1 .\" @(#) $Header: /tcpdump/master/tcpdump/Attic/tcpdump.1,v 1.192 2008-08-16 11:36:20 hannes 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 "07 January 2008"
27 tcpdump \- dump traffic on a network
32 .B \-AdDefIKlLnNOpqRStuUvxX
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 Set the operating system capture buffer size to \fIbuffer_size\fP.
198 Exit after receiving \fIcount\fP packets.
201 Before writing a raw packet to a savefile, check whether the file is
202 currently larger than \fIfile_size\fP and, if so, close the current
203 savefile and open a new one. Savefiles after the first savefile will
204 have the name specified with the
206 flag, with a number after it, starting at 1 and continuing upward.
207 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
208 not 1,048,576 bytes).
211 Dump the compiled packet-matching code in a human readable form to
212 standard output and stop.
215 Dump packet-matching code as a
220 Dump packet-matching code as decimal numbers (preceded with a count).
223 Print the list of the network interfaces available on the system and on
226 can capture packets. For each network interface, a number and an
227 interface name, possibly followed by a text description of the
228 interface, is printed. The interface name or the number can be supplied
231 flag to specify an interface on which to capture.
233 This can be useful on systems that don't have a command to list them
234 (e.g., Windows systems, or UNIX systems lacking
235 .BR "ifconfig \-a" );
236 the number can be useful on Windows 2000 and later systems, where the
237 interface name is a somewhat complex string.
241 flag will not be supported if
243 was built with an older version of
246 .B pcap_findalldevs()
250 Print the link-level header on each dump line.
253 Use \fIspi@ipaddr algo:secret\fP for decrypting IPsec ESP packets that
254 are addressed to \fIaddr\fP and contain Security Parameter Index value
255 \fIspi\fP. This combination may be repeated with comma or newline seperation.
257 Note that setting the secret for IPv4 ESP packets is supported at this time.
264 \fBcast128-cbc\fP, or
266 The default is \fBdes-cbc\fP.
267 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
268 with cryptography enabled.
270 \fIsecret\fP is the ASCII text for ESP secret key.
271 If preceeded by 0x, then a hex value will be read.
273 The option assumes RFC2406 ESP, not RFC1827 ESP.
274 The option is only for debugging purposes, and
275 the use of this option with a true `secret' key is discouraged.
276 By presenting IPsec secret key onto command line
277 you make it visible to others, via
281 In addition to the above syntax, the syntax \fIfile name\fP may be used
282 to have tcpdump read the provided file in. The file is opened upon
283 receiving the first ESP packet, so any special permissions that tcpdump
284 may have been given should already have been given up.
287 Print `foreign' IPv4 addresses numerically rather than symbolically
288 (this option is intended to get around serious brain damage in
289 Sun's NIS server \(em usually it hangs forever translating non-local
292 The test for `foreign' IPv4 addresses is done using the IPv4 address and
293 netmask of the interface on which capture is being done. If that
294 address or netmask are not available, available, either because the
295 interface on which capture is being done has no address or netmask or
296 because the capture is being done on the Linux "any" interface, which
297 can capture on more than one interface, this option will not work
301 Use \fIfile\fP as input for the filter expression.
302 An additional expression given on the command line is ignored.
305 If specified, rotates the dump file specified with the
307 option every \fIrotate_seconds\fP seconds.
308 Savefiles will have the name specified by
310 which should include a time format as defined by
312 If no time format is specified, each new file will overwrite the previous.
314 If used in conjunction with the
316 option, filenames will take the form of `\fIfile\fP<count>'.
319 Listen on \fIinterface\fP.
320 If unspecified, \fItcpdump\fP searches the system interface list for the
321 lowest numbered, configured up interface (excluding loopback).
322 Ties are broken by choosing the earliest match.
324 On Linux systems with 2.2 or later kernels, an
326 argument of ``any'' can be used to capture packets from all interfaces.
327 Note that captures on the ``any'' device will not be done in promiscuous
332 flag is supported, an interface number as printed by that flag can be
338 Put the interface in "monitor mode"; this is supported only on IEEE
339 802.11 Wi-Fi interfaces, and supported only on some operating systems.
341 Note that in monitor mode the adapter might disassociate from the
342 network with which it's associated, so that you will not be able to use
343 any wireless networks with that adapter. This could prevent accessing
344 files on a network server, or resolving host names or network addresses,
345 if you are capturing in monitor mode and are not connected to another
346 network with another adapter.
349 Don't attempt to verify TCP checksums. This is useful for interfaces
350 that perform the TCP checksum calculation in hardware; otherwise,
351 all outgoing TCP checksums will be flagged as bad.
354 Make stdout line buffered.
355 Useful if you want to see the data
359 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
360 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
363 List the known data link types for the interface and exit.
366 Load SMI MIB module definitions from file \fImodule\fR.
368 can be used several times to load several MIB modules into \fItcpdump\fP.
371 Use \fIsecret\fP as a shared secret for validating the digests found in
372 Routing Protocols (RSVP) and TCP segments with the TCP-MD5 option
373 (RFC 2385), if present.
376 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
379 Don't print domain name qualification of host names.
381 if you give this flag then \fItcpdump\fP will print ``nic''
382 instead of ``nic.ddn.mil''.
385 Do not run the packet-matching code optimizer.
387 if you suspect a bug in the optimizer.
390 \fIDon't\fP put the interface
391 into promiscuous mode.
392 Note that the interface might be in promiscuous
393 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
394 `ether host {local-hw-addr} or ether broadcast'.
397 Quick (quiet?) output.
398 Print less protocol information so output
402 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
403 If specified, \fItcpdump\fP will not print replay prevention field.
404 Since there is no protocol version field in ESP/AH specification,
405 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
408 Read packets from \fIfile\fR (which was created with the
411 Standard input is used if \fIfile\fR is ``-''.
414 Print absolute, rather than relative, TCP sequence numbers.
417 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
418 default of 68 (with SunOS's NIT, the minimum is actually 96).
419 68 bytes is adequate for IP, ICMP, TCP
420 and UDP but may truncate protocol information from name server and NFS
422 Packets truncated because of a limited snapshot
423 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
424 is the name of the protocol level at which the truncation has occurred.
425 Note that taking larger snapshots both increases
426 the amount of time it takes to process packets and, effectively,
427 decreases the amount of packet buffering.
428 This may cause packets to be
430 You should limit \fIsnaplen\fP to the smallest number that will
431 capture the protocol information you're interested in.
433 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
436 Force packets selected by "\fIexpression\fP" to be interpreted the
437 specified \fItype\fR.
438 Currently known types are
439 \fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
440 \fBcnfp\fR (Cisco NetFlow protocol),
441 \fBrpc\fR (Remote Procedure Call),
442 \fBrtp\fR (Real-Time Applications protocol),
443 \fBrtcp\fR (Real-Time Applications control protocol),
444 \fBsnmp\fR (Simple Network Management Protocol),
445 \fBtftp\fR (Trivial File Transfer Protocol),
446 \fBvat\fR (Visual Audio Tool),
448 \fBwb\fR (distributed White Board).
451 \fIDon't\fP print a timestamp on each dump line.
454 Print an unformatted timestamp on each dump line.
457 Print a delta (micro-second resolution) between current and previous line
461 Print a timestamp in default format proceeded by date on each dump line.
464 Print a delta (micro-second resolution) between current and first line
468 Print undecoded NFS handles.
471 Make output saved via the
473 option ``packet-buffered''; i.e., as each packet is saved, it will be
474 written to the output file, rather than being written only when the
479 flag will not be supported if
481 was built with an older version of
488 When parsing and printing, produce (slightly more) verbose output.
489 For example, the time to live,
490 identification, total length and options in an IP packet are printed.
491 Also enables additional packet integrity checks such as verifying the
492 IP and ICMP header checksum.
494 When writing to a file with the
496 option, report, every 10 seconds, the number of packets captured.
499 Even more verbose output.
500 For example, additional fields are
501 printed from NFS reply packets, and SMB packets are fully decoded.
504 Even more verbose output.
506 telnet \fBSB\fP ... \fBSE\fP options
510 Telnet options are printed in hex as well.
513 Write the raw packets to \fIfile\fR rather than parsing and printing
515 They can later be printed with the \-r option.
516 Standard output is used if \fIfile\fR is ``-''.
519 Used in conjunction with the
521 option, this will limit the number
522 of files created to the specified number, and begin overwriting files
523 from the beginning, thus creating a 'rotating' buffer.
524 In addition, it will name
525 the files with enough leading 0s to support the maximum number of
526 files, allowing them to sort correctly.
528 Used in conjunction with the
530 option, this will limit the number of rotated dump files that get
531 created, exiting with status 0 when reaching the limit. If used with
533 as well, the behavior will result in cyclical files per timeslice.
536 When parsing and printing,
537 in addition to printing the headers of each packet, print the data of
538 each packet (minus its link level header) in hex.
539 The smaller of the entire packet or
541 bytes will be printed. Note that this is the entire link-layer
542 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
543 will also be printed when the higher layer packet is shorter than the
547 When parsing and printing,
548 in addition to printing the headers of each packet, print the data of
551 its link level header, in hex.
554 When parsing and printing,
555 in addition to printing the headers of each packet, print the data of
556 each packet (minus its link level header) in hex and ASCII.
557 This is very handy for analysing new protocols.
560 When parsing and printing,
561 in addition to printing the headers of each packet, print the data of
564 its link level header, in hex and ASCII.
567 Set the data link type to use while capturing packets to \fIdatalinktype\fP.
570 Used in conjunction with the
574 options, this will make
580 is the savefile being closed after each rotation. For example, specifying
584 will compress each savefile using gzip or bzip2.
586 Note that tcpdump will run the command in parallel to the capture, using
587 the lowest priority so that this doesn't disturb the capture process.
589 And in case you would like to use a command that itself takes flags or
590 different arguments, you can always write a shell script that will take the
591 savefile name as the only argument, make the flags & arguments arrangements
592 and execute the command that you want.
595 Drops privileges (if root) and changes user ID to
597 and the group ID to the primary group of
600 This behavior can also be enabled by default at compile time.
601 .IP "\fI expression\fP"
603 selects which packets will be dumped.
604 If no \fIexpression\fP
605 is given, all packets on the net will be dumped.
607 only packets for which \fIexpression\fP is `true' will be dumped.
609 For the \fIexpression\fP syntax, see
612 Expression arguments can be passed to \fItcpdump\fP as either a single
613 argument or as multiple arguments, whichever is more convenient.
614 Generally, if the expression contains Shell metacharacters, it is
615 easier to pass it as a single, quoted argument.
616 Multiple arguments are concatenated with spaces before being parsed.
619 To print all packets arriving at or departing from \fIsundown\fP:
622 \fBtcpdump host sundown\fP
626 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
629 \fBtcpdump host helios and \\( hot or ace \\)\fP
633 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
636 \fBtcpdump ip host ace and not helios\fP
640 To print all traffic between local hosts and hosts at Berkeley:
644 tcpdump net ucb-ether
648 To print all ftp traffic through internet gateway \fIsnup\fP:
649 (note that the expression is quoted to prevent the shell from
650 (mis-)interpreting the parentheses):
654 tcpdump 'gateway snup and (port ftp or ftp-data)'
658 To print traffic neither sourced from nor destined for local hosts
659 (if you gateway to one other net, this stuff should never make it
660 onto your local net).
664 tcpdump ip and not net \fIlocalnet\fP
668 To print the start and end packets (the SYN and FIN packets) of each
669 TCP conversation that involves a non-local host.
673 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
677 To print all IPv4 HTTP packets to and from port 80, i.e. print only
678 packets that contain data, not, for example, SYN and FIN packets and
679 ACK-only packets. (IPv6 is left as an exercise for the reader.)
683 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
687 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
691 tcpdump 'gateway snup and ip[2:2] > 576'
695 To print IP broadcast or multicast packets that were
697 sent via Ethernet broadcast or multicast:
701 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
705 To print all ICMP packets that are not echo requests/replies (i.e., not
710 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
715 The output of \fItcpdump\fP is protocol dependent.
717 gives a brief description and examples of most of the formats.
725 If the '-e' option is given, the link level header is printed out.
726 On Ethernets, the source and destination addresses, protocol,
727 and packet length are printed.
729 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
730 the `frame control' field, the source and destination addresses,
731 and the packet length.
732 (The `frame control' field governs the
733 interpretation of the rest of the packet.
735 as those containing IP datagrams) are `async' packets, with a priority
736 value between 0 and 7; for example, `\fBasync4\fR'.
738 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
739 the LLC header is printed if it is \fInot\fR an ISO datagram or a
740 so-called SNAP packet.
742 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
743 the `access control' and `frame control' fields, the source and
744 destination addresses, and the packet length.
746 packets are assumed to contain an LLC packet.
747 Regardless of whether
748 the '-e' option is specified or not, the source routing information is
749 printed for source-routed packets.
751 On 802.11 networks, the '-e' option causes \fItcpdump\fP to print
752 the `frame control' fields, all of the addresses in the 802.11 header,
753 and the packet length.
755 packets are assumed to contain an LLC packet.
757 \fI(N.B.: The following description assumes familiarity with
758 the SLIP compression algorithm described in RFC-1144.)\fP
760 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
761 packet type, and compression information are printed out.
762 The packet type is printed first.
763 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
764 No further link information is printed for \fIip\fR packets.
765 For TCP packets, the connection identifier is printed following the type.
766 If the packet is compressed, its encoded header is printed out.
767 The special cases are printed out as
768 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
769 the sequence number (or sequence number and ack) has changed.
770 If it is not a special case,
771 zero or more changes are printed.
772 A change is indicated by U (urgent pointer), W (window), A (ack),
773 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
775 Finally, the amount of data in the packet and compressed header length
778 For example, the following line shows an outbound compressed TCP packet,
779 with an implicit connection identifier; the ack has changed by 6,
780 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
781 data and 6 bytes of compressed header:
784 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
790 Arp/rarp output shows the type of request and its arguments.
792 format is intended to be self explanatory.
793 Here is a short sample taken from the start of an `rlogin' from
794 host \fIrtsg\fP to host \fIcsam\fP:
798 \f(CWarp who-has csam tell rtsg
799 arp reply csam is-at CSAM\fR
803 The first line says that rtsg sent an arp packet asking
804 for the Ethernet address of internet host csam.
806 replies with its Ethernet address (in this example, Ethernet addresses
807 are in caps and internet addresses in lower case).
809 This would look less redundant if we had done \fItcpdump \-n\fP:
813 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
814 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
818 If we had done \fItcpdump \-e\fP, the fact that the first packet is
819 broadcast and the second is point-to-point would be visible:
823 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
824 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
828 For the first packet this says the Ethernet source address is RTSG, the
829 destination is the Ethernet broadcast address, the type field
830 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
834 \fI(N.B.:The following description assumes familiarity with
835 the TCP protocol described in RFC-793.
836 If you are not familiar
837 with the protocol, neither this description nor \fItcpdump\fP will
838 be of much use to you.)\fP
840 The general format of a tcp protocol line is:
844 \fIsrc > dst: flags data-seqno ack window urgent options\fP
848 \fISrc\fP and \fIdst\fP are the source and destination IP
850 \fIFlags\fP are some combination of S (SYN),
851 F (FIN), P (PUSH), R (RST), W (ECN CWR) or E (ECN-Echo), or a single
853 \fIData-seqno\fP describes the portion of sequence space covered
854 by the data in this packet (see example below).
855 \fIAck\fP is sequence number of the next data expected the other
856 direction on this connection.
857 \fIWindow\fP is the number of bytes of receive buffer space available
858 the other direction on this connection.
859 \fIUrg\fP indicates there is `urgent' data in the packet.
860 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
862 \fISrc, dst\fP and \fIflags\fP are always present.
864 depend on the contents of the packet's tcp protocol header and
865 are output only if appropriate.
867 Here is the opening portion of an rlogin from host \fIrtsg\fP to
872 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
873 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
874 rtsg.1023 > csam.login: . ack 1 win 4096
875 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
876 csam.login > rtsg.1023: . ack 2 win 4096
877 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
878 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
879 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
880 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
884 The first line says that tcp port 1023 on rtsg sent a packet
887 The \fBS\fP indicates that the \fISYN\fP flag was set.
888 The packet sequence number was 768512 and it contained no data.
889 (The notation is `first:last(nbytes)' which means `sequence
891 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
892 There was no piggy-backed ack, the available receive window was 4096
893 bytes and there was a max-segment-size option requesting an mss of
896 Csam replies with a similar packet except it includes a piggy-backed
898 Rtsg then acks csam's SYN.
901 The packet contained no data so there is no data sequence number.
902 Note that the ack sequence
903 number is a small integer (1).
904 The first time \fItcpdump\fP sees a
905 tcp `conversation', it prints the sequence number from the packet.
906 On subsequent packets of the conversation, the difference between
907 the current packet's sequence number and this initial sequence number
909 This means that sequence numbers after the
910 first can be interpreted
911 as relative byte positions in the conversation's data stream (with the
912 first data byte each direction being `1').
913 `-S' will override this
914 feature, causing the original sequence numbers to be output.
916 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
917 in the rtsg \(-> csam side of the conversation).
918 The PUSH flag is set in the packet.
919 On the 7th line, csam says it's received data sent by rtsg up to
920 but not including byte 21.
921 Most of this data is apparently sitting in the
922 socket buffer since csam's receive window has gotten 19 bytes smaller.
923 Csam also sends one byte of data to rtsg in this packet.
924 On the 8th and 9th lines,
925 csam sends two bytes of urgent, pushed data to rtsg.
927 If the snapshot was small enough that \fItcpdump\fP didn't capture
928 the full TCP header, it interprets as much of the header as it can
929 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
931 If the header contains a bogus option (one with a length
932 that's either too small or beyond the end of the header), \fItcpdump\fP
933 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
934 options (since it's impossible to tell where they start).
936 length indicates options are present but the IP datagram length is not
937 long enough for the options to actually be there, \fItcpdump\fP reports
938 it as ``[\fIbad hdr length\fP]''.
940 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
942 There are 8 bits in the control bits section of the TCP header:
944 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
946 Let's assume that we want to watch packets used in establishing
948 Recall that TCP uses a 3-way handshake protocol
949 when it initializes a new connection; the connection sequence with
950 regard to the TCP control bits is
956 2) Recipient responds with SYN, ACK
962 Now we're interested in capturing packets that have only the
963 SYN bit set (Step 1).
964 Note that we don't want packets from step 2
965 (SYN-ACK), just a plain initial SYN.
966 What we need is a correct filter
967 expression for \fItcpdump\fP.
969 Recall the structure of a TCP header without options:
973 -----------------------------------------------------------------
974 | source port | destination port |
975 -----------------------------------------------------------------
977 -----------------------------------------------------------------
978 | acknowledgment number |
979 -----------------------------------------------------------------
980 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
981 -----------------------------------------------------------------
982 | TCP checksum | urgent pointer |
983 -----------------------------------------------------------------
986 A TCP header usually holds 20 octets of data, unless options are
988 The first line of the graph contains octets 0 - 3, the
989 second line shows octets 4 - 7 etc.
991 Starting to count with 0, the relevant TCP control bits are contained
996 ----------------|---------------|---------------|----------------
997 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
998 ----------------|---------------|---------------|----------------
1002 Let's have a closer look at octet no. 13:
1012 These are the TCP control bits we are interested
1014 We have numbered the bits in this octet from 0 to 7, right to
1015 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1017 Recall that we want to capture packets with only SYN set.
1018 Let's see what happens to octet 13 if a TCP datagram arrives
1019 with the SYN bit set in its header:
1030 control bits section we see that only bit number 1 (SYN) is set.
1032 Assuming that octet number 13 is an 8-bit unsigned integer in
1033 network byte order, the binary value of this octet is
1037 and its decimal representation is
1041 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1044 We're almost done, because now we know that if only SYN is set,
1045 the value of the 13th octet in the TCP header, when interpreted
1046 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1048 This relationship can be expressed as
1054 We can use this expression as the filter for \fItcpdump\fP in order
1055 to watch packets which have only SYN set:
1058 tcpdump -i xl0 tcp[13] == 2
1061 The expression says "let the 13th octet of a TCP datagram have
1062 the decimal value 2", which is exactly what we want.
1064 Now, let's assume that we need to capture SYN packets, but we
1065 don't care if ACK or any other TCP control bit is set at the
1067 Let's see what happens to octet 13 when a TCP datagram
1068 with SYN-ACK set arrives:
1078 Now bits 1 and 4 are set in the 13th octet.
1084 which translates to decimal
1088 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1091 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1092 expression, because that would select only those packets that have
1093 SYN-ACK set, but not those with only SYN set.
1094 Remember that we don't care
1095 if ACK or any other control bit is set as long as SYN is set.
1097 In order to achieve our goal, we need to logically AND the
1098 binary value of octet 13 with some other value to preserve
1100 We know that we want SYN to be set in any case,
1101 so we'll logically AND the value in the 13th octet with
1102 the binary value of a SYN:
1106 00010010 SYN-ACK 00000010 SYN
1107 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1109 = 00000010 = 00000010
1112 We see that this AND operation delivers the same result
1113 regardless whether ACK or another TCP control bit is set.
1114 The decimal representation of the AND value as well as
1115 the result of this operation is 2 (binary 00000010),
1116 so we know that for packets with SYN set the following
1117 relation must hold true:
1119 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1121 This points us to the \fItcpdump\fP filter expression
1124 tcpdump -i xl0 'tcp[13] & 2 == 2'
1127 Note that you should use single quotes or a backslash
1128 in the expression to hide the AND ('&') special character
1134 UDP format is illustrated by this rwho packet:
1138 \f(CWactinide.who > broadcast.who: udp 84\fP
1142 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1143 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1145 The packet contained 84 bytes of user data.
1147 Some UDP services are recognized (from the source or destination
1148 port number) and the higher level protocol information printed.
1149 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1150 RPC calls (RFC-1050) to NFS.
1152 UDP Name Server Requests
1154 \fI(N.B.:The following description assumes familiarity with
1155 the Domain Service protocol described in RFC-1035.
1156 If you are not familiar
1157 with the protocol, the following description will appear to be written
1160 Name server requests are formatted as
1164 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1166 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1170 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1171 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1172 The query id was `3'.
1173 The `+' indicates the \fIrecursion desired\fP flag
1175 The query length was 37 bytes, not including the UDP and
1176 IP protocol headers.
1177 The query operation was the normal one, \fIQuery\fP,
1178 so the op field was omitted.
1179 If the op had been anything else, it would
1180 have been printed between the `3' and the `+'.
1181 Similarly, the qclass was the normal one,
1182 \fIC_IN\fP, and omitted.
1183 Any other qclass would have been printed
1184 immediately after the `A'.
1186 A few anomalies are checked and may result in extra fields enclosed in
1187 square brackets: If a query contains an answer, authority records or
1188 additional records section,
1193 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1194 is the appropriate count.
1195 If any of the response bits are set (AA, RA or rcode) or any of the
1196 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1197 is printed, where \fIx\fP is the hex value of header bytes two and three.
1199 UDP Name Server Responses
1201 Name server responses are formatted as
1205 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1207 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1208 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1212 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1213 with 3 answer records, 3 name server records and 7 additional records.
1214 The first answer record is type A (address) and its data is internet
1215 address 128.32.137.3.
1216 The total size of the response was 273 bytes,
1217 excluding UDP and IP headers.
1218 The op (Query) and response code
1219 (NoError) were omitted, as was the class (C_IN) of the A record.
1221 In the second example, \fIhelios\fP responds to query 2 with a
1222 response code of non-existent domain (NXDomain) with no answers,
1223 one name server and no authority records.
1224 The `*' indicates that
1225 the \fIauthoritative answer\fP bit was set.
1227 answers, no type, class or data were printed.
1229 Other flag characters that might appear are `\-' (recursion available,
1230 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1232 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1235 Note that name server requests and responses tend to be large and the
1236 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1238 Use the \fB\-s\fP flag to increase the snaplen if you
1239 need to seriously investigate name server traffic.
1241 has worked well for me.
1246 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1247 on UDP/137, UDP/138 and TCP/139.
1248 Some primitive decoding of IPX and
1249 NetBEUI SMB data is also done.
1251 By default a fairly minimal decode is done, with a much more detailed
1252 decode done if -v is used.
1253 Be warned that with -v a single SMB packet
1254 may take up a page or more, so only use -v if you really want all the
1257 For information on SMB packet formats and what all te fields mean see
1258 www.cifs.org or the pub/samba/specs/ directory on your favorite
1259 samba.org mirror site.
1260 The SMB patches were written by Andrew Tridgell
1264 NFS Requests and Replies
1266 Sun NFS (Network File System) requests and replies are printed as:
1270 \fIsrc.xid > dst.nfs: len op args\fP
1271 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1274 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1275 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1276 sushi.201b > wrl.nfs:
1277 144 lookup fh 9,74/4096.6878 "xcolors"
1278 wrl.nfs > sushi.201b:
1279 reply ok 128 lookup fh 9,74/4134.3150
1284 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1285 to \fIwrl\fP (note that the number following the src host is a
1286 transaction id, \fInot\fP the source port).
1287 The request was 112 bytes,
1288 excluding the UDP and IP headers.
1289 The operation was a \fIreadlink\fP
1290 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1291 (If one is lucky, as in this case, the file handle can be interpreted
1292 as a major,minor device number pair, followed by the inode number and
1294 \fIWrl\fP replies `ok' with the contents of the link.
1296 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1297 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1298 Note that the data printed
1299 depends on the operation type.
1300 The format is intended to be self
1301 explanatory if read in conjunction with
1302 an NFS protocol spec.
1304 If the \-v (verbose) flag is given, additional information is printed.
1310 sushi.1372a > wrl.nfs:
1311 148 read fh 21,11/12.195 8192 bytes @ 24576
1312 wrl.nfs > sushi.1372a:
1313 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1318 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1319 which have been omitted from this example.) In the first line,
1320 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1321 at byte offset 24576.
1322 \fIWrl\fP replies `ok'; the packet shown on the
1323 second line is the first fragment of the reply, and hence is only 1472
1324 bytes long (the other bytes will follow in subsequent fragments, but
1325 these fragments do not have NFS or even UDP headers and so might not be
1326 printed, depending on the filter expression used).
1327 Because the \-v flag
1328 is given, some of the file attributes (which are returned in addition
1329 to the file data) are printed: the file type (``REG'', for regular file),
1330 the file mode (in octal), the uid and gid, and the file size.
1332 If the \-v flag is given more than once, even more details are printed.
1334 Note that NFS requests are very large and much of the detail won't be printed
1335 unless \fIsnaplen\fP is increased.
1336 Try using `\fB\-s 192\fP' to watch
1339 NFS reply packets do not explicitly identify the RPC operation.
1341 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1342 replies using the transaction ID.
1343 If a reply does not closely follow the
1344 corresponding request, it might not be parsable.
1346 AFS Requests and Replies
1348 Transarc AFS (Andrew File System) requests and replies are printed
1354 \fIsrc.sport > dst.dport: rx packet-type\fP
1355 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1356 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1359 elvis.7001 > pike.afsfs:
1360 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1361 new fid 536876964/1/1 ".newsrc"
1362 pike.afsfs > elvis.7001: rx data fs reply rename
1367 In the first line, host elvis sends a RX packet to pike.
1369 a RX data packet to the fs (fileserver) service, and is the start of
1371 The RPC call was a rename, with the old directory file id
1372 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1373 file id of 536876964/1/1 and a new filename of `.newsrc'.
1375 responds with a RPC reply to the rename call (which was successful, because
1376 it was a data packet and not an abort packet).
1378 In general, all AFS RPCs are decoded at least by RPC call name.
1380 AFS RPCs have at least some of the arguments decoded (generally only
1381 the `interesting' arguments, for some definition of interesting).
1383 The format is intended to be self-describing, but it will probably
1384 not be useful to people who are not familiar with the workings of
1387 If the -v (verbose) flag is given twice, acknowledgement packets and
1388 additional header information is printed, such as the the RX call ID,
1389 call number, sequence number, serial number, and the RX packet flags.
1391 If the -v flag is given twice, additional information is printed,
1392 such as the the RX call ID, serial number, and the RX packet flags.
1393 The MTU negotiation information is also printed from RX ack packets.
1395 If the -v flag is given three times, the security index and service id
1398 Error codes are printed for abort packets, with the exception of Ubik
1399 beacon packets (because abort packets are used to signify a yes vote
1400 for the Ubik protocol).
1402 Note that AFS requests are very large and many of the arguments won't
1403 be printed unless \fIsnaplen\fP is increased.
1404 Try using `\fB-s 256\fP'
1405 to watch AFS traffic.
1407 AFS reply packets do not explicitly identify the RPC operation.
1409 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1410 replies using the call number and service ID.
1411 If a reply does not closely
1413 corresponding request, it might not be parsable.
1416 KIP AppleTalk (DDP in UDP)
1418 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1419 and dumped as DDP packets (i.e., all the UDP header information is
1423 is used to translate AppleTalk net and node numbers to names.
1424 Lines in this file have the form
1436 The first two lines give the names of AppleTalk networks.
1438 line gives the name of a particular host (a host is distinguished
1439 from a net by the 3rd octet in the number \-
1440 a net number \fImust\fP have two octets and a host number \fImust\fP
1441 have three octets.) The number and name should be separated by
1442 whitespace (blanks or tabs).
1445 file may contain blank lines or comment lines (lines starting with
1448 AppleTalk addresses are printed in the form
1454 \f(CW144.1.209.2 > icsd-net.112.220
1455 office.2 > icsd-net.112.220
1456 jssmag.149.235 > icsd-net.2\fR
1462 doesn't exist or doesn't contain an entry for some AppleTalk
1463 host/net number, addresses are printed in numeric form.)
1464 In the first example, NBP (DDP port 2) on net 144.1 node 209
1465 is sending to whatever is listening on port 220 of net icsd node 112.
1466 The second line is the same except the full name of the source node
1467 is known (`office').
1468 The third line is a send from port 235 on
1469 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1470 the broadcast address (255) is indicated by a net name with no host
1471 number \- for this reason it's a good idea to keep node names and
1472 net names distinct in /etc/atalk.names).
1474 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1475 packets have their contents interpreted.
1476 Other protocols just dump
1477 the protocol name (or number if no name is registered for the
1478 protocol) and packet size.
1480 \fBNBP packets\fP are formatted like the following examples:
1484 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1485 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1486 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1490 The first line is a name lookup request for laserwriters sent by net icsd host
1491 112 and broadcast on net jssmag.
1492 The nbp id for the lookup is 190.
1493 The second line shows a reply for this request (note that it has the
1494 same id) from host jssmag.209 saying that it has a laserwriter
1495 resource named "RM1140" registered on port 250.
1497 another reply to the same request saying host techpit has laserwriter
1498 "techpit" registered on port 186.
1500 \fBATP packet\fP formatting is demonstrated by the following example:
1504 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1505 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1506 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1507 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1508 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1509 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1510 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1511 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1512 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1513 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1514 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1515 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1516 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1517 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1521 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1522 up to 8 packets (the `<0-7>').
1523 The hex number at the end of the line
1524 is the value of the `userdata' field in the request.
1526 Helios responds with 8 512-byte packets.
1527 The `:digit' following the
1528 transaction id gives the packet sequence number in the transaction
1529 and the number in parens is the amount of data in the packet,
1530 excluding the atp header.
1531 The `*' on packet 7 indicates that the
1534 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1536 resends them then jssmag.209 releases the transaction.
1538 jssmag.209 initiates the next request.
1539 The `*' on the request
1540 indicates that XO (`exactly once') was \fInot\fP set.
1545 Fragmented Internet datagrams are printed as
1549 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1550 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1554 (The first form indicates there are more fragments.
1556 indicates this is the last fragment.)
1558 \fIId\fP is the fragment id.
1559 \fISize\fP is the fragment
1560 size (in bytes) excluding the IP header.
1561 \fIOffset\fP is this
1562 fragment's offset (in bytes) in the original datagram.
1564 The fragment information is output for each fragment.
1566 fragment contains the higher level protocol header and the frag
1567 info is printed after the protocol info.
1569 after the first contain no higher level protocol header and the
1570 frag info is printed after the source and destination addresses.
1571 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1572 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1576 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1577 arizona > rtsg: (frag 595a:204@328)
1578 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1582 There are a couple of things to note here: First, addresses in the
1583 2nd line don't include port numbers.
1584 This is because the TCP
1585 protocol information is all in the first fragment and we have no idea
1586 what the port or sequence numbers are when we print the later fragments.
1587 Second, the tcp sequence information in the first line is printed as if there
1588 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1589 the first frag and 204 in the second).
1590 If you are looking for holes
1591 in the sequence space or trying to match up acks
1592 with packets, this can fool you.
1594 A packet with the IP \fIdon't fragment\fP flag is marked with a
1595 trailing \fB(DF)\fP.
1599 By default, all output lines are preceded by a timestamp.
1601 is the current clock time in the form
1607 and is as accurate as the kernel's clock.
1608 The timestamp reflects the time the kernel first saw the packet.
1610 is made to account for the time lag between when the
1611 Ethernet interface removed the packet from the wire and when the kernel
1612 serviced the `new packet' interrupt.
1614 stty(1), pcap(3PCAP), pcap-filter(4), bpf(4), nit(4P)
1616 The original authors are:
1620 Steven McCanne, all of the
1621 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1623 It is currently being maintained by tcpdump.org.
1625 The current version is available via http:
1628 .I http://www.tcpdump.org/
1631 The original distribution is available via anonymous ftp:
1634 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1637 IPv6/IPsec support is added by WIDE/KAME project.
1638 This program uses Eric Young's SSLeay library, under specific configurations.
1640 Please send problems, bugs, questions, desirable enhancements, patches
1644 tcpdump-workers@lists.tcpdump.org
1647 NIT doesn't let you watch your own outbound traffic, BPF will.
1648 We recommend that you use the latter.
1650 On Linux systems with 2.0[.x] kernels:
1652 packets on the loopback device will be seen twice;
1654 packet filtering cannot be done in the kernel, so that all packets must
1655 be copied from the kernel in order to be filtered in user mode;
1657 all of a packet, not just the part that's within the snapshot length,
1658 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1659 asked to copy only part of a packet to userland, will not report the
1660 true length of the packet; this would cause most IP packets to get an
1664 capturing on some PPP devices won't work correctly.
1666 We recommend that you upgrade to a 2.2 or later kernel.
1668 Some attempt should be made to reassemble IP fragments or, at least
1669 to compute the right length for the higher level protocol.
1671 Name server inverse queries are not dumped correctly: the (empty)
1672 question section is printed rather than real query in the answer
1674 Some believe that inverse queries are themselves a bug and
1675 prefer to fix the program generating them rather than \fItcpdump\fP.
1677 A packet trace that crosses a daylight savings time change will give
1678 skewed time stamps (the time change is ignored).
1680 Filter expressions on fields other than those in Token Ring headers will
1681 not correctly handle source-routed Token Ring packets.
1683 Filter expressions on fields other than those in 802.11 headers will not
1684 correctly handle 802.11 data packets with both To DS and From DS set.
1687 should chase header chain, but at this moment it does not.
1688 .BR "ip6 protochain"
1689 is supplied for this behavior.
1691 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1692 does not work against IPv6 packets.
1693 It only looks at IPv4 packets.
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