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23 .TH TCPDUMP 1 "3 January 2001"
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/write 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 \fBcast128-cbc\fP, or
137 The default is \fBdes-cbc\fP.
138 The ability to decrypt packets is only present if \fItcpdump\fP 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 truly `secret' key is discouraged.
145 By presenting IPsec secret key onto command line
146 you make it visible to others, via
151 Print `foreign' internet addresses numerically rather than symbolically
152 (this option is intended to get around serious brain damage in
153 Sun's yp server \(em usually it hangs forever translating non-local
157 Use \fIfile\fP as input for the filter expression.
158 An additional expression given on the command line is ignored.
161 Listen on \fIinterface\fP.
162 If unspecified, \fItcpdump\fP searches the system interface list for the
163 lowest numbered, configured up interface (excluding loopback).
164 Ties are broken by choosing the earliest match.
166 On Linux systems with 2.2 or later kernels, an
168 argument of ``any'' can be used to capture packets from all interfaces.
169 Note that captures on the ``any'' device will not be done in promiscuous
173 Make stdout line buffered. Useful if you want to see the data
174 while capturing it. E.g.,
176 ``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
177 ``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
180 Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
183 Don't print domain name qualification of host names. E.g.,
184 if you give this flag then \fItcpdump\fP will print ``nic''
185 instead of ``nic.ddn.mil''.
188 Load SMI MIB module definitions from file \fImodule\fR. This option
189 can be used several times to load several MIB modules into \fItcpdump\fP.
192 Do not run the packet-matching code optimizer. This is useful only
193 if you suspect a bug in the optimizer.
196 \fIDon't\fP put the interface
197 into promiscuous mode. Note that the interface might be in promiscuous
198 mode for some other reason; hence, `-p' cannot be used as an abbreviation for
199 `ether host {local-hw-addr} or ether broadcast'.
202 Quick (quiet?) output. Print less protocol information so output
206 Read packets from \fIfile\fR (which was created with the -w option).
207 Standard input is used if \fIfile\fR is ``-''.
210 Snarf \fIsnaplen\fP bytes of data from each packet rather than the
211 default of 68 (with SunOS's NIT, the minimum is actually 96).
212 68 bytes is adequate for IP, ICMP, TCP
213 and UDP but may truncate protocol information from name server and NFS
214 packets (see below). Packets truncated because of a limited snapshot
215 are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
216 is the name of the protocol level at which the truncation has occurred.
217 Note that taking larger snapshots both increases
218 the amount of time it takes to process packets and, effectively,
219 decreases the amount of packet buffering. This may cause packets to be
220 lost. You should limit \fIsnaplen\fP to the smallest number that will
221 capture the protocol information you're interested in. Setting
222 \fIsnaplen\fP to 0 means use the required length to catch whole packets.
225 Force packets selected by "\fIexpression\fP" to be interpreted the
226 specified \fItype\fR. Currently known types are
227 \fBcnfp\fR (Cisco NetFlow protocol),
228 \fBrpc\fR (Remote Procedure Call),
229 \fBrtp\fR (Real-Time Applications protocol),
230 \fBrtcp\fR (Real-Time Applications control protocol),
231 \fBsnmp\fR (Simple Network Management Protocol),
232 \fBvat\fR (Visual Audio Tool),
234 \fBwb\fR (distributed White Board).
237 Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829).
238 If specified, \fItcpdump\fP will not print replay prevention field.
239 Since there is no protocol version field in ESP/AH specification,
240 \fItcpdump\fP cannot deduce the version of ESP/AH protocol.
243 Print absolute, rather than relative, TCP sequence numbers.
246 \fIDon't\fP print a timestamp on each dump line.
249 Print an unformatted timestamp on each dump line.
252 Print a delta (in micro-seconds) between current and previous line
256 Print a timestamp in default format proceeded by date on each dump line.
259 (Slightly more) verbose output. For example, the time to live,
260 identification, total length and options in an IP packet are printed.
261 Also enables additional packet integrity checks such as verifying the
262 IP and ICMP header checksum.
265 Even more verbose output. For example, additional fields are
266 printed from NFS reply packets, and SMB packets are fully decoded.
269 Even more verbose output. For example,
270 telnet \fBSB\fP ... \fBSE\fP options
271 are printed in full. With
273 telnet options are printed in hex as well.
276 Write the raw packets to \fIfile\fR rather than parsing and printing
277 them out. They can later be printed with the \-r option.
278 Standard output is used if \fIfile\fR is ``-''.
281 Print each packet (minus its link level header) in hex.
282 The smaller of the entire packet or
284 bytes will be printed.
287 When printing hex, print ascii too. Thus if
289 is also set, the packet is printed in hex/ascii.
290 This is very handy for analysing new protocols.
293 is not also set, some parts of some packets may be printed
295 .IP "\fI expression\fP"
297 selects which packets will be dumped. If no \fIexpression\fP
298 is given, all packets on the net will be dumped. Otherwise,
299 only packets for which \fIexpression\fP is `true' will be dumped.
301 The \fIexpression\fP consists of one or more
303 Primitives usually consist of an
305 (name or number) preceded by one or more qualifiers. There are three
306 different kinds of qualifier:
308 qualifiers say what kind of thing the id name or number refers to.
314 E.g., `host foo', `net 128.3', `port 20'. If there is no type
319 qualifiers specify a particular transfer direction to and/or from
321 Possible directions are
328 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
329 there is no dir qualifier,
332 For `null' link layers (i.e. point to point protocols such as slip) the
336 qualifiers can be used to specify a desired direction.
338 qualifiers restrict the match to a particular protocol. Possible
351 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
352 no proto qualifier, all protocols consistent with the type are
353 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
354 (except the latter is not legal syntax), `net bar' means `(ip or
355 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
357 [`fddi' is actually an alias for `ether'; the parser treats them
358 identically as meaning ``the data link level used on the specified
359 network interface.'' FDDI headers contain Ethernet-like source
360 and destination addresses, and often contain Ethernet-like packet
361 types, so you can filter on these FDDI fields just as with the
362 analogous Ethernet fields. FDDI headers also contain other fields,
363 but you cannot name them explicitly in a filter expression.
365 Similarly, `tr' is an alias for `ether'; the previous paragraph's
366 statements about FDDI headers also apply to Token Ring headers.]
368 In addition to the above, there are some special `primitive' keywords
369 that don't follow the pattern:
374 and arithmetic expressions. All of these are described below.
376 More complex filter expressions are built up by using the words
381 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
382 To save typing, identical qualifier lists can be omitted. E.g.,
383 `tcp dst port ftp or ftp-data or domain' is exactly the same as
384 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
386 Allowable primitives are:
387 .IP "\fBdst host \fIhost\fR"
388 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
389 which may be either an address or a name.
390 .IP "\fBsrc host \fIhost\fR"
391 True if the IPv4/v6 source field of the packet is \fIhost\fP.
392 .IP "\fBhost \fIhost\fP
393 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
394 Any of the above host expressions can be prepended with the keywords,
395 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
398 \fBip host \fIhost\fR
401 which is equivalent to:
404 \fBether proto \fI\\ip\fB and host \fIhost\fR
407 If \fIhost\fR is a name with multiple IP addresses, each address will
408 be checked for a match.
409 .IP "\fBether dst \fIehost\fP
410 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
411 may be either a name from /etc/ethers or a number (see
414 .IP "\fBether src \fIehost\fP
415 True if the ethernet source address is \fIehost\fP.
416 .IP "\fBether host \fIehost\fP
417 True if either the ethernet source or destination address is \fIehost\fP.
418 .IP "\fBgateway\fP \fIhost\fP
419 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
420 source or destination address was \fIhost\fP but neither the IP source
421 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
422 must be found in both /etc/hosts and /etc/ethers. (An equivalent
426 \fBether host \fIehost \fBand not host \fIhost\fR
429 which can be used with either names or numbers for \fIhost / ehost\fP.)
430 This syntax does not work in IPv6-enabled configuration at this moment.
431 .IP "\fBdst net \fInet\fR"
432 True if the IPv4/v6 destination address of the packet has a network
433 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
434 or a network number (see \fInetworks(4)\fP for details).
435 .IP "\fBsrc net \fInet\fR"
436 True if the IPv4/v6 source address of the packet has a network
438 .IP "\fBnet \fInet\fR"
439 True if either the IPv4/v6 source or destination address of the packet has a network
441 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
442 True if the IP address matches \fInet\fR with the specific netmask.
443 May be qualified with \fBsrc\fR or \fBdst\fR.
444 Note that this syntax is not valid for IPv6 \fInet\fR.
445 .IP "\fBnet \fInet\fR/\fIlen\fR"
446 True if the IPv4/v6 address matches \fInet\fR a netmask \fIlen\fR bits wide.
447 May be qualified with \fBsrc\fR or \fBdst\fR.
448 .IP "\fBdst port \fIport\fR"
449 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
450 destination port value of \fIport\fP.
451 The \fIport\fP can be a number or a name used in /etc/services (see
455 If a name is used, both the port
456 number and protocol are checked. If a number or ambiguous name is used,
457 only the port number is checked (e.g., \fBdst port 513\fR will print both
458 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
459 both tcp/domain and udp/domain traffic).
460 .IP "\fBsrc port \fIport\fR"
461 True if the packet has a source port value of \fIport\fP.
462 .IP "\fBport \fIport\fR"
463 True if either the source or destination port of the packet is \fIport\fP.
464 Any of the above port expressions can be prepended with the keywords,
465 \fBtcp\fP or \fBudp\fP, as in:
468 \fBtcp src port \fIport\fR
471 which matches only tcp packets whose source port is \fIport\fP.
472 .IP "\fBless \fIlength\fR"
473 True if the packet has a length less than or equal to \fIlength\fP.
474 This is equivalent to:
477 \fBlen <= \fIlength\fP.
480 .IP "\fBgreater \fIlength\fR"
481 True if the packet has a length greater than or equal to \fIlength\fP.
482 This is equivalent to:
485 \fBlen >= \fIlength\fP.
488 .IP "\fBip proto \fIprotocol\fR"
489 True if the packet is an IP packet (see
491 of protocol type \fIprotocol\fP.
492 \fIProtocol\fP can be a number or one of the names
493 \fIicmp\fP, \fIicmp6\fP, \fIigmp\fP, \fIigrp\fP, \fIpim\fP, \fIah\fP,
494 \fIesp\fP, \fIvrrp\fP, \fIudp\fP, or \fItcp\fP.
495 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
496 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
497 Note that this primitive does not chase the protocol header chain.
498 .IP "\fBip6 proto \fIprotocol\fR"
499 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
500 Note that this primitive does not chase the protocol header chain.
501 .IP "\fBip6 protochain \fIprotocol\fR"
502 True if the packet is IPv6 packet,
503 and contains protocol header with type \fIprotocol\fR
504 in its protocol header chain.
508 \fBip6 protochain 6\fR
511 matches any IPv6 packet with TCP protocol header in the protocol header chain.
512 The packet may contain, for example,
513 authentication header, routing header, or hop-by-hop option header,
514 between IPv6 header and TCP header.
515 The BPF code emitted by this primitive is complex and
516 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
517 so this can be somewhat slow.
518 .IP "\fBip protochain \fIprotocol\fR"
519 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
520 .IP "\fBether broadcast\fR"
521 True if the packet is an ethernet broadcast packet. The \fIether\fP
523 .IP "\fBip broadcast\fR"
524 True if the packet is an IP broadcast packet. It checks for both
525 the all-zeroes and all-ones broadcast conventions, and looks up
526 the local subnet mask.
527 .IP "\fBether multicast\fR"
528 True if the packet is an ethernet multicast packet. The \fIether\fP
530 This is shorthand for `\fBether[0] & 1 != 0\fP'.
531 .IP "\fBip multicast\fR"
532 True if the packet is an IP multicast packet.
533 .IP "\fBip6 multicast\fR"
534 True if the packet is an IPv6 multicast packet.
535 .IP "\fBether proto \fIprotocol\fR"
536 True if the packet is of ether type \fIprotocol\fR.
537 \fIProtocol\fP can be a number or one of the names
538 \fIip\fP, \fIip6\fP, \fIarp\fP, \fIrarp\fP, \fIatalk\fP, \fIaarp\fP,
539 \fIdecnet\fP, \fIsca\fP, \fIlat\fP, \fImopdl\fP, \fImoprc\fP,
540 \fIiso\fP, \fIstp\fP, \fIipx\fP, or \fInetbeui\fP.
541 Note these identifiers are also keywords
542 and must be escaped via backslash (\\).
544 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR') and Token Ring
545 (e.g., `\fBtr protocol arp\fR'), for most of those protocols, the
546 protocol identification comes from the 802.2 Logical Link Control (LLC)
547 header, which is usually layered on top of the FDDI or Token Ring
550 When filtering for most protocol identifiers on FDDI or Token Ring,
551 \fBtcpdump\fR checks only the protocol ID field of an LLC header in
552 so-called SNAP format with an Organizational Unit Identifier (OUI) of
553 0x000000, for encapsulated Ethernet; it doesn't check whether the packet
554 is in SNAP format with an OUI of 0x000000.
556 The exceptions are \fIiso\fP, for which it checks the DSAP (Destination
557 Service Access Point) and SSAP (Source Service Access Point) fields of
558 the LLC header, \fIstp\fP and \fInetbeui\fP, where it checks the DSAP of
559 the LLC header, and \fIatalk\fP, where it checks for a SNAP-format
560 packet with an OUI of 0x080007 and the Appletalk etype.
562 In the case of Ethernet, \fBtcpdump\fR checks the Ethernet type field
563 for most of those protocols; the exceptions are \fIiso\fP, \fIsap\fP,
564 and \fInetbeui\fP, for which it checks for an 802.3 frame and then
565 checks the LLC header as it does for FDDI and Token Ring, \fIatalk\fP,
566 where it checks both for the Appletalk etype in an Ethernet frame and
567 for a SNAP-format packet as it does for FDDI and Token Ring, \fIaarp\fP,
568 where it checks for the Appletalk ARP etype in either an Ethernet frame
569 or an 802.2 SNAP frame with an OUI of 0x000000, and \fIipx\fP, where it
570 checks for the IPX etype in an Ethernet frame, the IPX DSAP in the LLC
571 header, the 802.3 with no LLC header encapsulation of IPX, and the IPX
572 etype in a SNAP frame.]
573 .IP "\fBdecnet src \fIhost\fR"
574 True if the DECNET source address is
576 which may be an address of the form ``10.123'', or a DECNET host
577 name. [DECNET host name support is only available on Ultrix systems
578 that are configured to run DECNET.]
579 .IP "\fBdecnet dst \fIhost\fR"
580 True if the DECNET destination address is
582 .IP "\fBdecnet host \fIhost\fR"
583 True if either the DECNET source or destination address is
585 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBatalk\fR, \fBaarp\fR, \fBdecnet\fR, \fBiso\fR, \fBstp\fR, \fBipx\fR, \fInetbeui\fP"
589 \fBether proto \fIp\fR
592 where \fIp\fR is one of the above protocols.
593 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
597 \fBether proto \fIp\fR
600 where \fIp\fR is one of the above protocols.
602 \fItcpdump\fP does not currently know how to parse these protocols.
603 .IP "\fBvlan \fI[vlan_id]\fR"
604 True if the packet is an IEEE 802.1Q VLAN packet.
605 If \fI[vlan_id]\fR is specified, only true is the packet has the specified
607 Note that the first \fBvlan\fR keyword encountered in \fIexpression\fR
608 changes the decoding offsets for the remainder of \fIexpression\fR
609 on the assumption that the packet is a VLAN packet.
610 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
614 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
617 where \fIp\fR is one of the above protocols.
618 .IP "\fBiso proto \fIprotocol\fR"
619 True if the packet is an OSI packet of protocol type \fIprotocol\fP.
620 \fIProtocol\fP can be a number or one of the names
621 \fIclnp\fP, \fIesis\fP, or \fIisis\fP.
622 .IP "\fBclnp\fR, \fBesis\fR, \fBisis\fR"
629 where \fIp\fR is one of the above protocols.
630 Note that \fItcpdump\fR does an incomplete job of parsing these protocols.
631 .IP "\fIexpr relop expr\fR"
632 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
633 and \fIexpr\fR is an arithmetic expression composed of integer constants
634 (expressed in standard C syntax), the normal binary operators
635 [+, -, *, /, &, |], a length operator, and special packet data accessors.
637 data inside the packet, use the following syntax:
640 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
643 \fIProto\fR is one of \fBether, fddi, tr,
644 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
645 indicates the protocol layer for the index operation.
646 Note that \fItcp, udp\fR and other upper-layer protocol types only
647 apply to IPv4, not IPv6 (this will be fixed in the future).
648 The byte offset, relative to the indicated protocol layer, is
650 \fISize\fR is optional and indicates the number of bytes in the
651 field of interest; it can be either one, two, or four, and defaults to one.
652 The length operator, indicated by the keyword \fBlen\fP, gives the
653 length of the packet.
655 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
656 The expression `\fBip[0] & 0xf != 5\fP'
657 catches all IP packets with options. The expression
658 `\fBip[6:2] & 0x1fff = 0\fP'
659 catches only unfragmented datagrams and frag zero of fragmented datagrams.
660 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
662 For instance, \fBtcp[0]\fP always means the first
663 byte of the TCP \fIheader\fP, and never means the first byte of an
664 intervening fragment.
666 Some offsets and field values may be expressed as names rather than
667 as numeric values. The following protocol header field offsets are
668 available: \fBicmptype\fP (ICMP type field), \fBicmpcode\fP (ICMP
669 code field), and \fBtcpflags\fP (TCP flags field).
671 The following ICMP type field values are available: \fBicmp-echoreply\fP,
672 \fBicmp-unreach\fP, \fBicmp-sourcequench\fP, \fBicmp-redirect\fP,
673 \fBicmp-echo\fP, \fBicmp-routeradvert\fP, \fBicmp-routersolicit\fP,
674 \fBicmp-timxceed\fP, \fBicmp-paramprob\fP, \fBicmp-tstamp\fP,
675 \fBicmp-tstampreply\fP, \fBicmp-ireq\fP, \fBicmp-ireqreply\fP,
676 \fBicmp-maskreq\fP, \fBicmp-maskreply\fP.
678 The following TCP flags field values are available: \fBtcp-fin\fP,
679 \fBtcp-syn\fP, \fBtcp-rst\fP, \fBtcp-push\fP, \fBtcp-push\fP,
680 \fBtcp-ack\fP, \fBtcp-urg\fP.
682 Primitives may be combined using:
684 A parenthesized group of primitives and operators
685 (parentheses are special to the Shell and must be escaped).
687 Negation (`\fB!\fP' or `\fBnot\fP').
689 Concatenation (`\fB&&\fP' or `\fBand\fP').
691 Alternation (`\fB||\fP' or `\fBor\fP').
693 Negation has highest precedence.
694 Alternation and concatenation have equal precedence and associate
695 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
696 are now required for concatenation.
698 If an identifier is given without a keyword, the most recent keyword
703 \fBnot host vs and ace\fR
709 \fBnot host vs and host ace\fR
712 which should not be confused with
715 \fBnot ( host vs or ace )\fR
719 Expression arguments can be passed to \fItcpdump\fP as either a single
720 argument or as multiple arguments, whichever is more convenient.
721 Generally, if the expression contains Shell metacharacters, it is
722 easier to pass it as a single, quoted argument.
723 Multiple arguments are concatenated with spaces before being parsed.
726 To print all packets arriving at or departing from \fIsundown\fP:
729 \fBtcpdump host sundown\fP
733 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
736 \fBtcpdump host helios and \\( hot or ace \\)\fP
740 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
743 \fBtcpdump ip host ace and not helios\fP
747 To print all traffic between local hosts and hosts at Berkeley:
751 tcpdump net ucb-ether
755 To print all ftp traffic through internet gateway \fIsnup\fP:
756 (note that the expression is quoted to prevent the shell from
757 (mis-)interpreting the parentheses):
761 tcpdump 'gateway snup and (port ftp or ftp-data)'
765 To print traffic neither sourced from nor destined for local hosts
766 (if you gateway to one other net, this stuff should never make it
767 onto your local net).
771 tcpdump ip and not net \fIlocalnet\fP
775 To print the start and end packets (the SYN and FIN packets) of each
776 TCP conversation that involves a non-local host.
780 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
784 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
788 tcpdump 'gateway snup and ip[2:2] > 576'
792 To print IP broadcast or multicast packets that were
794 sent via ethernet broadcast or multicast:
798 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
802 To print all ICMP packets that are not echo requests/replies (i.e., not
807 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
812 The output of \fItcpdump\fP is protocol dependent. The following
813 gives a brief description and examples of most of the formats.
821 If the '-e' option is given, the link level header is printed out.
822 On ethernets, the source and destination addresses, protocol,
823 and packet length are printed.
825 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
826 the `frame control' field, the source and destination addresses,
827 and the packet length. (The `frame control' field governs the
828 interpretation of the rest of the packet. Normal packets (such
829 as those containing IP datagrams) are `async' packets, with a priority
830 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
831 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
832 the LLC header is printed if it is \fInot\fR an ISO datagram or a
833 so-called SNAP packet.
835 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
836 the `access control' and `frame control' fields, the source and
837 destination addresses, and the packet length. As on FDDI networks,
838 packets are assumed to contain an LLC packet. Regardless of whether
839 the '-e' option is specified or not, the source routing information is
840 printed for source-routed packets.
842 \fI(N.B.: The following description assumes familiarity with
843 the SLIP compression algorithm described in RFC-1144.)\fP
845 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
846 packet type, and compression information are printed out.
847 The packet type is printed first.
848 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
849 No further link information is printed for \fIip\fR packets.
850 For TCP packets, the connection identifier is printed following the type.
851 If the packet is compressed, its encoded header is printed out.
852 The special cases are printed out as
853 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
854 the sequence number (or sequence number and ack) has changed.
855 If it is not a special case,
856 zero or more changes are printed.
857 A change is indicated by U (urgent pointer), W (window), A (ack),
858 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
860 Finally, the amount of data in the packet and compressed header length
863 For example, the following line shows an outbound compressed TCP packet,
864 with an implicit connection identifier; the ack has changed by 6,
865 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
866 data and 6 bytes of compressed header:
869 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
875 Arp/rarp output shows the type of request and its arguments. The
876 format is intended to be self explanatory.
877 Here is a short sample taken from the start of an `rlogin' from
878 host \fIrtsg\fP to host \fIcsam\fP:
882 \f(CWarp who-has csam tell rtsg
883 arp reply csam is-at CSAM\fR
887 The first line says that rtsg sent an arp packet asking
888 for the ethernet address of internet host csam. Csam
889 replies with its ethernet address (in this example, ethernet addresses
890 are in caps and internet addresses in lower case).
892 This would look less redundant if we had done \fBtcpdump \-n\fP:
896 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
897 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
901 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
902 broadcast and the second is point-to-point would be visible:
906 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
907 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
911 For the first packet this says the ethernet source address is RTSG, the
912 destination is the ethernet broadcast address, the type field
913 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
917 \fI(N.B.:The following description assumes familiarity with
918 the TCP protocol described in RFC-793. If you are not familiar
919 with the protocol, neither this description nor \fItcpdump\fP will
920 be of much use to you.)\fP
922 The general format of a tcp protocol line is:
926 \fIsrc > dst: flags data-seqno ack window urgent options\fP
930 \fISrc\fP and \fIdst\fP are the source and destination IP
931 addresses and ports. \fIFlags\fP are some combination of S (SYN),
932 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
933 \fIData-seqno\fP describes the portion of sequence space covered
934 by the data in this packet (see example below).
935 \fIAck\fP is sequence number of the next data expected the other
936 direction on this connection.
937 \fIWindow\fP is the number of bytes of receive buffer space available
938 the other direction on this connection.
939 \fIUrg\fP indicates there is `urgent' data in the packet.
940 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
942 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
943 depend on the contents of the packet's tcp protocol header and
944 are output only if appropriate.
946 Here is the opening portion of an rlogin from host \fIrtsg\fP to
951 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
952 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
953 rtsg.1023 > csam.login: . ack 1 win 4096
954 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
955 csam.login > rtsg.1023: . ack 2 win 4096
956 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
957 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
958 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
959 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
963 The first line says that tcp port 1023 on rtsg sent a packet
965 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
966 The packet sequence number was 768512 and it contained no data.
967 (The notation is `first:last(nbytes)' which means `sequence
969 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
970 There was no piggy-backed ack, the available receive window was 4096
971 bytes and there was a max-segment-size option requesting an mss of
974 Csam replies with a similar packet except it includes a piggy-backed
975 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
977 The packet contained no data so there is no data sequence number.
978 Note that the ack sequence
979 number is a small integer (1). The first time \fItcpdump\fP sees a
980 tcp `conversation', it prints the sequence number from the packet.
981 On subsequent packets of the conversation, the difference between
982 the current packet's sequence number and this initial sequence number
983 is printed. This means that sequence numbers after the
984 first can be interpreted
985 as relative byte positions in the conversation's data stream (with the
986 first data byte each direction being `1'). `-S' will override this
987 feature, causing the original sequence numbers to be output.
989 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
990 in the rtsg \(-> csam side of the conversation).
991 The PUSH flag is set in the packet.
992 On the 7th line, csam says it's received data sent by rtsg up to
993 but not including byte 21. Most of this data is apparently sitting in the
994 socket buffer since csam's receive window has gotten 19 bytes smaller.
995 Csam also sends one byte of data to rtsg in this packet.
996 On the 8th and 9th lines,
997 csam sends two bytes of urgent, pushed data to rtsg.
999 If the snapshot was small enough that \fItcpdump\fP didn't capture
1000 the full TCP header, it interprets as much of the header as it can
1001 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
1002 be interpreted. If the header contains a bogus option (one with a length
1003 that's either too small or beyond the end of the header), \fItcpdump\fP
1004 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
1005 options (since it's impossible to tell where they start). If the header
1006 length indicates options are present but the IP datagram length is not
1007 long enough for the options to actually be there, \fItcpdump\fP reports
1008 it as ``[\fIbad hdr length\fP]''.
1010 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
1012 There are 6 bits in the control bits section of the TCP header:
1014 .I URG | ACK | PSH | RST | SYN | FIN
1016 Let's assume that we want to watch packets used in establishing
1017 a TCP connection. Recall that TCP uses a 3-way handshake protocol
1018 when it initializes a new connection; the connection sequence with
1019 regard to the TCP control bits is
1025 2) Recipient responds with SYN, ACK
1031 Now we're interested in capturing packets that have only the
1032 SYN bit set (Step 1). Note that we don't want packets from step 2
1033 (SYN-ACK), just a plain initial SYN. What we need is a correct filter
1034 expression for \fItcpdump\fP.
1036 Recall the structure of a TCP header without options:
1040 -----------------------------------------------------------------
1041 | source port | destination port |
1042 -----------------------------------------------------------------
1044 -----------------------------------------------------------------
1045 | acknowledgment number |
1046 -----------------------------------------------------------------
1047 | HL | reserved |U|A|P|R|S|F| window size |
1048 -----------------------------------------------------------------
1049 | TCP checksum | urgent pointer |
1050 -----------------------------------------------------------------
1053 A TCP header usually holds 20 octets of data, unless options are
1054 present. The fist line of the graph contains octets 0 - 3, the
1055 second line shows octets 4 - 7 etc.
1057 Starting to count with 0, the relevant TCP control bits are contained
1062 ----------------|---------------|---------------|----------------
1063 | HL | reserved |U|A|P|R|S|F| window size |
1064 ----------------|---------------|---------------|----------------
1065 | | 13th octet | | |
1068 Let's have a closer look at octet no. 13:
1078 We see that this octet contains 2 bytes from the reserved field.
1079 According to RFC 793 this field is reserved for future use and must
1080 be 0. The remaining 6 bits are the TCP control bits we are interested
1081 in. We have numbered the bits in this octet from 0 to 7, right to
1082 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1084 Recall that we want to capture packets with only SYN set.
1085 Let's see what happens to octet 13 if a TCP datagram arrives
1086 with the SYN bit set in its header:
1096 We already mentioned that bits number 7 and 6 belong to the
1097 reserved field, so they must must be 0. Looking at the
1098 control bits section we see that only bit number 1 (SYN) is set.
1100 Assuming that octet number 13 is an 8-bit unsigned integer in
1101 network byte order, the binary value of this octet is
1105 and its decimal representation is
1109 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1112 We're almost done, because now we know that if only SYN is set,
1113 the value of the 13th octet in the TCP header, when interpreted
1114 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1116 This relationship can be expressed as
1122 We can use this expression as the filter for \fItcpdump\fP in order
1123 to watch packets which have only SYN set:
1126 tcpdump -i xl0 tcp[13] == 2
1129 The expression says "let the 13th octet of a TCP datagram have
1130 the decimal value 2", which is exactly what we want.
1132 Now, let's assume that we need to capture SYN packets, but we
1133 don't care if ACK or any other TCP control bit is set at the
1134 same time. Let's see what happens to octet 13 when a TCP datagram
1135 with SYN-ACK set arrives:
1145 Now bits 1 and 4 are set in the 13th octet. The binary value of
1150 which translates to decimal
1154 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1157 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1158 expression, because that would select only those packets that have
1159 SYN-ACK set, but not those with only SYN set. Remember that we don't care
1160 if ACK or any other control bit is set as long as SYN is set.
1162 In order to achieve our goal, we need to logically AND the
1163 binary value of octet 13 with some other value to preserve
1164 the SYN bit. We know that we want SYN to be set in any case,
1165 so we'll logically AND the value in the 13th octet with
1166 the binary value of a SYN:
1170 00010010 SYN-ACK 00000010 SYN
1171 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1173 = 00000010 = 00000010
1176 We see that this AND operation delivers the same result
1177 regardless whether ACK or another TCP control bit is set.
1178 The decimal representation of the AND value as well as
1179 the result of this operation is 2 (binary 00000010),
1180 so we know that for packets with SYN set the following
1181 relation must hold true:
1183 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1185 This points us to the \fItcpdump\fP filter expression
1188 tcpdump -i xl0 'tcp[13] & 2 == 2'
1191 Note that you should use single quotes or a backslash
1192 in the expression to hide the AND ('&') special character
1198 UDP format is illustrated by this rwho packet:
1202 \f(CWactinide.who > broadcast.who: udp 84\fP
1206 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1207 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1208 broadcast address. The packet contained 84 bytes of user data.
1210 Some UDP services are recognized (from the source or destination
1211 port number) and the higher level protocol information printed.
1212 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1213 RPC calls (RFC-1050) to NFS.
1215 UDP Name Server Requests
1217 \fI(N.B.:The following description assumes familiarity with
1218 the Domain Service protocol described in RFC-1035. If you are not familiar
1219 with the protocol, the following description will appear to be written
1222 Name server requests are formatted as
1226 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1228 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1232 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1233 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1234 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
1235 was set. The query length was 37 bytes, not including the UDP and
1236 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
1237 so the op field was omitted. If the op had been anything else, it would
1238 have been printed between the `3' and the `+'.
1239 Similarly, the qclass was the normal one,
1240 \fIC_IN\fP, and omitted. Any other qclass would have been printed
1241 immediately after the `A'.
1243 A few anomalies are checked and may result in extra fields enclosed in
1244 square brackets: If a query contains an answer, name server or
1250 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1251 is the appropriate count.
1252 If any of the response bits are set (AA, RA or rcode) or any of the
1253 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1254 is printed, where \fIx\fP is the hex value of header bytes two and three.
1256 UDP Name Server Responses
1258 Name server responses are formatted as
1262 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1264 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1265 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1269 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1270 with 3 answer records, 3 name server records and 7 authority records.
1271 The first answer record is type A (address) and its data is internet
1272 address 128.32.137.3. The total size of the response was 273 bytes,
1273 excluding UDP and IP headers. The op (Query) and response code
1274 (NoError) were omitted, as was the class (C_IN) of the A record.
1276 In the second example, \fIhelios\fP responds to query 2 with a
1277 response code of non-existent domain (NXDomain) with no answers,
1278 one name server and no authority records. The `*' indicates that
1279 the \fIauthoritative answer\fP bit was set. Since there were no
1280 answers, no type, class or data were printed.
1282 Other flag characters that might appear are `\-' (recursion available,
1283 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
1284 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1287 Note that name server requests and responses tend to be large and the
1288 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1289 to print. Use the \fB\-s\fP flag to increase the snaplen if you
1290 need to seriously investigate name server traffic. `\fB\-s 128\fP'
1291 has worked well for me.
1296 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1297 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1298 NetBEUI SMB data is also done.
1300 By default a fairly minimal decode is done, with a much more detailed
1301 decode done if -v is used. Be warned that with -v a single SMB packet
1302 may take up a page or more, so only use -v if you really want all the
1305 If you are decoding SMB sessions containing unicode strings then you
1306 may wish to set the environment variable USE_UNICODE to 1. A patch to
1307 auto-detect unicode srings would be welcome.
1309 For information on SMB packet formats and what all te fields mean see
1310 www.cifs.org or the pub/samba/specs/ directory on your favourite
1311 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1315 NFS Requests and Replies
1317 Sun NFS (Network File System) requests and replies are printed as:
1321 \fIsrc.xid > dst.nfs: len op args\fP
1322 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1325 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1326 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1327 sushi.201b > wrl.nfs:
1328 144 lookup fh 9,74/4096.6878 "xcolors"
1329 wrl.nfs > sushi.201b:
1330 reply ok 128 lookup fh 9,74/4134.3150
1335 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1336 to \fIwrl\fP (note that the number following the src host is a
1337 transaction id, \fInot\fP the source port). The request was 112 bytes,
1338 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1339 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1340 (If one is lucky, as in this case, the file handle can be interpreted
1341 as a major,minor device number pair, followed by the inode number and
1343 \fIWrl\fP replies `ok' with the contents of the link.
1345 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1346 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1347 depends on the operation type. The format is intended to be self
1348 explanatory if read in conjunction with
1349 an NFS protocol spec.
1351 If the \-v (verbose) flag is given, additional information is printed.
1357 sushi.1372a > wrl.nfs:
1358 148 read fh 21,11/12.195 8192 bytes @ 24576
1359 wrl.nfs > sushi.1372a:
1360 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1365 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1366 which have been omitted from this example.) In the first line,
1367 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1368 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1369 second line is the first fragment of the reply, and hence is only 1472
1370 bytes long (the other bytes will follow in subsequent fragments, but
1371 these fragments do not have NFS or even UDP headers and so might not be
1372 printed, depending on the filter expression used). Because the \-v flag
1373 is given, some of the file attributes (which are returned in addition
1374 to the file data) are printed: the file type (``REG'', for regular file),
1375 the file mode (in octal), the uid and gid, and the file size.
1377 If the \-v flag is given more than once, even more details are printed.
1379 Note that NFS requests are very large and much of the detail won't be printed
1380 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1383 NFS reply packets do not explicitly identify the RPC operation. Instead,
1384 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1385 replies using the transaction ID. If a reply does not closely follow the
1386 corresponding request, it might not be parsable.
1388 AFS Requests and Replies
1390 Transarc AFS (Andrew File System) requests and replies are printed
1396 \fIsrc.sport > dst.dport: rx packet-type\fP
1397 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1398 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1401 elvis.7001 > pike.afsfs:
1402 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1403 new fid 536876964/1/1 ".newsrc"
1404 pike.afsfs > elvis.7001: rx data fs reply rename
1409 In the first line, host elvis sends a RX packet to pike. This was
1410 a RX data packet to the fs (fileserver) service, and is the start of
1411 an RPC call. The RPC call was a rename, with the old directory file id
1412 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1413 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1414 responds with a RPC reply to the rename call (which was successful, because
1415 it was a data packet and not an abort packet).
1417 In general, all AFS RPCs are decoded at least by RPC call name. Most
1418 AFS RPCs have at least some of the arguments decoded (generally only
1419 the `interesting' arguments, for some definition of interesting).
1421 The format is intended to be self-describing, but it will probably
1422 not be useful to people who are not familiar with the workings of
1425 If the -v (verbose) flag is given twice, acknowledgement packets and
1426 additional header information is printed, such as the the RX call ID,
1427 call number, sequence number, serial number, and the RX packet flags.
1429 If the -v flag is given twice, additional information is printed,
1430 such as the the RX call ID, serial number, and the RX packet flags.
1431 The MTU negotiation information is also printed from RX ack packets.
1433 If the -v flag is given three times, the security index and service id
1436 Error codes are printed for abort packets, with the exception of Ubik
1437 beacon packets (because abort packets are used to signify a yes vote
1438 for the Ubik protocol).
1440 Note that AFS requests are very large and many of the arguments won't
1441 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1442 to watch AFS traffic.
1444 AFS reply packets do not explicitly identify the RPC operation. Instead,
1445 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1446 replies using the call number and service ID. If a reply does not closely
1448 corresponding request, it might not be parsable.
1451 KIP Appletalk (DDP in UDP)
1453 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1454 and dumped as DDP packets (i.e., all the UDP header information is
1455 discarded). The file
1457 is used to translate appletalk net and node numbers to names.
1458 Lines in this file have the form
1470 The first two lines give the names of appletalk networks. The third
1471 line gives the name of a particular host (a host is distinguished
1472 from a net by the 3rd octet in the number \-
1473 a net number \fImust\fP have two octets and a host number \fImust\fP
1474 have three octets.) The number and name should be separated by
1475 whitespace (blanks or tabs).
1478 file may contain blank lines or comment lines (lines starting with
1481 Appletalk addresses are printed in the form
1487 \f(CW144.1.209.2 > icsd-net.112.220
1488 office.2 > icsd-net.112.220
1489 jssmag.149.235 > icsd-net.2\fR
1495 doesn't exist or doesn't contain an entry for some appletalk
1496 host/net number, addresses are printed in numeric form.)
1497 In the first example, NBP (DDP port 2) on net 144.1 node 209
1498 is sending to whatever is listening on port 220 of net icsd node 112.
1499 The second line is the same except the full name of the source node
1500 is known (`office'). The third line is a send from port 235 on
1501 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1502 the broadcast address (255) is indicated by a net name with no host
1503 number \- for this reason it's a good idea to keep node names and
1504 net names distinct in /etc/atalk.names).
1506 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1507 packets have their contents interpreted. Other protocols just dump
1508 the protocol name (or number if no name is registered for the
1509 protocol) and packet size.
1511 \fBNBP packets\fP are formatted like the following examples:
1515 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1516 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1517 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1521 The first line is a name lookup request for laserwriters sent by net icsd host
1522 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1523 The second line shows a reply for this request (note that it has the
1524 same id) from host jssmag.209 saying that it has a laserwriter
1525 resource named "RM1140" registered on port 250. The third line is
1526 another reply to the same request saying host techpit has laserwriter
1527 "techpit" registered on port 186.
1529 \fBATP packet\fP formatting is demonstrated by the following example:
1533 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1534 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1535 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1536 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1537 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1538 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1539 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1540 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1541 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1542 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1543 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1544 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1545 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1546 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1550 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1551 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1552 is the value of the `userdata' field in the request.
1554 Helios responds with 8 512-byte packets. The `:digit' following the
1555 transaction id gives the packet sequence number in the transaction
1556 and the number in parens is the amount of data in the packet,
1557 excluding the atp header. The `*' on packet 7 indicates that the
1560 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1561 resends them then jssmag.209 releases the transaction. Finally,
1562 jssmag.209 initiates the next request. The `*' on the request
1563 indicates that XO (`exactly once') was \fInot\fP set.
1568 Fragmented Internet datagrams are printed as
1572 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1573 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1577 (The first form indicates there are more fragments. The second
1578 indicates this is the last fragment.)
1580 \fIId\fP is the fragment id. \fISize\fP is the fragment
1581 size (in bytes) excluding the IP header. \fIOffset\fP is this
1582 fragment's offset (in bytes) in the original datagram.
1584 The fragment information is output for each fragment. The first
1585 fragment contains the higher level protocol header and the frag
1586 info is printed after the protocol info. Fragments
1587 after the first contain no higher level protocol header and the
1588 frag info is printed after the source and destination addresses.
1589 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1590 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1594 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1595 arizona > rtsg: (frag 595a:204@328)
1596 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1600 There are a couple of things to note here: First, addresses in the
1601 2nd line don't include port numbers. This is because the TCP
1602 protocol information is all in the first fragment and we have no idea
1603 what the port or sequence numbers are when we print the later fragments.
1604 Second, the tcp sequence information in the first line is printed as if there
1605 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1606 the first frag and 204 in the second). If you are looking for holes
1607 in the sequence space or trying to match up acks
1608 with packets, this can fool you.
1610 A packet with the IP \fIdon't fragment\fP flag is marked with a
1611 trailing \fB(DF)\fP.
1615 By default, all output lines are preceded by a timestamp. The timestamp
1616 is the current clock time in the form
1622 and is as accurate as the kernel's clock.
1623 The timestamp reflects the time the kernel first saw the packet. No attempt
1624 is made to account for the time lag between when the
1625 ethernet interface removed the packet from the wire and when the kernel
1626 serviced the `new packet' interrupt.
1628 traffic(1C), nit(4P), bpf(4), pcap(3)
1630 The original authors are:
1634 Steven McCanne, all of the
1635 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1637 It is currently being maintained by tcpdump.org.
1639 The current version is available via http:
1642 .I http://www.tcpdump.org/
1645 The original distribution is available via anonymous ftp:
1648 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1651 IPv6/IPsec support is added by WIDE/KAME project.
1652 This program uses Eric Young's SSLeay library, under specific configuration.
1654 Please send problems, bugs, questions, desirable enhancements, etc. to:
1657 tcpdump-workers@tcpdump.org
1660 Please send source code contributions, etc. to:
1666 NIT doesn't let you watch your own outbound traffic, BPF will.
1667 We recommend that you use the latter.
1669 On Linux systems with 2.0[.x] kernels:
1671 packets on the loopback device will be seen twice;
1673 packet filtering cannot be done in the kernel, so that all packets must
1674 be copied from the kernel in order to be filtered in user mode;
1676 all of a packet, not just the part that's within the snapshot length,
1677 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1678 asked to copy only part of a packet to userland, will not report the
1679 true length of the packet; this would cause most IP packets to get an
1683 We recommend that you upgrade to a 2.2 or later kernel.
1685 Some attempt should be made to reassemble IP fragments or, at least
1686 to compute the right length for the higher level protocol.
1688 Name server inverse queries are not dumped correctly: the (empty)
1689 question section is printed rather than real query in the answer
1690 section. Some believe that inverse queries are themselves a bug and
1691 prefer to fix the program generating them rather than \fItcpdump\fP.
1693 A packet trace that crosses a daylight savings time change will give
1694 skewed time stamps (the time change is ignored).
1696 Filter expressions that manipulate FDDI or Token Ring headers assume
1697 that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet
1698 packets. This is true for IP, ARP, and DECNET Phase IV, but is not true
1699 for protocols such as ISO CLNS. Therefore, the filter may inadvertently
1700 accept certain packets that do not properly match the filter expression.
1702 Filter expressions on fields other than those that manipulate Token Ring
1703 headers will not correctly handle source-routed Token Ring packets.
1706 should chase header chain, but at this moment it does not.
1707 .BR "ip6 protochain"
1708 is supplied for this behavior.
1710 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1711 does not work against IPv6 packets.
1712 It only looks at IPv4 packets.
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