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23 .TH TCPDUMP 1 "11 July 2014"
25 tcpdump \- dump traffic on a network
30 .B \-AbdDefhHIJKlLnNOpqRStuUvxX#
111 .I spi@ipaddr algo:secret,...
121 .I postrotate-command
129 .BI \-\-time\-stamp\-precision= tstamp_precision
142 \fITcpdump\fP prints out a description of the contents of packets on a
143 network interface that match the boolean \fIexpression\fP. It can also
146 flag, which causes it to save the packet data to a file for later
147 analysis, and/or with the
149 flag, which causes it to read from a saved packet file rather than to
150 read packets from a network interface. It can also be run with the
152 flag, which causes it to read a list of saved packet files. In all cases,
153 only packets that match
159 will, if not run with the
161 flag, continue capturing packets until it is interrupted by a SIGINT
162 signal (generated, for example, by typing your interrupt character,
163 typically control-C) or a SIGTERM signal (typically generated with the
165 command); if run with the
167 flag, it will capture packets until it is interrupted by a SIGINT or
168 SIGTERM signal or the specified number of packets have been processed.
172 finishes capturing packets, it will report counts of:
174 packets ``captured'' (this is the number of packets that
176 has received and processed);
178 packets ``received by filter'' (the meaning of this depends on the OS on
181 and possibly on the way the OS was configured - if a filter was
182 specified on the command line, on some OSes it counts packets regardless
183 of whether they were matched by the filter expression and, even if they
184 were matched by the filter expression, regardless of whether
186 has read and processed them yet, on other OSes it counts only packets that were
187 matched by the filter expression regardless of whether
189 has read and processed them yet, and on other OSes it counts only
190 packets that were matched by the filter expression and were processed by
193 packets ``dropped by kernel'' (this is the number of packets that were
194 dropped, due to a lack of buffer space, by the packet capture mechanism
197 is running, if the OS reports that information to applications; if not,
198 it will be reported as 0).
200 On platforms that support the SIGINFO signal, such as most BSDs
201 (including Mac OS X) and Digital/Tru64 UNIX, it will report those counts
202 when it receives a SIGINFO signal (generated, for example, by typing
203 your ``status'' character, typically control-T, although on some
204 platforms, such as Mac OS X, the ``status'' character is not set by
205 default, so you must set it with
207 in order to use it) and will continue capturing packets.
209 Reading packets from a network interface may require that you have
210 special privileges; see the
212 man page for details. Reading a saved packet file doesn't require
217 Print each packet (minus its link level header) in ASCII. Handy for
221 Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN
224 .BI \-B " buffer_size"
227 .BI \-\-buffer\-size= buffer_size
229 Set the operating system capture buffer size to \fIbuffer_size\fP, in
230 units of KiB (1024 bytes).
233 Exit after receiving \fIcount\fP packets.
236 Before writing a raw packet to a savefile, check whether the file is
237 currently larger than \fIfile_size\fP and, if so, close the current
238 savefile and open a new one. Savefiles after the first savefile will
239 have the name specified with the
241 flag, with a number after it, starting at 1 and continuing upward.
242 The units of \fIfile_size\fP are millions of bytes (1,000,000 bytes,
243 not 1,048,576 bytes).
246 Dump the compiled packet-matching code in a human readable form to
247 standard output and stop.
250 Dump packet-matching code as a
255 Dump packet-matching code as decimal numbers (preceded with a count).
260 .B \-\-list\-interfaces
262 Print the list of the network interfaces available on the system and on
265 can capture packets. For each network interface, a number and an
266 interface name, possibly followed by a text description of the
267 interface, is printed. The interface name or the number can be supplied
270 flag to specify an interface on which to capture.
272 This can be useful on systems that don't have a command to list them
273 (e.g., Windows systems, or UNIX systems lacking
274 .BR "ifconfig \-a" );
275 the number can be useful on Windows 2000 and later systems, where the
276 interface name is a somewhat complex string.
280 flag will not be supported if
282 was built with an older version of
285 .B pcap_findalldevs()
289 Print the link-level header on each dump line. This can be used, for
290 example, to print MAC layer addresses for protocols such as Ethernet and
294 Use \fIspi@ipaddr algo:secret\fP for decrypting IPsec ESP packets that
295 are addressed to \fIaddr\fP and contain Security Parameter Index value
296 \fIspi\fP. This combination may be repeated with comma or newline separation.
298 Note that setting the secret for IPv4 ESP packets is supported at this time.
305 \fBcast128-cbc\fP, or
307 The default is \fBdes-cbc\fP.
308 The ability to decrypt packets is only present if \fItcpdump\fP was compiled
309 with cryptography enabled.
311 \fIsecret\fP is the ASCII text for ESP secret key.
312 If preceded by 0x, then a hex value will be read.
314 The option assumes RFC2406 ESP, not RFC1827 ESP.
315 The option is only for debugging purposes, and
316 the use of this option with a true `secret' key is discouraged.
317 By presenting IPsec secret key onto command line
318 you make it visible to others, via
322 In addition to the above syntax, the syntax \fIfile name\fP may be used
323 to have tcpdump read the provided file in. The file is opened upon
324 receiving the first ESP packet, so any special permissions that tcpdump
325 may have been given should already have been given up.
328 Print `foreign' IPv4 addresses numerically rather than symbolically
329 (this option is intended to get around serious brain damage in
330 Sun's NIS server \(em usually it hangs forever translating non-local
333 The test for `foreign' IPv4 addresses is done using the IPv4 address and
334 netmask of the interface on which capture is being done. If that
335 address or netmask are not available, available, either because the
336 interface on which capture is being done has no address or netmask or
337 because the capture is being done on the Linux "any" interface, which
338 can capture on more than one interface, this option will not work
342 Use \fIfile\fP as input for the filter expression.
343 An additional expression given on the command line is ignored.
345 .BI \-G " rotate_seconds"
346 If specified, rotates the dump file specified with the
348 option every \fIrotate_seconds\fP seconds.
349 Savefiles will have the name specified by
351 which should include a time format as defined by
353 If no time format is specified, each new file will overwrite the previous.
355 If used in conjunction with the
357 option, filenames will take the form of `\fIfile\fP<count>'.
364 Print the tcpdump and libpcap version strings, print a usage message,
369 Print the tcpdump and libpcap version strings and exit.
372 Attempt to detect 802.11s draft mesh headers.
377 .BI \-\-interface= interface
379 Listen on \fIinterface\fP.
380 If unspecified, \fItcpdump\fP searches the system interface list for the
381 lowest numbered, configured up interface (excluding loopback), which may turn
382 out to be, for example, ``eth0''.
384 On Linux systems with 2.2 or later kernels, an
386 argument of ``any'' can be used to capture packets from all interfaces.
387 Note that captures on the ``any'' device will not be done in promiscuous
392 flag is supported, an interface number as printed by that flag can be
402 Put the interface in "monitor mode"; this is supported only on IEEE
403 802.11 Wi-Fi interfaces, and supported only on some operating systems.
405 Note that in monitor mode the adapter might disassociate from the
406 network with which it's associated, so that you will not be able to use
407 any wireless networks with that adapter. This could prevent accessing
408 files on a network server, or resolving host names or network addresses,
409 if you are capturing in monitor mode and are not connected to another
410 network with another adapter.
412 This flag will affect the output of the
416 isn't specified, only those link-layer types available when not in
417 monitor mode will be shown; if
419 is specified, only those link-layer types available when in monitor mode
422 .BI \-j " tstamp_type"
425 .BI \-\-time\-stamp\-type= tstamp_type
427 Set the time stamp type for the capture to \fItstamp_type\fP. The names
428 to use for the time stamp types are given in
429 .BR pcap-tstamp (@MAN_MISC_INFO@);
430 not all the types listed there will necessarily be valid for any given
436 .B \-\-list\-time\-stamp\-types
438 List the supported time stamp types for the interface and exit. If the
439 time stamp type cannot be set for the interface, no time stamp types are
442 .BI \-\-time\-stamp\-precision= tstamp_precision
443 When capturing, set the time stamp precision for the capture to
444 \fItstamp_precision\fP. Note that availability of high precision time
445 stamps (nanoseconds) and their actual accuracy is platform and hardware
446 dependent. Also note that when writing captures made with nanosecond
447 accuracy to a savefile, the time stamps are written with nanosecond
448 resolution, and the file is written with a different magic number, to
449 indicate that the time stamps are in seconds and nanoseconds; not all
450 programs that read pcap savefiles will be able to read those captures.
452 When reading a savefile, convert time stamps to the precision specified
453 by \fItimestamp_precision\fP, and display them with that resolution. If
454 the precision specified is less than the precision of time stamps in the
455 file, the conversion will lose precision.
457 The supported values for \fItimestamp_precision\fP are \fBmicro\fP for
458 microsecond resolution and \fBnano\fP for nanosecond resolution. The
459 default is microsecond resolution.
464 .B \-\-dont\-verify\-checksums
466 Don't attempt to verify IP, TCP, or UDP checksums. This is useful for
467 interfaces that perform some or all of those checksum calculation in
468 hardware; otherwise, all outgoing TCP checksums will be flagged as bad.
471 Make stdout line buffered.
472 Useful if you want to see the data
479 \fBtcpdump \-l | tee dat\fP
489 \fBtcpdump \-l > dat & tail \-f dat\fP
494 Note that on Windows,``line buffered'' means ``unbuffered'', so that
495 WinDump will write each character individually if
502 in its behavior, but it will cause output to be ``packet-buffered'', so
503 that the output is written to stdout at the end of each packet rather
504 than at the end of each line; this is buffered on all platforms,
510 .B \-\-list\-data\-link\-types
512 List the known data link types for the interface, in the specified mode,
513 and exit. The list of known data link types may be dependent on the
514 specified mode; for example, on some platforms, a Wi-Fi interface might
515 support one set of data link types when not in monitor mode (for
516 example, it might support only fake Ethernet headers, or might support
517 802.11 headers but not support 802.11 headers with radio information)
518 and another set of data link types when in monitor mode (for example, it
519 might support 802.11 headers, or 802.11 headers with radio information,
520 only in monitor mode).
523 Load SMI MIB module definitions from file \fImodule\fR.
525 can be used several times to load several MIB modules into \fItcpdump\fP.
528 Use \fIsecret\fP as a shared secret for validating the digests found in
529 TCP segments with the TCP-MD5 option (RFC 2385), if present.
532 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
535 Don't print domain name qualification of host names.
537 if you give this flag then \fItcpdump\fP will print ``nic''
538 instead of ``nic.ddn.mil''.
545 Print an optional packet number at the beginning of the line.
552 Do not run the packet-matching code optimizer.
554 if you suspect a bug in the optimizer.
559 .B \-\-no\-promiscuous\-mode
561 \fIDon't\fP put the interface
562 into promiscuous mode.
563 Note that the interface might be in promiscuous
564 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
565 `ether host {local-hw-addr} or ether broadcast'.
570 .BI \-\-direction= direction
572 Choose send/receive direction \fIdirection\fR for which packets should be
573 captured. Possible values are `in', `out' and `inout'. Not available
577 Quick (quiet?) output.
578 Print less protocol information so output
582 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
583 If specified, \fItcpdump\fP will not print replay prevention field.
584 Since there is no protocol version field in ESP/AH specification,
585 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
588 Read packets from \fIfile\fR (which was created with the
590 option or by other tools that write pcap or pcap-ng files).
591 Standard input is used if \fIfile\fR is ``-''.
596 .B \-\-absolute\-tcp\-sequence\-numbers
598 Print absolute, rather than relative, TCP sequence numbers.
603 .BI \-\-snapshot\-length= snaplen
605 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
606 default of 65535 bytes.
607 Packets truncated because of a limited snapshot
608 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
609 is the name of the protocol level at which the truncation has occurred.
610 Note that taking larger snapshots both increases
611 the amount of time it takes to process packets and, effectively,
612 decreases the amount of packet buffering.
613 This may cause packets to be
615 You should limit \fIsnaplen\fP to the smallest number that will
616 capture the protocol information you're interested in.
618 \fIsnaplen\fP to 0 sets it to the default of 65535,
619 for backwards compatibility with recent older versions of
623 Force packets selected by "\fIexpression\fP" to be interpreted the
624 specified \fItype\fR.
625 Currently known types are
626 \fBaodv\fR (Ad-hoc On-demand Distance Vector protocol),
627 \fBcarp\fR (Common Address Redundancy Protocol),
628 \fBcnfp\fR (Cisco NetFlow protocol),
629 \fBlmp\fR (Link Management Protocol),
630 \fBpgm\fR (Pragmatic General Multicast),
631 \fBpgm_zmtp1\fR (ZMTP/1.0 inside PGM/EPGM),
632 \fBradius\fR (RADIUS),
633 \fBrpc\fR (Remote Procedure Call),
634 \fBrtp\fR (Real-Time Applications protocol),
635 \fBrtcp\fR (Real-Time Applications control protocol),
636 \fBsnmp\fR (Simple Network Management Protocol),
637 \fBtftp\fR (Trivial File Transfer Protocol),
638 \fBvat\fR (Visual Audio Tool),
639 \fBwb\fR (distributed White Board),
640 \fBzmtp1\fR (ZeroMQ Message Transport Protocol 1.0)
642 \fBvxlan\fR (Virtual eXtensible Local Area Network).
644 Note that the \fBpgm\fR type above affects UDP interpretation only, the native
645 PGM is always recognised as IP protocol 113 regardless. UDP-encapsulated PGM is
646 often called "EPGM" or "PGM/UDP".
648 Note that the \fBpgm_zmtp1\fR type above affects interpretation of both native
649 PGM and UDP at once. During the native PGM decoding the application data of an
650 ODATA/RDATA packet would be decoded as a ZeroMQ datagram with ZMTP/1.0 frames.
651 During the UDP decoding in addition to that any UDP packet would be treated as
652 an encapsulated PGM packet.
655 \fIDon't\fP print a timestamp on each dump line.
658 Print an unformatted timestamp on each dump line.
661 Print a delta (micro-second resolution) between current and previous line
665 Print a timestamp in default format proceeded by date on each dump line.
668 Print a delta (micro-second resolution) between current and first line
672 Print undecoded NFS handles.
677 .B \-\-packet\-buffered
681 option is not specified, make the printed packet output
682 ``packet-buffered''; i.e., as the description of the contents of each
683 packet is printed, it will be written to the standard output, rather
684 than, when not writing to a terminal, being written only when the output
689 option is specified, make the saved raw packet output
690 ``packet-buffered''; i.e., as each packet is saved, it will be written
691 to the output file, rather than being written only when the output
696 flag will not be supported if
698 was built with an older version of
705 When parsing and printing, produce (slightly more) verbose output.
706 For example, the time to live,
707 identification, total length and options in an IP packet are printed.
708 Also enables additional packet integrity checks such as verifying the
709 IP and ICMP header checksum.
711 When writing to a file with the
713 option, report, every 10 seconds, the number of packets captured.
716 Even more verbose output.
717 For example, additional fields are
718 printed from NFS reply packets, and SMB packets are fully decoded.
721 Even more verbose output.
723 telnet \fBSB\fP ... \fBSE\fP options
727 Telnet options are printed in hex as well.
730 Read a list of filenames from \fIfile\fR. Standard input is used
731 if \fIfile\fR is ``-''.
734 Write the raw packets to \fIfile\fR rather than parsing and printing
736 They can later be printed with the \-r option.
737 Standard output is used if \fIfile\fR is ``-''.
739 This output will be buffered if written to a file or pipe, so a program
740 reading from the file or pipe may not see packets for an arbitrary
741 amount of time after they are received. Use the
743 flag to cause packets to be written as soon as they are received.
745 The MIME type \fIapplication/vnd.tcpdump.pcap\fP has been registered
746 with IANA for \fIpcap\fP files. The filename extension \fI.pcap\fP
747 appears to be the most commonly used along with \fI.cap\fP and
748 \fI.dmp\fP. \fITcpdump\fP itself doesn't check the extension when
749 reading capture files and doesn't add an extension when writing them
750 (it uses magic numbers in the file header instead). However, many
751 operating systems and applications will use the extension if it is
752 present and adding one (e.g. .pcap) is recommended.
755 .BR pcap-savefile (@MAN_FILE_FORMATS@)
756 for a description of the file format.
759 Used in conjunction with the
761 option, this will limit the number
762 of files created to the specified number, and begin overwriting files
763 from the beginning, thus creating a 'rotating' buffer.
764 In addition, it will name
765 the files with enough leading 0s to support the maximum number of
766 files, allowing them to sort correctly.
768 Used in conjunction with the
770 option, this will limit the number of rotated dump files that get
771 created, exiting with status 0 when reaching the limit. If used with
773 as well, the behavior will result in cyclical files per timeslice.
776 When parsing and printing,
777 in addition to printing the headers of each packet, print the data of
778 each packet (minus its link level header) in hex.
779 The smaller of the entire packet or
781 bytes will be printed. Note that this is the entire link-layer
782 packet, so for link layers that pad (e.g. Ethernet), the padding bytes
783 will also be printed when the higher layer packet is shorter than the
787 When parsing and printing,
788 in addition to printing the headers of each packet, print the data of
791 its link level header, in hex.
794 When parsing and printing,
795 in addition to printing the headers of each packet, print the data of
796 each packet (minus its link level header) in hex and ASCII.
797 This is very handy for analysing new protocols.
800 When parsing and printing,
801 in addition to printing the headers of each packet, print the data of
804 its link level header, in hex and ASCII.
806 .BI \-y " datalinktype"
809 .BI \-\-linktype= datalinktype
811 Set the data link type to use while capturing packets to \fIdatalinktype\fP.
813 .BI \-z " postrotate-command"
814 Used in conjunction with the
818 options, this will make
821 .I postrotate-command file
824 is the savefile being closed after each rotation. For example, specifying
828 will compress each savefile using gzip or bzip2.
830 Note that tcpdump will run the command in parallel to the capture, using
831 the lowest priority so that this doesn't disturb the capture process.
833 And in case you would like to use a command that itself takes flags or
834 different arguments, you can always write a shell script that will take the
835 savefile name as the only argument, make the flags & arguments arrangements
836 and execute the command that you want.
841 .BI \-\-relinquish\-privileges= user
845 is running as root, after opening the capture device or input savefile,
846 but before opening any savefiles for output, change the user ID to
848 and the group ID to the primary group of
851 This behavior can also be enabled by default at compile time.
852 .IP "\fI expression\fP"
854 selects which packets will be dumped.
855 If no \fIexpression\fP
856 is given, all packets on the net will be dumped.
858 only packets for which \fIexpression\fP is `true' will be dumped.
860 For the \fIexpression\fP syntax, see
861 .BR pcap-filter (@MAN_MISC_INFO@).
863 The \fIexpression\fP argument can be passed to \fItcpdump\fP as either a single
864 Shell argument, or as multiple Shell arguments, whichever is more convenient.
865 Generally, if the expression contains Shell metacharacters, such as
866 backslashes used to escape protocol names, it is easier to pass it as
867 a single, quoted argument rather than to escape the Shell
869 Multiple arguments are concatenated with spaces before being parsed.
872 To print all packets arriving at or departing from \fIsundown\fP:
875 \fBtcpdump host sundown\fP
879 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
882 \fBtcpdump host helios and \\( hot or ace \\)\fP
886 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
889 \fBtcpdump ip host ace and not helios\fP
893 To print all traffic between local hosts and hosts at Berkeley:
897 tcpdump net ucb-ether
901 To print all ftp traffic through internet gateway \fIsnup\fP:
902 (note that the expression is quoted to prevent the shell from
903 (mis-)interpreting the parentheses):
907 tcpdump 'gateway snup and (port ftp or ftp-data)'
911 To print traffic neither sourced from nor destined for local hosts
912 (if you gateway to one other net, this stuff should never make it
913 onto your local net).
917 tcpdump ip and not net \fIlocalnet\fP
921 To print the start and end packets (the SYN and FIN packets) of each
922 TCP conversation that involves a non-local host.
926 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
930 To print all IPv4 HTTP packets to and from port 80, i.e. print only
931 packets that contain data, not, for example, SYN and FIN packets and
932 ACK-only packets. (IPv6 is left as an exercise for the reader.)
936 tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
940 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
944 tcpdump 'gateway snup and ip[2:2] > 576'
948 To print IP broadcast or multicast packets that were
950 sent via Ethernet broadcast or multicast:
954 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
958 To print all ICMP packets that are not echo requests/replies (i.e., not
963 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
968 The output of \fItcpdump\fP is protocol dependent.
970 gives a brief description and examples of most of the formats.
978 If the '-e' option is given, the link level header is printed out.
979 On Ethernets, the source and destination addresses, protocol,
980 and packet length are printed.
982 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
983 the `frame control' field, the source and destination addresses,
984 and the packet length.
985 (The `frame control' field governs the
986 interpretation of the rest of the packet.
988 as those containing IP datagrams) are `async' packets, with a priority
989 value between 0 and 7; for example, `\fBasync4\fR'.
991 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
992 the LLC header is printed if it is \fInot\fR an ISO datagram or a
993 so-called SNAP packet.
995 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
996 the `access control' and `frame control' fields, the source and
997 destination addresses, and the packet length.
999 packets are assumed to contain an LLC packet.
1000 Regardless of whether
1001 the '-e' option is specified or not, the source routing information is
1002 printed for source-routed packets.
1004 On 802.11 networks, the '-e' option causes \fItcpdump\fP to print
1005 the `frame control' fields, all of the addresses in the 802.11 header,
1006 and the packet length.
1007 As on FDDI networks,
1008 packets are assumed to contain an LLC packet.
1010 \fI(N.B.: The following description assumes familiarity with
1011 the SLIP compression algorithm described in RFC-1144.)\fP
1013 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
1014 packet type, and compression information are printed out.
1015 The packet type is printed first.
1016 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
1017 No further link information is printed for \fIip\fR packets.
1018 For TCP packets, the connection identifier is printed following the type.
1019 If the packet is compressed, its encoded header is printed out.
1020 The special cases are printed out as
1021 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
1022 the sequence number (or sequence number and ack) has changed.
1023 If it is not a special case,
1024 zero or more changes are printed.
1025 A change is indicated by U (urgent pointer), W (window), A (ack),
1026 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
1027 or a new value (=n).
1028 Finally, the amount of data in the packet and compressed header length
1031 For example, the following line shows an outbound compressed TCP packet,
1032 with an implicit connection identifier; the ack has changed by 6,
1033 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
1034 data and 6 bytes of compressed header:
1037 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
1043 Arp/rarp output shows the type of request and its arguments.
1045 format is intended to be self explanatory.
1046 Here is a short sample taken from the start of an `rlogin' from
1047 host \fIrtsg\fP to host \fIcsam\fP:
1051 \f(CWarp who-has csam tell rtsg
1052 arp reply csam is-at CSAM\fR
1056 The first line says that rtsg sent an arp packet asking
1057 for the Ethernet address of internet host csam.
1059 replies with its Ethernet address (in this example, Ethernet addresses
1060 are in caps and internet addresses in lower case).
1062 This would look less redundant if we had done \fItcpdump \-n\fP:
1066 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
1067 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
1071 If we had done \fItcpdump \-e\fP, the fact that the first packet is
1072 broadcast and the second is point-to-point would be visible:
1076 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
1077 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
1081 For the first packet this says the Ethernet source address is RTSG, the
1082 destination is the Ethernet broadcast address, the type field
1083 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
1087 \fI(N.B.:The following description assumes familiarity with
1088 the TCP protocol described in RFC-793.
1089 If you are not familiar
1090 with the protocol, neither this description nor \fItcpdump\fP will
1091 be of much use to you.)\fP
1093 The general format of a tcp protocol line is:
1097 \fIsrc > dst: flags data-seqno ack window urgent options\fP
1101 \fISrc\fP and \fIdst\fP are the source and destination IP
1102 addresses and ports.
1103 \fIFlags\fP are some combination of S (SYN),
1104 F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or
1105 `.' (ACK), or `none' if no flags are set.
1106 \fIData-seqno\fP describes the portion of sequence space covered
1107 by the data in this packet (see example below).
1108 \fIAck\fP is sequence number of the next data expected the other
1109 direction on this connection.
1110 \fIWindow\fP is the number of bytes of receive buffer space available
1111 the other direction on this connection.
1112 \fIUrg\fP indicates there is `urgent' data in the packet.
1113 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
1115 \fISrc, dst\fP and \fIflags\fP are always present.
1117 depend on the contents of the packet's tcp protocol header and
1118 are output only if appropriate.
1120 Here is the opening portion of an rlogin from host \fIrtsg\fP to
1125 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
1126 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
1127 rtsg.1023 > csam.login: . ack 1 win 4096
1128 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
1129 csam.login > rtsg.1023: . ack 2 win 4096
1130 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
1131 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
1132 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
1133 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
1137 The first line says that tcp port 1023 on rtsg sent a packet
1140 The \fBS\fP indicates that the \fISYN\fP flag was set.
1141 The packet sequence number was 768512 and it contained no data.
1142 (The notation is `first:last(nbytes)' which means `sequence
1144 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
1145 There was no piggy-backed ack, the available receive window was 4096
1146 bytes and there was a max-segment-size option requesting an mss of
1149 Csam replies with a similar packet except it includes a piggy-backed
1151 Rtsg then acks csam's SYN.
1152 The `.' means the ACK flag was set.
1153 The packet contained no data so there is no data sequence number.
1154 Note that the ack sequence
1155 number is a small integer (1).
1156 The first time \fItcpdump\fP sees a
1157 tcp `conversation', it prints the sequence number from the packet.
1158 On subsequent packets of the conversation, the difference between
1159 the current packet's sequence number and this initial sequence number
1161 This means that sequence numbers after the
1162 first can be interpreted
1163 as relative byte positions in the conversation's data stream (with the
1164 first data byte each direction being `1').
1165 `-S' will override this
1166 feature, causing the original sequence numbers to be output.
1168 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
1169 in the rtsg \(-> csam side of the conversation).
1170 The PUSH flag is set in the packet.
1171 On the 7th line, csam says it's received data sent by rtsg up to
1172 but not including byte 21.
1173 Most of this data is apparently sitting in the
1174 socket buffer since csam's receive window has gotten 19 bytes smaller.
1175 Csam also sends one byte of data to rtsg in this packet.
1176 On the 8th and 9th lines,
1177 csam sends two bytes of urgent, pushed data to rtsg.
1179 If the snapshot was small enough that \fItcpdump\fP didn't capture
1180 the full TCP header, it interprets as much of the header as it can
1181 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
1183 If the header contains a bogus option (one with a length
1184 that's either too small or beyond the end of the header), \fItcpdump\fP
1185 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
1186 options (since it's impossible to tell where they start).
1188 length indicates options are present but the IP datagram length is not
1189 long enough for the options to actually be there, \fItcpdump\fP reports
1190 it as ``[\fIbad hdr length\fP]''.
1192 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
1194 There are 8 bits in the control bits section of the TCP header:
1196 .I CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
1198 Let's assume that we want to watch packets used in establishing
1200 Recall that TCP uses a 3-way handshake protocol
1201 when it initializes a new connection; the connection sequence with
1202 regard to the TCP control bits is
1208 2) Recipient responds with SYN, ACK
1214 Now we're interested in capturing packets that have only the
1215 SYN bit set (Step 1).
1216 Note that we don't want packets from step 2
1217 (SYN-ACK), just a plain initial SYN.
1218 What we need is a correct filter
1219 expression for \fItcpdump\fP.
1221 Recall the structure of a TCP header without options:
1225 -----------------------------------------------------------------
1226 | source port | destination port |
1227 -----------------------------------------------------------------
1229 -----------------------------------------------------------------
1230 | acknowledgment number |
1231 -----------------------------------------------------------------
1232 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1233 -----------------------------------------------------------------
1234 | TCP checksum | urgent pointer |
1235 -----------------------------------------------------------------
1238 A TCP header usually holds 20 octets of data, unless options are
1240 The first line of the graph contains octets 0 - 3, the
1241 second line shows octets 4 - 7 etc.
1243 Starting to count with 0, the relevant TCP control bits are contained
1248 ----------------|---------------|---------------|----------------
1249 | HL | rsvd |C|E|U|A|P|R|S|F| window size |
1250 ----------------|---------------|---------------|----------------
1251 | | 13th octet | | |
1254 Let's have a closer look at octet no. 13:
1264 These are the TCP control bits we are interested
1266 We have numbered the bits in this octet from 0 to 7, right to
1267 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1269 Recall that we want to capture packets with only SYN set.
1270 Let's see what happens to octet 13 if a TCP datagram arrives
1271 with the SYN bit set in its header:
1282 control bits section we see that only bit number 1 (SYN) is set.
1284 Assuming that octet number 13 is an 8-bit unsigned integer in
1285 network byte order, the binary value of this octet is
1289 and its decimal representation is
1293 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1296 We're almost done, because now we know that if only SYN is set,
1297 the value of the 13th octet in the TCP header, when interpreted
1298 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1300 This relationship can be expressed as
1306 We can use this expression as the filter for \fItcpdump\fP in order
1307 to watch packets which have only SYN set:
1310 tcpdump -i xl0 tcp[13] == 2
1313 The expression says "let the 13th octet of a TCP datagram have
1314 the decimal value 2", which is exactly what we want.
1316 Now, let's assume that we need to capture SYN packets, but we
1317 don't care if ACK or any other TCP control bit is set at the
1319 Let's see what happens to octet 13 when a TCP datagram
1320 with SYN-ACK set arrives:
1330 Now bits 1 and 4 are set in the 13th octet.
1336 which translates to decimal
1340 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1343 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1344 expression, because that would select only those packets that have
1345 SYN-ACK set, but not those with only SYN set.
1346 Remember that we don't care
1347 if ACK or any other control bit is set as long as SYN is set.
1349 In order to achieve our goal, we need to logically AND the
1350 binary value of octet 13 with some other value to preserve
1352 We know that we want SYN to be set in any case,
1353 so we'll logically AND the value in the 13th octet with
1354 the binary value of a SYN:
1358 00010010 SYN-ACK 00000010 SYN
1359 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1361 = 00000010 = 00000010
1364 We see that this AND operation delivers the same result
1365 regardless whether ACK or another TCP control bit is set.
1366 The decimal representation of the AND value as well as
1367 the result of this operation is 2 (binary 00000010),
1368 so we know that for packets with SYN set the following
1369 relation must hold true:
1371 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1373 This points us to the \fItcpdump\fP filter expression
1376 tcpdump -i xl0 'tcp[13] & 2 == 2'
1379 Some offsets and field values may be expressed as names
1380 rather than as numeric values. For example tcp[13] may
1381 be replaced with tcp[tcpflags]. The following TCP flag
1382 field values are also available: tcp-fin, tcp-syn, tcp-rst,
1383 tcp-push, tcp-act, tcp-urg.
1385 This can be demonstrated as:
1388 tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0'
1391 Note that you should use single quotes or a backslash
1392 in the expression to hide the AND ('&') special character
1398 UDP format is illustrated by this rwho packet:
1402 \f(CWactinide.who > broadcast.who: udp 84\fP
1406 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1407 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1409 The packet contained 84 bytes of user data.
1411 Some UDP services are recognized (from the source or destination
1412 port number) and the higher level protocol information printed.
1413 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1414 RPC calls (RFC-1050) to NFS.
1416 UDP Name Server Requests
1418 \fI(N.B.:The following description assumes familiarity with
1419 the Domain Service protocol described in RFC-1035.
1420 If you are not familiar
1421 with the protocol, the following description will appear to be written
1424 Name server requests are formatted as
1428 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1430 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1434 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1435 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1436 The query id was `3'.
1437 The `+' indicates the \fIrecursion desired\fP flag
1439 The query length was 37 bytes, not including the UDP and
1440 IP protocol headers.
1441 The query operation was the normal one, \fIQuery\fP,
1442 so the op field was omitted.
1443 If the op had been anything else, it would
1444 have been printed between the `3' and the `+'.
1445 Similarly, the qclass was the normal one,
1446 \fIC_IN\fP, and omitted.
1447 Any other qclass would have been printed
1448 immediately after the `A'.
1450 A few anomalies are checked and may result in extra fields enclosed in
1451 square brackets: If a query contains an answer, authority records or
1452 additional records section,
1457 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1458 is the appropriate count.
1459 If any of the response bits are set (AA, RA or rcode) or any of the
1460 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1461 is printed, where \fIx\fP is the hex value of header bytes two and three.
1463 UDP Name Server Responses
1465 Name server responses are formatted as
1469 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1471 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1472 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1476 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1477 with 3 answer records, 3 name server records and 7 additional records.
1478 The first answer record is type A (address) and its data is internet
1479 address 128.32.137.3.
1480 The total size of the response was 273 bytes,
1481 excluding UDP and IP headers.
1482 The op (Query) and response code
1483 (NoError) were omitted, as was the class (C_IN) of the A record.
1485 In the second example, \fIhelios\fP responds to query 2 with a
1486 response code of non-existent domain (NXDomain) with no answers,
1487 one name server and no authority records.
1488 The `*' indicates that
1489 the \fIauthoritative answer\fP bit was set.
1491 answers, no type, class or data were printed.
1493 Other flag characters that might appear are `\-' (recursion available,
1494 RA, \fInot\fP set) and `|' (truncated message, TC, set).
1496 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1501 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1502 on UDP/137, UDP/138 and TCP/139.
1503 Some primitive decoding of IPX and
1504 NetBEUI SMB data is also done.
1506 By default a fairly minimal decode is done, with a much more detailed
1507 decode done if -v is used.
1508 Be warned that with -v a single SMB packet
1509 may take up a page or more, so only use -v if you really want all the
1512 For information on SMB packet formats and what all the fields mean see
1513 www.cifs.org or the pub/samba/specs/ directory on your favorite
1514 samba.org mirror site.
1515 The SMB patches were written by Andrew Tridgell
1518 NFS Requests and Replies
1520 Sun NFS (Network File System) requests and replies are printed as:
1524 \fIsrc.sport > dst.nfs: NFS request xid xid len op args\fP
1525 \fIsrc.nfs > dst.dport: NFS reply xid xid reply stat len op results\fP
1528 sushi.1023 > wrl.nfs: NFS request xid 26377
1529 112 readlink fh 21,24/10.73165
1530 wrl.nfs > sushi.1023: NFS reply xid 26377
1531 reply ok 40 readlink "../var"
1532 sushi.1022 > wrl.nfs: NFS request xid 8219
1533 144 lookup fh 9,74/4096.6878 "xcolors"
1534 wrl.nfs > sushi.1022: NFS reply xid 8219
1535 reply ok 128 lookup fh 9,74/4134.3150
1540 In the first line, host \fIsushi\fP sends a transaction with id \fI26377\fP
1542 The request was 112 bytes,
1543 excluding the UDP and IP headers.
1544 The operation was a \fIreadlink\fP
1545 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1546 (If one is lucky, as in this case, the file handle can be interpreted
1547 as a major,minor device number pair, followed by the inode number and
1548 generation number.) In the second line, \fIwrl\fP replies `ok' with
1549 the same transaction id and the contents of the link.
1551 In the third line, \fIsushi\fP asks (using a new transaction id) \fIwrl\fP
1552 to lookup the name `\fIxcolors\fP' in directory file 9,74/4096.6878. In
1553 the fourth line, \fIwrl\fP sends a reply with the respective transaction id.
1555 Note that the data printed
1556 depends on the operation type.
1557 The format is intended to be self
1558 explanatory if read in conjunction with
1559 an NFS protocol spec.
1560 Also note that older versions of tcpdump printed NFS packets in a
1561 slightly different format: the transaction id (xid) would be printed
1562 instead of the non-NFS port number of the packet.
1564 If the \-v (verbose) flag is given, additional information is printed.
1570 sushi.1023 > wrl.nfs: NFS request xid 79658
1571 148 read fh 21,11/12.195 8192 bytes @ 24576
1572 wrl.nfs > sushi.1023: NFS reply xid 79658
1573 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1578 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1579 which have been omitted from this example.) In the first line,
1580 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1581 at byte offset 24576.
1582 \fIWrl\fP replies `ok'; the packet shown on the
1583 second line is the first fragment of the reply, and hence is only 1472
1584 bytes long (the other bytes will follow in subsequent fragments, but
1585 these fragments do not have NFS or even UDP headers and so might not be
1586 printed, depending on the filter expression used).
1587 Because the \-v flag
1588 is given, some of the file attributes (which are returned in addition
1589 to the file data) are printed: the file type (``REG'', for regular file),
1590 the file mode (in octal), the uid and gid, and the file size.
1592 If the \-v flag is given more than once, even more details are printed.
1594 Note that NFS requests are very large and much of the detail won't be printed
1595 unless \fIsnaplen\fP is increased.
1596 Try using `\fB\-s 192\fP' to watch
1599 NFS reply packets do not explicitly identify the RPC operation.
1601 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1602 replies using the transaction ID.
1603 If a reply does not closely follow the
1604 corresponding request, it might not be parsable.
1606 AFS Requests and Replies
1608 Transarc AFS (Andrew File System) requests and replies are printed
1614 \fIsrc.sport > dst.dport: rx packet-type\fP
1615 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1616 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1619 elvis.7001 > pike.afsfs:
1620 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1621 new fid 536876964/1/1 ".newsrc"
1622 pike.afsfs > elvis.7001: rx data fs reply rename
1627 In the first line, host elvis sends a RX packet to pike.
1629 a RX data packet to the fs (fileserver) service, and is the start of
1631 The RPC call was a rename, with the old directory file id
1632 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1633 file id of 536876964/1/1 and a new filename of `.newsrc'.
1635 responds with a RPC reply to the rename call (which was successful, because
1636 it was a data packet and not an abort packet).
1638 In general, all AFS RPCs are decoded at least by RPC call name.
1640 AFS RPCs have at least some of the arguments decoded (generally only
1641 the `interesting' arguments, for some definition of interesting).
1643 The format is intended to be self-describing, but it will probably
1644 not be useful to people who are not familiar with the workings of
1647 If the -v (verbose) flag is given twice, acknowledgement packets and
1648 additional header information is printed, such as the RX call ID,
1649 call number, sequence number, serial number, and the RX packet flags.
1651 If the -v flag is given twice, additional information is printed,
1652 such as the RX call ID, serial number, and the RX packet flags.
1653 The MTU negotiation information is also printed from RX ack packets.
1655 If the -v flag is given three times, the security index and service id
1658 Error codes are printed for abort packets, with the exception of Ubik
1659 beacon packets (because abort packets are used to signify a yes vote
1660 for the Ubik protocol).
1662 Note that AFS requests are very large and many of the arguments won't
1663 be printed unless \fIsnaplen\fP is increased.
1664 Try using `\fB-s 256\fP'
1665 to watch AFS traffic.
1667 AFS reply packets do not explicitly identify the RPC operation.
1669 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1670 replies using the call number and service ID.
1671 If a reply does not closely
1673 corresponding request, it might not be parsable.
1676 KIP AppleTalk (DDP in UDP)
1678 AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1679 and dumped as DDP packets (i.e., all the UDP header information is
1683 is used to translate AppleTalk net and node numbers to names.
1684 Lines in this file have the form
1696 The first two lines give the names of AppleTalk networks.
1698 line gives the name of a particular host (a host is distinguished
1699 from a net by the 3rd octet in the number \-
1700 a net number \fImust\fP have two octets and a host number \fImust\fP
1701 have three octets.) The number and name should be separated by
1702 whitespace (blanks or tabs).
1705 file may contain blank lines or comment lines (lines starting with
1708 AppleTalk addresses are printed in the form
1714 \f(CW144.1.209.2 > icsd-net.112.220
1715 office.2 > icsd-net.112.220
1716 jssmag.149.235 > icsd-net.2\fR
1722 doesn't exist or doesn't contain an entry for some AppleTalk
1723 host/net number, addresses are printed in numeric form.)
1724 In the first example, NBP (DDP port 2) on net 144.1 node 209
1725 is sending to whatever is listening on port 220 of net icsd node 112.
1726 The second line is the same except the full name of the source node
1727 is known (`office').
1728 The third line is a send from port 235 on
1729 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1730 the broadcast address (255) is indicated by a net name with no host
1731 number \- for this reason it's a good idea to keep node names and
1732 net names distinct in /etc/atalk.names).
1734 NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
1735 packets have their contents interpreted.
1736 Other protocols just dump
1737 the protocol name (or number if no name is registered for the
1738 protocol) and packet size.
1740 \fBNBP packets\fP are formatted like the following examples:
1744 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1745 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1746 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1750 The first line is a name lookup request for laserwriters sent by net icsd host
1751 112 and broadcast on net jssmag.
1752 The nbp id for the lookup is 190.
1753 The second line shows a reply for this request (note that it has the
1754 same id) from host jssmag.209 saying that it has a laserwriter
1755 resource named "RM1140" registered on port 250.
1757 another reply to the same request saying host techpit has laserwriter
1758 "techpit" registered on port 186.
1760 \fBATP packet\fP formatting is demonstrated by the following example:
1764 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1765 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1766 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1767 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1768 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1769 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1770 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1771 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1772 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1773 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1774 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1775 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1776 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1777 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1781 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1782 up to 8 packets (the `<0-7>').
1783 The hex number at the end of the line
1784 is the value of the `userdata' field in the request.
1786 Helios responds with 8 512-byte packets.
1787 The `:digit' following the
1788 transaction id gives the packet sequence number in the transaction
1789 and the number in parens is the amount of data in the packet,
1790 excluding the atp header.
1791 The `*' on packet 7 indicates that the
1794 Jssmag.209 then requests that packets 3 & 5 be retransmitted.
1796 resends them then jssmag.209 releases the transaction.
1798 jssmag.209 initiates the next request.
1799 The `*' on the request
1800 indicates that XO (`exactly once') was \fInot\fP set.
1805 Fragmented Internet datagrams are printed as
1809 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1810 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1814 (The first form indicates there are more fragments.
1816 indicates this is the last fragment.)
1818 \fIId\fP is the fragment id.
1819 \fISize\fP is the fragment
1820 size (in bytes) excluding the IP header.
1821 \fIOffset\fP is this
1822 fragment's offset (in bytes) in the original datagram.
1824 The fragment information is output for each fragment.
1826 fragment contains the higher level protocol header and the frag
1827 info is printed after the protocol info.
1829 after the first contain no higher level protocol header and the
1830 frag info is printed after the source and destination addresses.
1831 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1832 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1836 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1837 arizona > rtsg: (frag 595a:204@328)
1838 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1842 There are a couple of things to note here: First, addresses in the
1843 2nd line don't include port numbers.
1844 This is because the TCP
1845 protocol information is all in the first fragment and we have no idea
1846 what the port or sequence numbers are when we print the later fragments.
1847 Second, the tcp sequence information in the first line is printed as if there
1848 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1849 the first frag and 204 in the second).
1850 If you are looking for holes
1851 in the sequence space or trying to match up acks
1852 with packets, this can fool you.
1854 A packet with the IP \fIdon't fragment\fP flag is marked with a
1855 trailing \fB(DF)\fP.
1859 By default, all output lines are preceded by a timestamp.
1861 is the current clock time in the form
1867 and is as accurate as the kernel's clock.
1868 The timestamp reflects the time the kernel first saw the packet.
1870 is made to account for the time lag between when the
1871 Ethernet interface removed the packet from the wire and when the kernel
1872 serviced the `new packet' interrupt.
1874 stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(@MAN_FILE_FORMATS@),
1875 pcap-filter(@MAN_MISC_INFO@), pcap-tstamp(@MAN_MISC_INFO@)
1878 .I http://www.iana.org/assignments/media-types/application/vnd.tcpdump.pcap
1882 The original authors are:
1886 Steven McCanne, all of the
1887 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1889 It is currently being maintained by tcpdump.org.
1891 The current version is available via http:
1894 .I http://www.tcpdump.org/
1897 The original distribution is available via anonymous ftp:
1900 .I ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z
1903 IPv6/IPsec support is added by WIDE/KAME project.
1904 This program uses Eric Young's SSLeay library, under specific configurations.
1906 Please send problems, bugs, questions, desirable enhancements, patches
1910 tcpdump-workers@lists.tcpdump.org
1913 NIT doesn't let you watch your own outbound traffic, BPF will.
1914 We recommend that you use the latter.
1916 On Linux systems with 2.0[.x] kernels:
1918 packets on the loopback device will be seen twice;
1920 packet filtering cannot be done in the kernel, so that all packets must
1921 be copied from the kernel in order to be filtered in user mode;
1923 all of a packet, not just the part that's within the snapshot length,
1924 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1925 asked to copy only part of a packet to userland, will not report the
1926 true length of the packet; this would cause most IP packets to get an
1930 capturing on some PPP devices won't work correctly.
1932 We recommend that you upgrade to a 2.2 or later kernel.
1934 Some attempt should be made to reassemble IP fragments or, at least
1935 to compute the right length for the higher level protocol.
1937 Name server inverse queries are not dumped correctly: the (empty)
1938 question section is printed rather than real query in the answer
1940 Some believe that inverse queries are themselves a bug and
1941 prefer to fix the program generating them rather than \fItcpdump\fP.
1943 A packet trace that crosses a daylight savings time change will give
1944 skewed time stamps (the time change is ignored).
1946 Filter expressions on fields other than those in Token Ring headers will
1947 not correctly handle source-routed Token Ring packets.
1949 Filter expressions on fields other than those in 802.11 headers will not
1950 correctly handle 802.11 data packets with both To DS and From DS set.
1953 should chase header chain, but at this moment it does not.
1954 .BR "ip6 protochain"
1955 is supplied for this behavior.
1957 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1958 does not work against IPv6 packets.
1959 It only looks at IPv4 packets.