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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 (Slightly more) verbose output. For example, the time to live,
253 identification, total length and options in an IP packet are printed.
254 Also enables additional packet integrity checks such as verifying the
255 IP and ICMP header checksum.
258 Even more verbose output. For example, additional fields are
259 printed from NFS reply packets, and SMB packets are fully decoded.
262 Even more verbose output. For example,
263 telnet \fBSB\fP ... \fBSE\fP options
264 are printed in full. With
266 telnet options are printed in hex as well.
269 Write the raw packets to \fIfile\fR rather than parsing and printing
270 them out. They can later be printed with the \-r option.
271 Standard output is used if \fIfile\fR is ``-''.
274 Print each packet (minus its link level header) in hex.
275 The smaller of the entire packet or
277 bytes will be printed.
280 When printing hex, print ascii too. Thus if
282 is also set, the packet is printed in hex/ascii.
283 This is very handy for analysing new protocols.
286 is not also set, some parts of some packets may be printed
288 .IP "\fI expression\fP"
290 selects which packets will be dumped. If no \fIexpression\fP
291 is given, all packets on the net will be dumped. Otherwise,
292 only packets for which \fIexpression\fP is `true' will be dumped.
294 The \fIexpression\fP consists of one or more
296 Primitives usually consist of an
298 (name or number) preceded by one or more qualifiers. There are three
299 different kinds of qualifier:
301 qualifiers say what kind of thing the id name or number refers to.
307 E.g., `host foo', `net 128.3', `port 20'. If there is no type
312 qualifiers specify a particular transfer direction to and/or from
314 Possible directions are
321 E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
322 there is no dir qualifier,
325 For `null' link layers (i.e. point to point protocols such as slip) the
329 qualifiers can be used to specify a desired direction.
331 qualifiers restrict the match to a particular protocol. Possible
344 E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
345 no proto qualifier, all protocols consistent with the type are
346 assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
347 (except the latter is not legal syntax), `net bar' means `(ip or
348 arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
350 [`fddi' is actually an alias for `ether'; the parser treats them
351 identically as meaning ``the data link level used on the specified
352 network interface.'' FDDI headers contain Ethernet-like source
353 and destination addresses, and often contain Ethernet-like packet
354 types, so you can filter on these FDDI fields just as with the
355 analogous Ethernet fields. FDDI headers also contain other fields,
356 but you cannot name them explicitly in a filter expression.
358 Similarly, `tr' is an alias for `ether'; the previous paragraph's
359 statements about FDDI headers also apply to Token Ring headers.]
361 In addition to the above, there are some special `primitive' keywords
362 that don't follow the pattern:
367 and arithmetic expressions. All of these are described below.
369 More complex filter expressions are built up by using the words
374 to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
375 To save typing, identical qualifier lists can be omitted. E.g.,
376 `tcp dst port ftp or ftp-data or domain' is exactly the same as
377 `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
379 Allowable primitives are:
380 .IP "\fBdst host \fIhost\fR"
381 True if the IPv4/v6 destination field of the packet is \fIhost\fP,
382 which may be either an address or a name.
383 .IP "\fBsrc host \fIhost\fR"
384 True if the IPv4/v6 source field of the packet is \fIhost\fP.
385 .IP "\fBhost \fIhost\fP
386 True if either the IPv4/v6 source or destination of the packet is \fIhost\fP.
387 Any of the above host expressions can be prepended with the keywords,
388 \fBip\fP, \fBarp\fP, \fBrarp\fP, or \fBip6\fP as in:
391 \fBip host \fIhost\fR
394 which is equivalent to:
397 \fBether proto \fI\\ip\fB and host \fIhost\fR
400 If \fIhost\fR is a name with multiple IP addresses, each address will
401 be checked for a match.
402 .IP "\fBether dst \fIehost\fP
403 True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
404 may be either a name from /etc/ethers or a number (see
407 .IP "\fBether src \fIehost\fP
408 True if the ethernet source address is \fIehost\fP.
409 .IP "\fBether host \fIehost\fP
410 True if either the ethernet source or destination address is \fIehost\fP.
411 .IP "\fBgateway\fP \fIhost\fP
412 True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
413 source or destination address was \fIhost\fP but neither the IP source
414 nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
415 must be found in both /etc/hosts and /etc/ethers. (An equivalent
419 \fBether host \fIehost \fBand not host \fIhost\fR
422 which can be used with either names or numbers for \fIhost / ehost\fP.)
423 This syntax does not work in IPv6-enabled configuration at this moment.
424 .IP "\fBdst net \fInet\fR"
425 True if the IPv4/v6 destination address of the packet has a network
426 number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
427 or a network number (see \fInetworks(4)\fP for details).
428 .IP "\fBsrc net \fInet\fR"
429 True if the IPv4/v6 source address of the packet has a network
431 .IP "\fBnet \fInet\fR"
432 True if either the IPv4/v6 source or destination address of the packet has a network
434 .IP "\fBnet \fInet\fR \fBmask \fImask\fR"
435 True if the IP address matches \fInet\fR with the specific netmask.
436 May be qualified with \fBsrc\fR or \fBdst\fR.
437 Note that this syntax is not valid for IPv6 \fInet\fR.
438 .IP "\fBnet \fInet\fR/\fIlen\fR"
439 True if the IPv4/v6 address matches \fInet\fR a netmask \fIlen\fR bits wide.
440 May be qualified with \fBsrc\fR or \fBdst\fR.
441 .IP "\fBdst port \fIport\fR"
442 True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a
443 destination port value of \fIport\fP.
444 The \fIport\fP can be a number or a name used in /etc/services (see
448 If a name is used, both the port
449 number and protocol are checked. If a number or ambiguous name is used,
450 only the port number is checked (e.g., \fBdst port 513\fR will print both
451 tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
452 both tcp/domain and udp/domain traffic).
453 .IP "\fBsrc port \fIport\fR"
454 True if the packet has a source port value of \fIport\fP.
455 .IP "\fBport \fIport\fR"
456 True if either the source or destination port of the packet is \fIport\fP.
457 Any of the above port expressions can be prepended with the keywords,
458 \fBtcp\fP or \fBudp\fP, as in:
461 \fBtcp src port \fIport\fR
464 which matches only tcp packets whose source port is \fIport\fP.
465 .IP "\fBless \fIlength\fR"
466 True if the packet has a length less than or equal to \fIlength\fP.
467 This is equivalent to:
470 \fBlen <= \fIlength\fP.
473 .IP "\fBgreater \fIlength\fR"
474 True if the packet has a length greater than or equal to \fIlength\fP.
475 This is equivalent to:
478 \fBlen >= \fIlength\fP.
481 .IP "\fBip proto \fIprotocol\fR"
482 True if the packet is an IP packet (see
484 of protocol type \fIprotocol\fP.
485 \fIProtocol\fP can be a number or one of the names
486 \fIicmp\fP, \fIicmp6\fP, \fIigmp\fP, \fIigrp\fP, \fIpim\fP, \fIah\fP,
487 \fIesp\fP, \fIvrrp\fP, \fIudp\fP, or \fItcp\fP.
488 Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
489 keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
490 Note that this primitive does not chase the protocol header chain.
491 .IP "\fBip6 proto \fIprotocol\fR"
492 True if the packet is an IPv6 packet of protocol type \fIprotocol\fP.
493 Note that this primitive does not chase the protocol header chain.
494 .IP "\fBip6 protochain \fIprotocol\fR"
495 True if the packet is IPv6 packet,
496 and contains protocol header with type \fIprotocol\fR
497 in its protocol header chain.
501 \fBip6 protochain 6\fR
504 matches any IPv6 packet with TCP protocol header in the protocol header chain.
505 The packet may contain, for example,
506 authentication header, routing header, or hop-by-hop option header,
507 between IPv6 header and TCP header.
508 The BPF code emitted by this primitive is complex and
509 cannot be optimized by BPF optimizer code in \fItcpdump\fP,
510 so this can be somewhat slow.
511 .IP "\fBip protochain \fIprotocol\fR"
512 Equivalent to \fBip6 protochain \fIprotocol\fR, but this is for IPv4.
513 .IP "\fBether broadcast\fR"
514 True if the packet is an ethernet broadcast packet. The \fIether\fP
516 .IP "\fBip broadcast\fR"
517 True if the packet is an IP broadcast packet. It checks for both
518 the all-zeroes and all-ones broadcast conventions, and looks up
519 the local subnet mask.
520 .IP "\fBether multicast\fR"
521 True if the packet is an ethernet multicast packet. The \fIether\fP
523 This is shorthand for `\fBether[0] & 1 != 0\fP'.
524 .IP "\fBip multicast\fR"
525 True if the packet is an IP multicast packet.
526 .IP "\fBip6 multicast\fR"
527 True if the packet is an IPv6 multicast packet.
528 .IP "\fBether proto \fIprotocol\fR"
529 True if the packet is of ether type \fIprotocol\fR.
530 \fIProtocol\fP can be a number or one of the names
531 \fIip\fP, \fIip6\fP, \fIarp\fP, \fIrarp\fP, \fIatalk\fP, \fIaarp\fP,
532 \fIdecnet\fP, \fIsca\fP, \fIlat\fP, \fImopdl\fP, \fImoprc\fP,
533 \fIiso\fP, \fIstp\fP, \fIipx\fP, or \fInetbeui\fP.
534 Note these identifiers are also keywords
535 and must be escaped via backslash (\\).
537 [In the case of FDDI (e.g., `\fBfddi protocol arp\fR') and Token Ring
538 (e.g., `\fBtr protocol arp\fR'), for most of those protocols, the
539 protocol identification comes from the 802.2 Logical Link Control (LLC)
540 header, which is usually layered on top of the FDDI or Token Ring
543 When filtering for most protocol identifiers on FDDI or Token Ring,
544 \fBtcpdump\fR checks only the protocol ID field of an LLC header in
545 so-called SNAP format with an Organizational Unit Identifier (OUI) of
546 0x000000, for encapsulated Ethernet; it doesn't check whether the packet
547 is in SNAP format with an OUI of 0x000000.
549 The exceptions are \fIiso\fP, for which it checks the DSAP (Destination
550 Service Access Point) and SSAP (Source Service Access Point) fields of
551 the LLC header, \fIstp\fP and \fInetbeui\fP, where it checks the DSAP of
552 the LLC header, and \fIatalk\fP, where it checks for a SNAP-format
553 packet with an OUI of 0x080007 and the Appletalk etype.
555 In the case of Ethernet, \fBtcpdump\fR checks the Ethernet type field
556 for most of those protocols; the exceptions are \fIiso\fP, \fIsap\fP,
557 and \fInetbeui\fP, for which it checks for an 802.3 frame and then
558 checks the LLC header as it does for FDDI and Token Ring, \fIatalk\fP,
559 where it checks both for the Appletalk etype in an Ethernet frame and
560 for a SNAP-format packet as it does for FDDI and Token Ring, \fIaarp\fP,
561 where it checks for the Appletalk ARP etype in either an Ethernet frame
562 or an 802.2 SNAP frame with an OUI of 0x000000, and \fIipx\fP, where it
563 checks for the IPX etype in an Ethernet frame, the IPX DSAP in the LLC
564 header, the 802.3 with no LLC header encapsulation of IPX, and the IPX
565 etype in a SNAP frame.]
566 .IP "\fBdecnet src \fIhost\fR"
567 True if the DECNET source address is
569 which may be an address of the form ``10.123'', or a DECNET host
570 name. [DECNET host name support is only available on Ultrix systems
571 that are configured to run DECNET.]
572 .IP "\fBdecnet dst \fIhost\fR"
573 True if the DECNET destination address is
575 .IP "\fBdecnet host \fIhost\fR"
576 True if either the DECNET source or destination address is
578 .IP "\fBip\fR, \fBip6\fR, \fBarp\fR, \fBrarp\fR, \fBatalk\fR, \fBaarp\fR, \fBdecnet\fR, \fBiso\fR, \fBstp\fR, \fBipx\fR, \fInetbeui\fP"
582 \fBether proto \fIp\fR
585 where \fIp\fR is one of the above protocols.
586 .IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
590 \fBether proto \fIp\fR
593 where \fIp\fR is one of the above protocols.
595 \fItcpdump\fP does not currently know how to parse these protocols.
596 .IP "\fBvlan \fI[vlan_id]\fR"
597 True if the packet is an IEEE 802.1Q VLAN packet.
598 If \fI[vlan_id]\fR is specified, only true is the packet has the specified
600 Note that the first \fBvlan\fR keyword encountered in \fIexpression\fR
601 changes the decoding offsets for the remainder of \fIexpression\fR
602 on the assumption that the packet is a VLAN packet.
603 .IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
607 \fBip proto \fIp\fR\fB or ip6 proto \fIp\fR
610 where \fIp\fR is one of the above protocols.
611 .IP "\fBiso proto \fIprotocol\fR"
612 True if the packet is an OSI packet of protocol type \fIprotocol\fP.
613 \fIProtocol\fP can be a number or one of the names
614 \fIclnp\fP, \fIesis\fP, or \fIisis\fP.
615 .IP "\fBclnp\fR, \fBesis\fR, \fBisis\fR"
622 where \fIp\fR is one of the above protocols.
623 Note that \fItcpdump\fR does an incomplete job of parsing these protocols.
624 .IP "\fIexpr relop expr\fR"
625 True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
626 and \fIexpr\fR is an arithmetic expression composed of integer constants
627 (expressed in standard C syntax), the normal binary operators
628 [+, -, *, /, &, |], a length operator, and special packet data accessors.
630 data inside the packet, use the following syntax:
633 \fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
636 \fIProto\fR is one of \fBether, fddi, tr,
637 ip, arp, rarp, tcp, udp, icmp\fR or \fBip6\fR, and
638 indicates the protocol layer for the index operation.
639 Note that \fItcp, udp\fR and other upper-layer protocol types only
640 apply to IPv4, not IPv6 (this will be fixed in the future).
641 The byte offset, relative to the indicated protocol layer, is
643 \fISize\fR is optional and indicates the number of bytes in the
644 field of interest; it can be either one, two, or four, and defaults to one.
645 The length operator, indicated by the keyword \fBlen\fP, gives the
646 length of the packet.
648 For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
649 The expression `\fBip[0] & 0xf != 5\fP'
650 catches all IP packets with options. The expression
651 `\fBip[6:2] & 0x1fff = 0\fP'
652 catches only unfragmented datagrams and frag zero of fragmented datagrams.
653 This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
655 For instance, \fBtcp[0]\fP always means the first
656 byte of the TCP \fIheader\fP, and never means the first byte of an
657 intervening fragment.
659 Some offsets and field values may be expressed as names rather than
660 as numeric values. The following protocol header field offsets are
661 available: \fBicmptype\fP (ICMP type field), \fBicmpcode\fP (ICMP
662 code field), and \fBtcpflags\fP (TCP flags field).
664 The following ICMP type field values are available: \fBicmp-echoreply\fP,
665 \fBicmp-unreach\fP, \fBicmp-sourcequench\fP, \fBicmp-redirect\fP,
666 \fBicmp-echo\fP, \fBicmp-routeradvert\fP, \fBicmp-routersolicit\fP,
667 \fBicmp-timxceed\fP, \fBicmp-paramprob\fP, \fBicmp-tstamp\fP,
668 \fBicmp-tstampreply\fP, \fBicmp-ireq\fP, \fBicmp-ireqreply\fP,
669 \fBicmp-maskreq\fP, \fBicmp-maskreply\fP.
671 The following TCP flags field values are available: \fBtcp-fin\fP,
672 \fBtcp-syn\fP, \fBtcp-rst\fP, \fBtcp-push\fP, \fBtcp-push\fP,
673 \fBtcp-ack\fP, \fBtcp-urg\fP.
675 Primitives may be combined using:
677 A parenthesized group of primitives and operators
678 (parentheses are special to the Shell and must be escaped).
680 Negation (`\fB!\fP' or `\fBnot\fP').
682 Concatenation (`\fB&&\fP' or `\fBand\fP').
684 Alternation (`\fB||\fP' or `\fBor\fP').
686 Negation has highest precedence.
687 Alternation and concatenation have equal precedence and associate
688 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
689 are now required for concatenation.
691 If an identifier is given without a keyword, the most recent keyword
696 \fBnot host vs and ace\fR
702 \fBnot host vs and host ace\fR
705 which should not be confused with
708 \fBnot ( host vs or ace )\fR
712 Expression arguments can be passed to \fItcpdump\fP as either a single
713 argument or as multiple arguments, whichever is more convenient.
714 Generally, if the expression contains Shell metacharacters, it is
715 easier to pass it as a single, quoted argument.
716 Multiple arguments are concatenated with spaces before being parsed.
719 To print all packets arriving at or departing from \fIsundown\fP:
722 \fBtcpdump host sundown\fP
726 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
729 \fBtcpdump host helios and \\( hot or ace \\)\fP
733 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
736 \fBtcpdump ip host ace and not helios\fP
740 To print all traffic between local hosts and hosts at Berkeley:
744 tcpdump net ucb-ether
748 To print all ftp traffic through internet gateway \fIsnup\fP:
749 (note that the expression is quoted to prevent the shell from
750 (mis-)interpreting the parentheses):
754 tcpdump 'gateway snup and (port ftp or ftp-data)'
758 To print traffic neither sourced from nor destined for local hosts
759 (if you gateway to one other net, this stuff should never make it
760 onto your local net).
764 tcpdump ip and not net \fIlocalnet\fP
768 To print the start and end packets (the SYN and FIN packets) of each
769 TCP conversation that involves a non-local host.
773 tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net \fIlocalnet\fP'
777 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
781 tcpdump 'gateway snup and ip[2:2] > 576'
785 To print IP broadcast or multicast packets that were
787 sent via ethernet broadcast or multicast:
791 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
795 To print all ICMP packets that are not echo requests/replies (i.e., not
800 tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
805 The output of \fItcpdump\fP is protocol dependent. The following
806 gives a brief description and examples of most of the formats.
814 If the '-e' option is given, the link level header is printed out.
815 On ethernets, the source and destination addresses, protocol,
816 and packet length are printed.
818 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
819 the `frame control' field, the source and destination addresses,
820 and the packet length. (The `frame control' field governs the
821 interpretation of the rest of the packet. Normal packets (such
822 as those containing IP datagrams) are `async' packets, with a priority
823 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
824 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
825 the LLC header is printed if it is \fInot\fR an ISO datagram or a
826 so-called SNAP packet.
828 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
829 the `access control' and `frame control' fields, the source and
830 destination addresses, and the packet length. As on FDDI networks,
831 packets are assumed to contain an LLC packet. Regardless of whether
832 the '-e' option is specified or not, the source routing information is
833 printed for source-routed packets.
835 \fI(N.B.: The following description assumes familiarity with
836 the SLIP compression algorithm described in RFC-1144.)\fP
838 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
839 packet type, and compression information are printed out.
840 The packet type is printed first.
841 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
842 No further link information is printed for \fIip\fR packets.
843 For TCP packets, the connection identifier is printed following the type.
844 If the packet is compressed, its encoded header is printed out.
845 The special cases are printed out as
846 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
847 the sequence number (or sequence number and ack) has changed.
848 If it is not a special case,
849 zero or more changes are printed.
850 A change is indicated by U (urgent pointer), W (window), A (ack),
851 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
853 Finally, the amount of data in the packet and compressed header length
856 For example, the following line shows an outbound compressed TCP packet,
857 with an implicit connection identifier; the ack has changed by 6,
858 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
859 data and 6 bytes of compressed header:
862 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
868 Arp/rarp output shows the type of request and its arguments. The
869 format is intended to be self explanatory.
870 Here is a short sample taken from the start of an `rlogin' from
871 host \fIrtsg\fP to host \fIcsam\fP:
875 \f(CWarp who-has csam tell rtsg
876 arp reply csam is-at CSAM\fR
880 The first line says that rtsg sent an arp packet asking
881 for the ethernet address of internet host csam. Csam
882 replies with its ethernet address (in this example, ethernet addresses
883 are in caps and internet addresses in lower case).
885 This would look less redundant if we had done \fBtcpdump \-n\fP:
889 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
890 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
894 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
895 broadcast and the second is point-to-point would be visible:
899 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
900 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
904 For the first packet this says the ethernet source address is RTSG, the
905 destination is the ethernet broadcast address, the type field
906 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
910 \fI(N.B.:The following description assumes familiarity with
911 the TCP protocol described in RFC-793. If you are not familiar
912 with the protocol, neither this description nor \fItcpdump\fP will
913 be of much use to you.)\fP
915 The general format of a tcp protocol line is:
919 \fIsrc > dst: flags data-seqno ack window urgent options\fP
923 \fISrc\fP and \fIdst\fP are the source and destination IP
924 addresses and ports. \fIFlags\fP are some combination of S (SYN),
925 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
926 \fIData-seqno\fP describes the portion of sequence space covered
927 by the data in this packet (see example below).
928 \fIAck\fP is sequence number of the next data expected the other
929 direction on this connection.
930 \fIWindow\fP is the number of bytes of receive buffer space available
931 the other direction on this connection.
932 \fIUrg\fP indicates there is `urgent' data in the packet.
933 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
935 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
936 depend on the contents of the packet's tcp protocol header and
937 are output only if appropriate.
939 Here is the opening portion of an rlogin from host \fIrtsg\fP to
944 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
945 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
946 rtsg.1023 > csam.login: . ack 1 win 4096
947 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
948 csam.login > rtsg.1023: . ack 2 win 4096
949 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
950 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
951 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
952 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
956 The first line says that tcp port 1023 on rtsg sent a packet
958 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
959 The packet sequence number was 768512 and it contained no data.
960 (The notation is `first:last(nbytes)' which means `sequence
962 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
963 There was no piggy-backed ack, the available receive window was 4096
964 bytes and there was a max-segment-size option requesting an mss of
967 Csam replies with a similar packet except it includes a piggy-backed
968 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
970 The packet contained no data so there is no data sequence number.
971 Note that the ack sequence
972 number is a small integer (1). The first time \fItcpdump\fP sees a
973 tcp `conversation', it prints the sequence number from the packet.
974 On subsequent packets of the conversation, the difference between
975 the current packet's sequence number and this initial sequence number
976 is printed. This means that sequence numbers after the
977 first can be interpreted
978 as relative byte positions in the conversation's data stream (with the
979 first data byte each direction being `1'). `-S' will override this
980 feature, causing the original sequence numbers to be output.
982 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
983 in the rtsg \(-> csam side of the conversation).
984 The PUSH flag is set in the packet.
985 On the 7th line, csam says it's received data sent by rtsg up to
986 but not including byte 21. Most of this data is apparently sitting in the
987 socket buffer since csam's receive window has gotten 19 bytes smaller.
988 Csam also sends one byte of data to rtsg in this packet.
989 On the 8th and 9th lines,
990 csam sends two bytes of urgent, pushed data to rtsg.
992 If the snapshot was small enough that \fItcpdump\fP didn't capture
993 the full TCP header, it interprets as much of the header as it can
994 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
995 be interpreted. If the header contains a bogus option (one with a length
996 that's either too small or beyond the end of the header), \fItcpdump\fP
997 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
998 options (since it's impossible to tell where they start). If the header
999 length indicates options are present but the IP datagram length is not
1000 long enough for the options to actually be there, \fItcpdump\fP reports
1001 it as ``[\fIbad hdr length\fP]''.
1003 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
1005 There are 6 bits in the control bits section of the TCP header:
1007 .I URG | ACK | PSH | RST | SYN | FIN
1009 Let's assume that we want to watch packets used in establishing
1010 a TCP connection. Recall that TCP uses a 3-way handshake protocol
1011 when it initializes a new connection; the connection sequence with
1012 regard to the TCP control bits is
1018 2) Recipient responds with SYN, ACK
1024 Now we're interested in capturing packets that have only the
1025 SYN bit set (Step 1). Note that we don't want packets from step 2
1026 (SYN-ACK), just a plain initial SYN. What we need is a correct filter
1027 expression for \fItcpdump\fP.
1029 Recall the structure of a TCP header without options:
1033 -----------------------------------------------------------------
1034 | source port | destination port |
1035 -----------------------------------------------------------------
1037 -----------------------------------------------------------------
1038 | acknowledgment number |
1039 -----------------------------------------------------------------
1040 | HL | reserved |U|A|P|R|S|F| window size |
1041 -----------------------------------------------------------------
1042 | TCP checksum | urgent pointer |
1043 -----------------------------------------------------------------
1046 A TCP header usually holds 20 octets of data, unless options are
1047 present. The fist line of the graph contains octets 0 - 3, the
1048 second line shows octets 4 - 7 etc.
1050 Starting to count with 0, the relevant TCP control bits are contained
1055 ----------------|---------------|---------------|----------------
1056 | HL | reserved |U|A|P|R|S|F| window size |
1057 ----------------|---------------|---------------|----------------
1058 | | 13th octet | | |
1061 Let's have a closer look at octet no. 13:
1071 We see that this octet contains 2 bytes from the reserved field.
1072 According to RFC 793 this field is reserved for future use and must
1073 be 0. The remaining 6 bits are the TCP control bits we are interested
1074 in. We have numbered the bits in this octet from 0 to 7, right to
1075 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1077 Recall that we want to capture packets with only SYN set.
1078 Let's see what happens to octet 13 if a TCP datagram arrives
1079 with the SYN bit set in its header:
1089 We already mentioned that bits number 7 and 6 belong to the
1090 reserved field, so they must must be 0. Looking at the
1091 control bits section we see that only bit number 1 (SYN) is set.
1093 Assuming that octet number 13 is an 8-bit unsigned integer in
1094 network byte order, the binary value of this octet is
1098 and its decimal representation is
1102 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1105 We're almost done, because now we know that if only SYN is set,
1106 the value of the 13th octet in the TCP header, when interpreted
1107 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1109 This relationship can be expressed as
1115 We can use this expression as the filter for \fItcpdump\fP in order
1116 to watch packets which have only SYN set:
1119 tcpdump -i xl0 tcp[13] == 2
1122 The expression says "let the 13th octet of a TCP datagram have
1123 the decimal value 2", which is exactly what we want.
1125 Now, let's assume that we need to capture SYN packets, but we
1126 don't care if ACK or any other TCP control bit is set at the
1127 same time. Let's see what happens to octet 13 when a TCP datagram
1128 with SYN-ACK set arrives:
1138 Now bits 1 and 4 are set in the 13th octet. The binary value of
1143 which translates to decimal
1147 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1150 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1151 expression, because that would select only those packets that have
1152 SYN-ACK set, but not those with only SYN set. Remember that we don't care
1153 if ACK or any other control bit is set as long as SYN is set.
1155 In order to achieve our goal, we need to logically AND the
1156 binary value of octet 13 with some other value to preserve
1157 the SYN bit. We know that we want SYN to be set in any case,
1158 so we'll logically AND the value in the 13th octet with
1159 the binary value of a SYN:
1163 00010010 SYN-ACK 00000010 SYN
1164 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1166 = 00000010 = 00000010
1169 We see that this AND operation delivers the same result
1170 regardless whether ACK or another TCP control bit is set.
1171 The decimal representation of the AND value as well as
1172 the result of this operation is 2 (binary 00000010),
1173 so we know that for packets with SYN set the following
1174 relation must hold true:
1176 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1178 This points us to the \fItcpdump\fP filter expression
1181 tcpdump -i xl0 'tcp[13] & 2 == 2'
1184 Note that you should use single quotes or a backslash
1185 in the expression to hide the AND ('&') special character
1191 UDP format is illustrated by this rwho packet:
1195 \f(CWactinide.who > broadcast.who: udp 84\fP
1199 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1200 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1201 broadcast address. The packet contained 84 bytes of user data.
1203 Some UDP services are recognized (from the source or destination
1204 port number) and the higher level protocol information printed.
1205 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1206 RPC calls (RFC-1050) to NFS.
1208 UDP Name Server Requests
1210 \fI(N.B.:The following description assumes familiarity with
1211 the Domain Service protocol described in RFC-1035. If you are not familiar
1212 with the protocol, the following description will appear to be written
1215 Name server requests are formatted as
1219 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1221 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1225 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1226 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1227 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
1228 was set. The query length was 37 bytes, not including the UDP and
1229 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
1230 so the op field was omitted. If the op had been anything else, it would
1231 have been printed between the `3' and the `+'.
1232 Similarly, the qclass was the normal one,
1233 \fIC_IN\fP, and omitted. Any other qclass would have been printed
1234 immediately after the `A'.
1236 A few anomalies are checked and may result in extra fields enclosed in
1237 square brackets: If a query contains an answer, name server or
1243 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1244 is the appropriate count.
1245 If any of the response bits are set (AA, RA or rcode) or any of the
1246 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1247 is printed, where \fIx\fP is the hex value of header bytes two and three.
1249 UDP Name Server Responses
1251 Name server responses are formatted as
1255 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1257 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1258 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1262 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1263 with 3 answer records, 3 name server records and 7 authority records.
1264 The first answer record is type A (address) and its data is internet
1265 address 128.32.137.3. The total size of the response was 273 bytes,
1266 excluding UDP and IP headers. The op (Query) and response code
1267 (NoError) were omitted, as was the class (C_IN) of the A record.
1269 In the second example, \fIhelios\fP responds to query 2 with a
1270 response code of non-existent domain (NXDomain) with no answers,
1271 one name server and no authority records. The `*' indicates that
1272 the \fIauthoritative answer\fP bit was set. Since there were no
1273 answers, no type, class or data were printed.
1275 Other flag characters that might appear are `\-' (recursion available,
1276 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
1277 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1280 Note that name server requests and responses tend to be large and the
1281 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1282 to print. Use the \fB\-s\fP flag to increase the snaplen if you
1283 need to seriously investigate name server traffic. `\fB\-s 128\fP'
1284 has worked well for me.
1289 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1290 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1291 NetBEUI SMB data is also done.
1293 By default a fairly minimal decode is done, with a much more detailed
1294 decode done if -v is used. Be warned that with -v a single SMB packet
1295 may take up a page or more, so only use -v if you really want all the
1298 If you are decoding SMB sessions containing unicode strings then you
1299 may wish to set the environment variable USE_UNICODE to 1. A patch to
1300 auto-detect unicode srings would be welcome.
1302 For information on SMB packet formats and what all te fields mean see
1303 www.cifs.org or the pub/samba/specs/ directory on your favourite
1304 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1308 NFS Requests and Replies
1310 Sun NFS (Network File System) requests and replies are printed as:
1314 \fIsrc.xid > dst.nfs: len op args\fP
1315 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1318 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1319 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1320 sushi.201b > wrl.nfs:
1321 144 lookup fh 9,74/4096.6878 "xcolors"
1322 wrl.nfs > sushi.201b:
1323 reply ok 128 lookup fh 9,74/4134.3150
1328 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1329 to \fIwrl\fP (note that the number following the src host is a
1330 transaction id, \fInot\fP the source port). The request was 112 bytes,
1331 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1332 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1333 (If one is lucky, as in this case, the file handle can be interpreted
1334 as a major,minor device number pair, followed by the inode number and
1336 \fIWrl\fP replies `ok' with the contents of the link.
1338 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1339 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1340 depends on the operation type. The format is intended to be self
1341 explanatory if read in conjunction with
1342 an NFS protocol spec.
1344 If the \-v (verbose) flag is given, additional information is printed.
1350 sushi.1372a > wrl.nfs:
1351 148 read fh 21,11/12.195 8192 bytes @ 24576
1352 wrl.nfs > sushi.1372a:
1353 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1358 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1359 which have been omitted from this example.) In the first line,
1360 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1361 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1362 second line is the first fragment of the reply, and hence is only 1472
1363 bytes long (the other bytes will follow in subsequent fragments, but
1364 these fragments do not have NFS or even UDP headers and so might not be
1365 printed, depending on the filter expression used). Because the \-v flag
1366 is given, some of the file attributes (which are returned in addition
1367 to the file data) are printed: the file type (``REG'', for regular file),
1368 the file mode (in octal), the uid and gid, and the file size.
1370 If the \-v flag is given more than once, even more details are printed.
1372 Note that NFS requests are very large and much of the detail won't be printed
1373 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1376 NFS reply packets do not explicitly identify the RPC operation. Instead,
1377 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1378 replies using the transaction ID. If a reply does not closely follow the
1379 corresponding request, it might not be parsable.
1381 AFS Requests and Replies
1383 Transarc AFS (Andrew File System) requests and replies are printed
1389 \fIsrc.sport > dst.dport: rx packet-type\fP
1390 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1391 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1394 elvis.7001 > pike.afsfs:
1395 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1396 new fid 536876964/1/1 ".newsrc"
1397 pike.afsfs > elvis.7001: rx data fs reply rename
1402 In the first line, host elvis sends a RX packet to pike. This was
1403 a RX data packet to the fs (fileserver) service, and is the start of
1404 an RPC call. The RPC call was a rename, with the old directory file id
1405 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1406 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1407 responds with a RPC reply to the rename call (which was successful, because
1408 it was a data packet and not an abort packet).
1410 In general, all AFS RPCs are decoded at least by RPC call name. Most
1411 AFS RPCs have at least some of the arguments decoded (generally only
1412 the `interesting' arguments, for some definition of interesting).
1414 The format is intended to be self-describing, but it will probably
1415 not be useful to people who are not familiar with the workings of
1418 If the -v (verbose) flag is given twice, acknowledgement packets and
1419 additional header information is printed, such as the the RX call ID,
1420 call number, sequence number, serial number, and the RX packet flags.
1422 If the -v flag is given twice, additional information is printed,
1423 such as the the RX call ID, serial number, and the RX packet flags.
1424 The MTU negotiation information is also printed from RX ack packets.
1426 If the -v flag is given three times, the security index and service id
1429 Error codes are printed for abort packets, with the exception of Ubik
1430 beacon packets (because abort packets are used to signify a yes vote
1431 for the Ubik protocol).
1433 Note that AFS requests are very large and many of the arguments won't
1434 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1435 to watch AFS traffic.
1437 AFS reply packets do not explicitly identify the RPC operation. Instead,
1438 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1439 replies using the call number and service ID. If a reply does not closely
1441 corresponding request, it might not be parsable.
1444 KIP Appletalk (DDP in UDP)
1446 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1447 and dumped as DDP packets (i.e., all the UDP header information is
1448 discarded). The file
1450 is used to translate appletalk net and node numbers to names.
1451 Lines in this file have the form
1463 The first two lines give the names of appletalk networks. The third
1464 line gives the name of a particular host (a host is distinguished
1465 from a net by the 3rd octet in the number \-
1466 a net number \fImust\fP have two octets and a host number \fImust\fP
1467 have three octets.) The number and name should be separated by
1468 whitespace (blanks or tabs).
1471 file may contain blank lines or comment lines (lines starting with
1474 Appletalk addresses are printed in the form
1480 \f(CW144.1.209.2 > icsd-net.112.220
1481 office.2 > icsd-net.112.220
1482 jssmag.149.235 > icsd-net.2\fR
1488 doesn't exist or doesn't contain an entry for some appletalk
1489 host/net number, addresses are printed in numeric form.)
1490 In the first example, NBP (DDP port 2) on net 144.1 node 209
1491 is sending to whatever is listening on port 220 of net icsd node 112.
1492 The second line is the same except the full name of the source node
1493 is known (`office'). The third line is a send from port 235 on
1494 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1495 the broadcast address (255) is indicated by a net name with no host
1496 number \- for this reason it's a good idea to keep node names and
1497 net names distinct in /etc/atalk.names).
1499 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1500 packets have their contents interpreted. Other protocols just dump
1501 the protocol name (or number if no name is registered for the
1502 protocol) and packet size.
1504 \fBNBP packets\fP are formatted like the following examples:
1508 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1509 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1510 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1514 The first line is a name lookup request for laserwriters sent by net icsd host
1515 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1516 The second line shows a reply for this request (note that it has the
1517 same id) from host jssmag.209 saying that it has a laserwriter
1518 resource named "RM1140" registered on port 250. The third line is
1519 another reply to the same request saying host techpit has laserwriter
1520 "techpit" registered on port 186.
1522 \fBATP packet\fP formatting is demonstrated by the following example:
1526 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1527 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1528 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1529 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1530 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1531 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1532 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1533 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1534 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1535 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1536 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1537 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1538 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1539 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1543 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1544 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1545 is the value of the `userdata' field in the request.
1547 Helios responds with 8 512-byte packets. The `:digit' following the
1548 transaction id gives the packet sequence number in the transaction
1549 and the number in parens is the amount of data in the packet,
1550 excluding the atp header. The `*' on packet 7 indicates that the
1553 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1554 resends them then jssmag.209 releases the transaction. Finally,
1555 jssmag.209 initiates the next request. The `*' on the request
1556 indicates that XO (`exactly once') was \fInot\fP set.
1561 Fragmented Internet datagrams are printed as
1565 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1566 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1570 (The first form indicates there are more fragments. The second
1571 indicates this is the last fragment.)
1573 \fIId\fP is the fragment id. \fISize\fP is the fragment
1574 size (in bytes) excluding the IP header. \fIOffset\fP is this
1575 fragment's offset (in bytes) in the original datagram.
1577 The fragment information is output for each fragment. The first
1578 fragment contains the higher level protocol header and the frag
1579 info is printed after the protocol info. Fragments
1580 after the first contain no higher level protocol header and the
1581 frag info is printed after the source and destination addresses.
1582 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1583 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1587 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1588 arizona > rtsg: (frag 595a:204@328)
1589 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1593 There are a couple of things to note here: First, addresses in the
1594 2nd line don't include port numbers. This is because the TCP
1595 protocol information is all in the first fragment and we have no idea
1596 what the port or sequence numbers are when we print the later fragments.
1597 Second, the tcp sequence information in the first line is printed as if there
1598 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1599 the first frag and 204 in the second). If you are looking for holes
1600 in the sequence space or trying to match up acks
1601 with packets, this can fool you.
1603 A packet with the IP \fIdon't fragment\fP flag is marked with a
1604 trailing \fB(DF)\fP.
1608 By default, all output lines are preceded by a timestamp. The timestamp
1609 is the current clock time in the form
1615 and is as accurate as the kernel's clock.
1616 The timestamp reflects the time the kernel first saw the packet. No attempt
1617 is made to account for the time lag between when the
1618 ethernet interface removed the packet from the wire and when the kernel
1619 serviced the `new packet' interrupt.
1621 traffic(1C), nit(4P), bpf(4), pcap(3)
1623 The original authors are:
1627 Steven McCanne, all of the
1628 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1630 It is currently being maintained by tcpdump.org.
1632 The current version is available via http:
1635 .I http://www.tcpdump.org/
1638 The original distribution is available via anonymous ftp:
1641 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1644 IPv6/IPsec support is added by WIDE/KAME project.
1645 This program uses Eric Young's SSLeay library, under specific configuration.
1647 Please send problems, bugs, questions, desirable enhancements, etc. to:
1650 tcpdump-workers@tcpdump.org
1653 Please send source code contributions, etc. to:
1659 NIT doesn't let you watch your own outbound traffic, BPF will.
1660 We recommend that you use the latter.
1662 On Linux systems with 2.0[.x] kernels:
1664 packets on the loopback device will be seen twice;
1666 packet filtering cannot be done in the kernel, so that all packets must
1667 be copied from the kernel in order to be filtered in user mode;
1669 all of a packet, not just the part that's within the snapshot length,
1670 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1671 asked to copy only part of a packet to userland, will not report the
1672 true length of the packet; this would cause most IP packets to get an
1676 We recommend that you upgrade to a 2.2 or later kernel.
1678 Some attempt should be made to reassemble IP fragments or, at least
1679 to compute the right length for the higher level protocol.
1681 Name server inverse queries are not dumped correctly: the (empty)
1682 question section is printed rather than real query in the answer
1683 section. Some believe that inverse queries are themselves a bug and
1684 prefer to fix the program generating them rather than \fItcpdump\fP.
1686 A packet trace that crosses a daylight savings time change will give
1687 skewed time stamps (the time change is ignored).
1689 Filter expressions that manipulate FDDI or Token Ring headers assume
1690 that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet
1691 packets. This is true for IP, ARP, and DECNET Phase IV, but is not true
1692 for protocols such as ISO CLNS. Therefore, the filter may inadvertently
1693 accept certain packets that do not properly match the filter expression.
1695 Filter expressions on fields other than those that manipulate Token Ring
1696 headers will not correctly handle source-routed Token Ring packets.
1699 should chase header chain, but at this moment it does not.
1700 .BR "ip6 protochain"
1701 is supplied for this behavior.
1703 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1704 does not work against IPv6 packets.
1705 It only looks at IPv4 packets.
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