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23 .TH TCPDUMP 1 "30 June 1997"
25 tcpdump \- dump traffic on a network
79 \fITcpdump\fP prints out the headers of packets on a network interface
80 that match the boolean \fIexpression\fP.
82 .B Under SunOS with nit or bpf:
85 you must have read access to
89 .B Under Solaris with dlpi:
90 You must have read access to the network pseudo device, e.g.
92 .B Under HP-UX with dlpi:
93 You must be root or it must be installed setuid to root.
94 .B Under IRIX with snoop:
95 You must be root or it must be installed setuid to root.
97 You must be root or it must be installed setuid to root.
98 .B Under Ultrix and Digital UNIX:
99 Once the super-user has enabled promiscuous-mode operation using
104 You must have read access to
109 Attempt to convert network and broadcast addresses to names.
112 Exit after receiving \fIcount\fP packets.
115 Dump the compiled packet-matching code in a human readable form to
116 standard output and stop.
119 Dump packet-matching code as a
124 Dump packet-matching code as decimal numbers (preceded with a count).
127 Print the link-level header on each dump line.
130 Use \fIalgo:secret\fP for decrypting IPsec ESP packets. Algorithms may be
135 \fIcast128-cbc\fP, or
137 The default is \fIdes-cbc\fP.
138 The ability to decrypt packets is only present if tcpdump was compiled
139 with cryptography enabled.
140 \fIsecret\fP the ascii text for ESP secret key.
141 We cannot take arbitrary binary value at this moment.
142 The option assumes RFC2406 ESP, not RFC1827 ESP.
143 The option is only for debugging purposes, and
144 the use of this option with truely `secret' key is discouraged.
145 By presenting IPsec secret key onto command line
146 you make it visible to others, via ps(1) and other occasions.
149 Print `foreign' internet addresses numerically rather than symbolically
150 (this option is intended to get around serious brain damage in
151 Sun's yp server \(em usually it hangs forever translating non-local
155 Use \fIfile\fP as input for the filter expression.
156 An additional expression given on the command line is ignored.
159 Listen on \fIinterface\fP.
160 If unspecified, \fItcpdump\fP searches the system interface list for the
161 lowest numbered, configured up interface (excluding loopback).
162 Ties are broken by choosing the earliest match.
165 Make stdout line buffered. Useful if you want to see the data
166 while capturing it. E.g.,
168 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
169 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
172 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
175 Don't print domain name qualification of host names. E.g.,
176 if you give this flag then \fItcpdump\fP will print ``nic''
177 instead of ``nic.ddn.mil''.
180 Load SMI MIB module definitions from file \fImodule\fR. This option
181 can be used several times to load several MIB modules into tcpdump.
184 Do not run the packet-matching code optimizer. This is useful only
185 if you suspect a bug in the optimizer.
188 \fIDon't\fP put the interface
189 into promiscuous mode. Note that the interface might be in promiscuous
190 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
191 `ether host {local-hw-addr} or ether broadcast'.
194 Quick (quiet?) output. Print less protocol information so output
198 Read packets from \fIfile\fR (which was created with the -w option).
199 Standard input is used if \fIfile\fR is ``-''.
202 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
203 default of 68 (with SunOS's NIT, the minimum is actually 96).
204 68 bytes is adequate for IP, ICMP, TCP
205 and UDP but may truncate protocol information from name server and NFS
206 packets (see below). Packets truncated because of a limited snapshot
207 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
208 is the name of the protocol level at which the truncation has occurred.
209 Note that taking larger snapshots both increases
210 the amount of time it takes to process packets and, effectively,
211 decreases the amount of packet buffering. This may cause packets to be
212 lost. You should limit \fIsnaplen\fP to the smallest number that will
213 capture the protocol information you're interested in. Setting
214 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
217 Force packets selected by "\fIexpression\fP" to be interpreted the
218 specified \fItype\fR. Currently known types are
219 \fBcnfp\fR (Cisco NetFlow protocol),
220 \fBrpc\fR (Remote Procedure Call),
221 \fBrtp\fR (Real-Time Applications protocol),
222 \fBrtcp\fR (Real-Time Applications control protocol),
223 \fBsnmp\fR (Simple Network Management Protocol),
224 \fBvat\fR (Visual Audio Tool),
226 \fBwb\fR (distributed White Board).
229 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
230 If specified, \fItcpdump\fP will not print replay prevention field.
231 Since there is no protocol version field in ESP/AH specification,
232 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
235 Print absolute, rather than relative, TCP sequence numbers.
238 \fIDon't\fP print a timestamp on each dump line.
241 Print an unformatted timestamp on each dump line.
244 (Slightly more) verbose output. For example, the time to live
245 and type of service information in an IP packet is printed.
248 Even more verbose output. For example, additional fields are
249 printed from NFS reply packets.
252 Even more verbose output. For example,
253 telnet \fBSB\fP ... \fBSE\fP options
254 are printed in full. With
256 telnet options are printed in hex as well.
259 Write the raw packets to \fIfile\fR rather than parsing and printing
260 them out. They can later be printed with the \-r option.
261 Standard output is used if \fIfile\fR is ``-''.
264 Print each packet (minus its link level header) in hex.
265 The smaller of the entire packet or
267 bytes will be printed.
270 When printing hex, print ascii too. Thus if
272 is also set, the packet is printed in hex/ascii.
273 This is very handy for analysing new protocols.
276 is not also set, some parts of some packets may be printed
278 .IP "\fI expression\fP"
280 selects which packets will be dumped. If no \fIexpression\fP
281 is given, all packets on the net will be dumped. Otherwise,
282 only packets for which \fIexpression\fP is `true' will be dumped.
284 The \fIexpression\fP consists of one or more
286 Primitives usually consist of an
288 (name or number) preceded by one or more qualifiers. There are three
289 different kinds of qualifier:
291 qualifiers say what kind of thing the id name or number refers to.
297 E.g., `host foo', `net 128.3', `port 20'. If there is no type
302 qualifiers specify a particular transfer direction to and/or from
304 Possible directions are
311 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
312 there is no dir qualifier,
315 For `null' link layers (i.e. point to point protocols such as slip) the
319 qualifiers can be used to specify a desired direction.
321 qualifiers restrict the match to a particular protocol. Possible
339 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
340 no proto qualifier, all protocols consistent with the type are
341 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
342 (except the latter is not legal syntax), `net bar' means `(ip or
343 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
345 [`fddi' is actually an alias for `ether'; the parser treats them
346 identically as meaning ``the data link level used on the specified
347 network interface.'' FDDI headers contain Ethernet-like source
348 and destination addresses, and often contain Ethernet-like packet
349 types, so you can filter on these FDDI fields just as with the
350 analogous Ethernet fields. FDDI headers also contain other fields,
351 but you cannot name them explicitly in a filter expression.]
353 In addition to the above, there are some special `primitive' keywords
354 that don't follow the pattern:
359 and arithmetic expressions. All of these are described below.
361 More complex filter expressions are built up by using the words
366 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
367 To save typing, identical qualifier lists can be omitted. E.g.,
368 `tcp dst port ftp or ftp-data or domain' is exactly the same as
369 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
371 Allowable primitives are:
372 .IP "\fBdst host \fIhost\fR"
373 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
374 which may be either an address or a name.
375 .IP "\fBsrc host \fIhost\fR"
376 True if the IPv4/v6 source field of the packet is \fIhost\fP.
377 .IP "\fBhost \fIhost\fP
378 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
379 Any of the above host expressions can be prepended with the keywords,
380 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
383 \fBip host \fIhost\fR
386 which is equivalent to:
389 \fBether proto \fI\\ip\fB and host \fIhost\fR
392 If \fIhost\fR is a name with multiple IP addresses, each address will
393 be checked for a match.
394 .IP "\fBether dst \fIehost\fP
395 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
396 may be either a name from /etc/ethers or a number (see
399 .IP "\fBether src \fIehost\fP
400 True if the ethernet source address is \fIehost\fP.
401 .IP "\fBether host \fIehost\fP
402 True if either the ethernet source or destination address is \fIehost\fP.
403 .IP "\fBgateway\fP \fIhost\fP
404 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
405 source or destination address was \fIhost\fP but neither the IP source
406 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
407 must be found in both /etc/hosts and /etc/ethers. (An equivalent
411 \fBether host \fIehost \fBand not host \fIhost\fR
414 which can be used with either names or numbers for \fIhost / ehost\fP.)
415 This syntax does not work in IPv6-enabled configuration at this moment.
416 .IP "\fBdst net \fInet\fR"
417 True if the IPv4/v6 destination address of the packet has a network
418 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
419 or a network number (see \fInetworks(4)\fP for details).
420 .IP "\fBsrc net \fInet\fR"
421 True if the IPv4/v6 source address of the packet has a network
423 .IP "\fBnet \fInet\fR"
424 True if either the IPv4/v6 source or destination address of the packet has a network
426 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
427 True if the IP address matches \fInet\fR with the specific netmask.
428 May be qualified with \fBsrc\fR or \fBdst\fR.
429 Note that this syntax is not valid for IPv6 \fInet\fR.
430 .IP "\fBnet \fInet\fR/\fIlen\fR"
431 True if the IPv4/v6 address matches \fInet\fR a netmask \fIlen\fR bits wide.
432 May be qualified with \fBsrc\fR or \fBdst\fR.
433 .IP "\fBdst port \fIport\fR"
434 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
435 destination port value of \fIport\fP.
436 The \fIport\fP can be a number or a name used in /etc/services (see
440 If a name is used, both the port
441 number and protocol are checked. If a number or ambiguous name is used,
442 only the port number is checked (e.g., \fBdst port 513\fR will print both
443 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
444 both tcp/domain and udp/domain traffic).
445 .IP "\fBsrc port \fIport\fR"
446 True if the packet has a source port value of \fIport\fP.
447 .IP "\fBport \fIport\fR"
448 True if either the source or destination port of the packet is \fIport\fP.
449 Any of the above port expressions can be prepended with the keywords,
450 \fBtcp\fP or \fBudp\fP, as in:
453 \fBtcp src port \fIport\fR
456 which matches only tcp packets whose source port is \fIport\fP.
457 .IP "\fBless \fIlength\fR"
458 True if the packet has a length less than or equal to \fIlength\fP.
459 This is equivalent to:
462 \fBlen <= \fIlength\fP.
465 .IP "\fBgreater \fIlength\fR"
466 True if the packet has a length greater than or equal to \fIlength\fP.
467 This is equivalent to:
470 \fBlen >= \fIlength\fP.
473 .IP "\fBip proto \fIprotocol\fR"
474 True if the packet is an ip packet (see
476 of protocol type \fIprotocol\fP.
477 \fIProtocol\fP can be a number or one of the names
478 \fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
479 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
480 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
481 Note that this primitive does not chase protocol header chain.
482 .IP "\fBip6 proto \fIprotocol\fR"
483 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
484 Note that this primitive does not chase protocol header chain.
485 .IP "\fBip6 protochain \fIprotocol\fR"
486 True if the packet is IPv6 packet,
487 and contains protocol header with type \fIprotocol\fR
488 in its protocol header chain.
492 \fBip6 protochain 6\fR
495 matches any IPv6 packet with TCP protocol header in the protocol header chain.
496 The packet may contain, for example,
497 authentication header, routing header, or hop-by-hop option header,
498 between IPv6 header and TCP header.
499 The BPF code emitted by this primitive is complex and
500 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
501 so this can be somewhat slow.
502 .IP "\fBip protochain \fIprotocol\fR"
503 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
504 .IP "\fBether broadcast\fR"
505 True if the packet is an ethernet broadcast packet. The \fIether\fP
507 .IP "\fBip broadcast\fR"
508 True if the packet is an IP broadcast packet. It checks for both
509 the all-zeroes and all-ones broadcast conventions, and looks up
510 the local subnet mask.
511 .IP "\fBether multicast\fR"
512 True if the packet is an ethernet multicast packet. The \fIether\fP
514 This is shorthand for `\fBether[0] & 1 != 0\fP'.
515 .IP "\fBip multicast\fR"
516 True if the packet is an IP multicast packet.
517 .IP "\fBip6 multicast\fR"
518 True if the packet is an IPv6 multicast packet.
519 .IP "\fBether proto \fIprotocol\fR"
520 True if the packet is of ether type \fIprotocol\fR.
521 \fIProtocol\fP can be a number or a name like
522 \fIip\fP, \fIip6\fP, \fIarp\fP, or \fIrarp\fP.
523 Note these identifiers are also keywords
524 and must be escaped via backslash (\\).
525 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
526 protocol identification comes from the 802.2 Logical Link Control
527 (LLC) header, which is usually layered on top of the FDDI header.
528 \fITcpdump\fP assumes, when filtering on the protocol identifier,
529 that all FDDI packets include an LLC header, and that the LLC header
530 is in so-called SNAP format.]
531 .IP "\fBdecnet src \fIhost\fR"
532 True if the DECNET source address is
534 which may be an address of the form ``10.123'', or a DECNET host
535 name. [DECNET host name support is only available on Ultrix systems
536 that are configured to run DECNET.]
537 .IP "\fBdecnet dst \fIhost\fR"
538 True if the DECNET destination address is
540 .IP "\fBdecnet host \fIhost\fR"
541 True if either the DECNET source or destination address is
543 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR"
547 \fBether proto \fIp\fR
550 where \fIp\fR is one of the above protocols.
551 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
555 \fBether proto \fIp\fR
558 where \fIp\fR is one of the above protocols.
560 \fItcpdump\fP does not currently know how to parse these protocols.
561 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
565 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
568 where \fIp\fR is one of the above protocols.
569 .IP "\fIexpr relop expr\fR"
570 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
571 and \fIexpr\fR is an arithmetic expression composed of integer constants
572 (expressed in standard C syntax), the normal binary operators
573 [+, -, *, /, &, |], a length operator, and special packet data accessors.
575 data inside the packet, use the following syntax:
578 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
581 \fIProto\fR is one of \fBether, fddi,
582 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
583 indicates the protocol layer for the index operation.
584 Note that \fItcp, udp\fR and other upper-layer protocol types only
585 apply to IPv4, not IPv6 (this will be fixed in the future).
586 The byte offset, relative to the indicated protocol layer, is
588 \fISize\fR is optional and indicates the number of bytes in the
589 field of interest; it can be either one, two, or four, and defaults to one.
590 The length operator, indicated by the keyword \fBlen\fP, gives the
591 length of the packet.
593 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
594 The expression `\fBip[0] & 0xf != 5\fP'
595 catches all IP packets with options. The expression
596 `\fBip[6:2] & 0x1fff = 0\fP'
597 catches only unfragmented datagrams and frag zero of fragmented datagrams.
598 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
600 For instance, \fBtcp[0]\fP always means the first
601 byte of the TCP \fIheader\fP, and never means the first byte of an
602 intervening fragment.
604 Primitives may be combined using:
606 A parenthesized group of primitives and operators
607 (parentheses are special to the Shell and must be escaped).
609 Negation (`\fB!\fP' or `\fBnot\fP').
611 Concatenation (`\fB&&\fP' or `\fBand\fP').
613 Alternation (`\fB||\fP' or `\fBor\fP').
615 Negation has highest precedence.
616 Alternation and concatenation have equal precedence and associate
617 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
618 are now required for concatenation.
620 If an identifier is given without a keyword, the most recent keyword
625 \fBnot host vs and ace\fR
631 \fBnot host vs and host ace\fR
634 which should not be confused with
637 \fBnot ( host vs or ace )\fR
641 Expression arguments can be passed to tcpdump as either a single argument
642 or as multiple arguments, whichever is more convenient.
643 Generally, if the expression contains Shell metacharacters, it is
644 easier to pass it as a single, quoted argument.
645 Multiple arguments are concatenated with spaces before being parsed.
648 To print all packets arriving at or departing from \fIsundown\fP:
651 \fBtcpdump host sundown\fP
655 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
658 \fBtcpdump host helios and \\( hot or ace \\)\fP
662 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
665 \fBtcpdump ip host ace and not helios\fP
669 To print all traffic between local hosts and hosts at Berkeley:
673 tcpdump net ucb-ether
677 To print all ftp traffic through internet gateway \fIsnup\fP:
678 (note that the expression is quoted to prevent the shell from
679 (mis-)interpreting the parentheses):
683 tcpdump 'gateway snup and (port ftp or ftp-data)'
687 To print traffic neither sourced from nor destined for local hosts
688 (if you gateway to one other net, this stuff should never make it
689 onto your local net).
693 tcpdump ip and not net \fIlocalnet\fP
697 To print the start and end packets (the SYN and FIN packets) of each
698 TCP conversation that involves a non-local host.
702 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
706 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
710 tcpdump 'gateway snup and ip[2:2] > 576'
714 To print IP broadcast or multicast packets that were
716 sent via ethernet broadcast or multicast:
720 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
724 To print all ICMP packets that are not echo requests/replies (i.e., not
729 tcpdump 'icmp[0] != 8 and icmp[0] != 0"
734 The output of \fItcpdump\fP is protocol dependent. The following
735 gives a brief description and examples of most of the formats.
743 If the '-e' option is given, the link level header is printed out.
744 On ethernets, the source and destination addresses, protocol,
745 and packet length are printed.
747 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
748 the `frame control' field, the source and destination addresses,
749 and the packet length. (The `frame control' field governs the
750 interpretation of the rest of the packet. Normal packets (such
751 as those containing IP datagrams) are `async' packets, with a priority
752 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
753 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
754 the LLC header is printed if it is \fInot\fR an ISO datagram or a
755 so-called SNAP packet.
757 \fI(N.B.: The following description assumes familiarity with
758 the SLIP compression algorithm described in RFC-1144.)\fP
760 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
761 packet type, and compression information are printed out.
762 The packet type is printed first.
763 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
764 No further link information is printed for \fIip\fR packets.
765 For TCP packets, the connection identifier is printed following the type.
766 If the packet is compressed, its encoded header is printed out.
767 The special cases are printed out as
768 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
769 the sequence number (or sequence number and ack) has changed.
770 If it is not a special case,
771 zero or more changes are printed.
772 A change is indicated by U (urgent pointer), W (window), A (ack),
773 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
775 Finally, the amount of data in the packet and compressed header length
778 For example, the following line shows an outbound compressed TCP packet,
779 with an implicit connection identifier; the ack has changed by 6,
780 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
781 data and 6 bytes of compressed header:
784 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
790 Arp/rarp output shows the type of request and its arguments. The
791 format is intended to be self explanatory.
792 Here is a short sample taken from the start of an `rlogin' from
793 host \fIrtsg\fP to host \fIcsam\fP:
797 \f(CWarp who-has csam tell rtsg
798 arp reply csam is-at CSAM\fP
802 The first line says that rtsg sent an arp packet asking
803 for the ethernet address of internet host csam. Csam
804 replies with its ethernet address (in this example, ethernet addresses
805 are in caps and internet addresses in lower case).
807 This would look less redundant if we had done \fBtcpdump \-n\fP:
811 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
812 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
816 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
817 broadcast and the second is point-to-point would be visible:
821 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
822 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fP
826 For the first packet this says the ethernet source address is RTSG, the
827 destination is the ethernet broadcast address, the type field
828 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
832 \fI(N.B.:The following description assumes familiarity with
833 the TCP protocol described in RFC-793. If you are not familiar
834 with the protocol, neither this description nor tcpdump will
835 be of much use to you.)\fP
837 The general format of a tcp protocol line is:
841 \fIsrc > dst: flags data-seqno ack window urgent options\fP
845 \fISrc\fP and \fIdst\fP are the source and destination IP
846 addresses and ports. \fIFlags\fP are some combination of S (SYN),
847 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
848 \fIData-seqno\fP describes the portion of sequence space covered
849 by the data in this packet (see example below).
850 \fIAck\fP is sequence number of the next data expected the other
851 direction on this connection.
852 \fIWindow\fP is the number of bytes of receive buffer space available
853 the other direction on this connection.
854 \fIUrg\fP indicates there is `urgent' data in the packet.
855 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
857 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
858 depend on the contents of the packet's tcp protocol header and
859 are output only if appropriate.
861 Here is the opening portion of an rlogin from host \fIrtsg\fP to
866 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
867 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
868 rtsg.1023 > csam.login: . ack 1 win 4096
869 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
870 csam.login > rtsg.1023: . ack 2 win 4096
871 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
872 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
873 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
874 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fP\s+2
878 The first line says that tcp port 1023 on rtsg sent a packet
880 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
881 The packet sequence number was 768512 and it contained no data.
882 (The notation is `first:last(nbytes)' which means `sequence
884 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
885 There was no piggy-backed ack, the available receive window was 4096
886 bytes and there was a max-segment-size option requesting an mss of
889 Csam replies with a similar packet except it includes a piggy-backed
890 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
892 The packet contained no data so there is no data sequence number.
893 Note that the ack sequence
894 number is a small integer (1). The first time \fBtcpdump\fP sees a
895 tcp `conversation', it prints the sequence number from the packet.
896 On subsequent packets of the conversation, the difference between
897 the current packet's sequence number and this initial sequence number
898 is printed. This means that sequence numbers after the
899 first can be interpreted
900 as relative byte positions in the conversation's data stream (with the
901 first data byte each direction being `1'). `-S' will override this
902 feature, causing the original sequence numbers to be output.
904 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
905 in the rtsg \(-> csam side of the conversation).
906 The PUSH flag is set in the packet.
907 On the 7th line, csam says it's received data sent by rtsg up to
908 but not including byte 21. Most of this data is apparently sitting in the
909 socket buffer since csam's receive window has gotten 19 bytes smaller.
910 Csam also sends one byte of data to rtsg in this packet.
911 On the 8th and 9th lines,
912 csam sends two bytes of urgent, pushed data to rtsg.
914 If the snapshot was small enough that \fBtcpdump\fP didn't capture
915 the full TCP header, it interprets as much of the header as it can
916 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
917 be interpreted. If the header contains a bogus option (one with a length
918 that's either too small or beyond the end of the header), tcpdump reports
919 it as ``[\fIbad opt\fP]'' and does not interpret any further options (since
920 it's impossible to tell where they start). If the header length indicates
921 options are present but the IP datagram length is not long enough for the
922 options to actually be there, tcpdump reports it as ``[\fIbad hdr length\fP]''.
927 UDP format is illustrated by this rwho packet:
931 \f(CWactinide.who > broadcast.who: udp 84\fP
935 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
936 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
937 broadcast address. The packet contained 84 bytes of user data.
939 Some UDP services are recognized (from the source or destination
940 port number) and the higher level protocol information printed.
941 In particular, Domain Name service requests (RFC-1034/1035) and Sun
942 RPC calls (RFC-1050) to NFS.
944 UDP Name Server Requests
946 \fI(N.B.:The following description assumes familiarity with
947 the Domain Service protocol described in RFC-1035. If you are not familiar
948 with the protocol, the following description will appear to be written
951 Name server requests are formatted as
955 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
957 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fP
961 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
962 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
963 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
964 was set. The query length was 37 bytes, not including the UDP and
965 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
966 so the op field was omitted. If the op had been anything else, it would
967 have been printed between the `3' and the `+'.
968 Similarly, the qclass was the normal one,
969 \fIC_IN\fP, and omitted. Any other qclass would have been printed
970 immediately after the `A'.
972 A few anomalies are checked and may result in extra fields enclosed in
973 square brackets: If a query contains an answer, name server or
979 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
980 is the appropriate count.
981 If any of the response bits are set (AA, RA or rcode) or any of the
982 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
983 is printed, where \fIx\fP is the hex value of header bytes two and three.
985 UDP Name Server Responses
987 Name server responses are formatted as
991 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
993 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
994 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fP
998 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
999 with 3 answer records, 3 name server records and 7 authority records.
1000 The first answer record is type A (address) and its data is internet
1001 address 128.32.137.3. The total size of the response was 273 bytes,
1002 excluding UDP and IP headers. The op (Query) and response code
1003 (NoError) were omitted, as was the class (C_IN) of the A record.
1005 In the second example, \fIhelios\fP responds to query 2 with a
1006 response code of non-existent domain (NXDomain) with no answers,
1007 one name server and no authority records. The `*' indicates that
1008 the \fIauthoritative answer\fP bit was set. Since there were no
1009 answers, no type, class or data were printed.
1011 Other flag characters that might appear are `\-' (recursion available,
1012 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
1013 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1016 Note that name server requests and responses tend to be large and the
1017 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1018 to print. Use the \fB\-s\fP flag to increase the snaplen if you
1019 need to seriously investigate name server traffic. `\fB\-s 128\fP'
1020 has worked well for me.
1025 tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data
1026 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1027 NetBEUI SMB data is also done.
1029 By default a fairly minimal decode is done, with a much more detailed
1030 decode done if -v is used. Be warned that with -v a single SMB packet
1031 may take up a page or more, so only use -v if you really want all the
1034 If you are decoding SMB sessions containing unicode strings then you
1035 may wish to set the environment variable USE_UNICODE to 1. A patch to
1036 auto-detect unicode srings would be welcome.
1038 For information on SMB packet formats and what all te fields mean see
1039 www.cifs.org or the pub/samba/specs/ directory on your favourite
1040 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1044 NFS Requests and Replies
1046 Sun NFS (Network File System) requests and replies are printed as:
1050 \fIsrc.xid > dst.nfs: len op args\fP
1051 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1054 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1055 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1056 sushi.201b > wrl.nfs:
1057 144 lookup fh 9,74/4096.6878 "xcolors"
1058 wrl.nfs > sushi.201b:
1059 reply ok 128 lookup fh 9,74/4134.3150
1064 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1065 to \fIwrl\fP (note that the number following the src host is a
1066 transaction id, \fInot\fP the source port). The request was 112 bytes,
1067 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1068 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1069 (If one is lucky, as in this case, the file handle can be interpreted
1070 as a major,minor device number pair, followed by the inode number and
1072 \fIWrl\fP replies `ok' with the contents of the link.
1074 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1075 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1076 depends on the operation type. The format is intended to be self
1077 explanatory if read in conjunction with
1078 an NFS protocol spec.
1080 If the \-v (verbose) flag is given, additional information is printed.
1086 sushi.1372a > wrl.nfs:
1087 148 read fh 21,11/12.195 8192 bytes @ 24576
1088 wrl.nfs > sushi.1372a:
1089 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1094 (\-v also prints the IP header TTL, ID, and fragmentation fields,
1095 which have been omitted from this example.) In the first line,
1096 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1097 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1098 second line is the first fragment of the reply, and hence is only 1472
1099 bytes long (the other bytes will follow in subsequent fragments, but
1100 these fragments do not have NFS or even UDP headers and so might not be
1101 printed, depending on the filter expression used). Because the \-v flag
1102 is given, some of the file attributes (which are returned in addition
1103 to the file data) are printed: the file type (``REG'', for regular file),
1104 the file mode (in octal), the uid and gid, and the file size.
1106 If the \-v flag is given more than once, even more details are printed.
1108 Note that NFS requests are very large and much of the detail won't be printed
1109 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1112 NFS reply packets do not explicitly identify the RPC operation. Instead,
1113 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1114 replies using the transaction ID. If a reply does not closely follow the
1115 corresponding request, it might not be parsable.
1117 AFS Request and Replies
1119 Transarc AFS (Andrew File System) requests and replies are printed
1125 \fIsrc.sport > dst.dport: rx packet-type\fP
1126 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1127 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1130 elvis.7001 > pike.afsfs:
1131 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1132 new fid 536876964/1/1 ".newsrc"
1133 pike.afsfs > elvis.7001: rx data fs reply rename
1138 In the first line, host elvis sends a RX packet to pike. This was
1139 a RX data packet to the fs (fileserver) service, and is the start of
1140 an RPC call. The RPC call was a rename, with the old directory file id
1141 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1142 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1143 responds with a RPC reply to the rename call (which was successful, because
1144 it was a data packet and not an abort packet).
1146 In general, all AFS RPCs are decoded at least by RPC call name. Most
1147 AFS RPCs have at least some of the arguments decoded (generally only
1148 the `interesting' arguments, for some definition of interesting).
1150 The format is intended to be self-describing, but it will probably
1151 not be useful to people who are not familiar with the workings of
1154 If the -v (verbose) flag is given, the RX call number and sequence number
1157 If the -v flag is given twice, additional information is printed.
1158 such as the the RX call ID, serial number, and the RX packet flags.
1159 The MTU negotiation information is also printed from RX ack packets.
1161 If the -v flag three times, the security index and service id are printed.
1163 Error codes are printed for abort packets, with the exception of Ubik
1164 beacon packets (because abort packets are used to signify a yes vote
1165 for the Ubik protocol).
1167 Note that AFS requests are very large and many of the arguments won't
1168 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1169 to watch AFS traffic.
1171 AFS reply packets do not explicitly identify the RPC operation. Instead,
1172 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1173 replies using the call number and service ID. If a reply does not closely
1175 corresponding request, it might not be parsable.
1178 KIP Appletalk (DDP in UDP)
1180 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1181 and dumped as DDP packets (i.e., all the UDP header information is
1182 discarded). The file
1184 is used to translate appletalk net and node numbers to names.
1185 Lines in this file have the form
1197 The first two lines give the names of appletalk networks. The third
1198 line gives the name of a particular host (a host is distinguished
1199 from a net by the 3rd octet in the number \-
1200 a net number \fImust\fP have two octets and a host number \fImust\fP
1201 have three octets.) The number and name should be separated by
1202 whitespace (blanks or tabs).
1205 file may contain blank lines or comment lines (lines starting with
1208 Appletalk addresses are printed in the form
1214 \f(CW144.1.209.2 > icsd-net.112.220
1215 office.2 > icsd-net.112.220
1216 jssmag.149.235 > icsd-net.2\fP
1222 doesn't exist or doesn't contain an entry for some appletalk
1223 host/net number, addresses are printed in numeric form.)
1224 In the first example, NBP (DDP port 2) on net 144.1 node 209
1225 is sending to whatever is listening on port 220 of net icsd node 112.
1226 The second line is the same except the full name of the source node
1227 is known (`office'). The third line is a send from port 235 on
1228 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1229 the broadcast address (255) is indicated by a net name with no host
1230 number \- for this reason it's a good idea to keep node names and
1231 net names distinct in /etc/atalk.names).
1233 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1234 packets have their contents interpreted. Other protocols just dump
1235 the protocol name (or number if no name is registered for the
1236 protocol) and packet size.
1238 \fBNBP packets\fP are formatted like the following examples:
1242 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1243 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1244 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fP\s+2
1248 The first line is a name lookup request for laserwriters sent by net icsd host
1249 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1250 The second line shows a reply for this request (note that it has the
1251 same id) from host jssmag.209 saying that it has a laserwriter
1252 resource named "RM1140" registered on port 250. The third line is
1253 another reply to the same request saying host techpit has laserwriter
1254 "techpit" registered on port 186.
1256 \fBATP packet\fP formatting is demonstrated by the following example:
1260 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1261 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1262 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1263 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1264 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1265 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1266 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1267 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1268 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1269 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1270 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1271 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1272 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1273 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fP\s+2
1277 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1278 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1279 is the value of the `userdata' field in the request.
1281 Helios responds with 8 512-byte packets. The `:digit' following the
1282 transaction id gives the packet sequence number in the transaction
1283 and the number in parens is the amount of data in the packet,
1284 excluding the atp header. The `*' on packet 7 indicates that the
1287 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1288 resends them then jssmag.209 releases the transaction. Finally,
1289 jssmag.209 initiates the next request. The `*' on the request
1290 indicates that XO (`exactly once') was \fInot\fP set.
1295 Fragmented Internet datagrams are printed as
1299 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1300 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1304 (The first form indicates there are more fragments. The second
1305 indicates this is the last fragment.)
1307 \fIId\fP is the fragment id. \fISize\fP is the fragment
1308 size (in bytes) excluding the IP header. \fIOffset\fP is this
1309 fragment's offset (in bytes) in the original datagram.
1311 The fragment information is output for each fragment. The first
1312 fragment contains the higher level protocol header and the frag
1313 info is printed after the protocol info. Fragments
1314 after the first contain no higher level protocol header and the
1315 frag info is printed after the source and destination addresses.
1316 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1317 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1321 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1322 arizona > rtsg: (frag 595a:204@328)
1323 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1327 There are a couple of things to note here: First, addresses in the
1328 2nd line don't include port numbers. This is because the TCP
1329 protocol information is all in the first fragment and we have no idea
1330 what the port or sequence numbers are when we print the later fragments.
1331 Second, the tcp sequence information in the first line is printed as if there
1332 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1333 the first frag and 204 in the second). If you are looking for holes
1334 in the sequence space or trying to match up acks
1335 with packets, this can fool you.
1337 A packet with the IP \fIdon't fragment\fP flag is marked with a
1338 trailing \fB(DF)\fP.
1342 By default, all output lines are preceded by a timestamp. The timestamp
1343 is the current clock time in the form
1349 and is as accurate as the kernel's clock.
1350 The timestamp reflects the time the kernel first saw the packet. No attempt
1351 is made to account for the time lag between when the
1352 ethernet interface removed the packet from the wire and when the kernel
1353 serviced the `new packet' interrupt.
1355 traffic(1C), nit(4P), bpf(4), pcap(3)
1359 Steven McCanne, all of the
1360 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1362 The current version is available via anonymous ftp:
1365 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1368 IPv6/IPsec support is added by WIDE/KAME project.
1369 This program uses Eric Young's SSLeay library, under specific configuration.
1371 Please send bug reports to tcpdump@ee.lbl.gov.
1373 NIT doesn't let you watch your own outbound traffic, BPF will.
1374 We recommend that you use the latter.
1376 Some attempt should be made to reassemble IP fragments or, at least
1377 to compute the right length for the higher level protocol.
1379 Name server inverse queries are not dumped correctly: The (empty)
1380 question section is printed rather than real query in the answer
1381 section. Some believe that inverse queries are themselves a bug and
1382 prefer to fix the program generating them rather than tcpdump.
1384 Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
1386 Even if we were inclined to do anything to promote the use of
1387 Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
1388 networks so we'd would have no way of testing this code.
1390 A packet trace that crosses a daylight savings time change will give
1391 skewed time stamps (the time change is ignored).
1393 Filters expressions that manipulate FDDI headers assume that all FDDI
1394 packets are encapsulated Ethernet packets. This is true for IP, ARP,
1395 and DECNET Phase IV, but is not true for protocols such as ISO CLNS.
1396 Therefore, the filter may inadvertently accept certain packets that
1397 do not properly match the filter expression.
1400 should chase header chain, but at this moment it does not.
1401 .BR "ip6 protochain"
1402 is supplied for this behavior.
1404 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1405 does not work against IPv6 packets.
1406 It only looks at IPv4 packets.
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