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23 .TH TCPDUMP 1 "30 June 1997"
25 tcpdump \- dump traffic on a network
79 \fITcpdump\fP prints out the headers of packets on a network interface
80 that match the boolean \fIexpression\fP.
82 .B Under SunOS with nit or bpf:
85 you must have read access to
89 .B Under Solaris with dlpi:
90 You must have read access to the network pseudo device, e.g.
92 .B Under HP-UX with dlpi:
93 You must be root or it must be installed setuid to root.
94 .B Under IRIX with snoop:
95 You must be root or it must be installed setuid to root.
97 You must be root or it must be installed setuid to root.
98 .B Under Ultrix and Digital UNIX:
99 Once the super-user has enabled promiscuous-mode operation using
104 You must have read access to
109 Attempt to convert network and broadcast addresses to names.
112 Exit after receiving \fIcount\fP packets.
115 Dump the compiled packet-matching code in a human readable form to
116 standard output and stop.
119 Dump packet-matching code as a
124 Dump packet-matching code as decimal numbers (preceded with a count).
127 Print the link-level header on each dump line.
130 Use \fIalgo:secret\fP for decrypting IPsec ESP packets. Algorithms may be
135 \fIcast128-cbc\fP, or
137 The default is \fIdes-cbc\fP.
138 The ability to decrypt packets is only present if tcpdump was compiled
139 with cryptography enabled.
140 \fIsecret\fP the ascii text for ESP secret key.
141 We cannot take arbitrary binary value at this moment.
142 The option assumes RFC2406 ESP, not RFC1827 ESP.
143 The option is only for debugging purposes, and
144 the use of this option with truely `secret' key is discouraged.
145 By presenting IPsec secret key onto command line
146 you make it visible to others, via ps(1) and other occasions.
149 Print `foreign' internet addresses numerically rather than symbolically
150 (this option is intended to get around serious brain damage in
151 Sun's yp server \(em usually it hangs forever translating non-local
155 Use \fIfile\fP as input for the filter expression.
156 An additional expression given on the command line is ignored.
159 Listen on \fIinterface\fP.
160 If unspecified, \fItcpdump\fP searches the system interface list for the
161 lowest numbered, configured up interface (excluding loopback).
162 Ties are broken by choosing the earliest match.
165 Make stdout line buffered. Useful if you want to see the data
166 while capturing it. E.g.,
168 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
169 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
172 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
175 Don't print domain name qualification of host names. E.g.,
176 if you give this flag then \fItcpdump\fP will print ``nic''
177 instead of ``nic.ddn.mil''.
180 Load SMI MIB module definitions from file \fImodule\fR. This option
181 can be used several times to load several MIB modules into tcpdump.
184 Do not run the packet-matching code optimizer. This is useful only
185 if you suspect a bug in the optimizer.
188 \fIDon't\fP put the interface
189 into promiscuous mode. Note that the interface might be in promiscuous
190 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
191 `ether host {local-hw-addr} or ether broadcast'.
194 Quick (quiet?) output. Print less protocol information so output
198 Read packets from \fIfile\fR (which was created with the -w option).
199 Standard input is used if \fIfile\fR is ``-''.
202 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
203 default of 68 (with SunOS's NIT, the minimum is actually 96).
204 68 bytes is adequate for IP, ICMP, TCP
205 and UDP but may truncate protocol information from name server and NFS
206 packets (see below). Packets truncated because of a limited snapshot
207 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
208 is the name of the protocol level at which the truncation has occurred.
209 Note that taking larger snapshots both increases
210 the amount of time it takes to process packets and, effectively,
211 decreases the amount of packet buffering. This may cause packets to be
212 lost. You should limit \fIsnaplen\fP to the smallest number that will
213 capture the protocol information you're interested in. Setting
214 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
217 Force packets selected by "\fIexpression\fP" to be interpreted the
218 specified \fItype\fR. Currently known types are
219 \fBrpc\fR (Remote Procedure Call),
220 \fBrtp\fR (Real-Time Applications protocol),
221 \fBrtcp\fR (Real-Time Applications control protocol),
222 \fBsnmp\fR (Simple Network Management Protocol),
223 \fBvat\fR (Visual Audio Tool),
225 \fBwb\fR (distributed White Board).
228 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
229 If specified, \fItcpdump\fP will not print replay prevention field.
230 Since there is no protocol version field in ESP/AH specification,
231 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
234 Print absolute, rather than relative, TCP sequence numbers.
237 \fIDon't\fP print a timestamp on each dump line.
240 Print an unformatted timestamp on each dump line.
243 (Slightly more) verbose output. For example, the time to live
244 and type of service information in an IP packet is printed.
247 Even more verbose output. For example, additional fields are
248 printed from NFS reply packets.
251 Even more verbose output. For example,
252 telnet \fBSB\fP ... \fBSE\fP options
253 are printed in full. With
255 telnet options are printed in hex as well.
258 Write the raw packets to \fIfile\fR rather than parsing and printing
259 them out. They can later be printed with the \-r option.
260 Standard output is used if \fIfile\fR is ``-''.
263 Print each packet (minus its link level header) in hex.
264 The smaller of the entire packet or
266 bytes will be printed.
269 When printing hex, print ascii too. Thus if
271 is also set, the packet is printed in hex/ascii.
272 This is very handy for analysing new protocols.
275 is not also set, some parts of some packets may be printed
277 .IP "\fI expression\fP"
279 selects which packets will be dumped. If no \fIexpression\fP
280 is given, all packets on the net will be dumped. Otherwise,
281 only packets for which \fIexpression\fP is `true' will be dumped.
283 The \fIexpression\fP consists of one or more
285 Primitives usually consist of an
287 (name or number) preceded by one or more qualifiers. There are three
288 different kinds of qualifier:
290 qualifiers say what kind of thing the id name or number refers to.
296 E.g., `host foo', `net 128.3', `port 20'. If there is no type
301 qualifiers specify a particular transfer direction to and/or from
303 Possible directions are
310 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
311 there is no dir qualifier,
314 For `null' link layers (i.e. point to point protocols such as slip) the
318 qualifiers can be used to specify a desired direction.
320 qualifiers restrict the match to a particular protocol. Possible
338 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
339 no proto qualifier, all protocols consistent with the type are
340 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
341 (except the latter is not legal syntax), `net bar' means `(ip or
342 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
344 [`fddi' is actually an alias for `ether'; the parser treats them
345 identically as meaning ``the data link level used on the specified
346 network interface.'' FDDI headers contain Ethernet-like source
347 and destination addresses, and often contain Ethernet-like packet
348 types, so you can filter on these FDDI fields just as with the
349 analogous Ethernet fields. FDDI headers also contain other fields,
350 but you cannot name them explicitly in a filter expression.]
352 In addition to the above, there are some special `primitive' keywords
353 that don't follow the pattern:
358 and arithmetic expressions. All of these are described below.
360 More complex filter expressions are built up by using the words
365 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
366 To save typing, identical qualifier lists can be omitted. E.g.,
367 `tcp dst port ftp or ftp-data or domain' is exactly the same as
368 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
370 Allowable primitives are:
371 .IP "\fBdst host \fIhost\fR"
372 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
373 which may be either an address or a name.
374 .IP "\fBsrc host \fIhost\fR"
375 True if the IPv4/v6 source field of the packet is \fIhost\fP.
376 .IP "\fBhost \fIhost\fP
377 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
378 Any of the above host expressions can be prepended with the keywords,
379 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
382 \fBip host \fIhost\fR
385 which is equivalent to:
388 \fBether proto \fI\\ip\fB and host \fIhost\fR
391 If \fIhost\fR is a name with multiple IP addresses, each address will
392 be checked for a match.
393 .IP "\fBether dst \fIehost\fP
394 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
395 may be either a name from /etc/ethers or a number (see
398 .IP "\fBether src \fIehost\fP
399 True if the ethernet source address is \fIehost\fP.
400 .IP "\fBether host \fIehost\fP
401 True if either the ethernet source or destination address is \fIehost\fP.
402 .IP "\fBgateway\fP \fIhost\fP
403 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
404 source or destination address was \fIhost\fP but neither the IP source
405 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
406 must be found in both /etc/hosts and /etc/ethers. (An equivalent
410 \fBether host \fIehost \fBand not host \fIhost\fR
413 which can be used with either names or numbers for \fIhost / ehost\fP.)
414 This syntax does not work in IPv6-enabled configuration at this moment.
415 .IP "\fBdst net \fInet\fR"
416 True if the IPv4/v6 destination address of the packet has a network
417 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
418 or a network number (see \fInetworks(4)\fP for details).
419 .IP "\fBsrc net \fInet\fR"
420 True if the IPv4/v6 source address of the packet has a network
422 .IP "\fBnet \fInet\fR"
423 True if either the IPv4/v6 source or destination address of the packet has a network
425 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
426 True if the IP address matches \fInet\fR with the specific netmask.
427 May be qualified with \fBsrc\fR or \fBdst\fR.
428 Note that this syntax is not valid for IPv6 \fInet\fR.
429 .IP "\fBnet \fInet\fR/\fIlen\fR"
430 True if the IPv4/v6 address matches \fInet\fR a netmask \fIlen\fR bits wide.
431 May be qualified with \fBsrc\fR or \fBdst\fR.
432 .IP "\fBdst port \fIport\fR"
433 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
434 destination port value of \fIport\fP.
435 The \fIport\fP can be a number or a name used in /etc/services (see
439 If a name is used, both the port
440 number and protocol are checked. If a number or ambiguous name is used,
441 only the port number is checked (e.g., \fBdst port 513\fR will print both
442 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
443 both tcp/domain and udp/domain traffic).
444 .IP "\fBsrc port \fIport\fR"
445 True if the packet has a source port value of \fIport\fP.
446 .IP "\fBport \fIport\fR"
447 True if either the source or destination port of the packet is \fIport\fP.
448 Any of the above port expressions can be prepended with the keywords,
449 \fBtcp\fP or \fBudp\fP, as in:
452 \fBtcp src port \fIport\fR
455 which matches only tcp packets whose source port is \fIport\fP.
456 .IP "\fBless \fIlength\fR"
457 True if the packet has a length less than or equal to \fIlength\fP.
458 This is equivalent to:
461 \fBlen <= \fIlength\fP.
464 .IP "\fBgreater \fIlength\fR"
465 True if the packet has a length greater than or equal to \fIlength\fP.
466 This is equivalent to:
469 \fBlen >= \fIlength\fP.
472 .IP "\fBip proto \fIprotocol\fR"
473 True if the packet is an ip packet (see
475 of protocol type \fIprotocol\fP.
476 \fIProtocol\fP can be a number or one of the names
477 \fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
478 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
479 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
480 Note that this primitive does not chase protocol header chain.
481 .IP "\fBip6 proto \fIprotocol\fR"
482 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
483 Note that this primitive does not chase protocol header chain.
484 .IP "\fBip6 protochain \fIprotocol\fR"
485 True if the packet is IPv6 packet,
486 and contains protocol header with type \fIprotocol\fR
487 in its protocol header chain.
491 \fBip6 protochain 6\fR
494 matches any IPv6 packet with TCP protocol header in the protocol header chain.
495 The packet may contain, for example,
496 authentication header, routing header, or hop-by-hop option header,
497 between IPv6 header and TCP header.
498 The BPF code emitted by this primitive is complex and
499 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
500 so this can be somewhat slow.
501 .IP "\fBip protochain \fIprotocol\fR"
502 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
503 .IP "\fBether broadcast\fR"
504 True if the packet is an ethernet broadcast packet. The \fIether\fP
506 .IP "\fBip broadcast\fR"
507 True if the packet is an IP broadcast packet. It checks for both
508 the all-zeroes and all-ones broadcast conventions, and looks up
509 the local subnet mask.
510 .IP "\fBether multicast\fR"
511 True if the packet is an ethernet multicast packet. The \fIether\fP
513 This is shorthand for `\fBether[0] & 1 != 0\fP'.
514 .IP "\fBip multicast\fR"
515 True if the packet is an IP multicast packet.
516 .IP "\fBip6 multicast\fR"
517 True if the packet is an IPv6 multicast packet.
518 .IP "\fBether proto \fIprotocol\fR"
519 True if the packet is of ether type \fIprotocol\fR.
520 \fIProtocol\fP can be a number or a name like
521 \fIip\fP, \fIip6\fP, \fIarp\fP, or \fIrarp\fP.
522 Note these identifiers are also keywords
523 and must be escaped via backslash (\\).
524 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
525 protocol identification comes from the 802.2 Logical Link Control
526 (LLC) header, which is usually layered on top of the FDDI header.
527 \fITcpdump\fP assumes, when filtering on the protocol identifier,
528 that all FDDI packets include an LLC header, and that the LLC header
529 is in so-called SNAP format.]
530 .IP "\fBdecnet src \fIhost\fR"
531 True if the DECNET source address is
533 which may be an address of the form ``10.123'', or a DECNET host
534 name. [DECNET host name support is only available on Ultrix systems
535 that are configured to run DECNET.]
536 .IP "\fBdecnet dst \fIhost\fR"
537 True if the DECNET destination address is
539 .IP "\fBdecnet host \fIhost\fR"
540 True if either the DECNET source or destination address is
542 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR"
546 \fBether proto \fIp\fR
549 where \fIp\fR is one of the above protocols.
550 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
554 \fBether proto \fIp\fR
557 where \fIp\fR is one of the above protocols.
559 \fItcpdump\fP does not currently know how to parse these protocols.
560 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
564 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
567 where \fIp\fR is one of the above protocols.
568 .IP "\fIexpr relop expr\fR"
569 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
570 and \fIexpr\fR is an arithmetic expression composed of integer constants
571 (expressed in standard C syntax), the normal binary operators
572 [+, -, *, /, &, |], a length operator, and special packet data accessors.
574 data inside the packet, use the following syntax:
577 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
580 \fIProto\fR is one of \fBether, fddi,
581 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
582 indicates the protocol layer for the index operation.
583 Note that \fItcp, udp\fR and other upper-layer protocol types only
584 apply to IPv4, not IPv6 (this will be fixed in the future).
585 The byte offset, relative to the indicated protocol layer, is
587 \fISize\fR is optional and indicates the number of bytes in the
588 field of interest; it can be either one, two, or four, and defaults to one.
589 The length operator, indicated by the keyword \fBlen\fP, gives the
590 length of the packet.
592 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
593 The expression `\fBip[0] & 0xf != 5\fP'
594 catches all IP packets with options. The expression
595 `\fBip[6:2] & 0x1fff = 0\fP'
596 catches only unfragmented datagrams and frag zero of fragmented datagrams.
597 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
599 For instance, \fBtcp[0]\fP always means the first
600 byte of the TCP \fIheader\fP, and never means the first byte of an
601 intervening fragment.
603 Primitives may be combined using:
605 A parenthesized group of primitives and operators
606 (parentheses are special to the Shell and must be escaped).
608 Negation (`\fB!\fP' or `\fBnot\fP').
610 Concatenation (`\fB&&\fP' or `\fBand\fP').
612 Alternation (`\fB||\fP' or `\fBor\fP').
614 Negation has highest precedence.
615 Alternation and concatenation have equal precedence and associate
616 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
617 are now required for concatenation.
619 If an identifier is given without a keyword, the most recent keyword
624 \fBnot host vs and ace\fR
630 \fBnot host vs and host ace\fR
633 which should not be confused with
636 \fBnot ( host vs or ace )\fR
640 Expression arguments can be passed to tcpdump as either a single argument
641 or as multiple arguments, whichever is more convenient.
642 Generally, if the expression contains Shell metacharacters, it is
643 easier to pass it as a single, quoted argument.
644 Multiple arguments are concatenated with spaces before being parsed.
647 To print all packets arriving at or departing from \fIsundown\fP:
650 \fBtcpdump host sundown\fP
654 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
657 \fBtcpdump host helios and \\( hot or ace \\)\fP
661 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
664 \fBtcpdump ip host ace and not helios\fP
668 To print all traffic between local hosts and hosts at Berkeley:
672 tcpdump net ucb-ether
676 To print all ftp traffic through internet gateway \fIsnup\fP:
677 (note that the expression is quoted to prevent the shell from
678 (mis-)interpreting the parentheses):
682 tcpdump 'gateway snup and (port ftp or ftp-data)'
686 To print traffic neither sourced from nor destined for local hosts
687 (if you gateway to one other net, this stuff should never make it
688 onto your local net).
692 tcpdump ip and not net \fIlocalnet\fP
696 To print the start and end packets (the SYN and FIN packets) of each
697 TCP conversation that involves a non-local host.
701 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
705 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
709 tcpdump 'gateway snup and ip[2:2] > 576'
713 To print IP broadcast or multicast packets that were
715 sent via ethernet broadcast or multicast:
719 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
723 To print all ICMP packets that are not echo requests/replies (i.e., not
728 tcpdump 'icmp[0] != 8 and icmp[0] != 0"
733 The output of \fItcpdump\fP is protocol dependent. The following
734 gives a brief description and examples of most of the formats.
742 If the '-e' option is given, the link level header is printed out.
743 On ethernets, the source and destination addresses, protocol,
744 and packet length are printed.
746 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
747 the `frame control' field, the source and destination addresses,
748 and the packet length. (The `frame control' field governs the
749 interpretation of the rest of the packet. Normal packets (such
750 as those containing IP datagrams) are `async' packets, with a priority
751 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
752 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
753 the LLC header is printed if it is \fInot\fR an ISO datagram or a
754 so-called SNAP packet.
756 \fI(N.B.: The following description assumes familiarity with
757 the SLIP compression algorithm described in RFC-1144.)\fP
759 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
760 packet type, and compression information are printed out.
761 The packet type is printed first.
762 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
763 No further link information is printed for \fIip\fR packets.
764 For TCP packets, the connection identifier is printed following the type.
765 If the packet is compressed, its encoded header is printed out.
766 The special cases are printed out as
767 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
768 the sequence number (or sequence number and ack) has changed.
769 If it is not a special case,
770 zero or more changes are printed.
771 A change is indicated by U (urgent pointer), W (window), A (ack),
772 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
774 Finally, the amount of data in the packet and compressed header length
777 For example, the following line shows an outbound compressed TCP packet,
778 with an implicit connection identifier; the ack has changed by 6,
779 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
780 data and 6 bytes of compressed header:
783 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
789 Arp/rarp output shows the type of request and its arguments. The
790 format is intended to be self explanatory.
791 Here is a short sample taken from the start of an `rlogin' from
792 host \fIrtsg\fP to host \fIcsam\fP:
796 \f(CWarp who-has csam tell rtsg
797 arp reply csam is-at CSAM\fP
801 The first line says that rtsg sent an arp packet asking
802 for the ethernet address of internet host csam. Csam
803 replies with its ethernet address (in this example, ethernet addresses
804 are in caps and internet addresses in lower case).
806 This would look less redundant if we had done \fBtcpdump \-n\fP:
810 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
811 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
815 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
816 broadcast and the second is point-to-point would be visible:
820 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
821 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fP
825 For the first packet this says the ethernet source address is RTSG, the
826 destination is the ethernet broadcast address, the type field
827 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
831 \fI(N.B.:The following description assumes familiarity with
832 the TCP protocol described in RFC-793. If you are not familiar
833 with the protocol, neither this description nor tcpdump will
834 be of much use to you.)\fP
836 The general format of a tcp protocol line is:
840 \fIsrc > dst: flags data-seqno ack window urgent options\fP
844 \fISrc\fP and \fIdst\fP are the source and destination IP
845 addresses and ports. \fIFlags\fP are some combination of S (SYN),
846 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
847 \fIData-seqno\fP describes the portion of sequence space covered
848 by the data in this packet (see example below).
849 \fIAck\fP is sequence number of the next data expected the other
850 direction on this connection.
851 \fIWindow\fP is the number of bytes of receive buffer space available
852 the other direction on this connection.
853 \fIUrg\fP indicates there is `urgent' data in the packet.
854 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
856 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
857 depend on the contents of the packet's tcp protocol header and
858 are output only if appropriate.
860 Here is the opening portion of an rlogin from host \fIrtsg\fP to
865 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
866 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
867 rtsg.1023 > csam.login: . ack 1 win 4096
868 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
869 csam.login > rtsg.1023: . ack 2 win 4096
870 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
871 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
872 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
873 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fP\s+2
877 The first line says that tcp port 1023 on rtsg sent a packet
879 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
880 The packet sequence number was 768512 and it contained no data.
881 (The notation is `first:last(nbytes)' which means `sequence
883 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
884 There was no piggy-backed ack, the available receive window was 4096
885 bytes and there was a max-segment-size option requesting an mss of
888 Csam replies with a similar packet except it includes a piggy-backed
889 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
891 The packet contained no data so there is no data sequence number.
892 Note that the ack sequence
893 number is a small integer (1). The first time \fBtcpdump\fP sees a
894 tcp `conversation', it prints the sequence number from the packet.
895 On subsequent packets of the conversation, the difference between
896 the current packet's sequence number and this initial sequence number
897 is printed. This means that sequence numbers after the
898 first can be interpreted
899 as relative byte positions in the conversation's data stream (with the
900 first data byte each direction being `1'). `-S' will override this
901 feature, causing the original sequence numbers to be output.
903 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
904 in the rtsg \(-> csam side of the conversation).
905 The PUSH flag is set in the packet.
906 On the 7th line, csam says it's received data sent by rtsg up to
907 but not including byte 21. Most of this data is apparently sitting in the
908 socket buffer since csam's receive window has gotten 19 bytes smaller.
909 Csam also sends one byte of data to rtsg in this packet.
910 On the 8th and 9th lines,
911 csam sends two bytes of urgent, pushed data to rtsg.
913 If the snapshot was small enough that \fBtcpdump\fP didn't capture
914 the full TCP header, it interprets as much of the header as it can
915 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
916 be interpreted. If the header contains a bogus option (one with a length
917 that's either too small or beyond the end of the header), tcpdump reports
918 it as ``[\fIbad opt\fP]'' and does not interpret any further options (since
919 it's impossible to tell where they start). If the header length indicates
920 options are present but the IP datagram length is not long enough for the
921 options to actually be there, tcpdump reports it as ``[\fIbad hdr length\fP]''.
926 UDP format is illustrated by this rwho packet:
930 \f(CWactinide.who > broadcast.who: udp 84\fP
934 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
935 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
936 broadcast address. The packet contained 84 bytes of user data.
938 Some UDP services are recognized (from the source or destination
939 port number) and the higher level protocol information printed.
940 In particular, Domain Name service requests (RFC-1034/1035) and Sun
941 RPC calls (RFC-1050) to NFS.
943 UDP Name Server Requests
945 \fI(N.B.:The following description assumes familiarity with
946 the Domain Service protocol described in RFC-1035. If you are not familiar
947 with the protocol, the following description will appear to be written
950 Name server requests are formatted as
954 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
956 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fP
960 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
961 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
962 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
963 was set. The query length was 37 bytes, not including the UDP and
964 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
965 so the op field was omitted. If the op had been anything else, it would
966 have been printed between the `3' and the `+'.
967 Similarly, the qclass was the normal one,
968 \fIC_IN\fP, and omitted. Any other qclass would have been printed
969 immediately after the `A'.
971 A few anomalies are checked and may result in extra fields enclosed in
972 square brackets: If a query contains an answer, name server or
978 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
979 is the appropriate count.
980 If any of the response bits are set (AA, RA or rcode) or any of the
981 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
982 is printed, where \fIx\fP is the hex value of header bytes two and three.
984 UDP Name Server Responses
986 Name server responses are formatted as
990 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
992 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
993 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fP
997 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
998 with 3 answer records, 3 name server records and 7 authority records.
999 The first answer record is type A (address) and its data is internet
1000 address 128.32.137.3. The total size of the response was 273 bytes,
1001 excluding UDP and IP headers. The op (Query) and response code
1002 (NoError) were omitted, as was the class (C_IN) of the A record.
1004 In the second example, \fIhelios\fP responds to query 2 with a
1005 response code of non-existent domain (NXDomain) with no answers,
1006 one name server and no authority records. The `*' indicates that
1007 the \fIauthoritative answer\fP bit was set. Since there were no
1008 answers, no type, class or data were printed.
1010 Other flag characters that might appear are `\-' (recursion available,
1011 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
1012 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1015 Note that name server requests and responses tend to be large and the
1016 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1017 to print. Use the \fB\-s\fP flag to increase the snaplen if you
1018 need to seriously investigate name server traffic. `\fB\-s 128\fP'
1019 has worked well for me.
1024 tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data
1025 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1026 NetBEUI SMB data is also done.
1028 By default a fairly minimal decode is done, with a much more detailed
1029 decode done if -v is used. Be warned that with -v a single SMB packet
1030 may take up a page or more, so only use -v if you really want all the
1033 If you are decoding SMB sessions containing unicode strings then you
1034 may wish to set the environment variable USE_UNICODE to 1. A patch to
1035 auto-detect unicode srings would be welcome.
1037 For information on SMB packet formats and what all te fields mean see
1038 www.cifs.org or the pub/samba/specs/ directory on your favourite
1039 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1043 NFS Requests and Replies
1045 Sun NFS (Network File System) requests and replies are printed as:
1049 \fIsrc.xid > dst.nfs: len op args\fP
1050 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1053 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1054 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1055 sushi.201b > wrl.nfs:
1056 144 lookup fh 9,74/4096.6878 "xcolors"
1057 wrl.nfs > sushi.201b:
1058 reply ok 128 lookup fh 9,74/4134.3150
1063 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1064 to \fIwrl\fP (note that the number following the src host is a
1065 transaction id, \fInot\fP the source port). The request was 112 bytes,
1066 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1067 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1068 (If one is lucky, as in this case, the file handle can be interpreted
1069 as a major,minor device number pair, followed by the inode number and
1071 \fIWrl\fP replies `ok' with the contents of the link.
1073 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1074 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1075 depends on the operation type. The format is intended to be self
1076 explanatory if read in conjunction with
1077 an NFS protocol spec.
1079 If the \-v (verbose) flag is given, additional information is printed.
1085 sushi.1372a > wrl.nfs:
1086 148 read fh 21,11/12.195 8192 bytes @ 24576
1087 wrl.nfs > sushi.1372a:
1088 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1093 (\-v also prints the IP header TTL, ID, and fragmentation fields,
1094 which have been omitted from this example.) In the first line,
1095 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1096 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1097 second line is the first fragment of the reply, and hence is only 1472
1098 bytes long (the other bytes will follow in subsequent fragments, but
1099 these fragments do not have NFS or even UDP headers and so might not be
1100 printed, depending on the filter expression used). Because the \-v flag
1101 is given, some of the file attributes (which are returned in addition
1102 to the file data) are printed: the file type (``REG'', for regular file),
1103 the file mode (in octal), the uid and gid, and the file size.
1105 If the \-v flag is given more than once, even more details are printed.
1107 Note that NFS requests are very large and much of the detail won't be printed
1108 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1111 NFS reply packets do not explicitly identify the RPC operation. Instead,
1112 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1113 replies using the transaction ID. If a reply does not closely follow the
1114 corresponding request, it might not be parsable.
1116 AFS Request and Replies
1118 Transarc AFS (Andrew File System) requests and replies are printed
1124 \fIsrc.sport > dst.dport: rx packet-type\fP
1125 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1126 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1129 elvis.7001 > pike.afsfs:
1130 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1131 new fid 536876964/1/1 ".newsrc"
1132 pike.afsfs > elvis.7001: rx data fs reply rename
1137 In the first line, host elvis sends a RX packet to pike. This was
1138 a RX data packet to the fs (fileserver) service, and is the start of
1139 an RPC call. The RPC call was a rename, with the old directory file id
1140 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1141 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1142 responds with a RPC reply to the rename call (which was successful, because
1143 it was a data packet and not an abort packet).
1145 In general, all AFS RPCs are decoded at least by RPC call name. Most
1146 AFS RPCs have at least some of the arguments decoded (generally only
1147 the `interesting' arguments, for some definition of interesting).
1149 The format is intended to be self-describing, but it will probably
1150 not be useful to people who are not familiar with the workings of
1153 If the -v (verbose) flag is given, the RX call number and sequence number
1156 If the -v flag is given twice, additional information is printed.
1157 such as the the RX call ID, serial number, and the RX packet flags.
1158 The MTU negotiation information is also printed from RX ack packets.
1160 If the -v flag three times, the security index and service id are printed.
1162 Error codes are printed for abort packets, with the exception of Ubik
1163 beacon packets (because abort packets are used to signify a yes vote
1164 for the Ubik protocol).
1166 Note that AFS requests are very large and many of the arguments won't
1167 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1168 to watch AFS traffic.
1170 AFS reply packets do not explicitly identify the RPC operation. Instead,
1171 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1172 replies using the call number and service ID. If a reply does not closely
1174 corresponding request, it might not be parsable.
1177 KIP Appletalk (DDP in UDP)
1179 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1180 and dumped as DDP packets (i.e., all the UDP header information is
1181 discarded). The file
1183 is used to translate appletalk net and node numbers to names.
1184 Lines in this file have the form
1196 The first two lines give the names of appletalk networks. The third
1197 line gives the name of a particular host (a host is distinguished
1198 from a net by the 3rd octet in the number \-
1199 a net number \fImust\fP have two octets and a host number \fImust\fP
1200 have three octets.) The number and name should be separated by
1201 whitespace (blanks or tabs).
1204 file may contain blank lines or comment lines (lines starting with
1207 Appletalk addresses are printed in the form
1213 \f(CW144.1.209.2 > icsd-net.112.220
1214 office.2 > icsd-net.112.220
1215 jssmag.149.235 > icsd-net.2\fP
1221 doesn't exist or doesn't contain an entry for some appletalk
1222 host/net number, addresses are printed in numeric form.)
1223 In the first example, NBP (DDP port 2) on net 144.1 node 209
1224 is sending to whatever is listening on port 220 of net icsd node 112.
1225 The second line is the same except the full name of the source node
1226 is known (`office'). The third line is a send from port 235 on
1227 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1228 the broadcast address (255) is indicated by a net name with no host
1229 number \- for this reason it's a good idea to keep node names and
1230 net names distinct in /etc/atalk.names).
1232 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1233 packets have their contents interpreted. Other protocols just dump
1234 the protocol name (or number if no name is registered for the
1235 protocol) and packet size.
1237 \fBNBP packets\fP are formatted like the following examples:
1241 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1242 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1243 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fP\s+2
1247 The first line is a name lookup request for laserwriters sent by net icsd host
1248 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1249 The second line shows a reply for this request (note that it has the
1250 same id) from host jssmag.209 saying that it has a laserwriter
1251 resource named "RM1140" registered on port 250. The third line is
1252 another reply to the same request saying host techpit has laserwriter
1253 "techpit" registered on port 186.
1255 \fBATP packet\fP formatting is demonstrated by the following example:
1259 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1260 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1261 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1262 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1263 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1264 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1265 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1266 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1267 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1268 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1269 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1270 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1271 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1272 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fP\s+2
1276 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1277 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1278 is the value of the `userdata' field in the request.
1280 Helios responds with 8 512-byte packets. The `:digit' following the
1281 transaction id gives the packet sequence number in the transaction
1282 and the number in parens is the amount of data in the packet,
1283 excluding the atp header. The `*' on packet 7 indicates that the
1286 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1287 resends them then jssmag.209 releases the transaction. Finally,
1288 jssmag.209 initiates the next request. The `*' on the request
1289 indicates that XO (`exactly once') was \fInot\fP set.
1294 Fragmented Internet datagrams are printed as
1298 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1299 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1303 (The first form indicates there are more fragments. The second
1304 indicates this is the last fragment.)
1306 \fIId\fP is the fragment id. \fISize\fP is the fragment
1307 size (in bytes) excluding the IP header. \fIOffset\fP is this
1308 fragment's offset (in bytes) in the original datagram.
1310 The fragment information is output for each fragment. The first
1311 fragment contains the higher level protocol header and the frag
1312 info is printed after the protocol info. Fragments
1313 after the first contain no higher level protocol header and the
1314 frag info is printed after the source and destination addresses.
1315 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1316 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1320 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1321 arizona > rtsg: (frag 595a:204@328)
1322 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1326 There are a couple of things to note here: First, addresses in the
1327 2nd line don't include port numbers. This is because the TCP
1328 protocol information is all in the first fragment and we have no idea
1329 what the port or sequence numbers are when we print the later fragments.
1330 Second, the tcp sequence information in the first line is printed as if there
1331 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1332 the first frag and 204 in the second). If you are looking for holes
1333 in the sequence space or trying to match up acks
1334 with packets, this can fool you.
1336 A packet with the IP \fIdon't fragment\fP flag is marked with a
1337 trailing \fB(DF)\fP.
1341 By default, all output lines are preceded by a timestamp. The timestamp
1342 is the current clock time in the form
1348 and is as accurate as the kernel's clock.
1349 The timestamp reflects the time the kernel first saw the packet. No attempt
1350 is made to account for the time lag between when the
1351 ethernet interface removed the packet from the wire and when the kernel
1352 serviced the `new packet' interrupt.
1354 traffic(1C), nit(4P), bpf(4), pcap(3)
1358 Steven McCanne, all of the
1359 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1361 The current version is available via anonymous ftp:
1364 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1367 IPv6/IPsec support is added by WIDE/KAME project.
1368 This program uses Eric Young's SSLeay library, under specific configuration.
1370 Please send bug reports to tcpdump@ee.lbl.gov.
1372 NIT doesn't let you watch your own outbound traffic, BPF will.
1373 We recommend that you use the latter.
1375 Some attempt should be made to reassemble IP fragments or, at least
1376 to compute the right length for the higher level protocol.
1378 Name server inverse queries are not dumped correctly: The (empty)
1379 question section is printed rather than real query in the answer
1380 section. Some believe that inverse queries are themselves a bug and
1381 prefer to fix the program generating them rather than tcpdump.
1383 Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
1385 Even if we were inclined to do anything to promote the use of
1386 Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
1387 networks so we'd would have no way of testing this code.
1389 A packet trace that crosses a daylight savings time change will give
1390 skewed time stamps (the time change is ignored).
1392 Filters expressions that manipulate FDDI headers assume that all FDDI
1393 packets are encapsulated Ethernet packets. This is true for IP, ARP,
1394 and DECNET Phase IV, but is not true for protocols such as ISO CLNS.
1395 Therefore, the filter may inadvertently accept certain packets that
1396 do not properly match the filter expression.
1399 should chase header chain, but at this moment it does not.
1400 .BR "ip6 protochain"
1401 is supplied for this behavior.
1403 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1404 does not work against IPv6 packets.
1405 It only looks at IPv4 packets.
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