1 .\" @(#) $Header: /tcpdump/master/tcpdump/Attic/tcpdump.1,v 1.71 1999-12-13 18:06:15 mcr Exp $ (LBL)
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23 .TH TCPDUMP 1 "30 June 1997"
25 tcpdump \- dump traffic on a network
75 \fITcpdump\fP prints out the headers of packets on a network interface
76 that match the boolean \fIexpression\fP.
78 .B Under SunOS with nit or bpf:
81 you must have read access to
85 .B Under Solaris with dlpi:
86 You must have read access to the network pseudo device, e.g.
88 .B Under HP-UX with dlpi:
89 You must be root or it must be installed setuid to root.
90 .B Under IRIX with snoop:
91 You must be root or it must be installed setuid to root.
93 You must be root or it must be installed setuid to root.
94 .B Under Ultrix and Digital UNIX:
95 Once the super-user has enabled promiscuous-mode operation using
100 You must have read access to
105 Attempt to convert network and broadcast addresses to names.
108 Exit after receiving \fIcount\fP packets.
111 Dump the compiled packet-matching code in a human readable form to
112 standard output and stop.
115 Dump packet-matching code as a
120 Dump packet-matching code as decimal numbers (preceded with a count).
123 Print the link-level header on each dump line.
126 Print `foreign' internet addresses numerically rather than symbolically
127 (this option is intended to get around serious brain damage in
128 Sun's yp server \(em usually it hangs forever translating non-local
132 Use \fIfile\fP as input for the filter expression.
133 An additional expression given on the command line is ignored.
136 Listen on \fIinterface\fP.
137 If unspecified, \fItcpdump\fP searches the system interface list for the
138 lowest numbered, configured up interface (excluding loopback).
139 Ties are broken by choosing the earliest match.
142 Make stdout line buffered. Useful if you want to see the data
143 while capturing it. E.g.,
145 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
146 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
149 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
152 Don't print domain name qualification of host names. E.g.,
153 if you give this flag then \fItcpdump\fP will print ``nic''
154 instead of ``nic.ddn.mil''.
157 Load SMI MIB module definitions from file \fImodule\fR. This option
158 can be used several times to load several MIB modules into tcpdump.
161 Do not run the packet-matching code optimizer. This is useful only
162 if you suspect a bug in the optimizer.
165 \fIDon't\fP put the interface
166 into promiscuous mode. Note that the interface might be in promiscuous
167 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
168 `ether host {local-hw-addr} or ether broadcast'.
171 Quick (quiet?) output. Print less protocol information so output
175 Read packets from \fIfile\fR (which was created with the -w option).
176 Standard input is used if \fIfile\fR is ``-''.
179 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
180 default of 68 (with SunOS's NIT, the minimum is actually 96).
181 68 bytes is adequate for IP, ICMP, TCP
182 and UDP but may truncate protocol information from name server and NFS
183 packets (see below). Packets truncated because of a limited snapshot
184 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
185 is the name of the protocol level at which the truncation has occurred.
186 Note that taking larger snapshots both increases
187 the amount of time it takes to process packets and, effectively,
188 decreases the amount of packet buffering. This may cause packets to be
189 lost. You should limit \fIsnaplen\fP to the smallest number that will
190 capture the protocol information you're interested in.
193 Force packets selected by "\fIexpression\fP" to be interpreted the
194 specified \fItype\fR. Currently known types are
195 \fBrpc\fR (Remote Procedure Call),
196 \fBrtp\fR (Real-Time Applications protocol),
197 \fBrtcp\fR (Real-Time Applications control protocol),
198 \fBsnmp\fR (Simple Network Management Protocol),
199 \fBvat\fR (Visual Audio Tool),
201 \fBwb\fR (distributed White Board).
204 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
205 If specified, \fItcpdump\fP will not print replay prevention field.
206 Since there is no protocol version field in ESP/AH specification,
207 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
210 Print absolute, rather than relative, TCP sequence numbers.
213 \fIDon't\fP print a timestamp on each dump line.
216 Print an unformatted timestamp on each dump line.
219 (Slightly more) verbose output. For example, the time to live
220 and type of service information in an IP packet is printed.
223 Even more verbose output. For example, additional fields are
224 printed from NFS reply packets.
227 Write the raw packets to \fIfile\fR rather than parsing and printing
228 them out. They can later be printed with the \-r option.
229 Standard output is used if \fIfile\fR is ``-''.
232 Print each packet (minus its link level header) in hex.
233 The smaller of the entire packet or
235 bytes will be printed.
236 .IP "\fI expression\fP"
238 selects which packets will be dumped. If no \fIexpression\fP
239 is given, all packets on the net will be dumped. Otherwise,
240 only packets for which \fIexpression\fP is `true' will be dumped.
242 The \fIexpression\fP consists of one or more
244 Primitives usually consist of an
246 (name or number) preceded by one or more qualifiers. There are three
247 different kinds of qualifier:
249 qualifiers say what kind of thing the id name or number refers to.
255 E.g., `host foo', `net 128.3', `port 20'. If there is no type
260 qualifiers specify a particular transfer direction to and/or from
262 Possible directions are
269 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
270 there is no dir qualifier,
273 For `null' link layers (i.e. point to point protocols such as slip) the
277 qualifiers can be used to specify a desired direction.
279 qualifiers restrict the match to a particular protocol. Possible
297 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
298 no proto qualifier, all protocols consistent with the type are
299 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
300 (except the latter is not legal syntax), `net bar' means `(ip or
301 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
303 [`fddi' is actually an alias for `ether'; the parser treats them
304 identically as meaning ``the data link level used on the specified
305 network interface.'' FDDI headers contain Ethernet-like source
306 and destination addresses, and often contain Ethernet-like packet
307 types, so you can filter on these FDDI fields just as with the
308 analogous Ethernet fields. FDDI headers also contain other fields,
309 but you cannot name them explicitly in a filter expression.]
311 In addition to the above, there are some special `primitive' keywords
312 that don't follow the pattern:
317 and arithmetic expressions. All of these are described below.
319 More complex filter expressions are built up by using the words
324 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
325 To save typing, identical qualifier lists can be omitted. E.g.,
326 `tcp dst port ftp or ftp-data or domain' is exactly the same as
327 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
329 Allowable primitives are:
330 .IP "\fBdst host \fIhost\fR"
331 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
332 which may be either an address or a name.
333 .IP "\fBsrc host \fIhost\fR"
334 True if the IPv4/v6 source field of the packet is \fIhost\fP.
335 .IP "\fBhost \fIhost\fP
336 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
337 Any of the above host expressions can be prepended with the keywords,
338 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
341 \fBip host \fIhost\fR
344 which is equivalent to:
347 \fBether proto \fI\\ip\fB and host \fIhost\fR
350 If \fIhost\fR is a name with multiple IP addresses, each address will
351 be checked for a match.
352 .IP "\fBether dst \fIehost\fP
353 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
354 may be either a name from /etc/ethers or a number (see
357 .IP "\fBether src \fIehost\fP
358 True if the ethernet source address is \fIehost\fP.
359 .IP "\fBether host \fIehost\fP
360 True if either the ethernet source or destination address is \fIehost\fP.
361 .IP "\fBgateway\fP \fIhost\fP
362 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
363 source or destination address was \fIhost\fP but neither the IP source
364 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
365 must be found in both /etc/hosts and /etc/ethers. (An equivalent
369 \fBether host \fIehost \fBand not host \fIhost\fR
372 which can be used with either names or numbers for \fIhost / ehost\fP.)
373 This syntax does not work in IPv6-enabled configuration at this moment.
374 .IP "\fBdst net \fInet\fR"
375 True if the IPv4/v6 destination address of the packet has a network
376 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
377 or a network number (see \fInetworks(4)\fP for details).
378 .IP "\fBsrc net \fInet\fR"
379 True if the IPv4/v6 source address of the packet has a network
381 .IP "\fBnet \fInet\fR"
382 True if either the IPv4/v6 source or destination address of the packet has a network
384 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
385 True if the IP address matches \fInet\fR with the specific netmask.
386 May be qualified with \fBsrc\fR or \fBdst\fR.
387 Note that this syntax is not valid for IPv6 \fInet\fR.
388 .IP "\fBnet \fInet\fR/\fIlen\fR"
389 True if the IPv4/v6 address matches \fInet\fR a netmask \fIlen\fR bits wide.
390 May be qualified with \fBsrc\fR or \fBdst\fR.
391 .IP "\fBdst port \fIport\fR"
392 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
393 destination port value of \fIport\fP.
394 The \fIport\fP can be a number or a name used in /etc/services (see
398 If a name is used, both the port
399 number and protocol are checked. If a number or ambiguous name is used,
400 only the port number is checked (e.g., \fBdst port 513\fR will print both
401 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
402 both tcp/domain and udp/domain traffic).
403 .IP "\fBsrc port \fIport\fR"
404 True if the packet has a source port value of \fIport\fP.
405 .IP "\fBport \fIport\fR"
406 True if either the source or destination port of the packet is \fIport\fP.
407 Any of the above port expressions can be prepended with the keywords,
408 \fBtcp\fP or \fBudp\fP, as in:
411 \fBtcp src port \fIport\fR
414 which matches only tcp packets whose source port is \fIport\fP.
415 .IP "\fBless \fIlength\fR"
416 True if the packet has a length less than or equal to \fIlength\fP.
417 This is equivalent to:
420 \fBlen <= \fIlength\fP.
423 .IP "\fBgreater \fIlength\fR"
424 True if the packet has a length greater than or equal to \fIlength\fP.
425 This is equivalent to:
428 \fBlen >= \fIlength\fP.
431 .IP "\fBip proto \fIprotocol\fR"
432 True if the packet is an ip packet (see
434 of protocol type \fIprotocol\fP.
435 \fIProtocol\fP can be a number or one of the names
436 \fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
437 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
438 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
439 Note that this primitive does not chase protocol header chain.
440 .IP "\fBip6 proto \fIprotocol\fR"
441 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
442 Note that this primitive does not chase protocol header chain.
443 .IP "\fBip6 protochain \fIprotocol\fR"
444 True if the packet is IPv6 packet,
445 and contains protocol header with type \fIprotocol\fR
446 in its protocol header chain.
450 \fBip6 protochain 6\fR
453 matches any IPv6 packet with TCP protocol header in the protocol header chain.
454 The packet may contain, for example,
455 authentication header, routing header, or hop-by-hop option header,
456 between IPv6 header and TCP header.
457 The BPF code emitted by this primitive is complex and
458 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
459 so this can be somewhat slow.
460 .IP "\fBip protochain \fIprotocol\fR"
461 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
462 .IP "\fBether broadcast\fR"
463 True if the packet is an ethernet broadcast packet. The \fIether\fP
465 .IP "\fBip broadcast\fR"
466 True if the packet is an IP broadcast packet. It checks for both
467 the all-zeroes and all-ones broadcast conventions, and looks up
468 the local subnet mask.
469 .IP "\fBether multicast\fR"
470 True if the packet is an ethernet multicast packet. The \fIether\fP
472 This is shorthand for `\fBether[0] & 1 != 0\fP'.
473 .IP "\fBip multicast\fR"
474 True if the packet is an IP multicast packet.
475 .IP "\fBip6 multicast\fR"
476 True if the packet is an IPv6 multicast packet.
477 .IP "\fBether proto \fIprotocol\fR"
478 True if the packet is of ether type \fIprotocol\fR.
479 \fIProtocol\fP can be a number or a name like
480 \fIip\fP, \fIip6\fP, \fIarp\fP, or \fIrarp\fP.
481 Note these identifiers are also keywords
482 and must be escaped via backslash (\\).
483 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
484 protocol identification comes from the 802.2 Logical Link Control
485 (LLC) header, which is usually layered on top of the FDDI header.
486 \fITcpdump\fP assumes, when filtering on the protocol identifier,
487 that all FDDI packets include an LLC header, and that the LLC header
488 is in so-called SNAP format.]
489 .IP "\fBdecnet src \fIhost\fR"
490 True if the DECNET source address is
492 which may be an address of the form ``10.123'', or a DECNET host
493 name. [DECNET host name support is only available on Ultrix systems
494 that are configured to run DECNET.]
495 .IP "\fBdecnet dst \fIhost\fR"
496 True if the DECNET destination address is
498 .IP "\fBdecnet host \fIhost\fR"
499 True if either the DECNET source or destination address is
501 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR"
505 \fBether proto \fIp\fR
508 where \fIp\fR is one of the above protocols.
509 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
513 \fBether proto \fIp\fR
516 where \fIp\fR is one of the above protocols.
518 \fItcpdump\fP does not currently know how to parse these protocols.
519 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
523 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
526 where \fIp\fR is one of the above protocols.
527 .IP "\fIexpr relop expr\fR"
528 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
529 and \fIexpr\fR is an arithmetic expression composed of integer constants
530 (expressed in standard C syntax), the normal binary operators
531 [+, -, *, /, &, |], a length operator, and special packet data accessors.
533 data inside the packet, use the following syntax:
536 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
539 \fIProto\fR is one of \fBether, fddi,
540 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
541 indicates the protocol layer for the index operation.
542 Note that \fItcp, udp\fR and other upper-layer protocol types only
543 apply to IPv4, not IPv6 (this will be fixed in the future).
544 The byte offset, relative to the indicated protocol layer, is
546 \fISize\fR is optional and indicates the number of bytes in the
547 field of interest; it can be either one, two, or four, and defaults to one.
548 The length operator, indicated by the keyword \fBlen\fP, gives the
549 length of the packet.
551 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
552 The expression `\fBip[0] & 0xf != 5\fP'
553 catches all IP packets with options. The expression
554 `\fBip[6:2] & 0x1fff = 0\fP'
555 catches only unfragmented datagrams and frag zero of fragmented datagrams.
556 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
558 For instance, \fBtcp[0]\fP always means the first
559 byte of the TCP \fIheader\fP, and never means the first byte of an
560 intervening fragment.
562 Primitives may be combined using:
564 A parenthesized group of primitives and operators
565 (parentheses are special to the Shell and must be escaped).
567 Negation (`\fB!\fP' or `\fBnot\fP').
569 Concatenation (`\fB&&\fP' or `\fBand\fP').
571 Alternation (`\fB||\fP' or `\fBor\fP').
573 Negation has highest precedence.
574 Alternation and concatenation have equal precedence and associate
575 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
576 are now required for concatenation.
578 If an identifier is given without a keyword, the most recent keyword
583 \fBnot host vs and ace\fR
589 \fBnot host vs and host ace\fR
592 which should not be confused with
595 \fBnot ( host vs or ace )\fR
599 Expression arguments can be passed to tcpdump as either a single argument
600 or as multiple arguments, whichever is more convenient.
601 Generally, if the expression contains Shell metacharacters, it is
602 easier to pass it as a single, quoted argument.
603 Multiple arguments are concatenated with spaces before being parsed.
606 To print all packets arriving at or departing from \fIsundown\fP:
609 \fBtcpdump host sundown\fP
613 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
616 \fBtcpdump host helios and \\( hot or ace \\)\fP
620 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
623 \fBtcpdump ip host ace and not helios\fP
627 To print all traffic between local hosts and hosts at Berkeley:
631 tcpdump net ucb-ether
635 To print all ftp traffic through internet gateway \fIsnup\fP:
636 (note that the expression is quoted to prevent the shell from
637 (mis-)interpreting the parentheses):
641 tcpdump 'gateway snup and (port ftp or ftp-data)'
645 To print traffic neither sourced from nor destined for local hosts
646 (if you gateway to one other net, this stuff should never make it
647 onto your local net).
651 tcpdump ip and not net \fIlocalnet\fP
655 To print the start and end packets (the SYN and FIN packets) of each
656 TCP conversation that involves a non-local host.
660 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
664 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
668 tcpdump 'gateway snup and ip[2:2] > 576'
672 To print IP broadcast or multicast packets that were
674 sent via ethernet broadcast or multicast:
678 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
682 To print all ICMP packets that are not echo requests/replies (i.e., not
687 tcpdump 'icmp[0] != 8 and icmp[0] != 0"
692 The output of \fItcpdump\fP is protocol dependent. The following
693 gives a brief description and examples of most of the formats.
701 If the '-e' option is given, the link level header is printed out.
702 On ethernets, the source and destination addresses, protocol,
703 and packet length are printed.
705 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
706 the `frame control' field, the source and destination addresses,
707 and the packet length. (The `frame control' field governs the
708 interpretation of the rest of the packet. Normal packets (such
709 as those containing IP datagrams) are `async' packets, with a priority
710 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
711 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
712 the LLC header is printed if it is \fInot\fR an ISO datagram or a
713 so-called SNAP packet.
715 \fI(N.B.: The following description assumes familiarity with
716 the SLIP compression algorithm described in RFC-1144.)\fP
718 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
719 packet type, and compression information are printed out.
720 The packet type is printed first.
721 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
722 No further link information is printed for \fIip\fR packets.
723 For TCP packets, the connection identifier is printed following the type.
724 If the packet is compressed, its encoded header is printed out.
725 The special cases are printed out as
726 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
727 the sequence number (or sequence number and ack) has changed.
728 If it is not a special case,
729 zero or more changes are printed.
730 A change is indicated by U (urgent pointer), W (window), A (ack),
731 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
733 Finally, the amount of data in the packet and compressed header length
736 For example, the following line shows an outbound compressed TCP packet,
737 with an implicit connection identifier; the ack has changed by 6,
738 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
739 data and 6 bytes of compressed header:
742 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
748 Arp/rarp output shows the type of request and its arguments. The
749 format is intended to be self explanatory.
750 Here is a short sample taken from the start of an `rlogin' from
751 host \fIrtsg\fP to host \fIcsam\fP:
755 \f(CWarp who-has csam tell rtsg
756 arp reply csam is-at CSAM\fP
760 The first line says that rtsg sent an arp packet asking
761 for the ethernet address of internet host csam. Csam
762 replies with its ethernet address (in this example, ethernet addresses
763 are in caps and internet addresses in lower case).
765 This would look less redundant if we had done \fBtcpdump \-n\fP:
769 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
770 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
774 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
775 broadcast and the second is point-to-point would be visible:
779 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
780 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fP
784 For the first packet this says the ethernet source address is RTSG, the
785 destination is the ethernet broadcast address, the type field
786 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
790 \fI(N.B.:The following description assumes familiarity with
791 the TCP protocol described in RFC-793. If you are not familiar
792 with the protocol, neither this description nor tcpdump will
793 be of much use to you.)\fP
795 The general format of a tcp protocol line is:
799 \fIsrc > dst: flags data-seqno ack window urgent options\fP
803 \fISrc\fP and \fIdst\fP are the source and destination IP
804 addresses and ports. \fIFlags\fP are some combination of S (SYN),
805 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
806 \fIData-seqno\fP describes the portion of sequence space covered
807 by the data in this packet (see example below).
808 \fIAck\fP is sequence number of the next data expected the other
809 direction on this connection.
810 \fIWindow\fP is the number of bytes of receive buffer space available
811 the other direction on this connection.
812 \fIUrg\fP indicates there is `urgent' data in the packet.
813 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
815 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
816 depend on the contents of the packet's tcp protocol header and
817 are output only if appropriate.
819 Here is the opening portion of an rlogin from host \fIrtsg\fP to
824 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
825 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
826 rtsg.1023 > csam.login: . ack 1 win 4096
827 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
828 csam.login > rtsg.1023: . ack 2 win 4096
829 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
830 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
831 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
832 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fP\s+2
836 The first line says that tcp port 1023 on rtsg sent a packet
838 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
839 The packet sequence number was 768512 and it contained no data.
840 (The notation is `first:last(nbytes)' which means `sequence
842 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
843 There was no piggy-backed ack, the available receive window was 4096
844 bytes and there was a max-segment-size option requesting an mss of
847 Csam replies with a similar packet except it includes a piggy-backed
848 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
850 The packet contained no data so there is no data sequence number.
851 Note that the ack sequence
852 number is a small integer (1). The first time \fBtcpdump\fP sees a
853 tcp `conversation', it prints the sequence number from the packet.
854 On subsequent packets of the conversation, the difference between
855 the current packet's sequence number and this initial sequence number
856 is printed. This means that sequence numbers after the
857 first can be interpreted
858 as relative byte positions in the conversation's data stream (with the
859 first data byte each direction being `1'). `-S' will override this
860 feature, causing the original sequence numbers to be output.
862 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
863 in the rtsg \(-> csam side of the conversation).
864 The PUSH flag is set in the packet.
865 On the 7th line, csam says it's received data sent by rtsg up to
866 but not including byte 21. Most of this data is apparently sitting in the
867 socket buffer since csam's receive window has gotten 19 bytes smaller.
868 Csam also sends one byte of data to rtsg in this packet.
869 On the 8th and 9th lines,
870 csam sends two bytes of urgent, pushed data to rtsg.
872 If the snapshot was small enough that \fBtcpdump\fP didn't capture
873 the full TCP header, it interprets as much of the header as it can
874 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
875 be interpreted. If the header contains a bogus option (one with a length
876 that's either too small or beyond the end of the header), tcpdump reports
877 it as ``[\fIbad opt\fP]'' and does not interpret any further options (since
878 it's impossible to tell where they start). If the header length indicates
879 options are present but the IP datagram length is not long enough for the
880 options to actually be there, tcpdump reports it as ``[\fIbad hdr length\fP]''.
885 UDP format is illustrated by this rwho packet:
889 \f(CWactinide.who > broadcast.who: udp 84\fP
893 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
894 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
895 broadcast address. The packet contained 84 bytes of user data.
897 Some UDP services are recognized (from the source or destination
898 port number) and the higher level protocol information printed.
899 In particular, Domain Name service requests (RFC-1034/1035) and Sun
900 RPC calls (RFC-1050) to NFS.
902 UDP Name Server Requests
904 \fI(N.B.:The following description assumes familiarity with
905 the Domain Service protocol described in RFC-1035. If you are not familiar
906 with the protocol, the following description will appear to be written
909 Name server requests are formatted as
913 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
915 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fP
919 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
920 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
921 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
922 was set. The query length was 37 bytes, not including the UDP and
923 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
924 so the op field was omitted. If the op had been anything else, it would
925 have been printed between the `3' and the `+'.
926 Similarly, the qclass was the normal one,
927 \fIC_IN\fP, and omitted. Any other qclass would have been printed
928 immediately after the `A'.
930 A few anomalies are checked and may result in extra fields enclosed in
931 square brackets: If a query contains an answer, name server or
937 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
938 is the appropriate count.
939 If any of the response bits are set (AA, RA or rcode) or any of the
940 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
941 is printed, where \fIx\fP is the hex value of header bytes two and three.
943 UDP Name Server Responses
945 Name server responses are formatted as
949 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
951 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
952 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fP
956 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
957 with 3 answer records, 3 name server records and 7 authority records.
958 The first answer record is type A (address) and its data is internet
959 address 128.32.137.3. The total size of the response was 273 bytes,
960 excluding UDP and IP headers. The op (Query) and response code
961 (NoError) were omitted, as was the class (C_IN) of the A record.
963 In the second example, \fIhelios\fP responds to query 2 with a
964 response code of non-existent domain (NXDomain) with no answers,
965 one name server and no authority records. The `*' indicates that
966 the \fIauthoritative answer\fP bit was set. Since there were no
967 answers, no type, class or data were printed.
969 Other flag characters that might appear are `\-' (recursion available,
970 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
971 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
974 Note that name server requests and responses tend to be large and the
975 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
976 to print. Use the \fB\-s\fP flag to increase the snaplen if you
977 need to seriously investigate name server traffic. `\fB\-s 128\fP'
978 has worked well for me.
983 tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data
984 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
985 NetBEUI SMB data is also done.
987 By default a fairly minimal decode is done, with a much more detailed
988 decode done if -v is used. Be warned that with -v a single SMB packet
989 may take up a page or more, so only use -v if you really want all the
992 If you are decoding SMB sessions containing unicode strings then you
993 may wish to set the environment variable USE_UNICODE to 1. A patch to
994 auto-detect unicode srings would be welcome.
996 For information on SMB packet formats and what all te fields mean see
997 www.cifs.org or the pub/samba/specs/ directory on your favourite
998 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1002 NFS Requests and Replies
1004 Sun NFS (Network File System) requests and replies are printed as:
1008 \fIsrc.xid > dst.nfs: len op args\fP
1009 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1012 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1013 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1014 sushi.201b > wrl.nfs:
1015 144 lookup fh 9,74/4096.6878 "xcolors"
1016 wrl.nfs > sushi.201b:
1017 reply ok 128 lookup fh 9,74/4134.3150
1022 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1023 to \fIwrl\fP (note that the number following the src host is a
1024 transaction id, \fInot\fP the source port). The request was 112 bytes,
1025 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1026 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1027 (If one is lucky, as in this case, the file handle can be interpreted
1028 as a major,minor device number pair, followed by the inode number and
1030 \fIWrl\fP replies `ok' with the contents of the link.
1032 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1033 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1034 depends on the operation type. The format is intended to be self
1035 explanatory if read in conjunction with
1036 an NFS protocol spec.
1038 If the \-v (verbose) flag is given, additional information is printed.
1044 sushi.1372a > wrl.nfs:
1045 148 read fh 21,11/12.195 8192 bytes @ 24576
1046 wrl.nfs > sushi.1372a:
1047 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1052 (\-v also prints the IP header TTL, ID, and fragmentation fields,
1053 which have been omitted from this example.) In the first line,
1054 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1055 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1056 second line is the first fragment of the reply, and hence is only 1472
1057 bytes long (the other bytes will follow in subsequent fragments, but
1058 these fragments do not have NFS or even UDP headers and so might not be
1059 printed, depending on the filter expression used). Because the \-v flag
1060 is given, some of the file attributes (which are returned in addition
1061 to the file data) are printed: the file type (``REG'', for regular file),
1062 the file mode (in octal), the uid and gid, and the file size.
1064 If the \-v flag is given more than once, even more details are printed.
1066 Note that NFS requests are very large and much of the detail won't be printed
1067 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1070 NFS reply packets do not explicitly identify the RPC operation. Instead,
1071 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1072 replies using the transaction ID. If a reply does not closely follow the
1073 corresponding request, it might not be parsable.
1075 KIP Appletalk (DDP in UDP)
1077 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1078 and dumped as DDP packets (i.e., all the UDP header information is
1079 discarded). The file
1081 is used to translate appletalk net and node numbers to names.
1082 Lines in this file have the form
1094 The first two lines give the names of appletalk networks. The third
1095 line gives the name of a particular host (a host is distinguished
1096 from a net by the 3rd octet in the number \-
1097 a net number \fImust\fP have two octets and a host number \fImust\fP
1098 have three octets.) The number and name should be separated by
1099 whitespace (blanks or tabs).
1102 file may contain blank lines or comment lines (lines starting with
1105 Appletalk addresses are printed in the form
1111 \f(CW144.1.209.2 > icsd-net.112.220
1112 office.2 > icsd-net.112.220
1113 jssmag.149.235 > icsd-net.2\fP
1119 doesn't exist or doesn't contain an entry for some appletalk
1120 host/net number, addresses are printed in numeric form.)
1121 In the first example, NBP (DDP port 2) on net 144.1 node 209
1122 is sending to whatever is listening on port 220 of net icsd node 112.
1123 The second line is the same except the full name of the source node
1124 is known (`office'). The third line is a send from port 235 on
1125 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1126 the broadcast address (255) is indicated by a net name with no host
1127 number \- for this reason it's a good idea to keep node names and
1128 net names distinct in /etc/atalk.names).
1130 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1131 packets have their contents interpreted. Other protocols just dump
1132 the protocol name (or number if no name is registered for the
1133 protocol) and packet size.
1135 \fBNBP packets\fP are formatted like the following examples:
1139 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1140 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1141 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fP\s+2
1145 The first line is a name lookup request for laserwriters sent by net icsd host
1146 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1147 The second line shows a reply for this request (note that it has the
1148 same id) from host jssmag.209 saying that it has a laserwriter
1149 resource named "RM1140" registered on port 250. The third line is
1150 another reply to the same request saying host techpit has laserwriter
1151 "techpit" registered on port 186.
1153 \fBATP packet\fP formatting is demonstrated by the following example:
1157 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1158 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1159 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1160 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1161 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1162 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1163 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1164 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1165 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1166 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1167 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1168 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1169 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1170 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fP\s+2
1174 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1175 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1176 is the value of the `userdata' field in the request.
1178 Helios responds with 8 512-byte packets. The `:digit' following the
1179 transaction id gives the packet sequence number in the transaction
1180 and the number in parens is the amount of data in the packet,
1181 excluding the atp header. The `*' on packet 7 indicates that the
1184 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1185 resends them then jssmag.209 releases the transaction. Finally,
1186 jssmag.209 initiates the next request. The `*' on the request
1187 indicates that XO (`exactly once') was \fInot\fP set.
1192 Fragmented Internet datagrams are printed as
1196 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1197 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1201 (The first form indicates there are more fragments. The second
1202 indicates this is the last fragment.)
1204 \fIId\fP is the fragment id. \fISize\fP is the fragment
1205 size (in bytes) excluding the IP header. \fIOffset\fP is this
1206 fragment's offset (in bytes) in the original datagram.
1208 The fragment information is output for each fragment. The first
1209 fragment contains the higher level protocol header and the frag
1210 info is printed after the protocol info. Fragments
1211 after the first contain no higher level protocol header and the
1212 frag info is printed after the source and destination addresses.
1213 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1214 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1218 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1219 arizona > rtsg: (frag 595a:204@328)
1220 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1224 There are a couple of things to note here: First, addresses in the
1225 2nd line don't include port numbers. This is because the TCP
1226 protocol information is all in the first fragment and we have no idea
1227 what the port or sequence numbers are when we print the later fragments.
1228 Second, the tcp sequence information in the first line is printed as if there
1229 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1230 the first frag and 204 in the second). If you are looking for holes
1231 in the sequence space or trying to match up acks
1232 with packets, this can fool you.
1234 A packet with the IP \fIdon't fragment\fP flag is marked with a
1235 trailing \fB(DF)\fP.
1239 By default, all output lines are preceded by a timestamp. The timestamp
1240 is the current clock time in the form
1246 and is as accurate as the kernel's clock.
1247 The timestamp reflects the time the kernel first saw the packet. No attempt
1248 is made to account for the time lag between when the
1249 ethernet interface removed the packet from the wire and when the kernel
1250 serviced the `new packet' interrupt.
1252 traffic(1C), nit(4P), bpf(4), pcap(3)
1256 Steven McCanne, all of the
1257 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1259 The current version is available via anonymous ftp:
1262 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1265 IPv6/IPsec support is added by WIDE/KAME project.
1266 This program uses Eric Young's SSLeay library, under specific configuration.
1268 Please send bug reports to tcpdump@ee.lbl.gov.
1270 NIT doesn't let you watch your own outbound traffic, BPF will.
1271 We recommend that you use the latter.
1273 Some attempt should be made to reassemble IP fragments or, at least
1274 to compute the right length for the higher level protocol.
1276 Name server inverse queries are not dumped correctly: The (empty)
1277 question section is printed rather than real query in the answer
1278 section. Some believe that inverse queries are themselves a bug and
1279 prefer to fix the program generating them rather than tcpdump.
1281 Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
1283 Even if we were inclined to do anything to promote the use of
1284 Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
1285 networks so we'd would have no way of testing this code.
1287 A packet trace that crosses a daylight savings time change will give
1288 skewed time stamps (the time change is ignored).
1290 Filters expressions that manipulate FDDI headers assume that all FDDI
1291 packets are encapsulated Ethernet packets. This is true for IP, ARP,
1292 and DECNET Phase IV, but is not true for protocols such as ISO CLNS.
1293 Therefore, the filter may inadvertently accept certain packets that
1294 do not properly match the filter expression.
1297 should chase header chain, but at this moment it does not.
1301 should chase header chain too.
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