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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.
216 Force packets selected by "\fIexpression\fP" to be interpreted the
217 specified \fItype\fR. Currently known types are
218 \fBrpc\fR (Remote Procedure Call),
219 \fBrtp\fR (Real-Time Applications protocol),
220 \fBrtcp\fR (Real-Time Applications control protocol),
221 \fBsnmp\fR (Simple Network Management Protocol),
222 \fBvat\fR (Visual Audio Tool),
224 \fBwb\fR (distributed White Board).
227 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
228 If specified, \fItcpdump\fP will not print replay prevention field.
229 Since there is no protocol version field in ESP/AH specification,
230 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
233 Print absolute, rather than relative, TCP sequence numbers.
236 \fIDon't\fP print a timestamp on each dump line.
239 Print an unformatted timestamp on each dump line.
242 (Slightly more) verbose output. For example, the time to live
243 and type of service information in an IP packet is printed.
246 Even more verbose output. For example, additional fields are
247 printed from NFS reply packets.
250 Even more verbose output. For example,
251 telnet \fBSB\fP ... \fBSE\fP options
252 are printed in full. With
254 telnet options are printed in hex as well.
257 Write the raw packets to \fIfile\fR rather than parsing and printing
258 them out. They can later be printed with the \-r option.
259 Standard output is used if \fIfile\fR is ``-''.
262 Print each packet (minus its link level header) in hex.
263 The smaller of the entire packet or
265 bytes will be printed.
268 When printing hex, print ascii too. Thus if
270 is also set, the packet is printed in hex/ascii.
271 This is very handy for analysing new protocols.
274 is not also set, some parts of some packets may be printed
276 .IP "\fI expression\fP"
278 selects which packets will be dumped. If no \fIexpression\fP
279 is given, all packets on the net will be dumped. Otherwise,
280 only packets for which \fIexpression\fP is `true' will be dumped.
282 The \fIexpression\fP consists of one or more
284 Primitives usually consist of an
286 (name or number) preceded by one or more qualifiers. There are three
287 different kinds of qualifier:
289 qualifiers say what kind of thing the id name or number refers to.
295 E.g., `host foo', `net 128.3', `port 20'. If there is no type
300 qualifiers specify a particular transfer direction to and/or from
302 Possible directions are
309 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
310 there is no dir qualifier,
313 For `null' link layers (i.e. point to point protocols such as slip) the
317 qualifiers can be used to specify a desired direction.
319 qualifiers restrict the match to a particular protocol. Possible
337 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
338 no proto qualifier, all protocols consistent with the type are
339 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
340 (except the latter is not legal syntax), `net bar' means `(ip or
341 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
343 [`fddi' is actually an alias for `ether'; the parser treats them
344 identically as meaning ``the data link level used on the specified
345 network interface.'' FDDI headers contain Ethernet-like source
346 and destination addresses, and often contain Ethernet-like packet
347 types, so you can filter on these FDDI fields just as with the
348 analogous Ethernet fields. FDDI headers also contain other fields,
349 but you cannot name them explicitly in a filter expression.]
351 In addition to the above, there are some special `primitive' keywords
352 that don't follow the pattern:
357 and arithmetic expressions. All of these are described below.
359 More complex filter expressions are built up by using the words
364 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
365 To save typing, identical qualifier lists can be omitted. E.g.,
366 `tcp dst port ftp or ftp-data or domain' is exactly the same as
367 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
369 Allowable primitives are:
370 .IP "\fBdst host \fIhost\fR"
371 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
372 which may be either an address or a name.
373 .IP "\fBsrc host \fIhost\fR"
374 True if the IPv4/v6 source field of the packet is \fIhost\fP.
375 .IP "\fBhost \fIhost\fP
376 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
377 Any of the above host expressions can be prepended with the keywords,
378 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
381 \fBip host \fIhost\fR
384 which is equivalent to:
387 \fBether proto \fI\\ip\fB and host \fIhost\fR
390 If \fIhost\fR is a name with multiple IP addresses, each address will
391 be checked for a match.
392 .IP "\fBether dst \fIehost\fP
393 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
394 may be either a name from /etc/ethers or a number (see
397 .IP "\fBether src \fIehost\fP
398 True if the ethernet source address is \fIehost\fP.
399 .IP "\fBether host \fIehost\fP
400 True if either the ethernet source or destination address is \fIehost\fP.
401 .IP "\fBgateway\fP \fIhost\fP
402 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
403 source or destination address was \fIhost\fP but neither the IP source
404 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
405 must be found in both /etc/hosts and /etc/ethers. (An equivalent
409 \fBether host \fIehost \fBand not host \fIhost\fR
412 which can be used with either names or numbers for \fIhost / ehost\fP.)
413 This syntax does not work in IPv6-enabled configuration at this moment.
414 .IP "\fBdst net \fInet\fR"
415 True if the IPv4/v6 destination address of the packet has a network
416 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
417 or a network number (see \fInetworks(4)\fP for details).
418 .IP "\fBsrc net \fInet\fR"
419 True if the IPv4/v6 source address of the packet has a network
421 .IP "\fBnet \fInet\fR"
422 True if either the IPv4/v6 source or destination address of the packet has a network
424 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
425 True if the IP address matches \fInet\fR with the specific netmask.
426 May be qualified with \fBsrc\fR or \fBdst\fR.
427 Note that this syntax is not valid for IPv6 \fInet\fR.
428 .IP "\fBnet \fInet\fR/\fIlen\fR"
429 True if the IPv4/v6 address matches \fInet\fR a netmask \fIlen\fR bits wide.
430 May be qualified with \fBsrc\fR or \fBdst\fR.
431 .IP "\fBdst port \fIport\fR"
432 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
433 destination port value of \fIport\fP.
434 The \fIport\fP can be a number or a name used in /etc/services (see
438 If a name is used, both the port
439 number and protocol are checked. If a number or ambiguous name is used,
440 only the port number is checked (e.g., \fBdst port 513\fR will print both
441 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
442 both tcp/domain and udp/domain traffic).
443 .IP "\fBsrc port \fIport\fR"
444 True if the packet has a source port value of \fIport\fP.
445 .IP "\fBport \fIport\fR"
446 True if either the source or destination port of the packet is \fIport\fP.
447 Any of the above port expressions can be prepended with the keywords,
448 \fBtcp\fP or \fBudp\fP, as in:
451 \fBtcp src port \fIport\fR
454 which matches only tcp packets whose source port is \fIport\fP.
455 .IP "\fBless \fIlength\fR"
456 True if the packet has a length less than or equal to \fIlength\fP.
457 This is equivalent to:
460 \fBlen <= \fIlength\fP.
463 .IP "\fBgreater \fIlength\fR"
464 True if the packet has a length greater than or equal to \fIlength\fP.
465 This is equivalent to:
468 \fBlen >= \fIlength\fP.
471 .IP "\fBip proto \fIprotocol\fR"
472 True if the packet is an ip packet (see
474 of protocol type \fIprotocol\fP.
475 \fIProtocol\fP can be a number or one of the names
476 \fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
477 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
478 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
479 Note that this primitive does not chase protocol header chain.
480 .IP "\fBip6 proto \fIprotocol\fR"
481 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
482 Note that this primitive does not chase protocol header chain.
483 .IP "\fBip6 protochain \fIprotocol\fR"
484 True if the packet is IPv6 packet,
485 and contains protocol header with type \fIprotocol\fR
486 in its protocol header chain.
490 \fBip6 protochain 6\fR
493 matches any IPv6 packet with TCP protocol header in the protocol header chain.
494 The packet may contain, for example,
495 authentication header, routing header, or hop-by-hop option header,
496 between IPv6 header and TCP header.
497 The BPF code emitted by this primitive is complex and
498 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
499 so this can be somewhat slow.
500 .IP "\fBip protochain \fIprotocol\fR"
501 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
502 .IP "\fBether broadcast\fR"
503 True if the packet is an ethernet broadcast packet. The \fIether\fP
505 .IP "\fBip broadcast\fR"
506 True if the packet is an IP broadcast packet. It checks for both
507 the all-zeroes and all-ones broadcast conventions, and looks up
508 the local subnet mask.
509 .IP "\fBether multicast\fR"
510 True if the packet is an ethernet multicast packet. The \fIether\fP
512 This is shorthand for `\fBether[0] & 1 != 0\fP'.
513 .IP "\fBip multicast\fR"
514 True if the packet is an IP multicast packet.
515 .IP "\fBip6 multicast\fR"
516 True if the packet is an IPv6 multicast packet.
517 .IP "\fBether proto \fIprotocol\fR"
518 True if the packet is of ether type \fIprotocol\fR.
519 \fIProtocol\fP can be a number or a name like
520 \fIip\fP, \fIip6\fP, \fIarp\fP, or \fIrarp\fP.
521 Note these identifiers are also keywords
522 and must be escaped via backslash (\\).
523 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
524 protocol identification comes from the 802.2 Logical Link Control
525 (LLC) header, which is usually layered on top of the FDDI header.
526 \fITcpdump\fP assumes, when filtering on the protocol identifier,
527 that all FDDI packets include an LLC header, and that the LLC header
528 is in so-called SNAP format.]
529 .IP "\fBdecnet src \fIhost\fR"
530 True if the DECNET source address is
532 which may be an address of the form ``10.123'', or a DECNET host
533 name. [DECNET host name support is only available on Ultrix systems
534 that are configured to run DECNET.]
535 .IP "\fBdecnet dst \fIhost\fR"
536 True if the DECNET destination address is
538 .IP "\fBdecnet host \fIhost\fR"
539 True if either the DECNET source or destination address is
541 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR"
545 \fBether proto \fIp\fR
548 where \fIp\fR is one of the above protocols.
549 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
553 \fBether proto \fIp\fR
556 where \fIp\fR is one of the above protocols.
558 \fItcpdump\fP does not currently know how to parse these protocols.
559 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
563 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
566 where \fIp\fR is one of the above protocols.
567 .IP "\fIexpr relop expr\fR"
568 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
569 and \fIexpr\fR is an arithmetic expression composed of integer constants
570 (expressed in standard C syntax), the normal binary operators
571 [+, -, *, /, &, |], a length operator, and special packet data accessors.
573 data inside the packet, use the following syntax:
576 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
579 \fIProto\fR is one of \fBether, fddi,
580 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
581 indicates the protocol layer for the index operation.
582 Note that \fItcp, udp\fR and other upper-layer protocol types only
583 apply to IPv4, not IPv6 (this will be fixed in the future).
584 The byte offset, relative to the indicated protocol layer, is
586 \fISize\fR is optional and indicates the number of bytes in the
587 field of interest; it can be either one, two, or four, and defaults to one.
588 The length operator, indicated by the keyword \fBlen\fP, gives the
589 length of the packet.
591 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
592 The expression `\fBip[0] & 0xf != 5\fP'
593 catches all IP packets with options. The expression
594 `\fBip[6:2] & 0x1fff = 0\fP'
595 catches only unfragmented datagrams and frag zero of fragmented datagrams.
596 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
598 For instance, \fBtcp[0]\fP always means the first
599 byte of the TCP \fIheader\fP, and never means the first byte of an
600 intervening fragment.
602 Primitives may be combined using:
604 A parenthesized group of primitives and operators
605 (parentheses are special to the Shell and must be escaped).
607 Negation (`\fB!\fP' or `\fBnot\fP').
609 Concatenation (`\fB&&\fP' or `\fBand\fP').
611 Alternation (`\fB||\fP' or `\fBor\fP').
613 Negation has highest precedence.
614 Alternation and concatenation have equal precedence and associate
615 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
616 are now required for concatenation.
618 If an identifier is given without a keyword, the most recent keyword
623 \fBnot host vs and ace\fR
629 \fBnot host vs and host ace\fR
632 which should not be confused with
635 \fBnot ( host vs or ace )\fR
639 Expression arguments can be passed to tcpdump as either a single argument
640 or as multiple arguments, whichever is more convenient.
641 Generally, if the expression contains Shell metacharacters, it is
642 easier to pass it as a single, quoted argument.
643 Multiple arguments are concatenated with spaces before being parsed.
646 To print all packets arriving at or departing from \fIsundown\fP:
649 \fBtcpdump host sundown\fP
653 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
656 \fBtcpdump host helios and \\( hot or ace \\)\fP
660 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
663 \fBtcpdump ip host ace and not helios\fP
667 To print all traffic between local hosts and hosts at Berkeley:
671 tcpdump net ucb-ether
675 To print all ftp traffic through internet gateway \fIsnup\fP:
676 (note that the expression is quoted to prevent the shell from
677 (mis-)interpreting the parentheses):
681 tcpdump 'gateway snup and (port ftp or ftp-data)'
685 To print traffic neither sourced from nor destined for local hosts
686 (if you gateway to one other net, this stuff should never make it
687 onto your local net).
691 tcpdump ip and not net \fIlocalnet\fP
695 To print the start and end packets (the SYN and FIN packets) of each
696 TCP conversation that involves a non-local host.
700 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
704 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
708 tcpdump 'gateway snup and ip[2:2] > 576'
712 To print IP broadcast or multicast packets that were
714 sent via ethernet broadcast or multicast:
718 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
722 To print all ICMP packets that are not echo requests/replies (i.e., not
727 tcpdump 'icmp[0] != 8 and icmp[0] != 0"
732 The output of \fItcpdump\fP is protocol dependent. The following
733 gives a brief description and examples of most of the formats.
741 If the '-e' option is given, the link level header is printed out.
742 On ethernets, the source and destination addresses, protocol,
743 and packet length are printed.
745 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
746 the `frame control' field, the source and destination addresses,
747 and the packet length. (The `frame control' field governs the
748 interpretation of the rest of the packet. Normal packets (such
749 as those containing IP datagrams) are `async' packets, with a priority
750 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
751 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
752 the LLC header is printed if it is \fInot\fR an ISO datagram or a
753 so-called SNAP packet.
755 \fI(N.B.: The following description assumes familiarity with
756 the SLIP compression algorithm described in RFC-1144.)\fP
758 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
759 packet type, and compression information are printed out.
760 The packet type is printed first.
761 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
762 No further link information is printed for \fIip\fR packets.
763 For TCP packets, the connection identifier is printed following the type.
764 If the packet is compressed, its encoded header is printed out.
765 The special cases are printed out as
766 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
767 the sequence number (or sequence number and ack) has changed.
768 If it is not a special case,
769 zero or more changes are printed.
770 A change is indicated by U (urgent pointer), W (window), A (ack),
771 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
773 Finally, the amount of data in the packet and compressed header length
776 For example, the following line shows an outbound compressed TCP packet,
777 with an implicit connection identifier; the ack has changed by 6,
778 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
779 data and 6 bytes of compressed header:
782 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
788 Arp/rarp output shows the type of request and its arguments. The
789 format is intended to be self explanatory.
790 Here is a short sample taken from the start of an `rlogin' from
791 host \fIrtsg\fP to host \fIcsam\fP:
795 \f(CWarp who-has csam tell rtsg
796 arp reply csam is-at CSAM\fP
800 The first line says that rtsg sent an arp packet asking
801 for the ethernet address of internet host csam. Csam
802 replies with its ethernet address (in this example, ethernet addresses
803 are in caps and internet addresses in lower case).
805 This would look less redundant if we had done \fBtcpdump \-n\fP:
809 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
810 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
814 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
815 broadcast and the second is point-to-point would be visible:
819 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
820 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fP
824 For the first packet this says the ethernet source address is RTSG, the
825 destination is the ethernet broadcast address, the type field
826 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
830 \fI(N.B.:The following description assumes familiarity with
831 the TCP protocol described in RFC-793. If you are not familiar
832 with the protocol, neither this description nor tcpdump will
833 be of much use to you.)\fP
835 The general format of a tcp protocol line is:
839 \fIsrc > dst: flags data-seqno ack window urgent options\fP
843 \fISrc\fP and \fIdst\fP are the source and destination IP
844 addresses and ports. \fIFlags\fP are some combination of S (SYN),
845 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
846 \fIData-seqno\fP describes the portion of sequence space covered
847 by the data in this packet (see example below).
848 \fIAck\fP is sequence number of the next data expected the other
849 direction on this connection.
850 \fIWindow\fP is the number of bytes of receive buffer space available
851 the other direction on this connection.
852 \fIUrg\fP indicates there is `urgent' data in the packet.
853 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
855 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
856 depend on the contents of the packet's tcp protocol header and
857 are output only if appropriate.
859 Here is the opening portion of an rlogin from host \fIrtsg\fP to
864 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
865 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
866 rtsg.1023 > csam.login: . ack 1 win 4096
867 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
868 csam.login > rtsg.1023: . ack 2 win 4096
869 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
870 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
871 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
872 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fP\s+2
876 The first line says that tcp port 1023 on rtsg sent a packet
878 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
879 The packet sequence number was 768512 and it contained no data.
880 (The notation is `first:last(nbytes)' which means `sequence
882 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
883 There was no piggy-backed ack, the available receive window was 4096
884 bytes and there was a max-segment-size option requesting an mss of
887 Csam replies with a similar packet except it includes a piggy-backed
888 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
890 The packet contained no data so there is no data sequence number.
891 Note that the ack sequence
892 number is a small integer (1). The first time \fBtcpdump\fP sees a
893 tcp `conversation', it prints the sequence number from the packet.
894 On subsequent packets of the conversation, the difference between
895 the current packet's sequence number and this initial sequence number
896 is printed. This means that sequence numbers after the
897 first can be interpreted
898 as relative byte positions in the conversation's data stream (with the
899 first data byte each direction being `1'). `-S' will override this
900 feature, causing the original sequence numbers to be output.
902 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
903 in the rtsg \(-> csam side of the conversation).
904 The PUSH flag is set in the packet.
905 On the 7th line, csam says it's received data sent by rtsg up to
906 but not including byte 21. Most of this data is apparently sitting in the
907 socket buffer since csam's receive window has gotten 19 bytes smaller.
908 Csam also sends one byte of data to rtsg in this packet.
909 On the 8th and 9th lines,
910 csam sends two bytes of urgent, pushed data to rtsg.
912 If the snapshot was small enough that \fBtcpdump\fP didn't capture
913 the full TCP header, it interprets as much of the header as it can
914 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
915 be interpreted. If the header contains a bogus option (one with a length
916 that's either too small or beyond the end of the header), tcpdump reports
917 it as ``[\fIbad opt\fP]'' and does not interpret any further options (since
918 it's impossible to tell where they start). If the header length indicates
919 options are present but the IP datagram length is not long enough for the
920 options to actually be there, tcpdump reports it as ``[\fIbad hdr length\fP]''.
925 UDP format is illustrated by this rwho packet:
929 \f(CWactinide.who > broadcast.who: udp 84\fP
933 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
934 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
935 broadcast address. The packet contained 84 bytes of user data.
937 Some UDP services are recognized (from the source or destination
938 port number) and the higher level protocol information printed.
939 In particular, Domain Name service requests (RFC-1034/1035) and Sun
940 RPC calls (RFC-1050) to NFS.
942 UDP Name Server Requests
944 \fI(N.B.:The following description assumes familiarity with
945 the Domain Service protocol described in RFC-1035. If you are not familiar
946 with the protocol, the following description will appear to be written
949 Name server requests are formatted as
953 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
955 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fP
959 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
960 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
961 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
962 was set. The query length was 37 bytes, not including the UDP and
963 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
964 so the op field was omitted. If the op had been anything else, it would
965 have been printed between the `3' and the `+'.
966 Similarly, the qclass was the normal one,
967 \fIC_IN\fP, and omitted. Any other qclass would have been printed
968 immediately after the `A'.
970 A few anomalies are checked and may result in extra fields enclosed in
971 square brackets: If a query contains an answer, name server or
977 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
978 is the appropriate count.
979 If any of the response bits are set (AA, RA or rcode) or any of the
980 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
981 is printed, where \fIx\fP is the hex value of header bytes two and three.
983 UDP Name Server Responses
985 Name server responses are formatted as
989 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
991 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
992 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fP
996 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
997 with 3 answer records, 3 name server records and 7 authority records.
998 The first answer record is type A (address) and its data is internet
999 address 128.32.137.3. The total size of the response was 273 bytes,
1000 excluding UDP and IP headers. The op (Query) and response code
1001 (NoError) were omitted, as was the class (C_IN) of the A record.
1003 In the second example, \fIhelios\fP responds to query 2 with a
1004 response code of non-existent domain (NXDomain) with no answers,
1005 one name server and no authority records. The `*' indicates that
1006 the \fIauthoritative answer\fP bit was set. Since there were no
1007 answers, no type, class or data were printed.
1009 Other flag characters that might appear are `\-' (recursion available,
1010 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
1011 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1014 Note that name server requests and responses tend to be large and the
1015 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1016 to print. Use the \fB\-s\fP flag to increase the snaplen if you
1017 need to seriously investigate name server traffic. `\fB\-s 128\fP'
1018 has worked well for me.
1023 tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data
1024 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1025 NetBEUI SMB data is also done.
1027 By default a fairly minimal decode is done, with a much more detailed
1028 decode done if -v is used. Be warned that with -v a single SMB packet
1029 may take up a page or more, so only use -v if you really want all the
1032 If you are decoding SMB sessions containing unicode strings then you
1033 may wish to set the environment variable USE_UNICODE to 1. A patch to
1034 auto-detect unicode srings would be welcome.
1036 For information on SMB packet formats and what all te fields mean see
1037 www.cifs.org or the pub/samba/specs/ directory on your favourite
1038 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1042 NFS Requests and Replies
1044 Sun NFS (Network File System) requests and replies are printed as:
1048 \fIsrc.xid > dst.nfs: len op args\fP
1049 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1052 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1053 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1054 sushi.201b > wrl.nfs:
1055 144 lookup fh 9,74/4096.6878 "xcolors"
1056 wrl.nfs > sushi.201b:
1057 reply ok 128 lookup fh 9,74/4134.3150
1062 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1063 to \fIwrl\fP (note that the number following the src host is a
1064 transaction id, \fInot\fP the source port). The request was 112 bytes,
1065 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1066 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1067 (If one is lucky, as in this case, the file handle can be interpreted
1068 as a major,minor device number pair, followed by the inode number and
1070 \fIWrl\fP replies `ok' with the contents of the link.
1072 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1073 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1074 depends on the operation type. The format is intended to be self
1075 explanatory if read in conjunction with
1076 an NFS protocol spec.
1078 If the \-v (verbose) flag is given, additional information is printed.
1084 sushi.1372a > wrl.nfs:
1085 148 read fh 21,11/12.195 8192 bytes @ 24576
1086 wrl.nfs > sushi.1372a:
1087 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1092 (\-v also prints the IP header TTL, ID, and fragmentation fields,
1093 which have been omitted from this example.) In the first line,
1094 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1095 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1096 second line is the first fragment of the reply, and hence is only 1472
1097 bytes long (the other bytes will follow in subsequent fragments, but
1098 these fragments do not have NFS or even UDP headers and so might not be
1099 printed, depending on the filter expression used). Because the \-v flag
1100 is given, some of the file attributes (which are returned in addition
1101 to the file data) are printed: the file type (``REG'', for regular file),
1102 the file mode (in octal), the uid and gid, and the file size.
1104 If the \-v flag is given more than once, even more details are printed.
1106 Note that NFS requests are very large and much of the detail won't be printed
1107 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1110 NFS reply packets do not explicitly identify the RPC operation. Instead,
1111 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1112 replies using the transaction ID. If a reply does not closely follow the
1113 corresponding request, it might not be parsable.
1115 AFS Request and Replies
1117 Transarc AFS (Andrew File System) requests and replies are printed
1123 \fIsrc.sport > dst.dport: rx packet-type\fP
1124 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1125 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1128 elvis.7001 > pike.afsfs:
1129 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1130 new fid 536876964/1/1 ".newsrc"
1131 pike.afsfs > elvis.7001: rx data fs reply rename
1136 In the first line, host elvis sends a RX packet to pike. This was
1137 a RX data packet to the fs (fileserver) service, and is the start of
1138 an RPC call. The RPC call was a rename, with the old directory file id
1139 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1140 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1141 responds with a RPC reply to the rename call (which was successful, because
1142 it was a data packet and not an abort packet).
1144 In general, all AFS RPCs are decoded at least by RPC call name. Most
1145 AFS RPCs have at least some of the arguments decoded (generally only
1146 the `interesting' arguments, for some definition of interesting).
1148 The format is intended to be self-describing, but it will probably
1149 not be useful to people who are not familiar with the workings of
1152 If the -v (verbose) flag is given, the RX call number and sequence number
1155 If the -v flag is given twice, additional information is printed.
1156 such as the the RX call ID, serial number, and the RX packet flags.
1157 The MTU negotiation information is also printed from RX ack packets.
1159 If the -v flag three times, the security index and service id are printed.
1161 Error codes are printed for abort packets, with the exception of Ubik
1162 beacon packets (because abort packets are used to signify a yes vote
1163 for the Ubik protocol).
1165 Note that AFS requests are very large and many of the arguments won't
1166 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1167 to watch AFS traffic.
1169 AFS reply packets do not explicitly identify the RPC operation. Instead,
1170 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1171 replies using the call number and service ID. If a reply does not closely
1173 corresponding request, it might not be parsable.
1176 KIP Appletalk (DDP in UDP)
1178 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1179 and dumped as DDP packets (i.e., all the UDP header information is
1180 discarded). The file
1182 is used to translate appletalk net and node numbers to names.
1183 Lines in this file have the form
1195 The first two lines give the names of appletalk networks. The third
1196 line gives the name of a particular host (a host is distinguished
1197 from a net by the 3rd octet in the number \-
1198 a net number \fImust\fP have two octets and a host number \fImust\fP
1199 have three octets.) The number and name should be separated by
1200 whitespace (blanks or tabs).
1203 file may contain blank lines or comment lines (lines starting with
1206 Appletalk addresses are printed in the form
1212 \f(CW144.1.209.2 > icsd-net.112.220
1213 office.2 > icsd-net.112.220
1214 jssmag.149.235 > icsd-net.2\fP
1220 doesn't exist or doesn't contain an entry for some appletalk
1221 host/net number, addresses are printed in numeric form.)
1222 In the first example, NBP (DDP port 2) on net 144.1 node 209
1223 is sending to whatever is listening on port 220 of net icsd node 112.
1224 The second line is the same except the full name of the source node
1225 is known (`office'). The third line is a send from port 235 on
1226 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1227 the broadcast address (255) is indicated by a net name with no host
1228 number \- for this reason it's a good idea to keep node names and
1229 net names distinct in /etc/atalk.names).
1231 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1232 packets have their contents interpreted. Other protocols just dump
1233 the protocol name (or number if no name is registered for the
1234 protocol) and packet size.
1236 \fBNBP packets\fP are formatted like the following examples:
1240 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1241 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1242 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fP\s+2
1246 The first line is a name lookup request for laserwriters sent by net icsd host
1247 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1248 The second line shows a reply for this request (note that it has the
1249 same id) from host jssmag.209 saying that it has a laserwriter
1250 resource named "RM1140" registered on port 250. The third line is
1251 another reply to the same request saying host techpit has laserwriter
1252 "techpit" registered on port 186.
1254 \fBATP packet\fP formatting is demonstrated by the following example:
1258 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1259 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1260 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1261 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1262 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1263 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1264 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1265 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1266 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1267 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1268 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1269 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1270 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1271 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fP\s+2
1275 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1276 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1277 is the value of the `userdata' field in the request.
1279 Helios responds with 8 512-byte packets. The `:digit' following the
1280 transaction id gives the packet sequence number in the transaction
1281 and the number in parens is the amount of data in the packet,
1282 excluding the atp header. The `*' on packet 7 indicates that the
1285 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1286 resends them then jssmag.209 releases the transaction. Finally,
1287 jssmag.209 initiates the next request. The `*' on the request
1288 indicates that XO (`exactly once') was \fInot\fP set.
1293 Fragmented Internet datagrams are printed as
1297 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1298 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1302 (The first form indicates there are more fragments. The second
1303 indicates this is the last fragment.)
1305 \fIId\fP is the fragment id. \fISize\fP is the fragment
1306 size (in bytes) excluding the IP header. \fIOffset\fP is this
1307 fragment's offset (in bytes) in the original datagram.
1309 The fragment information is output for each fragment. The first
1310 fragment contains the higher level protocol header and the frag
1311 info is printed after the protocol info. Fragments
1312 after the first contain no higher level protocol header and the
1313 frag info is printed after the source and destination addresses.
1314 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1315 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1319 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1320 arizona > rtsg: (frag 595a:204@328)
1321 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1325 There are a couple of things to note here: First, addresses in the
1326 2nd line don't include port numbers. This is because the TCP
1327 protocol information is all in the first fragment and we have no idea
1328 what the port or sequence numbers are when we print the later fragments.
1329 Second, the tcp sequence information in the first line is printed as if there
1330 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1331 the first frag and 204 in the second). If you are looking for holes
1332 in the sequence space or trying to match up acks
1333 with packets, this can fool you.
1335 A packet with the IP \fIdon't fragment\fP flag is marked with a
1336 trailing \fB(DF)\fP.
1340 By default, all output lines are preceded by a timestamp. The timestamp
1341 is the current clock time in the form
1347 and is as accurate as the kernel's clock.
1348 The timestamp reflects the time the kernel first saw the packet. No attempt
1349 is made to account for the time lag between when the
1350 ethernet interface removed the packet from the wire and when the kernel
1351 serviced the `new packet' interrupt.
1353 traffic(1C), nit(4P), bpf(4), pcap(3)
1357 Steven McCanne, all of the
1358 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1360 The current version is available via anonymous ftp:
1363 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1366 IPv6/IPsec support is added by WIDE/KAME project.
1367 This program uses Eric Young's SSLeay library, under specific configuration.
1369 Please send bug reports to tcpdump@ee.lbl.gov.
1371 NIT doesn't let you watch your own outbound traffic, BPF will.
1372 We recommend that you use the latter.
1374 Some attempt should be made to reassemble IP fragments or, at least
1375 to compute the right length for the higher level protocol.
1377 Name server inverse queries are not dumped correctly: The (empty)
1378 question section is printed rather than real query in the answer
1379 section. Some believe that inverse queries are themselves a bug and
1380 prefer to fix the program generating them rather than tcpdump.
1382 Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
1384 Even if we were inclined to do anything to promote the use of
1385 Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
1386 networks so we'd would have no way of testing this code.
1388 A packet trace that crosses a daylight savings time change will give
1389 skewed time stamps (the time change is ignored).
1391 Filters expressions that manipulate FDDI headers assume that all FDDI
1392 packets are encapsulated Ethernet packets. This is true for IP, ARP,
1393 and DECNET Phase IV, but is not true for protocols such as ISO CLNS.
1394 Therefore, the filter may inadvertently accept certain packets that
1395 do not properly match the filter expression.
1398 should chase header chain, but at this moment it does not.
1399 .BR "ip6 protochain"
1400 is supplied for this behavior.
1402 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1403 does not work against IPv6 packets.
1404 It only looks at IPv4 packets.
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