 AppleTalk addresses are printed in the form
1564 \f(CW144.1.209.2 > icsd-net.112.220
1565 office.2 > icsd-net.112.220
1566 jssmag.149.235 > icsd-net.2\fR
1572 doesn't exist or doesn't contain an entry for some AppleTalk
1573 host/net number, addresses are printed in numeric form.)
1574 In the first example, NBP (DDP port 2) on net 144.1 node 209
1575 is sending to whatever is listening on port 220 of net icsd node 112.
1576 The second line is the same except the full name of the source node
1577 is known (`office').
1578 The third line is a send from port 235 on
1579 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1580 the broadcast address (255) is indicated by a net name with no host
1581 number \- for this reason it's a good idea to keep node names and
1582 net names distinct in /etc/atalk.names).
1584 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1585 packets have their contents interpreted.
1586 Other protocols just dump
1587 the protocol name (or number if no name is registered for the
1588 protocol) and packet size.
1590 \fBNBP packets\fP are formatted like the following examples:
1594 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1595 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1596 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1600 The first line is a name lookup request for laserwriters sent by net icsd host
1601 112 and broadcast on net jssmag.
1602 The nbp id for the lookup is 190.
1603 The second line shows a reply for this request (note that it has the
1604 same id) from host jssmag.209 saying that it has a laserwriter
1605 resource named "RM1140" registered on port 250.
1607 another reply to the same request saying host techpit has laserwriter
1608 "techpit" registered on port 186.
1610 \fBATP packet\fP formatting is demonstrated by the following example:
1614 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1615 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1616 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1617 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1618 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1619 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1620 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1621 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1622 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1623 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1624 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1625 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1626 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1627 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1631 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1632 up to 8 packets (the `<0-7>').
1633 The hex number at the end of the line
1634 is the value of the `userdata' field in the request.
1636 Helios responds with 8 512-byte packets.
1637 The `:digit' following the
1638 transaction id gives the packet sequence number in the transaction
1639 and the number in parens is the amount of data in the packet,
1640 excluding the atp header.
1641 The `*' on packet 7 indicates that the
1644 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1646 resends them then jssmag.209 releases the transaction.
1648 jssmag.209 initiates the next request.
1649 The `*' on the request
1650 indicates that XO (`exactly once') was \fInot\fP set.
1655 Fragmented Internet datagrams are printed as
1659 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1660 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1664 (The first form indicates there are more fragments.
1666 indicates this is the last fragment.)
1668 \fIId\fP is the fragment id.
1669 \fISize\fP is the fragment
1670 size (in bytes) excluding the IP header.
1671 \fIOffset\fP is this
1672 fragment's offset (in bytes) in the original datagram.
1674 The fragment information is output for each fragment.
1676 fragment contains the higher level protocol header and the frag
1677 info is printed after the protocol info.
1679 after the first contain no higher level protocol header and the
1680 frag info is printed after the source and destination addresses.
1681 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1682 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1686 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1687 arizona > rtsg: (frag 595a:204@328)
1688 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1692 There are a couple of things to note here: First, addresses in the
1693 2nd line don't include port numbers.
1694 This is because the TCP
1695 protocol information is all in the first fragment and we have no idea
1696 what the port or sequence numbers are when we print the later fragments.
1697 Second, the tcp sequence information in the first line is printed as if there
1698 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1699 the first frag and 204 in the second).
1700 If you are looking for holes
1701 in the sequence space or trying to match up acks
1702 with packets, this can fool you.
1704 A packet with the IP \fIdon't fragment\fP flag is marked with a
1705 trailing \fB(DF)\fP.
1709 By default, all output lines are preceded by a timestamp.
1711 is the current clock time in the form
1717 and is as accurate as the kernel's clock.
1718 The timestamp reflects the time the kernel first saw the packet.
1720 is made to account for the time lag between when the
1721 Ethernet interface removed the packet from the wire and when the kernel
1722 serviced the `new packet' interrupt.
1724 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@),
1725 pcap-filter(@MAN_MISC_INFO@), pcap-tstamp-type(@MAN_MISC_INFO@)
1728 .I http://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap
1732 The original authors are:
1736 Steven McCanne, all of the
1737 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1739 It is currently being maintained by tcpdump.org.
1741 The current version is available via http:
1744 .I http://www.tcpdump.org/
1747 The original distribution is available via anonymous ftp:
1750 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1753 IPv6/IPsec support is added by WIDE/KAME project.
1754 This program uses Eric Young's SSLeay library, under specific configurations.
1756 Please send problems, bugs, questions, desirable enhancements, patches
1760 tcpdump-workers@lists.tcpdump.org
1763 NIT doesn't let you watch your own outbound traffic, BPF will.
1764 We recommend that you use the latter.
1766 On Linux systems with 2.0[.x] kernels:
1768 packets on the loopback device will be seen twice;
1770 packet filtering cannot be done in the kernel, so that all packets must
1771 be copied from the kernel in order to be filtered in user mode;
1773 all of a packet, not just the part that's within the snapshot length,
1774 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1775 asked to copy only part of a packet to userland, will not report the
1776 true length of the packet; this would cause most IP packets to get an
1780 capturing on some PPP devices won't work correctly.
1782 We recommend that you upgrade to a 2.2 or later kernel.
1784 Some attempt should be made to reassemble IP fragments or, at least
1785 to compute the right length for the higher level protocol.
1787 Name server inverse queries are not dumped correctly: the (empty)
1788 question section is printed rather than real query in the answer
1790 Some believe that inverse queries are themselves a bug and
1791 prefer to fix the program generating them rather than \fItcpdump\fP.
1793 A packet trace that crosses a daylight savings time change will give
1794 skewed time stamps (the time change is ignored).
1796 Filter expressions on fields other than those in Token Ring headers will
1797 not correctly handle source-routed Token Ring packets.
1799 Filter expressions on fields other than those in 802.11 headers will not
1800 correctly handle 802.11 data packets with both To DS and From DS set.
1803 should chase header chain, but at this moment it does not.
1804 .BR "ip6 protochain"
1805 is supplied for this behavior.
1807 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1808 does not work against IPv6 packets.
1809 It only looks at IPv4 packets.
projects / tcpdump / blob