1 .\" @(#) $Header: /tcpdump/master/tcpdump/tcpdump.1.in,v 1.2 2008-11-09 23:35:03 mcr Exp $ (LBL)
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25 .TH TCPDUMP 1 "05 March 2009"
27 tcpdump \- dump traffic on a network
32 .B \-AbdDefhIJKlLnNOpqRStuUvxX
98 .I spi@ipaddr algo:secret,...
108 .I postrotate-command
122 \fITcpdump\fP prints out a description of the contents of packets on a
123 network interface that match the boolean \fIexpression\fP. It can also
126 flag, which causes it to save the packet data to a file for later
127 analysis, and/or with the
129 flag, which causes it to read from a saved packet file rather than to
130 read packets from a network interface. In all cases, only packets that
137 will, if not run with the
139 flag, continue capturing packets until it is interrupted by a SIGINT
140 signal (generated, for example, by typing your interrupt character,
141 typically control-C) or a SIGTERM signal (typically generated with the
143 command); if run with the
145 flag, it will capture packets until it is interrupted by a SIGINT or
146 SIGTERM signal or the specified number of packets have been processed.
150 finishes capturing packets, it will report counts of:
152 packets ``captured'' (this is the number of packets that
154 has received and processed);
156 packets ``received by filter'' (the meaning of this depends on the OS on
159 and possibly on the way the OS was configured - if a filter was
160 specified on the command line, on some OSes it counts packets regardless
161 of whether they were matched by the filter expression and, even if they
162 were matched by the filter expression, regardless of whether
164 has read and processed them yet, on other OSes it counts only packets that were
165 matched by the filter expression regardless of whether
167 has read and processed them yet, and on other OSes it counts only
168 packets that were matched by the filter expression and were processed by
171 packets ``dropped by kernel'' (this is the number of packets that were
172 dropped, due to a lack of buffer space, by the packet capture mechanism
175 is running, if the OS reports that information to applications; if not,
176 it will be reported as 0).
178 On platforms that support the SIGINFO signal, such as most BSDs
179 (including Mac OS X) and Digital/Tru64 UNIX, it will report those counts
180 when it receives a SIGINFO signal (generated, for example, by typing
181 your ``status'' character, typically control-T, although on some
182 platforms, such as Mac OS X, the ``status'' character is not set by
183 default, so you must set it with
185 in order to use it) and will continue capturing packets.
187 Reading packets from a network interface may require that you have
188 special privileges; see the
190 man page for details. Reading a saved packet file doesn't require
195 Print each packet (minus its link level header) in ASCII. Handy for
199 Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN
203 Set the operating system capture buffer size to \fIbuffer_size\fP.
206 Exit after receiving \fIcount\fP packets.
209 Before writing a raw packet to a savefile, check whether the file is
210 currently larger than \fIfile_size\fP and, if so, close the current
211 savefile and open a new one. Savefiles after the first savefile will
212 have the name specified with the
214 flag, with a number after it, starting at 1 and continuing upward.
215 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
216 not 1,048,576 bytes).
219 Dump the compiled packet-matching code in a human readable form to
220 standard output and stop.
223 Dump packet-matching code as a
228 Dump packet-matching code as decimal numbers (preceded with a count).
231 Print the list of the network interfaces available on the system and on
234 can capture packets. For each network interface, a number and an
235 interface name, possibly followed by a text description of the
236 interface, is printed. The interface name or the number can be supplied
239 flag to specify an interface on which to capture.
241 This can be useful on systems that don't have a command to list them
242 (e.g., Windows systems, or UNIX systems lacking
243 .BR "ifconfig \-a" );
244 the number can be useful on Windows 2000 and later systems, where the
245 interface name is a somewhat complex string.
249 flag will not be supported if
251 was built with an older version of
254 .B pcap_findalldevs()
258 Print the link-level header on each dump line.
261 Use \fIspi@ipaddr algo:secret\fP for decrypting IPsec ESP packets that
262 are addressed to \fIaddr\fP and contain Security Parameter Index value
263 \fIspi\fP. This combination may be repeated with comma or newline separation.
265 Note that setting the secret for IPv4 ESP packets is supported at this time.
272 \fBcast128-cbc\fP, or
274 The default is \fBdes-cbc\fP.
275 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
276 with cryptography enabled.
278 \fIsecret\fP is the ASCII text for ESP secret key.
279 If preceded by 0x, then a hex value will be read.
281 The option assumes RFC2406 ESP, not RFC1827 ESP.
282 The option is only for debugging purposes, and
283 the use of this option with a true `secret' key is discouraged.
284 By presenting IPsec secret key onto command line
285 you make it visible to others, via
289 In addition to the above syntax, the syntax \fIfile name\fP may be used
290 to have tcpdump read the provided file in. The file is opened upon
291 receiving the first ESP packet, so any special permissions that tcpdump
292 may have been given should already have been given up.
295 Print `foreign' IPv4 addresses numerically rather than symbolically
296 (this option is intended to get around serious brain damage in
297 Sun's NIS server \(em usually it hangs forever translating non-local
300 The test for `foreign' IPv4 addresses is done using the IPv4 address and
301 netmask of the interface on which capture is being done. If that
302 address or netmask are not available, available, either because the
303 interface on which capture is being done has no address or netmask or
304 because the capture is being done on the Linux "any" interface, which
305 can capture on more than one interface, this option will not work
309 Use \fIfile\fP as input for the filter expression.
310 An additional expression given on the command line is ignored.
313 If specified, rotates the dump file specified with the
315 option every \fIrotate_seconds\fP seconds.
316 Savefiles will have the name specified by
318 which should include a time format as defined by
320 If no time format is specified, each new file will overwrite the previous.
322 If used in conjunction with the
324 option, filenames will take the form of `\fIfile\fP<count>'.
327 Attempt to detect 802.11s draft mesh headers.
330 Listen on \fIinterface\fP.
331 If unspecified, \fItcpdump\fP searches the system interface list for the
332 lowest numbered, configured up interface (excluding loopback).
333 Ties are broken by choosing the earliest match.
335 On Linux systems with 2.2 or later kernels, an
337 argument of ``any'' can be used to capture packets from all interfaces.
338 Note that captures on the ``any'' device will not be done in promiscuous
343 flag is supported, an interface number as printed by that flag can be
349 Put the interface in "monitor mode"; this is supported only on IEEE
350 802.11 Wi-Fi interfaces, and supported only on some operating systems.
352 Note that in monitor mode the adapter might disassociate from the
353 network with which it's associated, so that you will not be able to use
354 any wireless networks with that adapter. This could prevent accessing
355 files on a network server, or resolving host names or network addresses,
356 if you are capturing in monitor mode and are not connected to another
357 network with another adapter.
359 This flag will affect the output of the
363 isn't specified, only those link-layer types available when not in
364 monitor mode will be shown; if
366 is specified, only those link-layer types available when in monitor mode
370 Set the time stamp type for the capture to \fItstamp_type\fP.
373 List the supported time stamp types for the interface and exit. If the
374 time stamp type cannot be set for the interface, no time stamp types are
378 Don't attempt to verify IP, TCP, or UDP checksums. This is useful for
379 interfaces that perform some or all of those checksum calculation in
380 hardware; otherwise, all outgoing TCP checksums will be flagged as bad.
383 Make stdout line buffered.
384 Useful if you want to see the data
388 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
389 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
392 List the known data link types for the interface, in the specified mode,
393 and exit. The list of known data link types may be dependent on the
394 specified mode; for example, on some platforms, a Wi-Fi interface might
395 support one set of data link types when not in monitor mode (for
396 example, it might support only fake Ethernet headers, or might support
397 802.11 headers but not support 802.11 headers with radio information)
398 and another set of data link types when in monitor mode (for example, it
399 might support 802.11 headers, or 802.11 headers with radio information,
400 only in monitor mode).
403 Load SMI MIB module definitions from file \fImodule\fR.
405 can be used several times to load several MIB modules into \fItcpdump\fP.
408 Use \fIsecret\fP as a shared secret for validating the digests found in
409 TCP segments with the TCP-MD5 option (RFC 2385), if present.
412 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
415 Don't print domain name qualification of host names.
417 if you give this flag then \fItcpdump\fP will print ``nic''
418 instead of ``nic.ddn.mil''.
421 Do not run the packet-matching code optimizer.
423 if you suspect a bug in the optimizer.
426 \fIDon't\fP put the interface
427 into promiscuous mode.
428 Note that the interface might be in promiscuous
429 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
430 `ether host {local-hw-addr} or ether broadcast'.
433 Quick (quiet?) output.
434 Print less protocol information so output
438 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
439 If specified, \fItcpdump\fP will not print replay prevention field.
440 Since there is no protocol version field in ESP/AH specification,
441 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
444 Read packets from \fIfile\fR (which was created with the
447 Standard input is used if \fIfile\fR is ``-''.
450 Print absolute, rather than relative, TCP sequence numbers.
453 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
454 default of 65535 bytes.
455 Packets truncated because of a limited snapshot
456 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
457 is the name of the protocol level at which the truncation has occurred.
458 Note that taking larger snapshots both increases
459 the amount of time it takes to process packets and, effectively,
460 decreases the amount of packet buffering.
461 This may cause packets to be
463 You should limit \fIsnaplen\fP to the smallest number that will
464 capture the protocol information you're interested in.
466 \fIsnaplen\fP to 0 sets it to the default of 65535,
467 for backwards compatibility with recent older versions of
471 Force packets selected by "\fIexpression\fP" to be interpreted the
472 specified \fItype\fR.
473 Currently known types are
474 \fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
475 \fBcnfp\fR (Cisco NetFlow protocol),
476 \fBrpc\fR (Remote Procedure Call),
477 \fBrtp\fR (Real-Time Applications protocol),
478 \fBrtcp\fR (Real-Time Applications control protocol),
479 \fBsnmp\fR (Simple Network Management Protocol),
480 \fBtftp\fR (Trivial File Transfer Protocol),
481 \fBvat\fR (Visual Audio Tool),
483 \fBwb\fR (distributed White Board).
486 \fIDon't\fP print a timestamp on each dump line.
489 Print an unformatted timestamp on each dump line.
492 Print a delta (micro-second resolution) between current and previous line
496 Print a timestamp in default format proceeded by date on each dump line.
499 Print a delta (micro-second resolution) between current and first line
503 Print undecoded NFS handles.
506 Make output saved via the
508 option ``packet-buffered''; i.e., as each packet is saved, it will be
509 written to the output file, rather than being written only when the
514 flag will not be supported if
516 was built with an older version of
523 When parsing and printing, produce (slightly more) verbose output.
524 For example, the time to live,
525 identification, total length and options in an IP packet are printed.
526 Also enables additional packet integrity checks such as verifying the
527 IP and ICMP header checksum.
529 When writing to a file with the
531 option, report, every 10 seconds, the number of packets captured.
534 Even more verbose output.
535 For example, additional fields are
536 printed from NFS reply packets, and SMB packets are fully decoded.
539 Even more verbose output.
541 telnet \fBSB\fP ... \fBSE\fP options
545 Telnet options are printed in hex as well.
548 Write the raw packets to \fIfile\fR rather than parsing and printing
550 They can later be printed with the \-r option.
551 Standard output is used if \fIfile\fR is ``-''.
553 .BR pcap-savefile (@MAN_FILE_FORMATS@)
554 for a description of the file format.
557 Used in conjunction with the
559 option, this will limit the number
560 of files created to the specified number, and begin overwriting files
561 from the beginning, thus creating a 'rotating' buffer.
562 In addition, it will name
563 the files with enough leading 0s to support the maximum number of
564 files, allowing them to sort correctly.
566 Used in conjunction with the
568 option, this will limit the number of rotated dump files that get
569 created, exiting with status 0 when reaching the limit. If used with
571 as well, the behavior will result in cyclical files per timeslice.
574 When parsing and printing,
575 in addition to printing the headers of each packet, print the data of
576 each packet (minus its link level header) in hex.
577 The smaller of the entire packet or
579 bytes will be printed. Note that this is the entire link-layer
580 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
581 will also be printed when the higher layer packet is shorter than the
585 When parsing and printing,
586 in addition to printing the headers of each packet, print the data of
589 its link level header, in hex.
592 When parsing and printing,
593 in addition to printing the headers of each packet, print the data of
594 each packet (minus its link level header) in hex and ASCII.
595 This is very handy for analysing new protocols.
598 When parsing and printing,
599 in addition to printing the headers of each packet, print the data of
602 its link level header, in hex and ASCII.
605 Set the data link type to use while capturing packets to \fIdatalinktype\fP.
608 Used in conjunction with the
612 options, this will make
618 is the savefile being closed after each rotation. For example, specifying
622 will compress each savefile using gzip or bzip2.
624 Note that tcpdump will run the command in parallel to the capture, using
625 the lowest priority so that this doesn't disturb the capture process.
627 And in case you would like to use a command that itself takes flags or
628 different arguments, you can always write a shell script that will take the
629 savefile name as the only argument, make the flags & arguments arrangements
630 and execute the command that you want.
633 Drops privileges (if root) and changes user ID to
635 and the group ID to the primary group of
638 This behavior can also be enabled by default at compile time.
639 .IP "\fI expression\fP"
641 selects which packets will be dumped.
642 If no \fIexpression\fP
643 is given, all packets on the net will be dumped.
645 only packets for which \fIexpression\fP is `true' will be dumped.
647 For the \fIexpression\fP syntax, see
648 .BR pcap-filter (@MAN_MISC_INFO@).
650 Expression arguments can be passed to \fItcpdump\fP as either a single
651 argument or as multiple arguments, whichever is more convenient.
652 Generally, if the expression contains Shell metacharacters, it is
653 easier to pass it as a single, quoted argument.
654 Multiple arguments are concatenated with spaces before being parsed.
657 To print all packets arriving at or departing from \fIsundown\fP:
660 \fBtcpdump host sundown\fP
664 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
667 \fBtcpdump host helios and \\( hot or ace \\)\fP
671 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
674 \fBtcpdump ip host ace and not helios\fP
678 To print all traffic between local hosts and hosts at Berkeley:
682 tcpdump net ucb-ether
686 To print all ftp traffic through internet gateway \fIsnup\fP:
687 (note that the expression is quoted to prevent the shell from
688 (mis-)interpreting the parentheses):
692 tcpdump 'gateway snup and (port ftp or ftp-data)'
696 To print traffic neither sourced from nor destined for local hosts
697 (if you gateway to one other net, this stuff should never make it
698 onto your local net).
702 tcpdump ip and not net \fIlocalnet\fP
706 To print the start and end packets (the SYN and FIN packets) of each
707 TCP conversation that involves a non-local host.
711 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
715 To print all IPv4 HTTP packets to and from port 80, i.e. print only
716 packets that contain data, not, for example, SYN and FIN packets and
717 ACK-only packets. (IPv6 is left as an exercise for the reader.)
721 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
725 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
729 tcpdump 'gateway snup and ip[2:2] > 576'
733 To print IP broadcast or multicast packets that were
735 sent via Ethernet broadcast or multicast:
739 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
743 To print all ICMP packets that are not echo requests/replies (i.e., not
748 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
753 The output of \fItcpdump\fP is protocol dependent.
755 gives a brief description and examples of most of the formats.
763 If the '-e' option is given, the link level header is printed out.
764 On Ethernets, the source and destination addresses, protocol,
765 and packet length are printed.
767 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
768 the `frame control' field, the source and destination addresses,
769 and the packet length.
770 (The `frame control' field governs the
771 interpretation of the rest of the packet.
773 as those containing IP datagrams) are `async' packets, with a priority
774 value between 0 and 7; for example, `\fBasync4\fR'.
776 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
777 the LLC header is printed if it is \fInot\fR an ISO datagram or a
778 so-called SNAP packet.
780 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
781 the `access control' and `frame control' fields, the source and
782 destination addresses, and the packet length.
784 packets are assumed to contain an LLC packet.
785 Regardless of whether
786 the '-e' option is specified or not, the source routing information is
787 printed for source-routed packets.
789 On 802.11 networks, the '-e' option causes \fItcpdump\fP to print
790 the `frame control' fields, all of the addresses in the 802.11 header,
791 and the packet length.
793 packets are assumed to contain an LLC packet.
795 \fI(N.B.: The following description assumes familiarity with
796 the SLIP compression algorithm described in RFC-1144.)\fP
798 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
799 packet type, and compression information are printed out.
800 The packet type is printed first.
801 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
802 No further link information is printed for \fIip\fR packets.
803 For TCP packets, the connection identifier is printed following the type.
804 If the packet is compressed, its encoded header is printed out.
805 The special cases are printed out as
806 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
807 the sequence number (or sequence number and ack) has changed.
808 If it is not a special case,
809 zero or more changes are printed.
810 A change is indicated by U (urgent pointer), W (window), A (ack),
811 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
813 Finally, the amount of data in the packet and compressed header length
816 For example, the following line shows an outbound compressed TCP packet,
817 with an implicit connection identifier; the ack has changed by 6,
818 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
819 data and 6 bytes of compressed header:
822 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
828 Arp/rarp output shows the type of request and its arguments.
830 format is intended to be self explanatory.
831 Here is a short sample taken from the start of an `rlogin' from
832 host \fIrtsg\fP to host \fIcsam\fP:
836 \f(CWarp who-has csam tell rtsg
837 arp reply csam is-at CSAM\fR
841 The first line says that rtsg sent an arp packet asking
842 for the Ethernet address of internet host csam.
844 replies with its Ethernet address (in this example, Ethernet addresses
845 are in caps and internet addresses in lower case).
847 This would look less redundant if we had done \fItcpdump \-n\fP:
851 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
852 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
856 If we had done \fItcpdump \-e\fP, the fact that the first packet is
857 broadcast and the second is point-to-point would be visible:
861 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
862 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
866 For the first packet this says the Ethernet source address is RTSG, the
867 destination is the Ethernet broadcast address, the type field
868 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
872 \fI(N.B.:The following description assumes familiarity with
873 the TCP protocol described in RFC-793.
874 If you are not familiar
875 with the protocol, neither this description nor \fItcpdump\fP will
876 be of much use to you.)\fP
878 The general format of a tcp protocol line is:
882 \fIsrc > dst: flags data-seqno ack window urgent options\fP
886 \fISrc\fP and \fIdst\fP are the source and destination IP
888 \fIFlags\fP are some combination of S (SYN),
889 F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or
890 `.' (ACK), or `none' if no flags are set.
891 \fIData-seqno\fP describes the portion of sequence space covered
892 by the data in this packet (see example below).
893 \fIAck\fP is sequence number of the next data expected the other
894 direction on this connection.
895 \fIWindow\fP is the number of bytes of receive buffer space available
896 the other direction on this connection.
897 \fIUrg\fP indicates there is `urgent' data in the packet.
898 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
900 \fISrc, dst\fP and \fIflags\fP are always present.
902 depend on the contents of the packet's tcp protocol header and
903 are output only if appropriate.
905 Here is the opening portion of an rlogin from host \fIrtsg\fP to
910 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
911 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
912 rtsg.1023 > csam.login: . ack 1 win 4096
913 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
914 csam.login > rtsg.1023: . ack 2 win 4096
915 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
916 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
917 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
918 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
922 The first line says that tcp port 1023 on rtsg sent a packet
925 The \fBS\fP indicates that the \fISYN\fP flag was set.
926 The packet sequence number was 768512 and it contained no data.
927 (The notation is `first:last(nbytes)' which means `sequence
929 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
930 There was no piggy-backed ack, the available receive window was 4096
931 bytes and there was a max-segment-size option requesting an mss of
934 Csam replies with a similar packet except it includes a piggy-backed
936 Rtsg then acks csam's SYN.
937 The `.' means the ACK flag was set.
938 The packet contained no data so there is no data sequence number.
939 Note that the ack sequence
940 number is a small integer (1).
941 The first time \fItcpdump\fP sees a
942 tcp `conversation', it prints the sequence number from the packet.
943 On subsequent packets of the conversation, the difference between
944 the current packet's sequence number and this initial sequence number
946 This means that sequence numbers after the
947 first can be interpreted
948 as relative byte positions in the conversation's data stream (with the
949 first data byte each direction being `1').
950 `-S' will override this
951 feature, causing the original sequence numbers to be output.
953 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
954 in the rtsg \(-> csam side of the conversation).
955 The PUSH flag is set in the packet.
956 On the 7th line, csam says it's received data sent by rtsg up to
957 but not including byte 21.
958 Most of this data is apparently sitting in the
959 socket buffer since csam's receive window has gotten 19 bytes smaller.
960 Csam also sends one byte of data to rtsg in this packet.
961 On the 8th and 9th lines,
962 csam sends two bytes of urgent, pushed data to rtsg.
964 If the snapshot was small enough that \fItcpdump\fP didn't capture
965 the full TCP header, it interprets as much of the header as it can
966 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
968 If the header contains a bogus option (one with a length
969 that's either too small or beyond the end of the header), \fItcpdump\fP
970 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
971 options (since it's impossible to tell where they start).
973 length indicates options are present but the IP datagram length is not
974 long enough for the options to actually be there, \fItcpdump\fP reports
975 it as ``[\fIbad hdr length\fP]''.
977 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
979 There are 8 bits in the control bits section of the TCP header:
981 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
983 Let's assume that we want to watch packets used in establishing
985 Recall that TCP uses a 3-way handshake protocol
986 when it initializes a new connection; the connection sequence with
987 regard to the TCP control bits is
993 2) Recipient responds with SYN, ACK
999 Now we're interested in capturing packets that have only the
1000 SYN bit set (Step 1).
1001 Note that we don't want packets from step 2
1002 (SYN-ACK), just a plain initial SYN.
1003 What we need is a correct filter
1004 expression for \fItcpdump\fP.
1006 Recall the structure of a TCP header without options:
1010 -----------------------------------------------------------------
1011 | source port | destination port |
1012 -----------------------------------------------------------------
1014 -----------------------------------------------------------------
1015 | acknowledgment number |
1016 -----------------------------------------------------------------
1017 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1018 -----------------------------------------------------------------
1019 | TCP checksum | urgent pointer |
1020 -----------------------------------------------------------------
1023 A TCP header usually holds 20 octets of data, unless options are
1025 The first line of the graph contains octets 0 - 3, the
1026 second line shows octets 4 - 7 etc.
1028 Starting to count with 0, the relevant TCP control bits are contained
1033 ----------------|---------------|---------------|----------------
1034 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1035 ----------------|---------------|---------------|----------------
1036 | | 13th octet | | |
1039 Let's have a closer look at octet no. 13:
1049 These are the TCP control bits we are interested
1051 We have numbered the bits in this octet from 0 to 7, right to
1052 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1054 Recall that we want to capture packets with only SYN set.
1055 Let's see what happens to octet 13 if a TCP datagram arrives
1056 with the SYN bit set in its header:
1067 control bits section we see that only bit number 1 (SYN) is set.
1069 Assuming that octet number 13 is an 8-bit unsigned integer in
1070 network byte order, the binary value of this octet is
1074 and its decimal representation is
1078 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1081 We're almost done, because now we know that if only SYN is set,
1082 the value of the 13th octet in the TCP header, when interpreted
1083 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1085 This relationship can be expressed as
1091 We can use this expression as the filter for \fItcpdump\fP in order
1092 to watch packets which have only SYN set:
1095 tcpdump -i xl0 tcp[13] == 2
1098 The expression says "let the 13th octet of a TCP datagram have
1099 the decimal value 2", which is exactly what we want.
1101 Now, let's assume that we need to capture SYN packets, but we
1102 don't care if ACK or any other TCP control bit is set at the
1104 Let's see what happens to octet 13 when a TCP datagram
1105 with SYN-ACK set arrives:
1115 Now bits 1 and 4 are set in the 13th octet.
1121 which translates to decimal
1125 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1128 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1129 expression, because that would select only those packets that have
1130 SYN-ACK set, but not those with only SYN set.
1131 Remember that we don't care
1132 if ACK or any other control bit is set as long as SYN is set.
1134 In order to achieve our goal, we need to logically AND the
1135 binary value of octet 13 with some other value to preserve
1137 We know that we want SYN to be set in any case,
1138 so we'll logically AND the value in the 13th octet with
1139 the binary value of a SYN:
1143 00010010 SYN-ACK 00000010 SYN
1144 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1146 = 00000010 = 00000010
1149 We see that this AND operation delivers the same result
1150 regardless whether ACK or another TCP control bit is set.
1151 The decimal representation of the AND value as well as
1152 the result of this operation is 2 (binary 00000010),
1153 so we know that for packets with SYN set the following
1154 relation must hold true:
1156 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1158 This points us to the \fItcpdump\fP filter expression
1161 tcpdump -i xl0 'tcp[13] & 2 == 2'
1164 Some offsets and field values may be expressed as names
1165 rather than as numeric values. For example tcp[13] may
1166 be replaced with tcp[tcpflags]. The following TCP flag
1167 field values are also available: tcp-fin, tcp-syn, tcp-rst,
1168 tcp-push, tcp-act, tcp-urg.
1170 This can be demonstrated as:
1173 tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
1176 Note that you should use single quotes or a backslash
1177 in the expression to hide the AND ('&') special character
1183 UDP format is illustrated by this rwho packet:
1187 \f(CWactinide.who > broadcast.who: udp 84\fP
1191 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1192 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1194 The packet contained 84 bytes of user data.
1196 Some UDP services are recognized (from the source or destination
1197 port number) and the higher level protocol information printed.
1198 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1199 RPC calls (RFC-1050) to NFS.
1201 UDP Name Server Requests
1203 \fI(N.B.:The following description assumes familiarity with
1204 the Domain Service protocol described in RFC-1035.
1205 If you are not familiar
1206 with the protocol, the following description will appear to be written
1209 Name server requests are formatted as
1213 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1215 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1219 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1220 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1221 The query id was `3'.
1222 The `+' indicates the \fIrecursion desired\fP flag
1224 The query length was 37 bytes, not including the UDP and
1225 IP protocol headers.
1226 The query operation was the normal one, \fIQuery\fP,
1227 so the op field was omitted.
1228 If the op had been anything else, it would
1229 have been printed between the `3' and the `+'.
1230 Similarly, the qclass was the normal one,
1231 \fIC_IN\fP, and omitted.
1232 Any other qclass would have been printed
1233 immediately after the `A'.
1235 A few anomalies are checked and may result in extra fields enclosed in
1236 square brackets: If a query contains an answer, authority records or
1237 additional records section,
1242 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1243 is the appropriate count.
1244 If any of the response bits are set (AA, RA or rcode) or any of the
1245 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1246 is printed, where \fIx\fP is the hex value of header bytes two and three.
1248 UDP Name Server Responses
1250 Name server responses are formatted as
1254 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1256 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1257 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1261 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1262 with 3 answer records, 3 name server records and 7 additional records.
1263 The first answer record is type A (address) and its data is internet
1264 address 128.32.137.3.
1265 The total size of the response was 273 bytes,
1266 excluding UDP and IP headers.
1267 The op (Query) and response code
1268 (NoError) were omitted, as was the class (C_IN) of the A record.
1270 In the second example, \fIhelios\fP responds to query 2 with a
1271 response code of non-existent domain (NXDomain) with no answers,
1272 one name server and no authority records.
1273 The `*' indicates that
1274 the \fIauthoritative answer\fP bit was set.
1276 answers, no type, class or data were printed.
1278 Other flag characters that might appear are `\-' (recursion available,
1279 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1281 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1287 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1288 on UDP/137, UDP/138 and TCP/139.
1289 Some primitive decoding of IPX and
1290 NetBEUI SMB data is also done.
1292 By default a fairly minimal decode is done, with a much more detailed
1293 decode done if -v is used.
1294 Be warned that with -v a single SMB packet
1295 may take up a page or more, so only use -v if you really want all the
1298 For information on SMB packet formats and what all te fields mean see
1299 www.cifs.org or the pub/samba/specs/ directory on your favorite
1300 samba.org mirror site.
1301 The SMB patches were written by Andrew Tridgell
1305 NFS Requests and Replies
1307 Sun NFS (Network File System) requests and replies are printed as:
1311 \fIsrc.xid > dst.nfs: len op args\fP
1312 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1315 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1316 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1317 sushi.201b > wrl.nfs:
1318 144 lookup fh 9,74/4096.6878 "xcolors"
1319 wrl.nfs > sushi.201b:
1320 reply ok 128 lookup fh 9,74/4134.3150
1325 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1326 to \fIwrl\fP (note that the number following the src host is a
1327 transaction id, \fInot\fP the source port).
1328 The request was 112 bytes,
1329 excluding the UDP and IP headers.
1330 The operation was a \fIreadlink\fP
1331 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1332 (If one is lucky, as in this case, the file handle can be interpreted
1333 as a major,minor device number pair, followed by the inode number and
1335 \fIWrl\fP replies `ok' with the contents of the link.
1337 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1338 `\fIxcolors\fP' in directory file 9,74/4096.6878.
1339 Note that the data printed
1340 depends on the operation type.
1341 The format is intended to be self
1342 explanatory if read in conjunction with
1343 an NFS protocol spec.
1345 If the \-v (verbose) flag is given, additional information is printed.
1351 sushi.1372a > wrl.nfs:
1352 148 read fh 21,11/12.195 8192 bytes @ 24576
1353 wrl.nfs > sushi.1372a:
1354 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1359 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1360 which have been omitted from this example.) In the first line,
1361 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1362 at byte offset 24576.
1363 \fIWrl\fP replies `ok'; the packet shown on the
1364 second line is the first fragment of the reply, and hence is only 1472
1365 bytes long (the other bytes will follow in subsequent fragments, but
1366 these fragments do not have NFS or even UDP headers and so might not be
1367 printed, depending on the filter expression used).
1368 Because the \-v flag
1369 is given, some of the file attributes (which are returned in addition
1370 to the file data) are printed: the file type (``REG'', for regular file),
1371 the file mode (in octal), the uid and gid, and the file size.
1373 If the \-v flag is given more than once, even more details are printed.
1375 Note that NFS requests are very large and much of the detail won't be printed
1376 unless \fIsnaplen\fP is increased.
1377 Try using `\fB\-s 192\fP' to watch
1380 NFS reply packets do not explicitly identify the RPC operation.
1382 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1383 replies using the transaction ID.
1384 If a reply does not closely follow the
1385 corresponding request, it might not be parsable.
1387 AFS Requests and Replies
1389 Transarc AFS (Andrew File System) requests and replies are printed
1395 \fIsrc.sport > dst.dport: rx packet-type\fP
1396 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1397 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1400 elvis.7001 > pike.afsfs:
1401 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1402 new fid 536876964/1/1 ".newsrc"
1403 pike.afsfs > elvis.7001: rx data fs reply rename
1408 In the first line, host elvis sends a RX packet to pike.
1410 a RX data packet to the fs (fileserver) service, and is the start of
1412 The RPC call was a rename, with the old directory file id
1413 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1414 file id of 536876964/1/1 and a new filename of `.newsrc'.
1416 responds with a RPC reply to the rename call (which was successful, because
1417 it was a data packet and not an abort packet).
1419 In general, all AFS RPCs are decoded at least by RPC call name.
1421 AFS RPCs have at least some of the arguments decoded (generally only
1422 the `interesting' arguments, for some definition of interesting).
1424 The format is intended to be self-describing, but it will probably
1425 not be useful to people who are not familiar with the workings of
1428 If the -v (verbose) flag is given twice, acknowledgement packets and
1429 additional header information is printed, such as the the RX call ID,
1430 call number, sequence number, serial number, and the RX packet flags.
1432 If the -v flag is given twice, additional information is printed,
1433 such as the the RX call ID, serial number, and the RX packet flags.
1434 The MTU negotiation information is also printed from RX ack packets.
1436 If the -v flag is given three times, the security index and service id
1439 Error codes are printed for abort packets, with the exception of Ubik
1440 beacon packets (because abort packets are used to signify a yes vote
1441 for the Ubik protocol).
1443 Note that AFS requests are very large and many of the arguments won't
1444 be printed unless \fIsnaplen\fP is increased.
1445 Try using `\fB-s 256\fP'
1446 to watch AFS traffic.
1448 AFS reply packets do not explicitly identify the RPC operation.
1450 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1451 replies using the call number and service ID.
1452 If a reply does not closely
1454 corresponding request, it might not be parsable.
1457 KIP AppleTalk (DDP in UDP)
1459 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1460 and dumped as DDP packets (i.e., all the UDP header information is
1464 is used to translate AppleTalk net and node numbers to names.
1465 Lines in this file have the form
1477 The first two lines give the names of AppleTalk networks.
1479 line gives the name of a particular host (a host is distinguished
1480 from a net by the 3rd octet in the number \-
1481 a net number \fImust\fP have two octets and a host number \fImust\fP
1482 have three octets.) The number and name should be separated by
1483 whitespace (blanks or tabs).
1486 file may contain blank lines or comment lines (lines starting with
1489 AppleTalk addresses are printed in the form
1495 \f(CW144.1.209.2 > icsd-net.112.220
1496 office.2 > icsd-net.112.220
1497 jssmag.149.235 > icsd-net.2\fR
1503 doesn't exist or doesn't contain an entry for some AppleTalk
1504 host/net number, addresses are printed in numeric form.)
1505 In the first example, NBP (DDP port 2) on net 144.1 node 209
1506 is sending to whatever is listening on port 220 of net icsd node 112.
1507 The second line is the same except the full name of the source node
1508 is known (`office').
1509 The third line is a send from port 235 on
1510 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1511 the broadcast address (255) is indicated by a net name with no host
1512 number \- for this reason it's a good idea to keep node names and
1513 net names distinct in /etc/atalk.names).
1515 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1516 packets have their contents interpreted.
1517 Other protocols just dump
1518 the protocol name (or number if no name is registered for the
1519 protocol) and packet size.
1521 \fBNBP packets\fP are formatted like the following examples:
1525 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1526 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1527 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1531 The first line is a name lookup request for laserwriters sent by net icsd host
1532 112 and broadcast on net jssmag.
1533 The nbp id for the lookup is 190.
1534 The second line shows a reply for this request (note that it has the
1535 same id) from host jssmag.209 saying that it has a laserwriter
1536 resource named "RM1140" registered on port 250.
1538 another reply to the same request saying host techpit has laserwriter
1539 "techpit" registered on port 186.
1541 \fBATP packet\fP formatting is demonstrated by the following example:
1545 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1546 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1547 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1548 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1549 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1550 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1551 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1552 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1553 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1554 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1555 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1556 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1557 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1558 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1562 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1563 up to 8 packets (the `<0-7>').
1564 The hex number at the end of the line
1565 is the value of the `userdata' field in the request.
1567 Helios responds with 8 512-byte packets.
1568 The `:digit' following the
1569 transaction id gives the packet sequence number in the transaction
1570 and the number in parens is the amount of data in the packet,
1571 excluding the atp header.
1572 The `*' on packet 7 indicates that the
1575 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1577 resends them then jssmag.209 releases the transaction.
1579 jssmag.209 initiates the next request.
1580 The `*' on the request
1581 indicates that XO (`exactly once') was \fInot\fP set.
1586 Fragmented Internet datagrams are printed as
1590 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1591 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1595 (The first form indicates there are more fragments.
1597 indicates this is the last fragment.)
1599 \fIId\fP is the fragment id.
1600 \fISize\fP is the fragment
1601 size (in bytes) excluding the IP header.
1602 \fIOffset\fP is this
1603 fragment's offset (in bytes) in the original datagram.
1605 The fragment information is output for each fragment.
1607 fragment contains the higher level protocol header and the frag
1608 info is printed after the protocol info.
1610 after the first contain no higher level protocol header and the
1611 frag info is printed after the source and destination addresses.
1612 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1613 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1617 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1618 arizona > rtsg: (frag 595a:204@328)
1619 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1623 There are a couple of things to note here: First, addresses in the
1624 2nd line don't include port numbers.
1625 This is because the TCP
1626 protocol information is all in the first fragment and we have no idea
1627 what the port or sequence numbers are when we print the later fragments.
1628 Second, the tcp sequence information in the first line is printed as if there
1629 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1630 the first frag and 204 in the second).
1631 If you are looking for holes
1632 in the sequence space or trying to match up acks
1633 with packets, this can fool you.
1635 A packet with the IP \fIdon't fragment\fP flag is marked with a
1636 trailing \fB(DF)\fP.
1640 By default, all output lines are preceded by a timestamp.
1642 is the current clock time in the form
1648 and is as accurate as the kernel's clock.
1649 The timestamp reflects the time the kernel first saw the packet.
1651 is made to account for the time lag between when the
1652 Ethernet interface removed the packet from the wire and when the kernel
1653 serviced the `new packet' interrupt.
1655 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@),
1656 pcap-filter(@MAN_MISC_INFO@)
1658 The original authors are:
1662 Steven McCanne, all of the
1663 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1665 It is currently being maintained by tcpdump.org.
1667 The current version is available via http:
1670 .I http://www.tcpdump.org/
1673 The original distribution is available via anonymous ftp:
1676 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1679 IPv6/IPsec support is added by WIDE/KAME project.
1680 This program uses Eric Young's SSLeay library, under specific configurations.
1682 Please send problems, bugs, questions, desirable enhancements, patches
1686 tcpdump-workers@lists.tcpdump.org
1689 NIT doesn't let you watch your own outbound traffic, BPF will.
1690 We recommend that you use the latter.
1692 On Linux systems with 2.0[.x] kernels:
1694 packets on the loopback device will be seen twice;
1696 packet filtering cannot be done in the kernel, so that all packets must
1697 be copied from the kernel in order to be filtered in user mode;
1699 all of a packet, not just the part that's within the snapshot length,
1700 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1701 asked to copy only part of a packet to userland, will not report the
1702 true length of the packet; this would cause most IP packets to get an
1706 capturing on some PPP devices won't work correctly.
1708 We recommend that you upgrade to a 2.2 or later kernel.
1710 Some attempt should be made to reassemble IP fragments or, at least
1711 to compute the right length for the higher level protocol.
1713 Name server inverse queries are not dumped correctly: the (empty)
1714 question section is printed rather than real query in the answer
1716 Some believe that inverse queries are themselves a bug and
1717 prefer to fix the program generating them rather than \fItcpdump\fP.
1719 A packet trace that crosses a daylight savings time change will give
1720 skewed time stamps (the time change is ignored).
1722 Filter expressions on fields other than those in Token Ring headers will
1723 not correctly handle source-routed Token Ring packets.
1725 Filter expressions on fields other than those in 802.11 headers will not
1726 correctly handle 802.11 data packets with both To DS and From DS set.
1729 should chase header chain, but at this moment it does not.
1730 .BR "ip6 protochain"
1731 is supplied for this behavior.
1733 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1734 does not work against IPv6 packets.
1735 It only looks at IPv4 packets.
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