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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.
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, \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 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 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, or \fIipx\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 those 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, where it checks the DSAP of the LLC
552 header, and \fIatalk\fP, where it checks for a SNAP-format packet with
553 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 and \fIsap\fP,
557 for which it checks for an 802.3 frame and then checks the LLC header as
558 it does for FDDI and Token Ring, \fIatalk\fP, where it checks both for
559 the Appletalk etype in an Ethernet frame and for a SNAP-format packet as
560 it does for FDDI and Token Ring, \fIaarp\fP, where it checks for the
561 Appletalk ARP etype in either an Ethernet frame or an 802.2 SNAP frame
562 with an OUI of 0x000000, and \fIipx\fP, where it checks for the IPX
563 etype in an Ethernet frame, the IPX DSAP in the LLC header, the
564 802.3 with no LLC header encapsulation of IPX, and the IPX etype in
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"
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 Primitives may be combined using:
661 A parenthesized group of primitives and operators
662 (parentheses are special to the Shell and must be escaped).
664 Negation (`\fB!\fP' or `\fBnot\fP').
666 Concatenation (`\fB&&\fP' or `\fBand\fP').
668 Alternation (`\fB||\fP' or `\fBor\fP').
670 Negation has highest precedence.
671 Alternation and concatenation have equal precedence and associate
672 left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
673 are now required for concatenation.
675 If an identifier is given without a keyword, the most recent keyword
680 \fBnot host vs and ace\fR
686 \fBnot host vs and host ace\fR
689 which should not be confused with
692 \fBnot ( host vs or ace )\fR
696 Expression arguments can be passed to \fItcpdump\fP as either a single
697 argument or as multiple arguments, whichever is more convenient.
698 Generally, if the expression contains Shell metacharacters, it is
699 easier to pass it as a single, quoted argument.
700 Multiple arguments are concatenated with spaces before being parsed.
703 To print all packets arriving at or departing from \fIsundown\fP:
706 \fBtcpdump host sundown\fP
710 To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
713 \fBtcpdump host helios and \\( hot or ace \\)\fP
717 To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
720 \fBtcpdump ip host ace and not helios\fP
724 To print all traffic between local hosts and hosts at Berkeley:
728 tcpdump net ucb-ether
732 To print all ftp traffic through internet gateway \fIsnup\fP:
733 (note that the expression is quoted to prevent the shell from
734 (mis-)interpreting the parentheses):
738 tcpdump 'gateway snup and (port ftp or ftp-data)'
742 To print traffic neither sourced from nor destined for local hosts
743 (if you gateway to one other net, this stuff should never make it
744 onto your local net).
748 tcpdump ip and not net \fIlocalnet\fP
752 To print the start and end packets (the SYN and FIN packets) of each
753 TCP conversation that involves a non-local host.
757 tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
761 To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
765 tcpdump 'gateway snup and ip[2:2] > 576'
769 To print IP broadcast or multicast packets that were
771 sent via ethernet broadcast or multicast:
775 tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
779 To print all ICMP packets that are not echo requests/replies (i.e., not
784 tcpdump 'icmp[0] != 8 and icmp[0] != 0'
789 The output of \fItcpdump\fP is protocol dependent. The following
790 gives a brief description and examples of most of the formats.
798 If the '-e' option is given, the link level header is printed out.
799 On ethernets, the source and destination addresses, protocol,
800 and packet length are printed.
802 On FDDI networks, the '-e' option causes \fItcpdump\fP to print
803 the `frame control' field, the source and destination addresses,
804 and the packet length. (The `frame control' field governs the
805 interpretation of the rest of the packet. Normal packets (such
806 as those containing IP datagrams) are `async' packets, with a priority
807 value between 0 and 7; for example, `\fBasync4\fR'. Such packets
808 are assumed to contain an 802.2 Logical Link Control (LLC) packet;
809 the LLC header is printed if it is \fInot\fR an ISO datagram or a
810 so-called SNAP packet.
812 On Token Ring networks, the '-e' option causes \fItcpdump\fP to print
813 the `access control' and `frame control' fields, the source and
814 destination addresses, and the packet length. As on FDDI networks,
815 packets are assumed to contain an LLC packet. Regardless of whether
816 the '-e' option is specified or not, the source routing information is
817 printed for source-routed packets.
819 \fI(N.B.: The following description assumes familiarity with
820 the SLIP compression algorithm described in RFC-1144.)\fP
822 On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
823 packet type, and compression information are printed out.
824 The packet type is printed first.
825 The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
826 No further link information is printed for \fIip\fR packets.
827 For TCP packets, the connection identifier is printed following the type.
828 If the packet is compressed, its encoded header is printed out.
829 The special cases are printed out as
830 \fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
831 the sequence number (or sequence number and ack) has changed.
832 If it is not a special case,
833 zero or more changes are printed.
834 A change is indicated by U (urgent pointer), W (window), A (ack),
835 S (sequence number), and I (packet ID), followed by a delta (+n or -n),
837 Finally, the amount of data in the packet and compressed header length
840 For example, the following line shows an outbound compressed TCP packet,
841 with an implicit connection identifier; the ack has changed by 6,
842 the sequence number by 49, and the packet ID by 6; there are 3 bytes of
843 data and 6 bytes of compressed header:
846 \fBO ctcp * A+6 S+49 I+6 3 (6)\fP
852 Arp/rarp output shows the type of request and its arguments. The
853 format is intended to be self explanatory.
854 Here is a short sample taken from the start of an `rlogin' from
855 host \fIrtsg\fP to host \fIcsam\fP:
859 \f(CWarp who-has csam tell rtsg
860 arp reply csam is-at CSAM\fR
864 The first line says that rtsg sent an arp packet asking
865 for the ethernet address of internet host csam. Csam
866 replies with its ethernet address (in this example, ethernet addresses
867 are in caps and internet addresses in lower case).
869 This would look less redundant if we had done \fBtcpdump \-n\fP:
873 \f(CWarp who-has 128.3.254.6 tell 128.3.254.68
874 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
878 If we had done \fBtcpdump \-e\fP, the fact that the first packet is
879 broadcast and the second is point-to-point would be visible:
883 \f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
884 CSAM RTSG 0806 64: arp reply csam is-at CSAM\fR
888 For the first packet this says the ethernet source address is RTSG, the
889 destination is the ethernet broadcast address, the type field
890 contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
894 \fI(N.B.:The following description assumes familiarity with
895 the TCP protocol described in RFC-793. If you are not familiar
896 with the protocol, neither this description nor \fItcpdump\fP will
897 be of much use to you.)\fP
899 The general format of a tcp protocol line is:
903 \fIsrc > dst: flags data-seqno ack window urgent options\fP
907 \fISrc\fP and \fIdst\fP are the source and destination IP
908 addresses and ports. \fIFlags\fP are some combination of S (SYN),
909 F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
910 \fIData-seqno\fP describes the portion of sequence space covered
911 by the data in this packet (see example below).
912 \fIAck\fP is sequence number of the next data expected the other
913 direction on this connection.
914 \fIWindow\fP is the number of bytes of receive buffer space available
915 the other direction on this connection.
916 \fIUrg\fP indicates there is `urgent' data in the packet.
917 \fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
919 \fISrc, dst\fP and \fIflags\fP are always present. The other fields
920 depend on the contents of the packet's tcp protocol header and
921 are output only if appropriate.
923 Here is the opening portion of an rlogin from host \fIrtsg\fP to
928 \s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
929 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
930 rtsg.1023 > csam.login: . ack 1 win 4096
931 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
932 csam.login > rtsg.1023: . ack 2 win 4096
933 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
934 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
935 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
936 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fR\s+2
940 The first line says that tcp port 1023 on rtsg sent a packet
942 on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
943 The packet sequence number was 768512 and it contained no data.
944 (The notation is `first:last(nbytes)' which means `sequence
946 up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
947 There was no piggy-backed ack, the available receive window was 4096
948 bytes and there was a max-segment-size option requesting an mss of
951 Csam replies with a similar packet except it includes a piggy-backed
952 ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
954 The packet contained no data so there is no data sequence number.
955 Note that the ack sequence
956 number is a small integer (1). The first time \fItcpdump\fP sees a
957 tcp `conversation', it prints the sequence number from the packet.
958 On subsequent packets of the conversation, the difference between
959 the current packet's sequence number and this initial sequence number
960 is printed. This means that sequence numbers after the
961 first can be interpreted
962 as relative byte positions in the conversation's data stream (with the
963 first data byte each direction being `1'). `-S' will override this
964 feature, causing the original sequence numbers to be output.
966 On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
967 in the rtsg \(-> csam side of the conversation).
968 The PUSH flag is set in the packet.
969 On the 7th line, csam says it's received data sent by rtsg up to
970 but not including byte 21. Most of this data is apparently sitting in the
971 socket buffer since csam's receive window has gotten 19 bytes smaller.
972 Csam also sends one byte of data to rtsg in this packet.
973 On the 8th and 9th lines,
974 csam sends two bytes of urgent, pushed data to rtsg.
976 If the snapshot was small enough that \fItcpdump\fP didn't capture
977 the full TCP header, it interprets as much of the header as it can
978 and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
979 be interpreted. If the header contains a bogus option (one with a length
980 that's either too small or beyond the end of the header), \fItcpdump\fP
981 reports it as ``[\fIbad opt\fP]'' and does not interpret any further
982 options (since it's impossible to tell where they start). If the header
983 length indicates options are present but the IP datagram length is not
984 long enough for the options to actually be there, \fItcpdump\fP reports
985 it as ``[\fIbad hdr length\fP]''.
987 .B Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
989 There are 6 bits in the control bits section of the TCP header:
991 .I URG | ACK | PSH | RST | SYN | FIN
993 Let's assume that we want to watch packets used in establishing
994 a TCP connection. Recall that TCP uses a 3-way handshake protocol
995 when it initializes a new connection; the connection sequence with
996 regard to the TCP control bits is
1002 2) Recipient responds with SYN, ACK
1008 Now we're interested in capturing packets that have only the
1009 SYN bit set (Step 1). Note that we don't want packets from step 2
1010 (SYN-ACK), just a plain initial SYN. What we need is a correct filter
1011 expression for \fItcpdump\fP.
1013 Recall the structure of a TCP header without options:
1017 -----------------------------------------------------------------
1018 | source port | destination port |
1019 -----------------------------------------------------------------
1021 -----------------------------------------------------------------
1022 | acknowledgment number |
1023 -----------------------------------------------------------------
1024 | HL | reserved |U|A|P|R|S|F| window size |
1025 -----------------------------------------------------------------
1026 | TCP checksum | urgent pointer |
1027 -----------------------------------------------------------------
1030 A TCP header usually holds 20 octets of data, unless options are
1031 present. The fist line of the graph contains octets 0 - 3, the
1032 second line shows octets 4 - 7 etc.
1034 Starting to count with 0, the relevant TCP control bits are contained
1039 ----------------|---------------|---------------|----------------
1040 | HL | reserved |U|A|P|R|S|F| window size |
1041 ----------------|---------------|---------------|----------------
1042 | | 13th octet | | |
1045 Let's have a closer look at octet no. 13:
1055 We see that this octet contains 2 bytes from the reserved field.
1056 According to RFC 793 this field is reserved for future use and must
1057 be 0. The remaining 6 bits are the TCP control bits we are interested
1058 in. We have numbered the bits in this octet from 0 to 7, right to
1059 left, so the PSH bit is bit number 3, while the URG bit is number 5.
1061 Recall that we want to capture packets with only SYN set.
1062 Let's see what happens to octet 13 if a TCP datagram arrives
1063 with the SYN bit set in its header:
1073 We already mentioned that bits number 7 and 6 belong to the
1074 reserved field, so they must must be 0. Looking at the
1075 control bits section we see that only bit number 1 (SYN) is set.
1077 Assuming that octet number 13 is an 8-bit unsigned integer in
1078 network byte order, the binary value of this octet is
1082 and its decimal representation is
1086 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
1089 We're almost done, because now we know that if only SYN is set,
1090 the value of the 13th octet in the TCP header, when interpreted
1091 as a 8-bit unsigned integer in network byte order, must be exactly 2.
1093 This relationship can be expressed as
1099 We can use this expression as the filter for \fItcpdump\fP in order
1100 to watch packets which have only SYN set:
1103 tcpdump -i xl0 tcp[13] == 2
1106 The expression says "let the 13th octet of a TCP datagram have
1107 the decimal value 2", which is exactly what we want.
1109 Now, let's assume that we need to capture SYN packets, but we
1110 don't care if ACK or any other TCP control bit is set at the
1111 same time. Let's see what happens to octet 13 when a TCP datagram
1112 with SYN-ACK set arrives:
1122 Now bits 1 and 4 are set in the 13th octet. The binary value of
1127 which translates to decimal
1131 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
1134 Now we can't just use 'tcp[13] == 18' in the \fItcpdump\fP filter
1135 expression, because that would select only those packets that have
1136 SYN-ACK set, but not those with only SYN set. Remember that we don't care
1137 if ACK or any other control bit is set as long as SYN is set.
1139 In order to achieve our goal, we need to logically AND the
1140 binary value of octet 13 with some other value to preserve
1141 the SYN bit. We know that we want SYN to be set in any case,
1142 so we'll logically AND the value in the 13th octet with
1143 the binary value of a SYN:
1147 00010010 SYN-ACK 00000010 SYN
1148 AND 00000010 (we want SYN) AND 00000010 (we want SYN)
1150 = 00000010 = 00000010
1153 We see that this AND operation delivers the same result
1154 regardless whether ACK or another TCP control bit is set.
1155 The decimal representation of the AND value as well as
1156 the result of this operation is 2 (binary 00000010),
1157 so we know that for packets with SYN set the following
1158 relation must hold true:
1160 ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
1162 This points us to the \fItcpdump\fP filter expression
1165 tcpdump -i xl0 'tcp[13] & 2 == 2'
1168 Note that you should use single quotes or a backslash
1169 in the expression to hide the AND ('&') special character
1175 UDP format is illustrated by this rwho packet:
1179 \f(CWactinide.who > broadcast.who: udp 84\fP
1183 This says that port \fIwho\fP on host \fIactinide\fP sent a udp
1184 datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
1185 broadcast address. The packet contained 84 bytes of user data.
1187 Some UDP services are recognized (from the source or destination
1188 port number) and the higher level protocol information printed.
1189 In particular, Domain Name service requests (RFC-1034/1035) and Sun
1190 RPC calls (RFC-1050) to NFS.
1192 UDP Name Server Requests
1194 \fI(N.B.:The following description assumes familiarity with
1195 the Domain Service protocol described in RFC-1035. If you are not familiar
1196 with the protocol, the following description will appear to be written
1199 Name server requests are formatted as
1203 \fIsrc > dst: id op? flags qtype qclass name (len)\fP
1205 \f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fR
1209 Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
1210 address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
1211 The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
1212 was set. The query length was 37 bytes, not including the UDP and
1213 IP protocol headers. The query operation was the normal one, \fIQuery\fP,
1214 so the op field was omitted. If the op had been anything else, it would
1215 have been printed between the `3' and the `+'.
1216 Similarly, the qclass was the normal one,
1217 \fIC_IN\fP, and omitted. Any other qclass would have been printed
1218 immediately after the `A'.
1220 A few anomalies are checked and may result in extra fields enclosed in
1221 square brackets: If a query contains an answer, name server or
1227 are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
1228 is the appropriate count.
1229 If any of the response bits are set (AA, RA or rcode) or any of the
1230 `must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
1231 is printed, where \fIx\fP is the hex value of header bytes two and three.
1233 UDP Name Server Responses
1235 Name server responses are formatted as
1239 \fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
1241 \f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
1242 helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fR
1246 In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
1247 with 3 answer records, 3 name server records and 7 authority records.
1248 The first answer record is type A (address) and its data is internet
1249 address 128.32.137.3. The total size of the response was 273 bytes,
1250 excluding UDP and IP headers. The op (Query) and response code
1251 (NoError) were omitted, as was the class (C_IN) of the A record.
1253 In the second example, \fIhelios\fP responds to query 2 with a
1254 response code of non-existent domain (NXDomain) with no answers,
1255 one name server and no authority records. The `*' indicates that
1256 the \fIauthoritative answer\fP bit was set. Since there were no
1257 answers, no type, class or data were printed.
1259 Other flag characters that might appear are `\-' (recursion available,
1260 RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
1261 `question' section doesn't contain exactly one entry, `[\fIn\fPq]'
1264 Note that name server requests and responses tend to be large and the
1265 default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
1266 to print. Use the \fB\-s\fP flag to increase the snaplen if you
1267 need to seriously investigate name server traffic. `\fB\-s 128\fP'
1268 has worked well for me.
1273 \fItcpdump\fP now includes fairly extensive SMB/CIFS/NBT decoding for data
1274 on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and
1275 NetBEUI SMB data is also done.
1277 By default a fairly minimal decode is done, with a much more detailed
1278 decode done if -v is used. Be warned that with -v a single SMB packet
1279 may take up a page or more, so only use -v if you really want all the
1282 If you are decoding SMB sessions containing unicode strings then you
1283 may wish to set the environment variable USE_UNICODE to 1. A patch to
1284 auto-detect unicode srings would be welcome.
1286 For information on SMB packet formats and what all te fields mean see
1287 www.cifs.org or the pub/samba/specs/ directory on your favourite
1288 samba.org mirror site. The SMB patches were written by Andrew Tridgell
1292 NFS Requests and Replies
1294 Sun NFS (Network File System) requests and replies are printed as:
1298 \fIsrc.xid > dst.nfs: len op args\fP
1299 \fIsrc.nfs > dst.xid: reply stat len op results\fP
1302 sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
1303 wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
1304 sushi.201b > wrl.nfs:
1305 144 lookup fh 9,74/4096.6878 "xcolors"
1306 wrl.nfs > sushi.201b:
1307 reply ok 128 lookup fh 9,74/4134.3150
1312 In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
1313 to \fIwrl\fP (note that the number following the src host is a
1314 transaction id, \fInot\fP the source port). The request was 112 bytes,
1315 excluding the UDP and IP headers. The operation was a \fIreadlink\fP
1316 (read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
1317 (If one is lucky, as in this case, the file handle can be interpreted
1318 as a major,minor device number pair, followed by the inode number and
1320 \fIWrl\fP replies `ok' with the contents of the link.
1322 In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
1323 `\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
1324 depends on the operation type. The format is intended to be self
1325 explanatory if read in conjunction with
1326 an NFS protocol spec.
1328 If the \-v (verbose) flag is given, additional information is printed.
1334 sushi.1372a > wrl.nfs:
1335 148 read fh 21,11/12.195 8192 bytes @ 24576
1336 wrl.nfs > sushi.1372a:
1337 reply ok 1472 read REG 100664 ids 417/0 sz 29388
1342 (\-v also prints the IP header TTL, ID, length, and fragmentation fields,
1343 which have been omitted from this example.) In the first line,
1344 \fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
1345 at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
1346 second line is the first fragment of the reply, and hence is only 1472
1347 bytes long (the other bytes will follow in subsequent fragments, but
1348 these fragments do not have NFS or even UDP headers and so might not be
1349 printed, depending on the filter expression used). Because the \-v flag
1350 is given, some of the file attributes (which are returned in addition
1351 to the file data) are printed: the file type (``REG'', for regular file),
1352 the file mode (in octal), the uid and gid, and the file size.
1354 If the \-v flag is given more than once, even more details are printed.
1356 Note that NFS requests are very large and much of the detail won't be printed
1357 unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
1360 NFS reply packets do not explicitly identify the RPC operation. Instead,
1361 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1362 replies using the transaction ID. If a reply does not closely follow the
1363 corresponding request, it might not be parsable.
1365 AFS Requests and Replies
1367 Transarc AFS (Andrew File System) requests and replies are printed
1373 \fIsrc.sport > dst.dport: rx packet-type\fP
1374 \fIsrc.sport > dst.dport: rx packet-type service call call-name args\fP
1375 \fIsrc.sport > dst.dport: rx packet-type service reply call-name args\fP
1378 elvis.7001 > pike.afsfs:
1379 rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
1380 new fid 536876964/1/1 ".newsrc"
1381 pike.afsfs > elvis.7001: rx data fs reply rename
1386 In the first line, host elvis sends a RX packet to pike. This was
1387 a RX data packet to the fs (fileserver) service, and is the start of
1388 an RPC call. The RPC call was a rename, with the old directory file id
1389 of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory
1390 file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike
1391 responds with a RPC reply to the rename call (which was successful, because
1392 it was a data packet and not an abort packet).
1394 In general, all AFS RPCs are decoded at least by RPC call name. Most
1395 AFS RPCs have at least some of the arguments decoded (generally only
1396 the `interesting' arguments, for some definition of interesting).
1398 The format is intended to be self-describing, but it will probably
1399 not be useful to people who are not familiar with the workings of
1402 If the -v (verbose) flag is given twice, acknowledgement packets and
1403 additional header information is printed, such as the the RX call ID,
1404 call number, sequence number, serial number, and the RX packet flags.
1406 If the -v flag is given twice, additional information is printed,
1407 such as the the RX call ID, serial number, and the RX packet flags.
1408 The MTU negotiation information is also printed from RX ack packets.
1410 If the -v flag is given three times, the security index and service id
1413 Error codes are printed for abort packets, with the exception of Ubik
1414 beacon packets (because abort packets are used to signify a yes vote
1415 for the Ubik protocol).
1417 Note that AFS requests are very large and many of the arguments won't
1418 be printed unless \fIsnaplen\fP is increased. Try using `\fB-s 256\fP'
1419 to watch AFS traffic.
1421 AFS reply packets do not explicitly identify the RPC operation. Instead,
1422 \fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
1423 replies using the call number and service ID. If a reply does not closely
1425 corresponding request, it might not be parsable.
1428 KIP Appletalk (DDP in UDP)
1430 Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
1431 and dumped as DDP packets (i.e., all the UDP header information is
1432 discarded). The file
1434 is used to translate appletalk net and node numbers to names.
1435 Lines in this file have the form
1447 The first two lines give the names of appletalk networks. The third
1448 line gives the name of a particular host (a host is distinguished
1449 from a net by the 3rd octet in the number \-
1450 a net number \fImust\fP have two octets and a host number \fImust\fP
1451 have three octets.) The number and name should be separated by
1452 whitespace (blanks or tabs).
1455 file may contain blank lines or comment lines (lines starting with
1458 Appletalk addresses are printed in the form
1464 \f(CW144.1.209.2 > icsd-net.112.220
1465 office.2 > icsd-net.112.220
1466 jssmag.149.235 > icsd-net.2\fR
1472 doesn't exist or doesn't contain an entry for some appletalk
1473 host/net number, addresses are printed in numeric form.)
1474 In the first example, NBP (DDP port 2) on net 144.1 node 209
1475 is sending to whatever is listening on port 220 of net icsd node 112.
1476 The second line is the same except the full name of the source node
1477 is known (`office'). The third line is a send from port 235 on
1478 net jssmag node 149 to broadcast on the icsd-net NBP port (note that
1479 the broadcast address (255) is indicated by a net name with no host
1480 number \- for this reason it's a good idea to keep node names and
1481 net names distinct in /etc/atalk.names).
1483 NBP (name binding protocol) and ATP (Appletalk transaction protocol)
1484 packets have their contents interpreted. Other protocols just dump
1485 the protocol name (or number if no name is registered for the
1486 protocol) and packet size.
1488 \fBNBP packets\fP are formatted like the following examples:
1492 \s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
1493 jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
1494 techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fR\s+2
1498 The first line is a name lookup request for laserwriters sent by net icsd host
1499 112 and broadcast on net jssmag. The nbp id for the lookup is 190.
1500 The second line shows a reply for this request (note that it has the
1501 same id) from host jssmag.209 saying that it has a laserwriter
1502 resource named "RM1140" registered on port 250. The third line is
1503 another reply to the same request saying host techpit has laserwriter
1504 "techpit" registered on port 186.
1506 \fBATP packet\fP formatting is demonstrated by the following example:
1510 \s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
1511 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
1512 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
1513 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
1514 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1515 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
1516 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1517 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
1518 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
1519 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
1520 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
1521 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
1522 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
1523 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fR\s+2
1527 Jssmag.209 initiates transaction id 12266 with host helios by requesting
1528 up to 8 packets (the `<0-7>'). The hex number at the end of the line
1529 is the value of the `userdata' field in the request.
1531 Helios responds with 8 512-byte packets. The `:digit' following the
1532 transaction id gives the packet sequence number in the transaction
1533 and the number in parens is the amount of data in the packet,
1534 excluding the atp header. The `*' on packet 7 indicates that the
1537 Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
1538 resends them then jssmag.209 releases the transaction. Finally,
1539 jssmag.209 initiates the next request. The `*' on the request
1540 indicates that XO (`exactly once') was \fInot\fP set.
1545 Fragmented Internet datagrams are printed as
1549 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
1550 \fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
1554 (The first form indicates there are more fragments. The second
1555 indicates this is the last fragment.)
1557 \fIId\fP is the fragment id. \fISize\fP is the fragment
1558 size (in bytes) excluding the IP header. \fIOffset\fP is this
1559 fragment's offset (in bytes) in the original datagram.
1561 The fragment information is output for each fragment. The first
1562 fragment contains the higher level protocol header and the frag
1563 info is printed after the protocol info. Fragments
1564 after the first contain no higher level protocol header and the
1565 frag info is printed after the source and destination addresses.
1566 For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
1567 over a CSNET connection that doesn't appear to handle 576 byte datagrams:
1571 \s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
1572 arizona > rtsg: (frag 595a:204@328)
1573 rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
1577 There are a couple of things to note here: First, addresses in the
1578 2nd line don't include port numbers. This is because the TCP
1579 protocol information is all in the first fragment and we have no idea
1580 what the port or sequence numbers are when we print the later fragments.
1581 Second, the tcp sequence information in the first line is printed as if there
1582 were 308 bytes of user data when, in fact, there are 512 bytes (308 in
1583 the first frag and 204 in the second). If you are looking for holes
1584 in the sequence space or trying to match up acks
1585 with packets, this can fool you.
1587 A packet with the IP \fIdon't fragment\fP flag is marked with a
1588 trailing \fB(DF)\fP.
1592 By default, all output lines are preceded by a timestamp. The timestamp
1593 is the current clock time in the form
1599 and is as accurate as the kernel's clock.
1600 The timestamp reflects the time the kernel first saw the packet. No attempt
1601 is made to account for the time lag between when the
1602 ethernet interface removed the packet from the wire and when the kernel
1603 serviced the `new packet' interrupt.
1605 traffic(1C), nit(4P), bpf(4), pcap(3)
1607 The original authors are:
1611 Steven McCanne, all of the
1612 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1614 It is currently being maintained by tcpdump.org.
1616 The current version is available via http:
1619 .I http://www.tcpdump.org/
1622 The original distribution is available via anonymous ftp:
1625 .I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
1628 IPv6/IPsec support is added by WIDE/KAME project.
1629 This program uses Eric Young's SSLeay library, under specific configuration.
1631 Please send problems, bugs, questions, desirable enhancements, etc. to:
1634 tcpdump-workers@tcpdump.org
1637 Please send source code contributions, etc. to:
1643 NIT doesn't let you watch your own outbound traffic, BPF will.
1644 We recommend that you use the latter.
1646 On Linux systems with 2.0[.x] kernels:
1648 packets on the loopback device will be seen twice;
1650 packet filtering cannot be done in the kernel, so that all packets must
1651 be copied from the kernel in order to be filtered in user mode;
1653 all of a packet, not just the part that's within the snapshot length,
1654 will be copied from the kernel (the 2.0[.x] packet capture mechanism, if
1655 asked to copy only part of a packet to userland, will not report the
1656 true length of the packet; this would cause most IP packets to get an
1660 We recommend that you upgrade to a 2.2 or later kernel.
1662 Some attempt should be made to reassemble IP fragments or, at least
1663 to compute the right length for the higher level protocol.
1665 Name server inverse queries are not dumped correctly: the (empty)
1666 question section is printed rather than real query in the answer
1667 section. Some believe that inverse queries are themselves a bug and
1668 prefer to fix the program generating them rather than \fItcpdump\fP.
1670 A packet trace that crosses a daylight savings time change will give
1671 skewed time stamps (the time change is ignored).
1673 Filter expressions that manipulate FDDI or Token Ring headers assume
1674 that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet
1675 packets. This is true for IP, ARP, and DECNET Phase IV, but is not true
1676 for protocols such as ISO CLNS. Therefore, the filter may inadvertently
1677 accept certain packets that do not properly match the filter expression.
1679 Filter expressions on fields other than those that manipulate Token Ring
1680 headers will not correctly handle source-routed Token Ring packets.
1683 should chase header chain, but at this moment it does not.
1684 .BR "ip6 protochain"
1685 is supplied for this behavior.
1687 Arithmetic expression against transport layer headers, like \fBtcp[0]\fP,
1688 does not work against IPv6 packets.
1689 It only looks at IPv4 packets.
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