1 .\" @(#) $Header: /tcpdump/master/tcpdump/Attic/tcpdump.1,v 1.67 1999-10-07 23:47:13 mcr Exp $ (LBL)
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23 .TH TCPDUMP 1 "30 June 1997"
25 tcpdump \- dump traffic on a network
68 \fITcpdump\fP prints out the headers of packets on a network interface
69 that match the boolean \fIexpression\fP.
71 .B Under SunOS with nit or bpf:
74 you must have read access to
78 .B Under Solaris with dlpi:
79 You must have read access to the network pseudo device, e.g.
81 .B Under HP-UX with dlpi:
82 You must be root or it must be installed setuid to root.
83 .B Under IRIX with snoop:
84 You must be root or it must be installed setuid to root.
86 You must be root or it must be installed setuid to root.
87 .B Under Ultrix and Digital UNIX:
88 Once the super-user has enabled promiscuous-mode operation using
93 You must have read access to
98 Attempt to convert network and broadcast addresses to names.
101 Exit after receiving \fIcount\fP packets.
104 Dump the compiled packet-matching code in a human readable form to
105 standard output and stop.
108 Dump packet-matching code as a
113 Dump packet-matching code as decimal numbers (preceded with a count).
116 Print the link-level header on each dump line.
119 Print `foreign' internet addresses numerically rather than symbolically
120 (this option is intended to get around serious brain damage in
121 Sun's yp server \(em usually it hangs forever translating non-local
125 Use \fIfile\fP as input for the filter expression.
126 An additional expression given on the command line is ignored.
129 Listen on \fIinterface\fP.
130 If unspecified, \fItcpdump\fP searches the system interface list for the
131 lowest numbered, configured up interface (excluding loopback).
132 Ties are broken by choosing the earliest match.
135 Make stdout line buffered. Useful if you want to see the data
136 while capturing it. E.g.,
138 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
139 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
142 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
145 Don't print domain name qualification of host names. E.g.,
146 if you give this flag then \fItcpdump\fP will print ``nic''
147 instead of ``nic.ddn.mil''.
150 Do not run the packet-matching code optimizer. This is useful only
151 if you suspect a bug in the optimizer.
154 \fIDon't\fP put the interface
155 into promiscuous mode. Note that the interface might be in promiscuous
156 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
157 `ether host {local-hw-addr} or ether broadcast'.
160 Quick (quiet?) output. Print less protocol information so output
164 Read packets from \fIfile\fR (which was created with the -w option).
165 Standard input is used if \fIfile\fR is ``-''.
168 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
169 default of 68 (with SunOS's NIT, the minimum is actually 96).
170 68 bytes is adequate for IP, ICMP, TCP
171 and UDP but may truncate protocol information from name server and NFS
172 packets (see below). Packets truncated because of a limited snapshot
173 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
174 is the name of the protocol level at which the truncation has occurred.
175 Note that taking larger snapshots both increases
176 the amount of time it takes to process packets and, effectively,
177 decreases the amount of packet buffering. This may cause packets to be
178 lost. You should limit \fIsnaplen\fP to the smallest number that will
179 capture the protocol information you're interested in.
182 Force packets selected by "\fIexpression\fP" to be interpreted the
183 specified \fItype\fR. Currently known types are
184 \fBrpc\fR (Remote Procedure Call),
185 \fBrtp\fR (Real-Time Applications protocol),
186 \fBrtcp\fR (Real-Time Applications control protocol),
187 \fBvat\fR (Visual Audio Tool),
189 \fBwb\fR (distributed White Board).
192 Print absolute, rather than relative, TCP sequence numbers.
195 \fIDon't\fP print a timestamp on each dump line.
198 Print an unformatted timestamp on each dump line.
201 (Slightly more) verbose output. For example, the time to live
202 and type of service information in an IP packet is printed.
205 Even more verbose output. For example, additional fields are
206 printed from NFS reply packets.
209 Write the raw packets to \fIfile\fR rather than parsing and printing
210 them out. They can later be printed with the \-r option.
211 Standard output is used if \fIfile\fR is ``-''.
214 Print each packet (minus its link level header) in hex.
215 The smaller of the entire packet or
217 bytes will be printed.
218 .IP "\fI expression\fP"
220 selects which packets will be dumped. If no \fIexpression\fP
221 is given, all packets on the net will be dumped. Otherwise,
222 only packets for which \fIexpression\fP is `true' will be dumped.
224 The \fIexpression\fP consists of one or more
226 Primitives usually consist of an
228 (name or number) preceded by one or more qualifiers. There are three
229 different kinds of qualifier:
231 qualifiers say what kind of thing the id name or number refers to.
237 E.g., `host foo', `net 128.3', `port 20'. If there is no type
242 qualifiers specify a particular transfer direction to and/or from
244 Possible directions are
251 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
252 there is no dir qualifier,
255 For `null' link layers (i.e. point to point protocols such as slip) the
259 qualifiers can be used to specify a desired direction.
261 qualifiers restrict the match to a particular protocol. Possible
276 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
277 no proto qualifier, all protocols consistent with the type are
278 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
279 (except the latter is not legal syntax), `net bar' means `(ip or
280 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
282 [`fddi' is actually an alias for `ether'; the parser treats them
283 identically as meaning ``the data link level used on the specified
284 network interface.'' FDDI headers contain Ethernet-like source
285 and destination addresses, and often contain Ethernet-like packet
286 types, so you can filter on these FDDI fields just as with the
287 analogous Ethernet fields. FDDI headers also contain other fields,
288 but you cannot name them explicitly in a filter expression.]
290 In addition to the above, there are some special `primitive' keywords
291 that don't follow the pattern:
296 and arithmetic expressions. All of these are described below.
298 More complex filter expressions are built up by using the words
303 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
304 To save typing, identical qualifier lists can be omitted. E.g.,
305 `tcp dst port ftp or ftp-data or domain' is exactly the same as
306 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
308 Allowable primitives are:
309 .IP "\fBdst host \fIhost\fR"
310 True if the IP destination field of the packet is \fIhost\fP,
311 which may be either an address or a name.
312 .IP "\fBsrc host \fIhost\fR"
313 True if the IP source field of the packet is \fIhost\fP.
314 .IP "\fBhost \fIhost\fP
315 True if either the IP source or destination of the packet is \fIhost\fP.
316 Any of the above host expressions can be prepended with the keywords,
317 \fBip\fP, \fBarp\fP, or \fBrarp\fP as in:
320 \fBip host \fIhost\fR
323 which is equivalent to:
326 \fBether proto \fI\\ip\fB and host \fIhost\fR
329 If \fIhost\fR is a name with multiple IP addresses, each address will
330 be checked for a match.
331 .IP "\fBether dst \fIehost\fP
332 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
333 may be either a name from /etc/ethers or a number (see
336 .IP "\fBether src \fIehost\fP
337 True if the ethernet source address is \fIehost\fP.
338 .IP "\fBether host \fIehost\fP
339 True if either the ethernet source or destination address is \fIehost\fP.
340 .IP "\fBgateway\fP \fIhost\fP
341 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
342 source or destination address was \fIhost\fP but neither the IP source
343 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
344 must be found in both /etc/hosts and /etc/ethers. (An equivalent
348 \fBether host \fIehost \fBand not host \fIhost\fR
351 which can be used with either names or numbers for \fIhost / ehost\fP.)
352 .IP "\fBdst net \fInet\fR"
353 True if the IP destination address of the packet has a network
354 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
355 or a network number (see \fInetworks(4)\fP for details).
356 .IP "\fBsrc net \fInet\fR"
357 True if the IP source address of the packet has a network
359 .IP "\fBnet \fInet\fR"
360 True if either the IP source or destination address of the packet has a network
362 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
363 True if the IP address matches \fInet\fR with the specific netmask.
364 May be qualified with \fBsrc\fR or \fBdst\fR.
365 .IP "\fBnet \fInet\fR/\fIlen\fR"
366 True if the IP address matches \fInet\fR a netmask \fIlen\fR bits wide.
367 May be qualified with \fBsrc\fR or \fBdst\fR.
368 .IP "\fBdst port \fIport\fR"
369 True if the packet is ip/tcp or ip/udp and has a
370 destination port value of \fIport\fP.
371 The \fIport\fP can be a number or a name used in /etc/services (see
375 If a name is used, both the port
376 number and protocol are checked. If a number or ambiguous name is used,
377 only the port number is checked (e.g., \fBdst port 513\fR will print both
378 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
379 both tcp/domain and udp/domain traffic).
380 .IP "\fBsrc port \fIport\fR"
381 True if the packet has a source port value of \fIport\fP.
382 .IP "\fBport \fIport\fR"
383 True if either the source or destination port of the packet is \fIport\fP.
384 Any of the above port expressions can be prepended with the keywords,
385 \fBtcp\fP or \fBudp\fP, as in:
388 \fBtcp src port \fIport\fR
391 which matches only tcp packets whose source port is \fIport\fP.
392 .IP "\fBless \fIlength\fR"
393 True if the packet has a length less than or equal to \fIlength\fP.
394 This is equivalent to:
397 \fBlen <= \fIlength\fP.
400 .IP "\fBgreater \fIlength\fR"
401 True if the packet has a length greater than or equal to \fIlength\fP.
402 This is equivalent to:
405 \fBlen >= \fIlength\fP.
408 .IP "\fBip proto \fIprotocol\fR"
409 True if the packet is an ip packet (see
411 of protocol type \fIprotocol\fP.
412 \fIProtocol\fP can be a number or one of the names
413 \fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
414 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
415 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
416 .IP "\fBether broadcast\fR"
417 True if the packet is an ethernet broadcast packet. The \fIether\fP
419 .IP "\fBip broadcast\fR"
420 True if the packet is an IP broadcast packet. It checks for both
421 the all-zeroes and all-ones broadcast conventions, and looks up
422 the local subnet mask.
423 .IP "\fBether multicast\fR"
424 True if the packet is an ethernet multicast packet. The \fIether\fP
426 This is shorthand for `\fBether[0] & 1 != 0\fP'.
427 .IP "\fBip multicast\fR"
428 True if the packet is an IP multicast packet.
429 .IP "\fBether proto \fIprotocol\fR"
430 True if the packet is of ether type \fIprotocol\fR.
431 \fIProtocol\fP can be a number or a name like
432 \fIip\fP, \fIarp\fP, or \fIrarp\fP.
433 Note these identifiers are also keywords
434 and must be escaped via backslash (\\).
435 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
436 protocol identification comes from the 802.2 Logical Link Control
437 (LLC) header, which is usually layered on top of the FDDI header.
438 \fITcpdump\fP assumes, when filtering on the protocol identifier,
439 that all FDDI packets include an LLC header, and that the LLC header
440 is in so-called SNAP format.]
441 .IP "\fBdecnet src \fIhost\fR"
442 True if the DECNET source address is
444 which may be an address of the form ``10.123'', or a DECNET host
445 name. [DECNET host name support is only available on Ultrix systems
446 that are configured to run DECNET.]
447 .IP "\fBdecnet dst \fIhost\fR"
448 True if the DECNET destination address is
450 .IP "\fBdecnet host \fIhost\fR"
451 True if either the DECNET source or destination address is
453 .IP "\fBip\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR"
457 \fBether proto \fIp\fR
460 where \fIp\fR is one of the above protocols.
461 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
465 \fBether proto \fIp\fR
468 where \fIp\fR is one of the above protocols.
470 \fItcpdump\fP does not currently know how to parse these protocols.
471 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
478 where \fIp\fR is one of the above protocols.
479 .IP "\fIexpr relop expr\fR"
480 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
481 and \fIexpr\fR is an arithmetic expression composed of integer constants
482 (expressed in standard C syntax), the normal binary operators
483 [+, -, *, /, &, |], a length operator, and special packet data accessors.
485 data inside the packet, use the following syntax:
488 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
491 \fIProto\fR is one of \fBether, fddi,
492 ip, arp, rarp, tcp, udp, \fRor \fBicmp\fR, and
493 indicates the protocol layer for the index operation.
494 The byte offset, relative to the indicated protocol layer, is
496 \fISize\fR is optional and indicates the number of bytes in the
497 field of interest; it can be either one, two, or four, and defaults to one.
498 The length operator, indicated by the keyword \fBlen\fP, gives the
499 length of the packet.
501 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
502 The expression `\fBip[0] & 0xf != 5\fP'
503 catches all IP packets with options. The expression
504 `\fBip[6:2] & 0x1fff = 0\fP'
505 catches only unfragmented datagrams and frag zero of fragmented datagrams.
506 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
508 For instance, \fBtcp[0]\fP always means the first
509 byte of the TCP \fIheader\fP, and never means the first byte of an
510 intervening fragment.
512 Primitives may be combined using:
514 A parenthesized group of primitives and operators
515 (parentheses are special to the Shell and must be escaped).
517 Negation (`\fB!\fP' or `\fBnot\fP').
519 Concatenation (`\fB&&\fP' or `\fBand\fP').
521 Alternation (`\fB||\fP' or `\fBor\fP').
523 Negation has highest precedence.
524 Alternation and concatenation have equal precedence and associate
525 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
526 are now required for concatenation.
528 If an identifier is given without a keyword, the most recent keyword
533 \fBnot host vs and ace\fR
539 \fBnot host vs and host ace\fR
542 which should not be confused with
545 \fBnot ( host vs or ace )\fR
549 Expression arguments can be passed to tcpdump as either a single argument
550 or as multiple arguments, whichever is more convenient.
551 Generally, if the expression contains Shell metacharacters, it is
552 easier to pass it as a single, quoted argument.
553 Multiple arguments are concatenated with spaces before being parsed.
556 To print all packets arriving at or departing from \fIsundown\fP:
559 \fBtcpdump host sundown\fP
563 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
566 \fBtcpdump host helios and \\( hot or ace \\)\fP
570 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
573 \fBtcpdump ip host ace and not helios\fP
577 To print all traffic between local hosts and hosts at Berkeley:
581 tcpdump net ucb-ether
585 To print all ftp traffic through internet gateway \fIsnup\fP:
586 (note that the expression is quoted to prevent the shell from
587 (mis-)interpreting the parentheses):
591 tcpdump 'gateway snup and (port ftp or ftp-data)'
595 To print traffic neither sourced from nor destined for local hosts
596 (if you gateway to one other net, this stuff should never make it
597 onto your local net).
601 tcpdump ip and not net \fIlocalnet\fP
605 To print the start and end packets (the SYN and FIN packets) of each
606 TCP conversation that involves a non-local host.
610 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
614 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
618 tcpdump 'gateway snup and ip[2:2] > 576'
622 To print IP broadcast or multicast packets that were
624 sent via ethernet broadcast or multicast:
628 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
632 To print all ICMP packets that are not echo requests/replies (i.e., not
637 tcpdump 'icmp[0] != 8 and icmp[0] != 0"
642 The output of \fItcpdump\fP is protocol dependent. The following
643 gives a brief description and examples of most of the formats.
651 If the '-e' option is given, the link level header is printed out.
652 On ethernets, the source and destination addresses, protocol,
653 and packet length are printed.
655 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
656 the `frame control' field, the source and destination addresses,
657 and the packet length. (The `frame control' field governs the
658 interpretation of the rest of the packet. Normal packets (such
659 as those containing IP datagrams) are `async' packets, with a priority
660 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
661 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
662 the LLC header is printed if it is \fInot\fR an ISO datagram or a
663 so-called SNAP packet.
665 \fI(N.B.: The following description assumes familiarity with
666 the SLIP compression algorithm described in RFC-1144.)\fP
668 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
669 packet type, and compression information are printed out.
670 The packet type is printed first.
671 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
672 No further link information is printed for \fIip\fR packets.
673 For TCP packets, the connection identifier is printed following the type.
674 If the packet is compressed, its encoded header is printed out.
675 The special cases are printed out as
676 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
677 the sequence number (or sequence number and ack) has changed.
678 If it is not a special case,
679 zero or more changes are printed.
680 A change is indicated by U (urgent pointer), W (window), A (ack),
681 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
683 Finally, the amount of data in the packet and compressed header length
686 For example, the following line shows an outbound compressed TCP packet,
687 with an implicit connection identifier; the ack has changed by 6,
688 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
689 data and 6 bytes of compressed header:
692 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
698 Arp/rarp output shows the type of request and its arguments. The
699 format is intended to be self explanatory.
700 Here is a short sample taken from the start of an `rlogin' from
701 host \fIrtsg\fP to host \fIcsam\fP:
705 \f(CWarp who-has csam tell rtsg
706 arp reply csam is-at CSAM\fP
710 The first line says that rtsg sent an arp packet asking
711 for the ethernet address of internet host csam. Csam
712 replies with its ethernet address (in this example, ethernet addresses
713 are in caps and internet addresses in lower case).
715 This would look less redundant if we had done \fBtcpdump \-n\fP:
719 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
720 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
724 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
725 broadcast and the second is point-to-point would be visible:
729 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
730 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fP
734 For the first packet this says the ethernet source address is RTSG, the
735 destination is the ethernet broadcast address, the type field
736 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
740 \fI(N.B.:The following description assumes familiarity with
741 the TCP protocol described in RFC-793. If you are not familiar
742 with the protocol, neither this description nor tcpdump will
743 be of much use to you.)\fP
745 The general format of a tcp protocol line is:
749 \fIsrc > dst: flags data-seqno ack window urgent options\fP
753 \fISrc\fP and \fIdst\fP are the source and destination IP
754 addresses and ports. \fIFlags\fP are some combination of S (SYN),
755 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
756 \fIData-seqno\fP describes the portion of sequence space covered
757 by the data in this packet (see example below).
758 \fIAck\fP is sequence number of the next data expected the other
759 direction on this connection.
760 \fIWindow\fP is the number of bytes of receive buffer space available
761 the other direction on this connection.
762 \fIUrg\fP indicates there is `urgent' data in the packet.
763 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
765 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
766 depend on the contents of the packet's tcp protocol header and
767 are output only if appropriate.
769 Here is the opening portion of an rlogin from host \fIrtsg\fP to
774 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
775 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
776 rtsg.1023 > csam.login: . ack 1 win 4096
777 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
778 csam.login > rtsg.1023: . ack 2 win 4096
779 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
780 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
781 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
782 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fP\s+2
786 The first line says that tcp port 1023 on rtsg sent a packet
788 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
789 The packet sequence number was 768512 and it contained no data.
790 (The notation is `first:last(nbytes)' which means `sequence
792 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
793 There was no piggy-backed ack, the available receive window was 4096
794 bytes and there was a max-segment-size option requesting an mss of
797 Csam replies with a similar packet except it includes a piggy-backed
798 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
800 The packet contained no data so there is no data sequence number.
801 Note that the ack sequence
802 number is a small integer (1). The first time \fBtcpdump\fP sees a
803 tcp `conversation', it prints the sequence number from the packet.
804 On subsequent packets of the conversation, the difference between
805 the current packet's sequence number and this initial sequence number
806 is printed. This means that sequence numbers after the
807 first can be interpreted
808 as relative byte positions in the conversation's data stream (with the
809 first data byte each direction being `1'). `-S' will override this
810 feature, causing the original sequence numbers to be output.
812 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
813 in the rtsg \(-> csam side of the conversation).
814 The PUSH flag is set in the packet.
815 On the 7th line, csam says it's received data sent by rtsg up to
816 but not including byte 21. Most of this data is apparently sitting in the
817 socket buffer since csam's receive window has gotten 19 bytes smaller.
818 Csam also sends one byte of data to rtsg in this packet.
819 On the 8th and 9th lines,
820 csam sends two bytes of urgent, pushed data to rtsg.
822 If the snapshot was small enough that \fBtcpdump\fP didn't capture
823 the full TCP header, it interprets as much of the header as it can
824 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
825 be interpreted. If the header contains a bogus option (one with a length
826 that's either too small or beyond the end of the header), tcpdump reports
827 it as ``[\fIbad opt\fP]'' and does not interpret any further options (since
828 it's impossible to tell where they start). If the header length indicates
829 options are present but the IP datagram length is not long enough for the
830 options to actually be there, tcpdump reports it as ``[\fIbad hdr length\fP]''.
835 UDP format is illustrated by this rwho packet:
839 \f(CWactinide.who > broadcast.who: udp 84\fP
843 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
844 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
845 broadcast address. The packet contained 84 bytes of user data.
847 Some UDP services are recognized (from the source or destination
848 port number) and the higher level protocol information printed.
849 In particular, Domain Name service requests (RFC-1034/1035) and Sun
850 RPC calls (RFC-1050) to NFS.
852 UDP Name Server Requests
854 \fI(N.B.:The following description assumes familiarity with
855 the Domain Service protocol described in RFC-1035. If you are not familiar
856 with the protocol, the following description will appear to be written
859 Name server requests are formatted as
863 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
865 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fP
869 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
870 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
871 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
872 was set. The query length was 37 bytes, not including the UDP and
873 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
874 so the op field was omitted. If the op had been anything else, it would
875 have been printed between the `3' and the `+'.
876 Similarly, the qclass was the normal one,
877 \fIC_IN\fP, and omitted. Any other qclass would have been printed
878 immediately after the `A'.
880 A few anomalies are checked and may result in extra fields enclosed in
881 square brackets: If a query contains an answer, name server or
887 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
888 is the appropriate count.
889 If any of the response bits are set (AA, RA or rcode) or any of the
890 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
891 is printed, where \fIx\fP is the hex value of header bytes two and three.
893 UDP Name Server Responses
895 Name server responses are formatted as
899 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
901 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
902 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fP
906 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
907 with 3 answer records, 3 name server records and 7 authority records.
908 The first answer record is type A (address) and its data is internet
909 address 128.32.137.3. The total size of the response was 273 bytes,
910 excluding UDP and IP headers. The op (Query) and response code
911 (NoError) were omitted, as was the class (C_IN) of the A record.
913 In the second example, \fIhelios\fP responds to query 2 with a
914 response code of non-existent domain (NXDomain) with no answers,
915 one name server and no authority records. The `*' indicates that
916 the \fIauthoritative answer\fP bit was set. Since there were no
917 answers, no type, class or data were printed.
919 Other flag characters that might appear are `\-' (recursion available,
920 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
921 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
924 Note that name server requests and responses tend to be large and the
925 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
926 to print. Use the \fB\-s\fP flag to increase the snaplen if you
927 need to seriously investigate name server traffic. `\fB\-s 128\fP'
928 has worked well for me.
931 NFS Requests and Replies
933 Sun NFS (Network File System) requests and replies are printed as:
937 \fIsrc.xid > dst.nfs: len op args\fP
938 \fIsrc.nfs > dst.xid: reply stat len op results\fP
941 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
942 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
943 sushi.201b > wrl.nfs:
944 144 lookup fh 9,74/4096.6878 "xcolors"
945 wrl.nfs > sushi.201b:
946 reply ok 128 lookup fh 9,74/4134.3150
951 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
952 to \fIwrl\fP (note that the number following the src host is a
953 transaction id, \fInot\fP the source port). The request was 112 bytes,
954 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
955 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
956 (If one is lucky, as in this case, the file handle can be interpreted
957 as a major,minor device number pair, followed by the inode number and
959 \fIWrl\fP replies `ok' with the contents of the link.
961 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
962 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
963 depends on the operation type. The format is intended to be self
964 explanatory if read in conjunction with
965 an NFS protocol spec.
967 If the \-v (verbose) flag is given, additional information is printed.
973 sushi.1372a > wrl.nfs:
974 148 read fh 21,11/12.195 8192 bytes @ 24576
975 wrl.nfs > sushi.1372a:
976 reply ok 1472 read REG 100664 ids 417/0 sz 29388
981 (\-v also prints the IP header TTL, ID, and fragmentation fields,
982 which have been omitted from this example.) In the first line,
983 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
984 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
985 second line is the first fragment of the reply, and hence is only 1472
986 bytes long (the other bytes will follow in subsequent fragments, but
987 these fragments do not have NFS or even UDP headers and so might not be
988 printed, depending on the filter expression used). Because the \-v flag
989 is given, some of the file attributes (which are returned in addition
990 to the file data) are printed: the file type (``REG'', for regular file),
991 the file mode (in octal), the uid and gid, and the file size.
993 If the \-v flag is given more than once, even more details are printed.
995 Note that NFS requests are very large and much of the detail won't be printed
996 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
999 NFS reply packets do not explicitly identify the RPC operation. Instead,
1000 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1001 replies using the transaction ID. If a reply does not closely follow the
1002 corresponding request, it might not be parsable.
1004 KIP Appletalk (DDP in UDP)
1006 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1007 and dumped as DDP packets (i.e., all the UDP header information is
1008 discarded). The file
1010 is used to translate appletalk net and node numbers to names.
1011 Lines in this file have the form
1023 The first two lines give the names of appletalk networks. The third
1024 line gives the name of a particular host (a host is distinguished
1025 from a net by the 3rd octet in the number \-
1026 a net number \fImust\fP have two octets and a host number \fImust\fP
1027 have three octets.) The number and name should be separated by
1028 whitespace (blanks or tabs).
1031 file may contain blank lines or comment lines (lines starting with
1034 Appletalk addresses are printed in the form
1040 \f(CW144.1.209.2 > icsd-net.112.220
1041 office.2 > icsd-net.112.220
1042 jssmag.149.235 > icsd-net.2\fP
1048 doesn't exist or doesn't contain an entry for some appletalk
1049 host/net number, addresses are printed in numeric form.)
1050 In the first example, NBP (DDP port 2) on net 144.1 node 209
1051 is sending to whatever is listening on port 220 of net icsd node 112.
1052 The second line is the same except the full name of the source node
1053 is known (`office'). The third line is a send from port 235 on
1054 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1055 the broadcast address (255) is indicated by a net name with no host
1056 number \- for this reason it's a good idea to keep node names and
1057 net names distinct in /etc/atalk.names).
1059 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1060 packets have their contents interpreted. Other protocols just dump
1061 the protocol name (or number if no name is registered for the
1062 protocol) and packet size.
1064 \fBNBP packets\fP are formatted like the following examples:
1068 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1069 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1070 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fP\s+2
1074 The first line is a name lookup request for laserwriters sent by net icsd host
1075 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1076 The second line shows a reply for this request (note that it has the
1077 same id) from host jssmag.209 saying that it has a laserwriter
1078 resource named "RM1140" registered on port 250. The third line is
1079 another reply to the same request saying host techpit has laserwriter
1080 "techpit" registered on port 186.
1082 \fBATP packet\fP formatting is demonstrated by the following example:
1086 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1087 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1088 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1089 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1090 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1091 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1092 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1093 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1094 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1095 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1096 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1097 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1098 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1099 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fP\s+2
1103 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1104 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1105 is the value of the `userdata' field in the request.
1107 Helios responds with 8 512-byte packets. The `:digit' following the
1108 transaction id gives the packet sequence number in the transaction
1109 and the number in parens is the amount of data in the packet,
1110 excluding the atp header. The `*' on packet 7 indicates that the
1113 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1114 resends them then jssmag.209 releases the transaction. Finally,
1115 jssmag.209 initiates the next request. The `*' on the request
1116 indicates that XO (`exactly once') was \fInot\fP set.
1121 Fragmented Internet datagrams are printed as
1125 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1126 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1130 (The first form indicates there are more fragments. The second
1131 indicates this is the last fragment.)
1133 \fIId\fP is the fragment id. \fISize\fP is the fragment
1134 size (in bytes) excluding the IP header. \fIOffset\fP is this
1135 fragment's offset (in bytes) in the original datagram.
1137 The fragment information is output for each fragment. The first
1138 fragment contains the higher level protocol header and the frag
1139 info is printed after the protocol info. Fragments
1140 after the first contain no higher level protocol header and the
1141 frag info is printed after the source and destination addresses.
1142 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1143 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1147 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1148 arizona > rtsg: (frag 595a:204@328)
1149 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1153 There are a couple of things to note here: First, addresses in the
1154 2nd line don't include port numbers. This is because the TCP
1155 protocol information is all in the first fragment and we have no idea
1156 what the port or sequence numbers are when we print the later fragments.
1157 Second, the tcp sequence information in the first line is printed as if there
1158 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1159 the first frag and 204 in the second). If you are looking for holes
1160 in the sequence space or trying to match up acks
1161 with packets, this can fool you.
1163 A packet with the IP \fIdon't fragment\fP flag is marked with a
1164 trailing \fB(DF)\fP.
1168 By default, all output lines are preceded by a timestamp. The timestamp
1169 is the current clock time in the form
1175 and is as accurate as the kernel's clock.
1176 The timestamp reflects the time the kernel first saw the packet. No attempt
1177 is made to account for the time lag between when the
1178 ethernet interface removed the packet from the wire and when the kernel
1179 serviced the `new packet' interrupt.
1181 traffic(1C), nit(4P), bpf(4), pcap(3)
1185 Steven McCanne, all of the
1186 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1188 The current version is available via anonymous ftp:
1191 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1194 Please send bug reports to tcpdump@ee.lbl.gov.
1196 NIT doesn't let you watch your own outbound traffic, BPF will.
1197 We recommend that you use the latter.
1199 Some attempt should be made to reassemble IP fragments or, at least
1200 to compute the right length for the higher level protocol.
1202 Name server inverse queries are not dumped correctly: The (empty)
1203 question section is printed rather than real query in the answer
1204 section. Some believe that inverse queries are themselves a bug and
1205 prefer to fix the program generating them rather than tcpdump.
1207 Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
1209 Even if we were inclined to do anything to promote the use of
1210 Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
1211 networks so we'd would have no way of testing this code.
1213 A packet trace that crosses a daylight savings time change will give
1214 skewed time stamps (the time change is ignored).
1216 Filters expressions that manipulate FDDI headers assume that all FDDI
1217 packets are encapsulated Ethernet packets. This is true for IP, ARP,
1218 and DECNET Phase IV, but is not true for protocols such as ISO CLNS.
1219 Therefore, the filter may inadvertently accept certain packets that
1220 do not properly match the filter expression.
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