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23 .TH TCPDUMP 1 "30 June 1997"
25 tcpdump \- dump traffic on a network
75 \fITcpdump\fP prints out the headers of packets on a network interface
76 that match the boolean \fIexpression\fP.
78 .B Under SunOS with nit or bpf:
81 you must have read access to
85 .B Under Solaris with dlpi:
86 You must have read access to the network pseudo device, e.g.
88 .B Under HP-UX with dlpi:
89 You must be root or it must be installed setuid to root.
90 .B Under IRIX with snoop:
91 You must be root or it must be installed setuid to root.
93 You must be root or it must be installed setuid to root.
94 .B Under Ultrix and Digital UNIX:
95 Once the super-user has enabled promiscuous-mode operation using
100 You must have read access to
105 Attempt to convert network and broadcast addresses to names.
108 Exit after receiving \fIcount\fP packets.
111 Dump the compiled packet-matching code in a human readable form to
112 standard output and stop.
115 Dump packet-matching code as a
120 Dump packet-matching code as decimal numbers (preceded with a count).
123 Print the link-level header on each dump line.
126 Print `foreign' internet addresses numerically rather than symbolically
127 (this option is intended to get around serious brain damage in
128 Sun's yp server \(em usually it hangs forever translating non-local
132 Use \fIfile\fP as input for the filter expression.
133 An additional expression given on the command line is ignored.
136 Listen on \fIinterface\fP.
137 If unspecified, \fItcpdump\fP searches the system interface list for the
138 lowest numbered, configured up interface (excluding loopback).
139 Ties are broken by choosing the earliest match.
142 Make stdout line buffered. Useful if you want to see the data
143 while capturing it. E.g.,
145 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
146 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
149 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
152 Don't print domain name qualification of host names. E.g.,
153 if you give this flag then \fItcpdump\fP will print ``nic''
154 instead of ``nic.ddn.mil''.
157 Load SMI MIB module definitions from file \fImodule\fR. This option
158 can be used several times to load several MIB modules into tcpdump.
161 Do not run the packet-matching code optimizer. This is useful only
162 if you suspect a bug in the optimizer.
165 \fIDon't\fP put the interface
166 into promiscuous mode. Note that the interface might be in promiscuous
167 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
168 `ether host {local-hw-addr} or ether broadcast'.
171 Quick (quiet?) output. Print less protocol information so output
175 Read packets from \fIfile\fR (which was created with the -w option).
176 Standard input is used if \fIfile\fR is ``-''.
179 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
180 default of 68 (with SunOS's NIT, the minimum is actually 96).
181 68 bytes is adequate for IP, ICMP, TCP
182 and UDP but may truncate protocol information from name server and NFS
183 packets (see below). Packets truncated because of a limited snapshot
184 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
185 is the name of the protocol level at which the truncation has occurred.
186 Note that taking larger snapshots both increases
187 the amount of time it takes to process packets and, effectively,
188 decreases the amount of packet buffering. This may cause packets to be
189 lost. You should limit \fIsnaplen\fP to the smallest number that will
190 capture the protocol information you're interested in.
193 Force packets selected by "\fIexpression\fP" to be interpreted the
194 specified \fItype\fR. Currently known types are
195 \fBrpc\fR (Remote Procedure Call),
196 \fBrtp\fR (Real-Time Applications protocol),
197 \fBrtcp\fR (Real-Time Applications control protocol),
198 \fBsnmp\fR (Simple Network Management Protocol),
199 \fBvat\fR (Visual Audio Tool),
201 \fBwb\fR (distributed White Board).
204 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
205 If specified, \fItcpdump\fP will not print replay prevention field.
206 Since there is no protocol version field in ESP/AH specification,
207 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
210 Print absolute, rather than relative, TCP sequence numbers.
213 \fIDon't\fP print a timestamp on each dump line.
216 Print an unformatted timestamp on each dump line.
219 (Slightly more) verbose output. For example, the time to live
220 and type of service information in an IP packet is printed.
223 Even more verbose output. For example, additional fields are
224 printed from NFS reply packets.
227 Even more verbose output. For example,
228 telnet \fBSB\fP ... \fBSE\fP options
229 are printed in full. With
231 telnet options are printed in hex as well.
234 Write the raw packets to \fIfile\fR rather than parsing and printing
235 them out. They can later be printed with the \-r option.
236 Standard output is used if \fIfile\fR is ``-''.
239 Print each packet (minus its link level header) in hex.
240 The smaller of the entire packet or
242 bytes will be printed.
245 When printing hex, print ascii too. Thus if
247 is also set, the packet is printed in hex/ascii.
248 This is very handy for analysing new protocols.
251 is not also set, some parts of some packets may be printed
253 .IP "\fI expression\fP"
255 selects which packets will be dumped. If no \fIexpression\fP
256 is given, all packets on the net will be dumped. Otherwise,
257 only packets for which \fIexpression\fP is `true' will be dumped.
259 The \fIexpression\fP consists of one or more
261 Primitives usually consist of an
263 (name or number) preceded by one or more qualifiers. There are three
264 different kinds of qualifier:
266 qualifiers say what kind of thing the id name or number refers to.
272 E.g., `host foo', `net 128.3', `port 20'. If there is no type
277 qualifiers specify a particular transfer direction to and/or from
279 Possible directions are
286 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
287 there is no dir qualifier,
290 For `null' link layers (i.e. point to point protocols such as slip) the
294 qualifiers can be used to specify a desired direction.
296 qualifiers restrict the match to a particular protocol. Possible
314 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
315 no proto qualifier, all protocols consistent with the type are
316 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
317 (except the latter is not legal syntax), `net bar' means `(ip or
318 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
320 [`fddi' is actually an alias for `ether'; the parser treats them
321 identically as meaning ``the data link level used on the specified
322 network interface.'' FDDI headers contain Ethernet-like source
323 and destination addresses, and often contain Ethernet-like packet
324 types, so you can filter on these FDDI fields just as with the
325 analogous Ethernet fields. FDDI headers also contain other fields,
326 but you cannot name them explicitly in a filter expression.]
328 In addition to the above, there are some special `primitive' keywords
329 that don't follow the pattern:
334 and arithmetic expressions. All of these are described below.
336 More complex filter expressions are built up by using the words
341 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
342 To save typing, identical qualifier lists can be omitted. E.g.,
343 `tcp dst port ftp or ftp-data or domain' is exactly the same as
344 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
346 Allowable primitives are:
347 .IP "\fBdst host \fIhost\fR"
348 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
349 which may be either an address or a name.
350 .IP "\fBsrc host \fIhost\fR"
351 True if the IPv4/v6 source field of the packet is \fIhost\fP.
352 .IP "\fBhost \fIhost\fP
353 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
354 Any of the above host expressions can be prepended with the keywords,
355 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
358 \fBip host \fIhost\fR
361 which is equivalent to:
364 \fBether proto \fI\\ip\fB and host \fIhost\fR
367 If \fIhost\fR is a name with multiple IP addresses, each address will
368 be checked for a match.
369 .IP "\fBether dst \fIehost\fP
370 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
371 may be either a name from /etc/ethers or a number (see
374 .IP "\fBether src \fIehost\fP
375 True if the ethernet source address is \fIehost\fP.
376 .IP "\fBether host \fIehost\fP
377 True if either the ethernet source or destination address is \fIehost\fP.
378 .IP "\fBgateway\fP \fIhost\fP
379 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
380 source or destination address was \fIhost\fP but neither the IP source
381 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
382 must be found in both /etc/hosts and /etc/ethers. (An equivalent
386 \fBether host \fIehost \fBand not host \fIhost\fR
389 which can be used with either names or numbers for \fIhost / ehost\fP.)
390 This syntax does not work in IPv6-enabled configuration at this moment.
391 .IP "\fBdst net \fInet\fR"
392 True if the IPv4/v6 destination address of the packet has a network
393 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
394 or a network number (see \fInetworks(4)\fP for details).
395 .IP "\fBsrc net \fInet\fR"
396 True if the IPv4/v6 source address of the packet has a network
398 .IP "\fBnet \fInet\fR"
399 True if either the IPv4/v6 source or destination address of the packet has a network
401 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
402 True if the IP address matches \fInet\fR with the specific netmask.
403 May be qualified with \fBsrc\fR or \fBdst\fR.
404 Note that this syntax is not valid for IPv6 \fInet\fR.
405 .IP "\fBnet \fInet\fR/\fIlen\fR"
406 True if the IPv4/v6 address matches \fInet\fR a netmask \fIlen\fR bits wide.
407 May be qualified with \fBsrc\fR or \fBdst\fR.
408 .IP "\fBdst port \fIport\fR"
409 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
410 destination port value of \fIport\fP.
411 The \fIport\fP can be a number or a name used in /etc/services (see
415 If a name is used, both the port
416 number and protocol are checked. If a number or ambiguous name is used,
417 only the port number is checked (e.g., \fBdst port 513\fR will print both
418 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
419 both tcp/domain and udp/domain traffic).
420 .IP "\fBsrc port \fIport\fR"
421 True if the packet has a source port value of \fIport\fP.
422 .IP "\fBport \fIport\fR"
423 True if either the source or destination port of the packet is \fIport\fP.
424 Any of the above port expressions can be prepended with the keywords,
425 \fBtcp\fP or \fBudp\fP, as in:
428 \fBtcp src port \fIport\fR
431 which matches only tcp packets whose source port is \fIport\fP.
432 .IP "\fBless \fIlength\fR"
433 True if the packet has a length less than or equal to \fIlength\fP.
434 This is equivalent to:
437 \fBlen <= \fIlength\fP.
440 .IP "\fBgreater \fIlength\fR"
441 True if the packet has a length greater than or equal to \fIlength\fP.
442 This is equivalent to:
445 \fBlen >= \fIlength\fP.
448 .IP "\fBip proto \fIprotocol\fR"
449 True if the packet is an ip packet (see
451 of protocol type \fIprotocol\fP.
452 \fIProtocol\fP can be a number or one of the names
453 \fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
454 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
455 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
456 Note that this primitive does not chase protocol header chain.
457 .IP "\fBip6 proto \fIprotocol\fR"
458 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
459 Note that this primitive does not chase protocol header chain.
460 .IP "\fBip6 protochain \fIprotocol\fR"
461 True if the packet is IPv6 packet,
462 and contains protocol header with type \fIprotocol\fR
463 in its protocol header chain.
467 \fBip6 protochain 6\fR
470 matches any IPv6 packet with TCP protocol header in the protocol header chain.
471 The packet may contain, for example,
472 authentication header, routing header, or hop-by-hop option header,
473 between IPv6 header and TCP header.
474 The BPF code emitted by this primitive is complex and
475 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
476 so this can be somewhat slow.
477 .IP "\fBip protochain \fIprotocol\fR"
478 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
479 .IP "\fBether broadcast\fR"
480 True if the packet is an ethernet broadcast packet. The \fIether\fP
482 .IP "\fBip broadcast\fR"
483 True if the packet is an IP broadcast packet. It checks for both
484 the all-zeroes and all-ones broadcast conventions, and looks up
485 the local subnet mask.
486 .IP "\fBether multicast\fR"
487 True if the packet is an ethernet multicast packet. The \fIether\fP
489 This is shorthand for `\fBether[0] & 1 != 0\fP'.
490 .IP "\fBip multicast\fR"
491 True if the packet is an IP multicast packet.
492 .IP "\fBip6 multicast\fR"
493 True if the packet is an IPv6 multicast packet.
494 .IP "\fBether proto \fIprotocol\fR"
495 True if the packet is of ether type \fIprotocol\fR.
496 \fIProtocol\fP can be a number or a name like
497 \fIip\fP, \fIip6\fP, \fIarp\fP, or \fIrarp\fP.
498 Note these identifiers are also keywords
499 and must be escaped via backslash (\\).
500 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
501 protocol identification comes from the 802.2 Logical Link Control
502 (LLC) header, which is usually layered on top of the FDDI header.
503 \fITcpdump\fP assumes, when filtering on the protocol identifier,
504 that all FDDI packets include an LLC header, and that the LLC header
505 is in so-called SNAP format.]
506 .IP "\fBdecnet src \fIhost\fR"
507 True if the DECNET source address is
509 which may be an address of the form ``10.123'', or a DECNET host
510 name. [DECNET host name support is only available on Ultrix systems
511 that are configured to run DECNET.]
512 .IP "\fBdecnet dst \fIhost\fR"
513 True if the DECNET destination address is
515 .IP "\fBdecnet host \fIhost\fR"
516 True if either the DECNET source or destination address is
518 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR"
522 \fBether proto \fIp\fR
525 where \fIp\fR is one of the above protocols.
526 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
530 \fBether proto \fIp\fR
533 where \fIp\fR is one of the above protocols.
535 \fItcpdump\fP does not currently know how to parse these protocols.
536 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
540 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
543 where \fIp\fR is one of the above protocols.
544 .IP "\fIexpr relop expr\fR"
545 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
546 and \fIexpr\fR is an arithmetic expression composed of integer constants
547 (expressed in standard C syntax), the normal binary operators
548 [+, -, *, /, &, |], a length operator, and special packet data accessors.
550 data inside the packet, use the following syntax:
553 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
556 \fIProto\fR is one of \fBether, fddi,
557 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
558 indicates the protocol layer for the index operation.
559 Note that \fItcp, udp\fR and other upper-layer protocol types only
560 apply to IPv4, not IPv6 (this will be fixed in the future).
561 The byte offset, relative to the indicated protocol layer, is
563 \fISize\fR is optional and indicates the number of bytes in the
564 field of interest; it can be either one, two, or four, and defaults to one.
565 The length operator, indicated by the keyword \fBlen\fP, gives the
566 length of the packet.
568 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
569 The expression `\fBip[0] & 0xf != 5\fP'
570 catches all IP packets with options. The expression
571 `\fBip[6:2] & 0x1fff = 0\fP'
572 catches only unfragmented datagrams and frag zero of fragmented datagrams.
573 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
575 For instance, \fBtcp[0]\fP always means the first
576 byte of the TCP \fIheader\fP, and never means the first byte of an
577 intervening fragment.
579 Primitives may be combined using:
581 A parenthesized group of primitives and operators
582 (parentheses are special to the Shell and must be escaped).
584 Negation (`\fB!\fP' or `\fBnot\fP').
586 Concatenation (`\fB&&\fP' or `\fBand\fP').
588 Alternation (`\fB||\fP' or `\fBor\fP').
590 Negation has highest precedence.
591 Alternation and concatenation have equal precedence and associate
592 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
593 are now required for concatenation.
595 If an identifier is given without a keyword, the most recent keyword
600 \fBnot host vs and ace\fR
606 \fBnot host vs and host ace\fR
609 which should not be confused with
612 \fBnot ( host vs or ace )\fR
616 Expression arguments can be passed to tcpdump as either a single argument
617 or as multiple arguments, whichever is more convenient.
618 Generally, if the expression contains Shell metacharacters, it is
619 easier to pass it as a single, quoted argument.
620 Multiple arguments are concatenated with spaces before being parsed.
623 To print all packets arriving at or departing from \fIsundown\fP:
626 \fBtcpdump host sundown\fP
630 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
633 \fBtcpdump host helios and \\( hot or ace \\)\fP
637 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
640 \fBtcpdump ip host ace and not helios\fP
644 To print all traffic between local hosts and hosts at Berkeley:
648 tcpdump net ucb-ether
652 To print all ftp traffic through internet gateway \fIsnup\fP:
653 (note that the expression is quoted to prevent the shell from
654 (mis-)interpreting the parentheses):
658 tcpdump 'gateway snup and (port ftp or ftp-data)'
662 To print traffic neither sourced from nor destined for local hosts
663 (if you gateway to one other net, this stuff should never make it
664 onto your local net).
668 tcpdump ip and not net \fIlocalnet\fP
672 To print the start and end packets (the SYN and FIN packets) of each
673 TCP conversation that involves a non-local host.
677 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
681 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
685 tcpdump 'gateway snup and ip[2:2] > 576'
689 To print IP broadcast or multicast packets that were
691 sent via ethernet broadcast or multicast:
695 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
699 To print all ICMP packets that are not echo requests/replies (i.e., not
704 tcpdump 'icmp[0] != 8 and icmp[0] != 0"
709 The output of \fItcpdump\fP is protocol dependent. The following
710 gives a brief description and examples of most of the formats.
718 If the '-e' option is given, the link level header is printed out.
719 On ethernets, the source and destination addresses, protocol,
720 and packet length are printed.
722 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
723 the `frame control' field, the source and destination addresses,
724 and the packet length. (The `frame control' field governs the
725 interpretation of the rest of the packet. Normal packets (such
726 as those containing IP datagrams) are `async' packets, with a priority
727 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
728 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
729 the LLC header is printed if it is \fInot\fR an ISO datagram or a
730 so-called SNAP packet.
732 \fI(N.B.: The following description assumes familiarity with
733 the SLIP compression algorithm described in RFC-1144.)\fP
735 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
736 packet type, and compression information are printed out.
737 The packet type is printed first.
738 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
739 No further link information is printed for \fIip\fR packets.
740 For TCP packets, the connection identifier is printed following the type.
741 If the packet is compressed, its encoded header is printed out.
742 The special cases are printed out as
743 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
744 the sequence number (or sequence number and ack) has changed.
745 If it is not a special case,
746 zero or more changes are printed.
747 A change is indicated by U (urgent pointer), W (window), A (ack),
748 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
750 Finally, the amount of data in the packet and compressed header length
753 For example, the following line shows an outbound compressed TCP packet,
754 with an implicit connection identifier; the ack has changed by 6,
755 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
756 data and 6 bytes of compressed header:
759 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
765 Arp/rarp output shows the type of request and its arguments. The
766 format is intended to be self explanatory.
767 Here is a short sample taken from the start of an `rlogin' from
768 host \fIrtsg\fP to host \fIcsam\fP:
772 \f(CWarp who-has csam tell rtsg
773 arp reply csam is-at CSAM\fR
777 The first line says that rtsg sent an arp packet asking
778 for the ethernet address of internet host csam. Csam
779 replies with its ethernet address (in this example, ethernet addresses
780 are in caps and internet addresses in lower case).
782 This would look less redundant if we had done \fBtcpdump \-n\fP:
786 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
787 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
791 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
792 broadcast and the second is point-to-point would be visible:
796 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
797 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
801 For the first packet this says the ethernet source address is RTSG, the
802 destination is the ethernet broadcast address, the type field
803 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
807 \fI(N.B.:The following description assumes familiarity with
808 the TCP protocol described in RFC-793. If you are not familiar
809 with the protocol, neither this description nor tcpdump will
810 be of much use to you.)\fP
812 The general format of a tcp protocol line is:
816 \fIsrc > dst: flags data-seqno ack window urgent options\fP
820 \fISrc\fP and \fIdst\fP are the source and destination IP
821 addresses and ports. \fIFlags\fP are some combination of S (SYN),
822 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
823 \fIData-seqno\fP describes the portion of sequence space covered
824 by the data in this packet (see example below).
825 \fIAck\fP is sequence number of the next data expected the other
826 direction on this connection.
827 \fIWindow\fP is the number of bytes of receive buffer space available
828 the other direction on this connection.
829 \fIUrg\fP indicates there is `urgent' data in the packet.
830 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
832 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
833 depend on the contents of the packet's tcp protocol header and
834 are output only if appropriate.
836 Here is the opening portion of an rlogin from host \fIrtsg\fP to
841 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
842 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
843 rtsg.1023 > csam.login: . ack 1 win 4096
844 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
845 csam.login > rtsg.1023: . ack 2 win 4096
846 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
847 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
848 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
849 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
853 The first line says that tcp port 1023 on rtsg sent a packet
855 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
856 The packet sequence number was 768512 and it contained no data.
857 (The notation is `first:last(nbytes)' which means `sequence
859 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
860 There was no piggy-backed ack, the available receive window was 4096
861 bytes and there was a max-segment-size option requesting an mss of
864 Csam replies with a similar packet except it includes a piggy-backed
865 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
867 The packet contained no data so there is no data sequence number.
868 Note that the ack sequence
869 number is a small integer (1). The first time \fBtcpdump\fP sees a
870 tcp `conversation', it prints the sequence number from the packet.
871 On subsequent packets of the conversation, the difference between
872 the current packet's sequence number and this initial sequence number
873 is printed. This means that sequence numbers after the
874 first can be interpreted
875 as relative byte positions in the conversation's data stream (with the
876 first data byte each direction being `1'). `-S' will override this
877 feature, causing the original sequence numbers to be output.
879 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
880 in the rtsg \(-> csam side of the conversation).
881 The PUSH flag is set in the packet.
882 On the 7th line, csam says it's received data sent by rtsg up to
883 but not including byte 21. Most of this data is apparently sitting in the
884 socket buffer since csam's receive window has gotten 19 bytes smaller.
885 Csam also sends one byte of data to rtsg in this packet.
886 On the 8th and 9th lines,
887 csam sends two bytes of urgent, pushed data to rtsg.
889 If the snapshot was small enough that \fBtcpdump\fP didn't capture
890 the full TCP header, it interprets as much of the header as it can
891 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
892 be interpreted. If the header contains a bogus option (one with a length
893 that's either too small or beyond the end of the header), tcpdump reports
894 it as ``[\fIbad opt\fP]'' and does not interpret any further options (since
895 it's impossible to tell where they start). If the header length indicates
896 options are present but the IP datagram length is not long enough for the
897 options to actually be there, tcpdump reports it as ``[\fIbad hdr length\fP]''.
902 UDP format is illustrated by this rwho packet:
906 \f(CWactinide.who > broadcast.who: udp 84\fP
910 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
911 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
912 broadcast address. The packet contained 84 bytes of user data.
914 Some UDP services are recognized (from the source or destination
915 port number) and the higher level protocol information printed.
916 In particular, Domain Name service requests (RFC-1034/1035) and Sun
917 RPC calls (RFC-1050) to NFS.
919 UDP Name Server Requests
921 \fI(N.B.:The following description assumes familiarity with
922 the Domain Service protocol described in RFC-1035. If you are not familiar
923 with the protocol, the following description will appear to be written
926 Name server requests are formatted as
930 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
932 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
936 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
937 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
938 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
939 was set. The query length was 37 bytes, not including the UDP and
940 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
941 so the op field was omitted. If the op had been anything else, it would
942 have been printed between the `3' and the `+'.
943 Similarly, the qclass was the normal one,
944 \fIC_IN\fP, and omitted. Any other qclass would have been printed
945 immediately after the `A'.
947 A few anomalies are checked and may result in extra fields enclosed in
948 square brackets: If a query contains an answer, name server or
954 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
955 is the appropriate count.
956 If any of the response bits are set (AA, RA or rcode) or any of the
957 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
958 is printed, where \fIx\fP is the hex value of header bytes two and three.
960 UDP Name Server Responses
962 Name server responses are formatted as
966 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
968 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
969 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
973 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
974 with 3 answer records, 3 name server records and 7 authority records.
975 The first answer record is type A (address) and its data is internet
976 address 128.32.137.3. The total size of the response was 273 bytes,
977 excluding UDP and IP headers. The op (Query) and response code
978 (NoError) were omitted, as was the class (C_IN) of the A record.
980 In the second example, \fIhelios\fP responds to query 2 with a
981 response code of non-existent domain (NXDomain) with no answers,
982 one name server and no authority records. The `*' indicates that
983 the \fIauthoritative answer\fP bit was set. Since there were no
984 answers, no type, class or data were printed.
986 Other flag characters that might appear are `\-' (recursion available,
987 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
988 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
991 Note that name server requests and responses tend to be large and the
992 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
993 to print. Use the \fB\-s\fP flag to increase the snaplen if you
994 need to seriously investigate name server traffic. `\fB\-s 128\fP'
995 has worked well for me.
1000 tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data
1001 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1002 NetBEUI SMB data is also done.
1004 By default a fairly minimal decode is done, with a much more detailed
1005 decode done if -v is used. Be warned that with -v a single SMB packet
1006 may take up a page or more, so only use -v if you really want all the
1009 If you are decoding SMB sessions containing unicode strings then you
1010 may wish to set the environment variable USE_UNICODE to 1. A patch to
1011 auto-detect unicode srings would be welcome.
1013 For information on SMB packet formats and what all te fields mean see
1014 www.cifs.org or the pub/samba/specs/ directory on your favourite
1015 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1019 NFS Requests and Replies
1021 Sun NFS (Network File System) requests and replies are printed as:
1025 \fIsrc.xid > dst.nfs: len op args\fP
1026 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1029 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1030 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1031 sushi.201b > wrl.nfs:
1032 144 lookup fh 9,74/4096.6878 "xcolors"
1033 wrl.nfs > sushi.201b:
1034 reply ok 128 lookup fh 9,74/4134.3150
1039 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1040 to \fIwrl\fP (note that the number following the src host is a
1041 transaction id, \fInot\fP the source port). The request was 112 bytes,
1042 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1043 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1044 (If one is lucky, as in this case, the file handle can be interpreted
1045 as a major,minor device number pair, followed by the inode number and
1047 \fIWrl\fP replies `ok' with the contents of the link.
1049 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1050 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1051 depends on the operation type. The format is intended to be self
1052 explanatory if read in conjunction with
1053 an NFS protocol spec.
1055 If the \-v (verbose) flag is given, additional information is printed.
1061 sushi.1372a > wrl.nfs:
1062 148 read fh 21,11/12.195 8192 bytes @ 24576
1063 wrl.nfs > sushi.1372a:
1064 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1069 (\-v also prints the IP header TTL, ID, and fragmentation fields,
1070 which have been omitted from this example.) In the first line,
1071 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1072 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1073 second line is the first fragment of the reply, and hence is only 1472
1074 bytes long (the other bytes will follow in subsequent fragments, but
1075 these fragments do not have NFS or even UDP headers and so might not be
1076 printed, depending on the filter expression used). Because the \-v flag
1077 is given, some of the file attributes (which are returned in addition
1078 to the file data) are printed: the file type (``REG'', for regular file),
1079 the file mode (in octal), the uid and gid, and the file size.
1081 If the \-v flag is given more than once, even more details are printed.
1083 Note that NFS requests are very large and much of the detail won't be printed
1084 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1087 NFS reply packets do not explicitly identify the RPC operation. Instead,
1088 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1089 replies using the transaction ID. If a reply does not closely follow the
1090 corresponding request, it might not be parsable.
1092 AFS Requests and Replies
1094 Transarc AFS (Andrew File System) requests and replies are printed
1100 \fIsrc.sport > dst.dport: rx packet-type\fP
1101 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1102 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1105 elvis.7001 > pike.afsfs:
1106 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1107 new fid 536876964/1/1 ".newsrc"
1108 pike.afsfs > elvis.7001: rx data fs reply rename
1113 In the first line, host elvis sends a RX packet to pike. This was
1114 a RX data packet to the fs (fileserver) service, and is the start of
1115 an RPC call. The RPC call was a rename, with the old directory file id
1116 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1117 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1118 responds with a RPC reply to the rename call (which was successful, because
1119 it was a data packet and not an abort packet).
1121 In general, all AFS RPCs are decoded at least by RPC call name. Most
1122 AFS RPCs have at least some of the arguments decoded (generally only
1123 the `interesting' arguments, for some definition of interesting).
1125 The format is intended to be self-describing, but it will probably
1126 not be useful to people who are not familiar with the workings of
1129 If the -v (verbose) flag is given twice, additional information is printed,
1130 such as the the RX call ID, call number, sequence number, serial number,
1131 and the RX packet flags.
1133 If the -v flag is given again, the security index and service id are printed.
1135 Error codes are printed for abort packets, with the exception of Ubik
1136 beacon packets (because abort packets are used to signify a yes vote
1137 for the Ubik protocol).
1139 Note that AFS requests are very large and many of the arguments won't
1140 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1141 to watch AFS traffic.
1143 AFS reply packets do not explicitly identify the RPC operation. Instead,
1144 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1145 replies using the call number and service ID. If a reply does not closely
1147 corresponding request, it might not be parsable.
1150 KIP Appletalk (DDP in UDP)
1152 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1153 and dumped as DDP packets (i.e., all the UDP header information is
1154 discarded). The file
1156 is used to translate appletalk net and node numbers to names.
1157 Lines in this file have the form
1169 The first two lines give the names of appletalk networks. The third
1170 line gives the name of a particular host (a host is distinguished
1171 from a net by the 3rd octet in the number \-
1172 a net number \fImust\fP have two octets and a host number \fImust\fP
1173 have three octets.) The number and name should be separated by
1174 whitespace (blanks or tabs).
1177 file may contain blank lines or comment lines (lines starting with
1180 Appletalk addresses are printed in the form
1186 \f(CW144.1.209.2 > icsd-net.112.220
1187 office.2 > icsd-net.112.220
1188 jssmag.149.235 > icsd-net.2\fR
1194 doesn't exist or doesn't contain an entry for some appletalk
1195 host/net number, addresses are printed in numeric form.)
1196 In the first example, NBP (DDP port 2) on net 144.1 node 209
1197 is sending to whatever is listening on port 220 of net icsd node 112.
1198 The second line is the same except the full name of the source node
1199 is known (`office'). The third line is a send from port 235 on
1200 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1201 the broadcast address (255) is indicated by a net name with no host
1202 number \- for this reason it's a good idea to keep node names and
1203 net names distinct in /etc/atalk.names).
1205 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1206 packets have their contents interpreted. Other protocols just dump
1207 the protocol name (or number if no name is registered for the
1208 protocol) and packet size.
1210 \fBNBP packets\fP are formatted like the following examples:
1214 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1215 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1216 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1220 The first line is a name lookup request for laserwriters sent by net icsd host
1221 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1222 The second line shows a reply for this request (note that it has the
1223 same id) from host jssmag.209 saying that it has a laserwriter
1224 resource named "RM1140" registered on port 250. The third line is
1225 another reply to the same request saying host techpit has laserwriter
1226 "techpit" registered on port 186.
1228 \fBATP packet\fP formatting is demonstrated by the following example:
1232 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1233 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1234 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1235 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1236 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1237 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1238 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1239 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1240 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1241 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1242 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1243 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1244 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1245 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1249 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1250 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1251 is the value of the `userdata' field in the request.
1253 Helios responds with 8 512-byte packets. The `:digit' following the
1254 transaction id gives the packet sequence number in the transaction
1255 and the number in parens is the amount of data in the packet,
1256 excluding the atp header. The `*' on packet 7 indicates that the
1259 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1260 resends them then jssmag.209 releases the transaction. Finally,
1261 jssmag.209 initiates the next request. The `*' on the request
1262 indicates that XO (`exactly once') was \fInot\fP set.
1267 Fragmented Internet datagrams are printed as
1271 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1272 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1276 (The first form indicates there are more fragments. The second
1277 indicates this is the last fragment.)
1279 \fIId\fP is the fragment id. \fISize\fP is the fragment
1280 size (in bytes) excluding the IP header. \fIOffset\fP is this
1281 fragment's offset (in bytes) in the original datagram.
1283 The fragment information is output for each fragment. The first
1284 fragment contains the higher level protocol header and the frag
1285 info is printed after the protocol info. Fragments
1286 after the first contain no higher level protocol header and the
1287 frag info is printed after the source and destination addresses.
1288 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1289 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1293 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1294 arizona > rtsg: (frag 595a:204@328)
1295 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1299 There are a couple of things to note here: First, addresses in the
1300 2nd line don't include port numbers. This is because the TCP
1301 protocol information is all in the first fragment and we have no idea
1302 what the port or sequence numbers are when we print the later fragments.
1303 Second, the tcp sequence information in the first line is printed as if there
1304 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1305 the first frag and 204 in the second). If you are looking for holes
1306 in the sequence space or trying to match up acks
1307 with packets, this can fool you.
1309 A packet with the IP \fIdon't fragment\fP flag is marked with a
1310 trailing \fB(DF)\fP.
1314 By default, all output lines are preceded by a timestamp. The timestamp
1315 is the current clock time in the form
1321 and is as accurate as the kernel's clock.
1322 The timestamp reflects the time the kernel first saw the packet. No attempt
1323 is made to account for the time lag between when the
1324 ethernet interface removed the packet from the wire and when the kernel
1325 serviced the `new packet' interrupt.
1327 traffic(1C), nit(4P), bpf(4), pcap(3)
1329 The original authors are:
1333 Steven McCanne, all of the
1334 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1336 It is currently being maintained by tcpdump.org.
1339 .I http://www.tcpdump.org/
1342 IPv6/IPsec support is added by WIDE/KAME project.
1343 This program uses Eric Young's SSLeay library, under specific configuration.
1345 Please send problems, bugs, questions, desirable enhancements, etc. to:
1348 tcpdump-workers@tcpdump.org
1351 Please send source code contributions, etc. to:
1357 NIT doesn't let you watch your own outbound traffic, BPF will.
1358 We recommend that you use the latter.
1360 Some attempt should be made to reassemble IP fragments or, at least
1361 to compute the right length for the higher level protocol.
1363 Name server inverse queries are not dumped correctly: The (empty)
1364 question section is printed rather than real query in the answer
1365 section. Some believe that inverse queries are themselves a bug and
1366 prefer to fix the program generating them rather than tcpdump.
1368 Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
1370 Even if we were inclined to do anything to promote the use of
1371 Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
1372 networks so we'd would have no way of testing this code.
1374 A packet trace that crosses a daylight savings time change will give
1375 skewed time stamps (the time change is ignored).
1377 Filters expressions that manipulate FDDI headers assume that all FDDI
1378 packets are encapsulated Ethernet packets. This is true for IP, ARP,
1379 and DECNET Phase IV, but is not true for protocols such as ISO CLNS.
1380 Therefore, the filter may inadvertently accept certain packets that
1381 do not properly match the filter expression.
1384 should chase header chain, but at this moment it does not.
1388 should chase header chain too.
1390 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1391 does not work against IPv6 packets.
1392 It only looks at IPv4 packets.
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