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25 .TH TCPDUMP 1 "30 June 1997"
27 tcpdump \- dump traffic on a network
77 \fITcpdump\fP prints out the headers of packets on a network interface
78 that match the boolean \fIexpression\fP.
80 .B Under SunOS with nit or bpf:
83 you must have read access to
87 .B Under Solaris with dlpi:
88 You must have read access to the network pseudo device, e.g.
90 .B Under HP-UX with dlpi:
91 You must be root or it must be installed setuid to root.
92 .B Under IRIX with snoop:
93 You must be root or it must be installed setuid to root.
95 You must be root or it must be installed setuid to root.
96 .B Under Ultrix and Digital UNIX:
97 Once the super-user has enabled promiscuous-mode operation using
102 You must have read access to
107 Attempt to convert network and broadcast addresses to names.
110 Exit after receiving \fIcount\fP packets.
113 Dump the compiled packet-matching code in a human readable form to
114 standard output and stop.
117 Dump packet-matching code as a
122 Dump packet-matching code as decimal numbers (preceded with a count).
125 Print the link-level header on each dump line.
128 Print `foreign' internet addresses numerically rather than symbolically
129 (this option is intended to get around serious brain damage in
130 Sun's yp server \(em usually it hangs forever translating non-local
134 Use \fIfile\fP as input for the filter expression.
135 An additional expression given on the command line is ignored.
138 Listen on \fIinterface\fP.
139 If unspecified, \fItcpdump\fP searches the system interface list for the
140 lowest numbered, configured up interface (excluding loopback).
141 Ties are broken by choosing the earliest match.
144 Make stdout line buffered. Useful if you want to see the data
145 while capturing it. E.g.,
147 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
148 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
151 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
154 Don't print domain name qualification of host names. E.g.,
155 if you give this flag then \fItcpdump\fP will print ``nic''
156 instead of ``nic.ddn.mil''.
159 Load SMI MIB module definitions from file \fImodule\fR. This option
160 can be used several times to load several MIB modules into tcpdump.
163 Do not run the packet-matching code optimizer. This is useful only
164 if you suspect a bug in the optimizer.
167 \fIDon't\fP put the interface
168 into promiscuous mode. Note that the interface might be in promiscuous
169 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
170 `ether host {local-hw-addr} or ether broadcast'.
173 Quick (quiet?) output. Print less protocol information so output
177 Read packets from \fIfile\fR (which was created with the -w option).
178 Standard input is used if \fIfile\fR is ``-''.
181 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
182 default of 68 (with SunOS's NIT, the minimum is actually 96).
183 68 bytes is adequate for IP, ICMP, TCP
184 and UDP but may truncate protocol information from name server and NFS
185 packets (see below). Packets truncated because of a limited snapshot
186 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
187 is the name of the protocol level at which the truncation has occurred.
188 Note that taking larger snapshots both increases
189 the amount of time it takes to process packets and, effectively,
190 decreases the amount of packet buffering. This may cause packets to be
191 lost. You should limit \fIsnaplen\fP to the smallest number that will
192 capture the protocol information you're interested in.
195 Force packets selected by "\fIexpression\fP" to be interpreted the
196 specified \fItype\fR. Currently known types are
197 \fBrpc\fR (Remote Procedure Call),
198 \fBrtp\fR (Real-Time Applications protocol),
199 \fBrtcp\fR (Real-Time Applications control protocol),
200 \fBsnmp\fR (Simple Network Management Protocol),
201 \fBvat\fR (Visual Audio Tool),
203 \fBwb\fR (distributed White Board).
206 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
207 If specified, \fItcpdump\fP will not print replay prevention field.
208 Since there is no protocol version field in ESP/AH specification,
209 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
212 Print absolute, rather than relative, TCP sequence numbers.
215 \fIDon't\fP print a timestamp on each dump line.
218 Print an unformatted timestamp on each dump line.
221 (Slightly more) verbose output. For example, the time to live
222 and type of service information in an IP packet is printed.
225 Even more verbose output. For example, additional fields are
226 printed from NFS reply packets.
229 Even more verbose output. For example,
230 telnet \fBSB\fP ... \fBSE\fP options
231 are printed in full. With
233 telnet options are printed in hex as well.
236 Write the raw packets to \fIfile\fR rather than parsing and printing
237 them out. They can later be printed with the \-r option.
238 Standard output is used if \fIfile\fR is ``-''.
241 Print each packet (minus its link level header) in hex.
242 The smaller of the entire packet or
244 bytes will be printed.
247 When printing hex, print ascii too. Thus if
249 is also set, the packet is printed in hex/ascii.
250 This is very handy for analysing new protocols.
253 is not also set, some parts of some packets may be printed
255 .IP "\fI expression\fP"
257 selects which packets will be dumped. If no \fIexpression\fP
258 is given, all packets on the net will be dumped. Otherwise,
259 only packets for which \fIexpression\fP is `true' will be dumped.
261 The \fIexpression\fP consists of one or more
263 Primitives usually consist of an
265 (name or number) preceded by one or more qualifiers. There are three
266 different kinds of qualifier:
268 qualifiers say what kind of thing the id name or number refers to.
274 E.g., `host foo', `net 128.3', `port 20'. If there is no type
279 qualifiers specify a particular transfer direction to and/or from
281 Possible directions are
288 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
289 there is no dir qualifier,
292 For `null' link layers (i.e. point to point protocols such as slip) the
296 qualifiers can be used to specify a desired direction.
298 qualifiers restrict the match to a particular protocol. Possible
319 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
320 no proto qualifier, all protocols consistent with the type are
321 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
322 (except the latter is not legal syntax), `net bar' means `(ip or
323 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
325 [`fddi' is actually an alias for `ether'; the parser treats them
326 identically as meaning ``the data link level used on the specified
327 network interface.'' FDDI headers contain Ethernet-like source
328 and destination addresses, and often contain Ethernet-like packet
329 types, so you can filter on these FDDI fields just as with the
330 analogous Ethernet fields. FDDI headers also contain other fields,
331 but you cannot name them explicitly in a filter expression.]
333 In addition to the above, there are some special `primitive' keywords
334 that don't follow the pattern:
339 and arithmetic expressions. All of these are described below.
341 More complex filter expressions are built up by using the words
346 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
347 To save typing, identical qualifier lists can be omitted. E.g.,
348 `tcp dst port ftp or ftp-data or domain' is exactly the same as
349 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
351 Allowable primitives are:
352 .IP "\fBdst host \fIhost\fR"
353 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
354 which may be either an address or a name.
355 .IP "\fBsrc host \fIhost\fR"
356 True if the IPv4/v6 source field of the packet is \fIhost\fP.
357 .IP "\fBhost \fIhost\fP
358 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
359 Any of the above host expressions can be prepended with the keywords,
360 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
363 \fBip host \fIhost\fR
366 which is equivalent to:
369 \fBether proto \fI\\ip\fB and host \fIhost\fR
372 If \fIhost\fR is a name with multiple IP addresses, each address will
373 be checked for a match.
374 .IP "\fBether dst \fIehost\fP
375 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
376 may be either a name from /etc/ethers or a number (see
379 .IP "\fBether src \fIehost\fP
380 True if the ethernet source address is \fIehost\fP.
381 .IP "\fBether host \fIehost\fP
382 True if either the ethernet source or destination address is \fIehost\fP.
383 .IP "\fBgateway\fP \fIhost\fP
384 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
385 source or destination address was \fIhost\fP but neither the IP source
386 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
387 must be found in both /etc/hosts and /etc/ethers. (An equivalent
391 \fBether host \fIehost \fBand not host \fIhost\fR
394 which can be used with either names or numbers for \fIhost / ehost\fP.)
395 This syntax does not work in IPv6-enabled configuration at this moment.
396 .IP "\fBdst net \fInet\fR"
397 True if the IPv4/v6 destination address of the packet has a network
398 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
399 or a network number (see \fInetworks(4)\fP for details).
400 .IP "\fBsrc net \fInet\fR"
401 True if the IPv4/v6 source address of the packet has a network
403 .IP "\fBnet \fInet\fR"
404 True if either the IPv4/v6 source or destination address of the packet has a network
406 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
407 True if the IP address matches \fInet\fR with the specific netmask.
408 May be qualified with \fBsrc\fR or \fBdst\fR.
409 Note that this syntax is not valid for IPv6 \fInet\fR.
410 .IP "\fBnet \fInet\fR/\fIlen\fR"
411 True if the IPv4/v6 address matches \fInet\fR a netmask \fIlen\fR bits wide.
412 May be qualified with \fBsrc\fR or \fBdst\fR.
413 .IP "\fBdst port \fIport\fR"
414 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
415 destination port value of \fIport\fP.
416 The \fIport\fP can be a number or a name used in /etc/services (see
420 If a name is used, both the port
421 number and protocol are checked. If a number or ambiguous name is used,
422 only the port number is checked (e.g., \fBdst port 513\fR will print both
423 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
424 both tcp/domain and udp/domain traffic).
425 .IP "\fBsrc port \fIport\fR"
426 True if the packet has a source port value of \fIport\fP.
427 .IP "\fBport \fIport\fR"
428 True if either the source or destination port of the packet is \fIport\fP.
429 Any of the above port expressions can be prepended with the keywords,
430 \fBtcp\fP or \fBudp\fP, as in:
433 \fBtcp src port \fIport\fR
436 which matches only tcp packets whose source port is \fIport\fP.
437 .IP "\fBless \fIlength\fR"
438 True if the packet has a length less than or equal to \fIlength\fP.
439 This is equivalent to:
442 \fBlen <= \fIlength\fP.
445 .IP "\fBgreater \fIlength\fR"
446 True if the packet has a length greater than or equal to \fIlength\fP.
447 This is equivalent to:
450 \fBlen >= \fIlength\fP.
453 .IP "\fBip proto \fIprotocol\fR"
454 True if the packet is an ip packet (see
456 of protocol type \fIprotocol\fP.
457 \fIProtocol\fP can be a number or one of the names
458 \fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
459 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
460 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
461 Note that this primitive does not chase protocol header chain.
462 .IP "\fBip6 proto \fIprotocol\fR"
463 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
464 Note that this primitive does not chase protocol header chain.
465 .IP "\fBip6 protochain \fIprotocol\fR"
466 True if the packet is IPv6 packet,
467 and contains protocol header with type \fIprotocol\fR
468 in its protocol header chain.
472 \fBip6 protochain 6\fR
475 matches any IPv6 packet with TCP protocol header in the protocol header chain.
476 The packet may contain, for example,
477 authentication header, routing header, or hop-by-hop option header,
478 between IPv6 header and TCP header.
479 The BPF code emitted by this primitive is complex and
480 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
481 so this can be somewhat slow.
482 .IP "\fBip protochain \fIprotocol\fR"
483 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
484 .IP "\fBether broadcast\fR"
485 True if the packet is an ethernet broadcast packet. The \fIether\fP
487 .IP "\fBip broadcast\fR"
488 True if the packet is an IP broadcast packet. It checks for both
489 the all-zeroes and all-ones broadcast conventions, and looks up
490 the local subnet mask.
491 .IP "\fBether multicast\fR"
492 True if the packet is an ethernet multicast packet. The \fIether\fP
494 This is shorthand for `\fBether[0] & 1 != 0\fP'.
495 .IP "\fBip multicast\fR"
496 True if the packet is an IP multicast packet.
497 .IP "\fBip6 multicast\fR"
498 True if the packet is an IPv6 multicast packet.
499 .IP "\fBether proto \fIprotocol\fR"
500 True if the packet is of ether type \fIprotocol\fR.
501 \fIProtocol\fP can be a number or a name like
502 \fIip\fP, \fIip6\fP, \fIarp\fP, or \fIrarp\fP.
503 Note these identifiers are also keywords
504 and must be escaped via backslash (\\).
505 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
506 protocol identification comes from the 802.2 Logical Link Control
507 (LLC) header, which is usually layered on top of the FDDI header.
508 \fITcpdump\fP assumes, when filtering on the protocol identifier,
509 that all FDDI packets include an LLC header, and that the LLC header
510 is in so-called SNAP format.]
511 .IP "\fBdecnet src \fIhost\fR"
512 True if the DECNET source address is
514 which may be an address of the form ``10.123'', or a DECNET host
515 name. [DECNET host name support is only available on Ultrix systems
516 that are configured to run DECNET.]
517 .IP "\fBdecnet dst \fIhost\fR"
518 True if the DECNET destination address is
520 .IP "\fBdecnet host \fIhost\fR"
521 True if either the DECNET source or destination address is
523 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR, \fBiso\fR"
527 \fBether proto \fIp\fR
530 where \fIp\fR is one of the above protocols.
531 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
535 \fBether proto \fIp\fR
538 where \fIp\fR is one of the above protocols.
540 \fItcpdump\fP does not currently know how to parse these protocols.
541 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
545 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
548 where \fIp\fR is one of the above protocols.
549 .IP "\fBesis\fR, \fBisis\fR"
556 where \fIp\fR is one of the above protocols.
557 Note that \fItcpdump\fR does an incomplete job of parsing these protocols.
558 .IP "\fIexpr relop expr\fR"
559 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
560 and \fIexpr\fR is an arithmetic expression composed of integer constants
561 (expressed in standard C syntax), the normal binary operators
562 [+, -, *, /, &, |], a length operator, and special packet data accessors.
564 data inside the packet, use the following syntax:
567 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
570 \fIProto\fR is one of \fBether, fddi,
571 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
572 indicates the protocol layer for the index operation.
573 Note that \fItcp, udp\fR and other upper-layer protocol types only
574 apply to IPv4, not IPv6 (this will be fixed in the future).
575 The byte offset, relative to the indicated protocol layer, is
577 \fISize\fR is optional and indicates the number of bytes in the
578 field of interest; it can be either one, two, or four, and defaults to one.
579 The length operator, indicated by the keyword \fBlen\fP, gives the
580 length of the packet.
582 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
583 The expression `\fBip[0] & 0xf != 5\fP'
584 catches all IP packets with options. The expression
585 `\fBip[6:2] & 0x1fff = 0\fP'
586 catches only unfragmented datagrams and frag zero of fragmented datagrams.
587 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
589 For instance, \fBtcp[0]\fP always means the first
590 byte of the TCP \fIheader\fP, and never means the first byte of an
591 intervening fragment.
593 Primitives may be combined using:
595 A parenthesized group of primitives and operators
596 (parentheses are special to the Shell and must be escaped).
598 Negation (`\fB!\fP' or `\fBnot\fP').
600 Concatenation (`\fB&&\fP' or `\fBand\fP').
602 Alternation (`\fB||\fP' or `\fBor\fP').
604 Negation has highest precedence.
605 Alternation and concatenation have equal precedence and associate
606 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
607 are now required for concatenation.
609 If an identifier is given without a keyword, the most recent keyword
614 \fBnot host vs and ace\fR
620 \fBnot host vs and host ace\fR
623 which should not be confused with
626 \fBnot ( host vs or ace )\fR
630 Expression arguments can be passed to tcpdump as either a single argument
631 or as multiple arguments, whichever is more convenient.
632 Generally, if the expression contains Shell metacharacters, it is
633 easier to pass it as a single, quoted argument.
634 Multiple arguments are concatenated with spaces before being parsed.
637 To print all packets arriving at or departing from \fIsundown\fP:
640 \fBtcpdump host sundown\fP
644 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
647 \fBtcpdump host helios and \\( hot or ace \\)\fP
651 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
654 \fBtcpdump ip host ace and not helios\fP
658 To print all traffic between local hosts and hosts at Berkeley:
662 tcpdump net ucb-ether
666 To print all ftp traffic through internet gateway \fIsnup\fP:
667 (note that the expression is quoted to prevent the shell from
668 (mis-)interpreting the parentheses):
672 tcpdump 'gateway snup and (port ftp or ftp-data)'
676 To print traffic neither sourced from nor destined for local hosts
677 (if you gateway to one other net, this stuff should never make it
678 onto your local net).
682 tcpdump ip and not net \fIlocalnet\fP
686 To print the start and end packets (the SYN and FIN packets) of each
687 TCP conversation that involves a non-local host.
691 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
695 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
699 tcpdump 'gateway snup and ip[2:2] > 576'
703 To print IP broadcast or multicast packets that were
705 sent via ethernet broadcast or multicast:
709 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
713 To print all ICMP packets that are not echo requests/replies (i.e., not
718 tcpdump 'icmp[0] != 8 and icmp[0] != 0'
723 The output of \fItcpdump\fP is protocol dependent. The following
724 gives a brief description and examples of most of the formats.
732 If the '-e' option is given, the link level header is printed out.
733 On ethernets, the source and destination addresses, protocol,
734 and packet length are printed.
736 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
737 the `frame control' field, the source and destination addresses,
738 and the packet length. (The `frame control' field governs the
739 interpretation of the rest of the packet. Normal packets (such
740 as those containing IP datagrams) are `async' packets, with a priority
741 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
742 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
743 the LLC header is printed if it is \fInot\fR an ISO datagram or a
744 so-called SNAP packet.
746 \fI(N.B.: The following description assumes familiarity with
747 the SLIP compression algorithm described in RFC-1144.)\fP
749 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
750 packet type, and compression information are printed out.
751 The packet type is printed first.
752 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
753 No further link information is printed for \fIip\fR packets.
754 For TCP packets, the connection identifier is printed following the type.
755 If the packet is compressed, its encoded header is printed out.
756 The special cases are printed out as
757 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
758 the sequence number (or sequence number and ack) has changed.
759 If it is not a special case,
760 zero or more changes are printed.
761 A change is indicated by U (urgent pointer), W (window), A (ack),
762 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
764 Finally, the amount of data in the packet and compressed header length
767 For example, the following line shows an outbound compressed TCP packet,
768 with an implicit connection identifier; the ack has changed by 6,
769 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
770 data and 6 bytes of compressed header:
773 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
779 Arp/rarp output shows the type of request and its arguments. The
780 format is intended to be self explanatory.
781 Here is a short sample taken from the start of an `rlogin' from
782 host \fIrtsg\fP to host \fIcsam\fP:
786 \f(CWarp who-has csam tell rtsg
787 arp reply csam is-at CSAM\fP
791 The first line says that rtsg sent an arp packet asking
792 for the ethernet address of internet host csam. Csam
793 replies with its ethernet address (in this example, ethernet addresses
794 are in caps and internet addresses in lower case).
796 This would look less redundant if we had done \fBtcpdump \-n\fP:
800 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
801 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
805 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
806 broadcast and the second is point-to-point would be visible:
810 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
811 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fP
815 For the first packet this says the ethernet source address is RTSG, the
816 destination is the ethernet broadcast address, the type field
817 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
821 \fI(N.B.:The following description assumes familiarity with
822 the TCP protocol described in RFC-793. If you are not familiar
823 with the protocol, neither this description nor tcpdump will
824 be of much use to you.)\fP
826 The general format of a tcp protocol line is:
830 \fIsrc > dst: flags data-seqno ack window urgent options\fP
834 \fISrc\fP and \fIdst\fP are the source and destination IP
835 addresses and ports. \fIFlags\fP are some combination of S (SYN),
836 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
837 \fIData-seqno\fP describes the portion of sequence space covered
838 by the data in this packet (see example below).
839 \fIAck\fP is sequence number of the next data expected the other
840 direction on this connection.
841 \fIWindow\fP is the number of bytes of receive buffer space available
842 the other direction on this connection.
843 \fIUrg\fP indicates there is `urgent' data in the packet.
844 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
846 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
847 depend on the contents of the packet's tcp protocol header and
848 are output only if appropriate.
850 Here is the opening portion of an rlogin from host \fIrtsg\fP to
855 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
856 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
857 rtsg.1023 > csam.login: . ack 1 win 4096
858 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
859 csam.login > rtsg.1023: . ack 2 win 4096
860 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
861 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
862 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
863 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fP\s+2
867 The first line says that tcp port 1023 on rtsg sent a packet
869 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
870 The packet sequence number was 768512 and it contained no data.
871 (The notation is `first:last(nbytes)' which means `sequence
873 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
874 There was no piggy-backed ack, the available receive window was 4096
875 bytes and there was a max-segment-size option requesting an mss of
878 Csam replies with a similar packet except it includes a piggy-backed
879 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
881 The packet contained no data so there is no data sequence number.
882 Note that the ack sequence
883 number is a small integer (1). The first time \fBtcpdump\fP sees a
884 tcp `conversation', it prints the sequence number from the packet.
885 On subsequent packets of the conversation, the difference between
886 the current packet's sequence number and this initial sequence number
887 is printed. This means that sequence numbers after the
888 first can be interpreted
889 as relative byte positions in the conversation's data stream (with the
890 first data byte each direction being `1'). `-S' will override this
891 feature, causing the original sequence numbers to be output.
893 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
894 in the rtsg \(-> csam side of the conversation).
895 The PUSH flag is set in the packet.
896 On the 7th line, csam says it's received data sent by rtsg up to
897 but not including byte 21. Most of this data is apparently sitting in the
898 socket buffer since csam's receive window has gotten 19 bytes smaller.
899 Csam also sends one byte of data to rtsg in this packet.
900 On the 8th and 9th lines,
901 csam sends two bytes of urgent, pushed data to rtsg.
903 If the snapshot was small enough that \fBtcpdump\fP didn't capture
904 the full TCP header, it interprets as much of the header as it can
905 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
906 be interpreted. If the header contains a bogus option (one with a length
907 that's either too small or beyond the end of the header), tcpdump reports
908 it as ``[\fIbad opt\fP]'' and does not interpret any further options (since
909 it's impossible to tell where they start). If the header length indicates
910 options are present but the IP datagram length is not long enough for the
911 options to actually be there, tcpdump reports it as ``[\fIbad hdr length\fP]''.
916 UDP format is illustrated by this rwho packet:
920 \f(CWactinide.who > broadcast.who: udp 84\fP
924 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
925 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
926 broadcast address. The packet contained 84 bytes of user data.
928 Some UDP services are recognized (from the source or destination
929 port number) and the higher level protocol information printed.
930 In particular, Domain Name service requests (RFC-1034/1035) and Sun
931 RPC calls (RFC-1050) to NFS.
933 UDP Name Server Requests
935 \fI(N.B.:The following description assumes familiarity with
936 the Domain Service protocol described in RFC-1035. If you are not familiar
937 with the protocol, the following description will appear to be written
940 Name server requests are formatted as
944 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
946 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fP
950 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
951 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
952 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
953 was set. The query length was 37 bytes, not including the UDP and
954 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
955 so the op field was omitted. If the op had been anything else, it would
956 have been printed between the `3' and the `+'.
957 Similarly, the qclass was the normal one,
958 \fIC_IN\fP, and omitted. Any other qclass would have been printed
959 immediately after the `A'.
961 A few anomalies are checked and may result in extra fields enclosed in
962 square brackets: If a query contains an answer, name server or
968 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
969 is the appropriate count.
970 If any of the response bits are set (AA, RA or rcode) or any of the
971 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
972 is printed, where \fIx\fP is the hex value of header bytes two and three.
974 UDP Name Server Responses
976 Name server responses are formatted as
980 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
982 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
983 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fP
987 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
988 with 3 answer records, 3 name server records and 7 authority records.
989 The first answer record is type A (address) and its data is internet
990 address 128.32.137.3. The total size of the response was 273 bytes,
991 excluding UDP and IP headers. The op (Query) and response code
992 (NoError) were omitted, as was the class (C_IN) of the A record.
994 In the second example, \fIhelios\fP responds to query 2 with a
995 response code of non-existent domain (NXDomain) with no answers,
996 one name server and no authority records. The `*' indicates that
997 the \fIauthoritative answer\fP bit was set. Since there were no
998 answers, no type, class or data were printed.
1000 Other flag characters that might appear are `\-' (recursion available,
1001 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
1002 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1005 Note that name server requests and responses tend to be large and the
1006 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1007 to print. Use the \fB\-s\fP flag to increase the snaplen if you
1008 need to seriously investigate name server traffic. `\fB\-s 128\fP'
1009 has worked well for me.
1014 tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data
1015 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1016 NetBEUI SMB data is also done.
1018 By default a fairly minimal decode is done, with a much more detailed
1019 decode done if -v is used. Be warned that with -v a single SMB packet
1020 may take up a page or more, so only use -v if you really want all the
1023 If you are decoding SMB sessions containing unicode strings then you
1024 may wish to set the environment variable USE_UNICODE to 1. A patch to
1025 auto-detect unicode srings would be welcome.
1027 For information on SMB packet formats and what all te fields mean see
1028 www.cifs.org or the pub/samba/specs/ directory on your favourite
1029 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1033 NFS Requests and Replies
1035 Sun NFS (Network File System) requests and replies are printed as:
1039 \fIsrc.xid > dst.nfs: len op args\fP
1040 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1043 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1044 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1045 sushi.201b > wrl.nfs:
1046 144 lookup fh 9,74/4096.6878 "xcolors"
1047 wrl.nfs > sushi.201b:
1048 reply ok 128 lookup fh 9,74/4134.3150
1053 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1054 to \fIwrl\fP (note that the number following the src host is a
1055 transaction id, \fInot\fP the source port). The request was 112 bytes,
1056 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1057 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1058 (If one is lucky, as in this case, the file handle can be interpreted
1059 as a major,minor device number pair, followed by the inode number and
1061 \fIWrl\fP replies `ok' with the contents of the link.
1063 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1064 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1065 depends on the operation type. The format is intended to be self
1066 explanatory if read in conjunction with
1067 an NFS protocol spec.
1069 If the \-v (verbose) flag is given, additional information is printed.
1075 sushi.1372a > wrl.nfs:
1076 148 read fh 21,11/12.195 8192 bytes @ 24576
1077 wrl.nfs > sushi.1372a:
1078 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1083 (\-v also prints the IP header TTL, ID, and fragmentation fields,
1084 which have been omitted from this example.) In the first line,
1085 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1086 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1087 second line is the first fragment of the reply, and hence is only 1472
1088 bytes long (the other bytes will follow in subsequent fragments, but
1089 these fragments do not have NFS or even UDP headers and so might not be
1090 printed, depending on the filter expression used). Because the \-v flag
1091 is given, some of the file attributes (which are returned in addition
1092 to the file data) are printed: the file type (``REG'', for regular file),
1093 the file mode (in octal), the uid and gid, and the file size.
1095 If the \-v flag is given more than once, even more details are printed.
1097 Note that NFS requests are very large and much of the detail won't be printed
1098 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1101 NFS reply packets do not explicitly identify the RPC operation. Instead,
1102 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1103 replies using the transaction ID. If a reply does not closely follow the
1104 corresponding request, it might not be parsable.
1106 AFS Request and Replies
1108 Transarc AFS (Andrew File System) requests and replies are printed
1114 \fIsrc.sport > dst.dport: rx packet-type\fP
1115 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1116 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1119 elvis.7001 > pike.afsfs:
1120 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1121 new fid 536876964/1/1 ".newsrc"
1122 pike.afsfs > elvis.7001: rx data fs reply rename
1127 In the first line, host elvis sends a RX packet to pike. This was
1128 a RX data packet to the fs (fileserver) service, and is the start of
1129 an RPC call. The RPC call was a rename, with the old directory file id
1130 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1131 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1132 responds with a RPC reply to the rename call (which was successful, because
1133 it was a data packet and not an abort packet).
1135 In general, all AFS RPCs are decoded at least by RPC call name. Most
1136 AFS RPCs have at least some of the arguments decoded (generally only
1137 the `interesting' arguments, for some definition of interesting).
1139 The format is intended to be self-describing, but it will probably
1140 not be useful to people who are not familiar with the workings of
1143 If the -v (verbose) flag is given twice, additional information is printed,
1144 such as the the RX call ID, call number, sequence number, serial number,
1145 and the RX packet flags.
1147 If the -v flag is given again, the security index and service id are printed.
1149 Error codes are printed for abort packets, with the exception of Ubik
1150 beacon packets (because abort packets are used to signify a yes vote
1151 for the Ubik protocol).
1153 Note that AFS requests are very large and many of the arguments won't
1154 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1155 to watch AFS traffic.
1157 AFS reply packets do not explicitly identify the RPC operation. Instead,
1158 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1159 replies using the call number and service ID. If a reply does not closely
1161 corresponding request, it might not be parsable.
1164 KIP Appletalk (DDP in UDP)
1166 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1167 and dumped as DDP packets (i.e., all the UDP header information is
1168 discarded). The file
1170 is used to translate appletalk net and node numbers to names.
1171 Lines in this file have the form
1183 The first two lines give the names of appletalk networks. The third
1184 line gives the name of a particular host (a host is distinguished
1185 from a net by the 3rd octet in the number \-
1186 a net number \fImust\fP have two octets and a host number \fImust\fP
1187 have three octets.) The number and name should be separated by
1188 whitespace (blanks or tabs).
1191 file may contain blank lines or comment lines (lines starting with
1194 Appletalk addresses are printed in the form
1200 \f(CW144.1.209.2 > icsd-net.112.220
1201 office.2 > icsd-net.112.220
1202 jssmag.149.235 > icsd-net.2\fP
1208 doesn't exist or doesn't contain an entry for some appletalk
1209 host/net number, addresses are printed in numeric form.)
1210 In the first example, NBP (DDP port 2) on net 144.1 node 209
1211 is sending to whatever is listening on port 220 of net icsd node 112.
1212 The second line is the same except the full name of the source node
1213 is known (`office'). The third line is a send from port 235 on
1214 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1215 the broadcast address (255) is indicated by a net name with no host
1216 number \- for this reason it's a good idea to keep node names and
1217 net names distinct in /etc/atalk.names).
1219 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1220 packets have their contents interpreted. Other protocols just dump
1221 the protocol name (or number if no name is registered for the
1222 protocol) and packet size.
1224 \fBNBP packets\fP are formatted like the following examples:
1228 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1229 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1230 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fP\s+2
1234 The first line is a name lookup request for laserwriters sent by net icsd host
1235 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1236 The second line shows a reply for this request (note that it has the
1237 same id) from host jssmag.209 saying that it has a laserwriter
1238 resource named "RM1140" registered on port 250. The third line is
1239 another reply to the same request saying host techpit has laserwriter
1240 "techpit" registered on port 186.
1242 \fBATP packet\fP formatting is demonstrated by the following example:
1246 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1247 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1248 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1249 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1250 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1251 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1252 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1253 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1254 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1255 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1256 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1257 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1258 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1259 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fP\s+2
1263 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1264 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1265 is the value of the `userdata' field in the request.
1267 Helios responds with 8 512-byte packets. The `:digit' following the
1268 transaction id gives the packet sequence number in the transaction
1269 and the number in parens is the amount of data in the packet,
1270 excluding the atp header. The `*' on packet 7 indicates that the
1273 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1274 resends them then jssmag.209 releases the transaction. Finally,
1275 jssmag.209 initiates the next request. The `*' on the request
1276 indicates that XO (`exactly once') was \fInot\fP set.
1281 Fragmented Internet datagrams are printed as
1285 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1286 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1290 (The first form indicates there are more fragments. The second
1291 indicates this is the last fragment.)
1293 \fIId\fP is the fragment id. \fISize\fP is the fragment
1294 size (in bytes) excluding the IP header. \fIOffset\fP is this
1295 fragment's offset (in bytes) in the original datagram.
1297 The fragment information is output for each fragment. The first
1298 fragment contains the higher level protocol header and the frag
1299 info is printed after the protocol info. Fragments
1300 after the first contain no higher level protocol header and the
1301 frag info is printed after the source and destination addresses.
1302 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1303 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1307 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1308 arizona > rtsg: (frag 595a:204@328)
1309 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1313 There are a couple of things to note here: First, addresses in the
1314 2nd line don't include port numbers. This is because the TCP
1315 protocol information is all in the first fragment and we have no idea
1316 what the port or sequence numbers are when we print the later fragments.
1317 Second, the tcp sequence information in the first line is printed as if there
1318 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1319 the first frag and 204 in the second). If you are looking for holes
1320 in the sequence space or trying to match up acks
1321 with packets, this can fool you.
1323 A packet with the IP \fIdon't fragment\fP flag is marked with a
1324 trailing \fB(DF)\fP.
1328 By default, all output lines are preceded by a timestamp. The timestamp
1329 is the current clock time in the form
1335 and is as accurate as the kernel's clock.
1336 The timestamp reflects the time the kernel first saw the packet. No attempt
1337 is made to account for the time lag between when the
1338 ethernet interface removed the packet from the wire and when the kernel
1339 serviced the `new packet' interrupt.
1345 Steven McCanne, all of the
1346 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1348 The current version is available via anonymous ftp:
1351 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1354 IPv6/IPsec support is added by WIDE/KAME project.
1355 This program uses Eric Young's SSLeay library, under specific configuration.
1357 Please send bug reports to tcpdump@ee.lbl.gov.
1359 NIT doesn't let you watch your own outbound traffic, BPF will.
1360 We recommend that you use the latter.
1362 Some attempt should be made to reassemble IP fragments or, at least
1363 to compute the right length for the higher level protocol.
1365 Name server inverse queries are not dumped correctly: The (empty)
1366 question section is printed rather than real query in the answer
1367 section. Some believe that inverse queries are themselves a bug and
1368 prefer to fix the program generating them rather than tcpdump.
1370 Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
1372 Even if we were inclined to do anything to promote the use of
1373 Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
1374 networks so we'd would have no way of testing this code.
1376 A packet trace that crosses a daylight savings time change will give
1377 skewed time stamps (the time change is ignored).
1379 Filters expressions that manipulate FDDI headers assume that all FDDI
1380 packets are encapsulated Ethernet packets. This is true for IP, ARP,
1381 and DECNET Phase IV, but is not true for protocols such as ISO CLNS.
1382 Therefore, the filter may inadvertently accept certain packets that
1383 do not properly match the filter expression.
1386 should chase header chain, but at this moment it does not.
1390 should chase header chain too.
1392 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1393 does not work against IPv6 packets.
1394 It only looks at IPv4 packets.
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