1233 lines
41 KiB
Groff
1233 lines
41 KiB
Groff
.\" @(#) $Header: tcpdump.1,v 1.67 97/06/30 16:31:50 leres Exp $ (LBL)
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.\"
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.\" Copyright (c) 1987, 1988, 1989, 1990, 1991, 1992, 1994, 1995, 1996, 1997
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.\" The Regents of the University of California. All rights reserved.
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that: (1) source code distributions
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.\" retain the above copyright notice and this paragraph in its entirety, (2)
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.\" distributions including binary code include the above copyright notice and
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.\" this paragraph in its entirety in the documentation or other materials
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.\" provided with the distribution, and (3) all advertising materials mentioning
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.\" features or use of this software display the following acknowledgement:
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.\" ``This product includes software developed by the University of California,
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.\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
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.\" the University nor the names of its contributors may be used to endorse
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.\" or promote products derived from this software without specific prior
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.\" written permission.
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.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
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.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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.\"
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.TH TCPDUMP 1 "30 June 1997"
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.SH NAME
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tcpdump \- dump traffic on a network
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.SH SYNOPSIS
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.na
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.B tcpdump
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[
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.B \-adeflnNOpqStvx
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] [
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.B \-c
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.I count
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] [
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.B \-F
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.I file
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]
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.br
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.ti +8
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[
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.B \-i
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.I interface
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] [
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.B \-r
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.I file
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]
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[
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.B \-s
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.I snaplen
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]
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.br
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.ti +8
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[
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.B \-T
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.I type
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]
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[
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.B \-w
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.I file
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]
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[
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.I expression
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]
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.br
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.ad
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.SH DESCRIPTION
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.LP
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\fITcpdump\fP prints out the headers of packets on a network interface
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that match the boolean \fIexpression\fP.
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.LP
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.B Under SunOS with nit or bpf:
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To run
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.I tcpdump
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you must have read access to
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.I /dev/nit
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or
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.IR /dev/bpf* .
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.B Under Solaris with dlpi:
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You must have read access to the network pseudo device, e.g.
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.IR /dev/le .
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.B Under HP-UX with dlpi:
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You must be root or it must be installed setuid to root.
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.B Under IRIX with snoop:
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You must be root or it must be installed setuid to root.
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.B Under Linux:
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You must be root or it must be installed setuid to root.
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.B Under Ultrix and Digital UNIX:
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Once the super-user has enabled promiscuous-mode operation using
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.IR pfconfig (8),
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any user may run
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.BR tcpdump .
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.B Under BSD:
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You must have read access to
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.IR /dev/bpf* .
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.SH OPTIONS
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.TP
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.B \-a
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Attempt to convert network and broadcast addresses to names.
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.TP
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.B \-c
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Exit after receiving \fIcount\fP packets.
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.TP
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.B \-d
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Dump the compiled packet-matching code in a human readable form to
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standard output and stop.
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.TP
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.B \-dd
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Dump packet-matching code as a
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.B C
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program fragment.
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.TP
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.B \-ddd
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Dump packet-matching code as decimal numbers (preceded with a count).
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.TP
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.B \-e
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Print the link-level header on each dump line.
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.TP
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.B \-f
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Print `foreign' internet addresses numerically rather than symbolically
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(this option is intended to get around serious brain damage in
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Sun's yp server \(em usually it hangs forever translating non-local
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internet numbers).
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.TP
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.B \-F
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Use \fIfile\fP as input for the filter expression.
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An additional expression given on the command line is ignored.
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.TP
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.B \-i
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Listen on \fIinterface\fP.
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If unspecified, \fItcpdump\fP searches the system interface list for the
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lowest numbered, configured up interface (excluding loopback).
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Ties are broken by choosing the earliest match.
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.TP
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.B \-l
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Make stdout line buffered. Useful if you want to see the data
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while capturing it. E.g.,
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.br
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``tcpdump\ \ \-l\ \ |\ \ tee dat'' or
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``tcpdump\ \ \-l \ \ > dat\ \ &\ \ tail\ \ \-f\ \ dat''.
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.TP
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.B \-n
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Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
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.TP
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.B \-N
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Don't print domain name qualification of host names. E.g.,
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if you give this flag then \fItcpdump\fP will print ``nic''
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instead of ``nic.ddn.mil''.
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.TP
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.B \-O
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Do not run the packet-matching code optimizer. This is useful only
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if you suspect a bug in the optimizer.
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.TP
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.B \-p
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\fIDon't\fP put the interface
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into promiscuous mode. Note that the interface might be in promiscuous
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mode for some other reason; hence, `-p' cannot be used as an abbreviation for
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`ether host {local-hw-addr} or ether broadcast'.
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.TP
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.B \-q
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Quick (quiet?) output. Print less protocol information so output
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lines are shorter.
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.TP
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.B \-r
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Read packets from \fIfile\fR (which was created with the -w option).
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Standard input is used if \fIfile\fR is ``-''.
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.TP
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.B \-s
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Snarf \fIsnaplen\fP bytes of data from each packet rather than the
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default of 68 (with SunOS's NIT, the minimum is actually 96).
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68 bytes is adequate for IP, ICMP, TCP
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and UDP but may truncate protocol information from name server and NFS
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packets (see below). Packets truncated because of a limited snapshot
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are indicated in the output with ``[|\fIproto\fP]'', where \fIproto\fP
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is the name of the protocol level at which the truncation has occurred.
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Note that taking larger snapshots both increases
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the amount of time it takes to process packets and, effectively,
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decreases the amount of packet buffering. This may cause packets to be
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lost. You should limit \fIsnaplen\fP to the smallest number that will
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capture the protocol information you're interested in.
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.TP
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.B \-T
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Force packets selected by "\fIexpression\fP" to be interpreted the
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specified \fItype\fR. Currently known types are
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\fBrpc\fR (Remote Procedure Call),
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\fBrtp\fR (Real-Time Applications protocol),
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\fBrtcp\fR (Real-Time Applications control protocol),
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\fBvat\fR (Visual Audio Tool),
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and
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\fBwb\fR (distributed White Board).
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.TP
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.B \-S
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Print absolute, rather than relative, TCP sequence numbers.
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.TP
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.B \-t
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\fIDon't\fP print a timestamp on each dump line.
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.TP
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.B \-tt
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Print an unformatted timestamp on each dump line.
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.TP
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.B \-v
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(Slightly more) verbose output. For example, the time to live
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and type of service information in an IP packet is printed.
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.TP
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.B \-vv
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Even more verbose output. For example, additional fields are
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printed from NFS reply packets.
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.TP
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.B \-w
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Write the raw packets to \fIfile\fR rather than parsing and printing
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them out. They can later be printed with the \-r option.
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Standard output is used if \fIfile\fR is ``-''.
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.TP
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.B \-x
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Print each packet (minus its link level header) in hex.
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The smaller of the entire packet or
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.I snaplen
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bytes will be printed.
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.IP "\fI expression\fP"
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.RS
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selects which packets will be dumped. If no \fIexpression\fP
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is given, all packets on the net will be dumped. Otherwise,
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only packets for which \fIexpression\fP is `true' will be dumped.
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.LP
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The \fIexpression\fP consists of one or more
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.I primitives.
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Primitives usually consist of an
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.I id
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(name or number) preceded by one or more qualifiers. There are three
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different kinds of qualifier:
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.IP \fItype\fP
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qualifiers say what kind of thing the id name or number refers to.
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Possible types are
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.BR host ,
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.B net
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and
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.BR port .
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E.g., `host foo', `net 128.3', `port 20'. If there is no type
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qualifier,
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.B host
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is assumed.
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.IP \fIdir\fP
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qualifiers specify a particular transfer direction to and/or from
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.I id.
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Possible directions are
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.BR src ,
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.BR dst ,
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.B "src or dst"
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and
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.B "src and"
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.BR dst .
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E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
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there is no dir qualifier,
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.B "src or dst"
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is assumed.
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For `null' link layers (i.e. point to point protocols such as slip) the
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.B inbound
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and
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.B outbound
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qualifiers can be used to specify a desired direction.
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.IP \fIproto\fP
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qualifiers restrict the match to a particular protocol. Possible
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protos are:
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.BR ether ,
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.BR fddi ,
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.BR ip ,
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.BR arp ,
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.BR rarp ,
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.BR decnet ,
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.BR lat ,
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.BR sca ,
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.BR moprc ,
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.BR mopdl ,
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.BR iso ,
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.BR esis ,
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.BR isis ,
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.B tcp
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and
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.BR udp .
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E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
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no proto qualifier, all protocols consistent with the type are
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assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
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(except the latter is not legal syntax), `net bar' means `(ip or
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arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
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.LP
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[`fddi' is actually an alias for `ether'; the parser treats them
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identically as meaning ``the data link level used on the specified
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network interface.'' FDDI headers contain Ethernet-like source
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and destination addresses, and often contain Ethernet-like packet
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types, so you can filter on these FDDI fields just as with the
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analogous Ethernet fields. FDDI headers also contain other fields,
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but you cannot name them explicitly in a filter expression.]
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.LP
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In addition to the above, there are some special `primitive' keywords
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that don't follow the pattern:
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.BR gateway ,
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.BR broadcast ,
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.BR less ,
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.B greater
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and arithmetic expressions. All of these are described below.
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.LP
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More complex filter expressions are built up by using the words
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.BR and ,
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.B or
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and
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.B not
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to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
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To save typing, identical qualifier lists can be omitted. E.g.,
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`tcp dst port ftp or ftp-data or domain' is exactly the same as
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`tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
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.LP
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Allowable primitives are:
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.IP "\fBdst host \fIhost\fR"
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True if the IP destination field of the packet is \fIhost\fP,
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which may be either an address or a name.
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.IP "\fBsrc host \fIhost\fR"
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True if the IP source field of the packet is \fIhost\fP.
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.IP "\fBhost \fIhost\fP
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True if either the IP source or destination of the packet is \fIhost\fP.
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Any of the above host expressions can be prepended with the keywords,
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\fBip\fP, \fBarp\fP, or \fBrarp\fP as in:
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.in +.5i
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.nf
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\fBip host \fIhost\fR
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.fi
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.in -.5i
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which is equivalent to:
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.in +.5i
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.nf
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\fBether proto \fI\\ip\fB and host \fIhost\fR
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.fi
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.in -.5i
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If \fIhost\fR is a name with multiple IP addresses, each address will
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be checked for a match.
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.IP "\fBether dst \fIehost\fP
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True if the ethernet destination address is \fIehost\fP. \fIEhost\fP
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may be either a name from /etc/ethers or a number (see
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.IR ethers (3N)
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for numeric format).
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.IP "\fBether src \fIehost\fP
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True if the ethernet source address is \fIehost\fP.
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.IP "\fBether host \fIehost\fP
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True if either the ethernet source or destination address is \fIehost\fP.
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.IP "\fBgateway\fP \fIhost\fP
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True if the packet used \fIhost\fP as a gateway. I.e., the ethernet
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source or destination address was \fIhost\fP but neither the IP source
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nor the IP destination was \fIhost\fP. \fIHost\fP must be a name and
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must be found in both /etc/hosts and /etc/ethers. (An equivalent
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expression is
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.in +.5i
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.nf
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\fBether host \fIehost \fBand not host \fIhost\fR
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.fi
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.in -.5i
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which can be used with either names or numbers for \fIhost / ehost\fP.)
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.IP "\fBdst net \fInet\fR"
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True if the IP destination address of the packet has a network
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number of \fInet\fP. \fINet\fP may be either a name from /etc/networks
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or a network number (see \fInetworks(4)\fP for details).
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.IP "\fBsrc net \fInet\fR"
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True if the IP source address of the packet has a network
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number of \fInet\fP.
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.IP "\fBnet \fInet\fR"
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True if either the IP source or destination address of the packet has a network
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number of \fInet\fP.
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.IP "\fBnet \fInet\fR \fBmask \fImask\fR"
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True if the IP address matches \fInet\fR with the specific netmask.
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May be qualified with \fBsrc\fR or \fBdst\fR.
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.IP "\fBnet \fInet\fR/\fIlen\fR"
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True if the IP address matches \fInet\fR a netmask \fIlen\fR bits wide.
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May be qualified with \fBsrc\fR or \fBdst\fR.
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.IP "\fBdst port \fIport\fR"
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True if the packet is ip/tcp or ip/udp and has a
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destination port value of \fIport\fP.
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The \fIport\fP can be a number or a name used in /etc/services (see
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.IR tcp (4P)
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and
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.IR udp (4P)).
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If a name is used, both the port
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number and protocol are checked. If a number or ambiguous name is used,
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only the port number is checked (e.g., \fBdst port 513\fR will print both
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tcp/login traffic and udp/who traffic, and \fBport domain\fR will print
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both tcp/domain and udp/domain traffic).
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.IP "\fBsrc port \fIport\fR"
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True if the packet has a source port value of \fIport\fP.
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.IP "\fBport \fIport\fR"
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True if either the source or destination port of the packet is \fIport\fP.
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Any of the above port expressions can be prepended with the keywords,
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\fBtcp\fP or \fBudp\fP, as in:
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.in +.5i
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.nf
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\fBtcp src port \fIport\fR
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.fi
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.in -.5i
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which matches only tcp packets whose source port is \fIport\fP.
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.IP "\fBless \fIlength\fR"
|
|
True if the packet has a length less than or equal to \fIlength\fP.
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This is equivalent to:
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.in +.5i
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.nf
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\fBlen <= \fIlength\fP.
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.fi
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.in -.5i
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.IP "\fBgreater \fIlength\fR"
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True if the packet has a length greater than or equal to \fIlength\fP.
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This is equivalent to:
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.in +.5i
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.nf
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\fBlen >= \fIlength\fP.
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.fi
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.in -.5i
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.IP "\fBip proto \fIprotocol\fR"
|
|
True if the packet is an ip packet (see
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.IR ip (4P))
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of protocol type \fIprotocol\fP.
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|
\fIProtocol\fP can be a number or one of the names
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\fIicmp\fP, \fIigrp\fP, \fIudp\fP, \fInd\fP, or \fItcp\fP.
|
|
Note that the identifiers \fItcp\fP, \fIudp\fP, and \fIicmp\fP are also
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|
keywords and must be escaped via backslash (\\), which is \\\\ in the C-shell.
|
|
.IP "\fBether broadcast\fR"
|
|
True if the packet is an ethernet broadcast packet. The \fIether\fP
|
|
keyword is optional.
|
|
.IP "\fBip broadcast\fR"
|
|
True if the packet is an IP broadcast packet. It checks for both
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the all-zeroes and all-ones broadcast conventions, and looks up
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the local subnet mask.
|
|
.IP "\fBether multicast\fR"
|
|
True if the packet is an ethernet multicast packet. The \fIether\fP
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|
keyword is optional.
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This is shorthand for `\fBether[0] & 1 != 0\fP'.
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|
.IP "\fBip multicast\fR"
|
|
True if the packet is an IP multicast packet.
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|
.IP "\fBether proto \fIprotocol\fR"
|
|
True if the packet is of ether type \fIprotocol\fR.
|
|
\fIProtocol\fP can be a number or a name like
|
|
\fIip\fP, \fIarp\fP, or \fIrarp\fP.
|
|
Note these identifiers are also keywords
|
|
and must be escaped via backslash (\\).
|
|
[In the case of FDDI (e.g., `\fBfddi protocol arp\fR'), the
|
|
protocol identification comes from the 802.2 Logical Link Control
|
|
(LLC) header, which is usually layered on top of the FDDI header.
|
|
\fITcpdump\fP assumes, when filtering on the protocol identifier,
|
|
that all FDDI packets include an LLC header, and that the LLC header
|
|
is in so-called SNAP format.]
|
|
.IP "\fBdecnet src \fIhost\fR"
|
|
True if the DECNET source address is
|
|
.IR host ,
|
|
which may be an address of the form ``10.123'', or a DECNET host
|
|
name. [DECNET host name support is only available on Ultrix systems
|
|
that are configured to run DECNET.]
|
|
.IP "\fBdecnet dst \fIhost\fR"
|
|
True if the DECNET destination address is
|
|
.IR host .
|
|
.IP "\fBdecnet host \fIhost\fR"
|
|
True if either the DECNET source or destination address is
|
|
.IR host .
|
|
.IP "\fBip\fR, \fBarp\fR, \fBrarp\fR, \fBdecnet\fR, \fBiso\fR"
|
|
Abbreviations for:
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|
.in +.5i
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|
.nf
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|
\fBether proto \fIp\fR
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|
.fi
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|
.in -.5i
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|
where \fIp\fR is one of the above protocols.
|
|
.IP "\fBlat\fR, \fBmoprc\fR, \fBmopdl\fR"
|
|
Abbreviations for:
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|
.in +.5i
|
|
.nf
|
|
\fBether proto \fIp\fR
|
|
.fi
|
|
.in -.5i
|
|
where \fIp\fR is one of the above protocols.
|
|
Note that
|
|
\fItcpdump\fP does not currently know how to parse these protocols.
|
|
.IP "\fBtcp\fR, \fBudp\fR, \fBicmp\fR"
|
|
Abbreviations for:
|
|
.in +.5i
|
|
.nf
|
|
\fBip proto \fIp\fR
|
|
.fi
|
|
.in -.5i
|
|
where \fIp\fR is one of the above protocols.
|
|
.IP "\fBesis\fR, \fBisis\fR"
|
|
Abbreviations for:
|
|
.in +.5i
|
|
.nf
|
|
\fBiso proto \fIp\fR
|
|
.fi
|
|
.in -.5i
|
|
where \fIp\fR is one of the above protocols.
|
|
Note that \fItcpdump\fR does an incomplete job of parsing these protocols.
|
|
.IP "\fIexpr relop expr\fR"
|
|
True if the relation holds, where \fIrelop\fR is one of >, <, >=, <=, =, !=,
|
|
and \fIexpr\fR is an arithmetic expression composed of integer constants
|
|
(expressed in standard C syntax), the normal binary operators
|
|
[+, -, *, /, &, |], a length operator, and special packet data accessors.
|
|
To access
|
|
data inside the packet, use the following syntax:
|
|
.in +.5i
|
|
.nf
|
|
\fIproto\fB [ \fIexpr\fB : \fIsize\fB ]\fR
|
|
.fi
|
|
.in -.5i
|
|
\fIProto\fR is one of \fBether, fddi,
|
|
ip, arp, rarp, tcp, udp, \fRor \fBicmp\fR, and
|
|
indicates the protocol layer for the index operation.
|
|
The byte offset, relative to the indicated protocol layer, is
|
|
given by \fIexpr\fR.
|
|
\fISize\fR is optional and indicates the number of bytes in the
|
|
field of interest; it can be either one, two, or four, and defaults to one.
|
|
The length operator, indicated by the keyword \fBlen\fP, gives the
|
|
length of the packet.
|
|
|
|
For example, `\fBether[0] & 1 != 0\fP' catches all multicast traffic.
|
|
The expression `\fBip[0] & 0xf != 5\fP'
|
|
catches all IP packets with options. The expression
|
|
`\fBip[6:2] & 0x1fff = 0\fP'
|
|
catches only unfragmented datagrams and frag zero of fragmented datagrams.
|
|
This check is implicitly applied to the \fBtcp\fP and \fBudp\fP
|
|
index operations.
|
|
For instance, \fBtcp[0]\fP always means the first
|
|
byte of the TCP \fIheader\fP, and never means the first byte of an
|
|
intervening fragment.
|
|
.LP
|
|
Primitives may be combined using:
|
|
.IP
|
|
A parenthesized group of primitives and operators
|
|
(parentheses are special to the Shell and must be escaped).
|
|
.IP
|
|
Negation (`\fB!\fP' or `\fBnot\fP').
|
|
.IP
|
|
Concatenation (`\fB&&\fP' or `\fBand\fP').
|
|
.IP
|
|
Alternation (`\fB||\fP' or `\fBor\fP').
|
|
.LP
|
|
Negation has highest precedence.
|
|
Alternation and concatenation have equal precedence and associate
|
|
left to right. Note that explicit \fBand\fR tokens, not juxtaposition,
|
|
are now required for concatenation.
|
|
.LP
|
|
If an identifier is given without a keyword, the most recent keyword
|
|
is assumed.
|
|
For example,
|
|
.in +.5i
|
|
.nf
|
|
\fBnot host vs and ace\fR
|
|
.fi
|
|
.in -.5i
|
|
is short for
|
|
.in +.5i
|
|
.nf
|
|
\fBnot host vs and host ace\fR
|
|
.fi
|
|
.in -.5i
|
|
which should not be confused with
|
|
.in +.5i
|
|
.nf
|
|
\fBnot ( host vs or ace )\fR
|
|
.fi
|
|
.in -.5i
|
|
.LP
|
|
Expression arguments can be passed to tcpdump as either a single argument
|
|
or as multiple arguments, whichever is more convenient.
|
|
Generally, if the expression contains Shell metacharacters, it is
|
|
easier to pass it as a single, quoted argument.
|
|
Multiple arguments are concatenated with spaces before being parsed.
|
|
.SH EXAMPLES
|
|
.LP
|
|
To print all packets arriving at or departing from \fIsundown\fP:
|
|
.RS
|
|
.nf
|
|
\fBtcpdump host sundown\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
To print traffic between \fIhelios\fR and either \fIhot\fR or \fIace\fR:
|
|
.RS
|
|
.nf
|
|
\fBtcpdump host helios and \\( hot or ace \\)\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
To print all IP packets between \fIace\fR and any host except \fIhelios\fR:
|
|
.RS
|
|
.nf
|
|
\fBtcpdump ip host ace and not helios\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
To print all traffic between local hosts and hosts at Berkeley:
|
|
.RS
|
|
.nf
|
|
.B
|
|
tcpdump net ucb-ether
|
|
.fi
|
|
.RE
|
|
.LP
|
|
To print all ftp traffic through internet gateway \fIsnup\fP:
|
|
(note that the expression is quoted to prevent the shell from
|
|
(mis-)interpreting the parentheses):
|
|
.RS
|
|
.nf
|
|
.B
|
|
tcpdump 'gateway snup and (port ftp or ftp-data)'
|
|
.fi
|
|
.RE
|
|
.LP
|
|
To print traffic neither sourced from nor destined for local hosts
|
|
(if you gateway to one other net, this stuff should never make it
|
|
onto your local net).
|
|
.RS
|
|
.nf
|
|
.B
|
|
tcpdump ip and not net \fIlocalnet\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
To print the start and end packets (the SYN and FIN packets) of each
|
|
TCP conversation that involves a non-local host.
|
|
.RS
|
|
.nf
|
|
.B
|
|
tcpdump 'tcp[13] & 3 != 0 and not src and dst net \fIlocalnet\fP'
|
|
.fi
|
|
.RE
|
|
.LP
|
|
To print IP packets longer than 576 bytes sent through gateway \fIsnup\fP:
|
|
.RS
|
|
.nf
|
|
.B
|
|
tcpdump 'gateway snup and ip[2:2] > 576'
|
|
.fi
|
|
.RE
|
|
.LP
|
|
To print IP broadcast or multicast packets that were
|
|
.I not
|
|
sent via ethernet broadcast or multicast:
|
|
.RS
|
|
.nf
|
|
.B
|
|
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
|
|
.fi
|
|
.RE
|
|
.LP
|
|
To print all ICMP packets that are not echo requests/replies (i.e., not
|
|
ping packets):
|
|
.RS
|
|
.nf
|
|
.B
|
|
tcpdump 'icmp[0] != 8 and icmp[0] != 0'
|
|
.fi
|
|
.RE
|
|
.SH OUTPUT FORMAT
|
|
.LP
|
|
The output of \fItcpdump\fP is protocol dependent. The following
|
|
gives a brief description and examples of most of the formats.
|
|
.de HD
|
|
.sp 1.5
|
|
.B
|
|
..
|
|
.HD
|
|
Link Level Headers
|
|
.LP
|
|
If the '-e' option is given, the link level header is printed out.
|
|
On ethernets, the source and destination addresses, protocol,
|
|
and packet length are printed.
|
|
.LP
|
|
On FDDI networks, the '-e' option causes \fItcpdump\fP to print
|
|
the `frame control' field, the source and destination addresses,
|
|
and the packet length. (The `frame control' field governs the
|
|
interpretation of the rest of the packet. Normal packets (such
|
|
as those containing IP datagrams) are `async' packets, with a priority
|
|
value between 0 and 7; for example, `\fBasync4\fR'. Such packets
|
|
are assumed to contain an 802.2 Logical Link Control (LLC) packet;
|
|
the LLC header is printed if it is \fInot\fR an ISO datagram or a
|
|
so-called SNAP packet.
|
|
.LP
|
|
\fI(N.B.: The following description assumes familiarity with
|
|
the SLIP compression algorithm described in RFC-1144.)\fP
|
|
.LP
|
|
On SLIP links, a direction indicator (``I'' for inbound, ``O'' for outbound),
|
|
packet type, and compression information are printed out.
|
|
The packet type is printed first.
|
|
The three types are \fIip\fP, \fIutcp\fP, and \fIctcp\fP.
|
|
No further link information is printed for \fIip\fR packets.
|
|
For TCP packets, the connection identifier is printed following the type.
|
|
If the packet is compressed, its encoded header is printed out.
|
|
The special cases are printed out as
|
|
\fB*S+\fIn\fR and \fB*SA+\fIn\fR, where \fIn\fR is the amount by which
|
|
the sequence number (or sequence number and ack) has changed.
|
|
If it is not a special case,
|
|
zero or more changes are printed.
|
|
A change is indicated by U (urgent pointer), W (window), A (ack),
|
|
S (sequence number), and I (packet ID), followed by a delta (+n or -n),
|
|
or a new value (=n).
|
|
Finally, the amount of data in the packet and compressed header length
|
|
are printed.
|
|
.LP
|
|
For example, the following line shows an outbound compressed TCP packet,
|
|
with an implicit connection identifier; the ack has changed by 6,
|
|
the sequence number by 49, and the packet ID by 6; there are 3 bytes of
|
|
data and 6 bytes of compressed header:
|
|
.RS
|
|
.nf
|
|
\fBO ctcp * A+6 S+49 I+6 3 (6)\fP
|
|
.fi
|
|
.RE
|
|
.HD
|
|
ARP/RARP Packets
|
|
.LP
|
|
Arp/rarp output shows the type of request and its arguments. The
|
|
format is intended to be self explanatory.
|
|
Here is a short sample taken from the start of an `rlogin' from
|
|
host \fIrtsg\fP to host \fIcsam\fP:
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\f(CWarp who-has csam tell rtsg
|
|
arp reply csam is-at CSAM\fP
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
The first line says that rtsg sent an arp packet asking
|
|
for the ethernet address of internet host csam. Csam
|
|
replies with its ethernet address (in this example, ethernet addresses
|
|
are in caps and internet addresses in lower case).
|
|
.LP
|
|
This would look less redundant if we had done \fBtcpdump \-n\fP:
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\f(CWarp who-has 128.3.254.6 tell 128.3.254.68
|
|
arp reply 128.3.254.6 is-at 02:07:01:00:01:c4\fP
|
|
.fi
|
|
.RE
|
|
.LP
|
|
If we had done \fBtcpdump \-e\fP, the fact that the first packet is
|
|
broadcast and the second is point-to-point would be visible:
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\f(CWRTSG Broadcast 0806 64: arp who-has csam tell rtsg
|
|
CSAM RTSG 0806 64: arp reply csam is-at CSAM\fP
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
For the first packet this says the ethernet source address is RTSG, the
|
|
destination is the ethernet broadcast address, the type field
|
|
contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
|
|
.HD
|
|
TCP Packets
|
|
.LP
|
|
\fI(N.B.:The following description assumes familiarity with
|
|
the TCP protocol described in RFC-793. If you are not familiar
|
|
with the protocol, neither this description nor tcpdump will
|
|
be of much use to you.)\fP
|
|
.LP
|
|
The general format of a tcp protocol line is:
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\fIsrc > dst: flags data-seqno ack window urgent options\fP
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
\fISrc\fP and \fIdst\fP are the source and destination IP
|
|
addresses and ports. \fIFlags\fP are some combination of S (SYN),
|
|
F (FIN), P (PUSH) or R (RST) or a single `.' (no flags).
|
|
\fIData-seqno\fP describes the portion of sequence space covered
|
|
by the data in this packet (see example below).
|
|
\fIAck\fP is sequence number of the next data expected the other
|
|
direction on this connection.
|
|
\fIWindow\fP is the number of bytes of receive buffer space available
|
|
the other direction on this connection.
|
|
\fIUrg\fP indicates there is `urgent' data in the packet.
|
|
\fIOptions\fP are tcp options enclosed in angle brackets (e.g., <mss 1024>).
|
|
.LP
|
|
\fISrc, dst\fP and \fIflags\fP are always present. The other fields
|
|
depend on the contents of the packet's tcp protocol header and
|
|
are output only if appropriate.
|
|
.LP
|
|
Here is the opening portion of an rlogin from host \fIrtsg\fP to
|
|
host \fIcsam\fP.
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\s-2\f(CWrtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
|
|
csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
|
|
rtsg.1023 > csam.login: . ack 1 win 4096
|
|
rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
|
|
csam.login > rtsg.1023: . ack 2 win 4096
|
|
rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
|
|
csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
|
|
csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
|
|
csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1\fP\s+2
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
The first line says that tcp port 1023 on rtsg sent a packet
|
|
to port \fIlogin\fP
|
|
on csam. The \fBS\fP indicates that the \fISYN\fP flag was set.
|
|
The packet sequence number was 768512 and it contained no data.
|
|
(The notation is `first:last(nbytes)' which means `sequence
|
|
numbers \fIfirst\fP
|
|
up to but not including \fIlast\fP which is \fInbytes\fP bytes of user data'.)
|
|
There was no piggy-backed ack, the available receive window was 4096
|
|
bytes and there was a max-segment-size option requesting an mss of
|
|
1024 bytes.
|
|
.LP
|
|
Csam replies with a similar packet except it includes a piggy-backed
|
|
ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no
|
|
flags were set.
|
|
The packet contained no data so there is no data sequence number.
|
|
Note that the ack sequence
|
|
number is a small integer (1). The first time \fBtcpdump\fP sees a
|
|
tcp `conversation', it prints the sequence number from the packet.
|
|
On subsequent packets of the conversation, the difference between
|
|
the current packet's sequence number and this initial sequence number
|
|
is printed. This means that sequence numbers after the
|
|
first can be interpreted
|
|
as relative byte positions in the conversation's data stream (with the
|
|
first data byte each direction being `1'). `-S' will override this
|
|
feature, causing the original sequence numbers to be output.
|
|
.LP
|
|
On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20
|
|
in the rtsg \(-> csam side of the conversation).
|
|
The PUSH flag is set in the packet.
|
|
On the 7th line, csam says it's received data sent by rtsg up to
|
|
but not including byte 21. Most of this data is apparently sitting in the
|
|
socket buffer since csam's receive window has gotten 19 bytes smaller.
|
|
Csam also sends one byte of data to rtsg in this packet.
|
|
On the 8th and 9th lines,
|
|
csam sends two bytes of urgent, pushed data to rtsg.
|
|
.LP
|
|
If the snapshot was small enough that \fBtcpdump\fP didn't capture
|
|
the full TCP header, it interprets as much of the header as it can
|
|
and then reports ``[|\fItcp\fP]'' to indicate the remainder could not
|
|
be interpreted. If the header contains a bogus option (one with a length
|
|
that's either too small or beyond the end of the header), tcpdump reports
|
|
it as ``[\fIbad opt\fP]'' and does not interpret any further options (since
|
|
it's impossible to tell where they start). If the header length indicates
|
|
options are present but the IP datagram length is not long enough for the
|
|
options to actually be there, tcpdump reports it as ``[\fIbad hdr length\fP]''.
|
|
.HD
|
|
.B
|
|
UDP Packets
|
|
.LP
|
|
UDP format is illustrated by this rwho packet:
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\f(CWactinide.who > broadcast.who: udp 84\fP
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
This says that port \fIwho\fP on host \fIactinide\fP sent a udp
|
|
datagram to port \fIwho\fP on host \fIbroadcast\fP, the Internet
|
|
broadcast address. The packet contained 84 bytes of user data.
|
|
.LP
|
|
Some UDP services are recognized (from the source or destination
|
|
port number) and the higher level protocol information printed.
|
|
In particular, Domain Name service requests (RFC-1034/1035) and Sun
|
|
RPC calls (RFC-1050) to NFS.
|
|
.HD
|
|
UDP Name Server Requests
|
|
.LP
|
|
\fI(N.B.:The following description assumes familiarity with
|
|
the Domain Service protocol described in RFC-1035. If you are not familiar
|
|
with the protocol, the following description will appear to be written
|
|
in greek.)\fP
|
|
.LP
|
|
Name server requests are formatted as
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\fIsrc > dst: id op? flags qtype qclass name (len)\fP
|
|
.sp .5
|
|
\f(CWh2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)\fP
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
Host \fIh2opolo\fP asked the domain server on \fIhelios\fP for an
|
|
address record (qtype=A) associated with the name \fIucbvax.berkeley.edu.\fP
|
|
The query id was `3'. The `+' indicates the \fIrecursion desired\fP flag
|
|
was set. The query length was 37 bytes, not including the UDP and
|
|
IP protocol headers. The query operation was the normal one, \fIQuery\fP,
|
|
so the op field was omitted. If the op had been anything else, it would
|
|
have been printed between the `3' and the `+'.
|
|
Similarly, the qclass was the normal one,
|
|
\fIC_IN\fP, and omitted. Any other qclass would have been printed
|
|
immediately after the `A'.
|
|
.LP
|
|
A few anomalies are checked and may result in extra fields enclosed in
|
|
square brackets: If a query contains an answer, name server or
|
|
authority section,
|
|
.IR ancount ,
|
|
.IR nscount ,
|
|
or
|
|
.I arcount
|
|
are printed as `[\fIn\fPa]', `[\fIn\fPn]' or `[\fIn\fPau]' where \fIn\fP
|
|
is the appropriate count.
|
|
If any of the response bits are set (AA, RA or rcode) or any of the
|
|
`must be zero' bits are set in bytes two and three, `[b2&3=\fIx\fP]'
|
|
is printed, where \fIx\fP is the hex value of header bytes two and three.
|
|
.HD
|
|
UDP Name Server Responses
|
|
.LP
|
|
Name server responses are formatted as
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\fIsrc > dst: id op rcode flags a/n/au type class data (len)\fP
|
|
.sp .5
|
|
\f(CWhelios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
|
|
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)\fP
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
In the first example, \fIhelios\fP responds to query id 3 from \fIh2opolo\fP
|
|
with 3 answer records, 3 name server records and 7 authority records.
|
|
The first answer record is type A (address) and its data is internet
|
|
address 128.32.137.3. The total size of the response was 273 bytes,
|
|
excluding UDP and IP headers. The op (Query) and response code
|
|
(NoError) were omitted, as was the class (C_IN) of the A record.
|
|
.LP
|
|
In the second example, \fIhelios\fP responds to query 2 with a
|
|
response code of non-existent domain (NXDomain) with no answers,
|
|
one name server and no authority records. The `*' indicates that
|
|
the \fIauthoritative answer\fP bit was set. Since there were no
|
|
answers, no type, class or data were printed.
|
|
.LP
|
|
Other flag characters that might appear are `\-' (recursion available,
|
|
RA, \fInot\fP set) and `|' (truncated message, TC, set). If the
|
|
`question' section doesn't contain exactly one entry, `[\fIn\fPq]'
|
|
is printed.
|
|
.LP
|
|
Note that name server requests and responses tend to be large and the
|
|
default \fIsnaplen\fP of 68 bytes may not capture enough of the packet
|
|
to print. Use the \fB\-s\fP flag to increase the snaplen if you
|
|
need to seriously investigate name server traffic. `\fB\-s 128\fP'
|
|
has worked well for me.
|
|
|
|
.HD
|
|
NFS Requests and Replies
|
|
.LP
|
|
Sun NFS (Network File System) requests and replies are printed as:
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\fIsrc.xid > dst.nfs: len op args\fP
|
|
\fIsrc.nfs > dst.xid: reply stat len op results\fP
|
|
.sp .5
|
|
\f(CW
|
|
sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
|
|
wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
|
|
sushi.201b > wrl.nfs:
|
|
144 lookup fh 9,74/4096.6878 "xcolors"
|
|
wrl.nfs > sushi.201b:
|
|
reply ok 128 lookup fh 9,74/4134.3150
|
|
\fP
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
In the first line, host \fIsushi\fP sends a transaction with id \fI6709\fP
|
|
to \fIwrl\fP (note that the number following the src host is a
|
|
transaction id, \fInot\fP the source port). The request was 112 bytes,
|
|
excluding the UDP and IP headers. The operation was a \fIreadlink\fP
|
|
(read symbolic link) on file handle (\fIfh\fP) 21,24/10.731657119.
|
|
(If one is lucky, as in this case, the file handle can be interpreted
|
|
as a major,minor device number pair, followed by the inode number and
|
|
generation number.)
|
|
\fIWrl\fP replies `ok' with the contents of the link.
|
|
.LP
|
|
In the third line, \fIsushi\fP asks \fIwrl\fP to lookup the name
|
|
`\fIxcolors\fP' in directory file 9,74/4096.6878. Note that the data printed
|
|
depends on the operation type. The format is intended to be self
|
|
explanatory if read in conjunction with
|
|
an NFS protocol spec.
|
|
.LP
|
|
If the \-v (verbose) flag is given, additional information is printed.
|
|
For example:
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\f(CW
|
|
sushi.1372a > wrl.nfs:
|
|
148 read fh 21,11/12.195 8192 bytes @ 24576
|
|
wrl.nfs > sushi.1372a:
|
|
reply ok 1472 read REG 100664 ids 417/0 sz 29388
|
|
\fP
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
(\-v also prints the IP header TTL, ID, and fragmentation fields,
|
|
which have been omitted from this example.) In the first line,
|
|
\fIsushi\fP asks \fIwrl\fP to read 8192 bytes from file 21,11/12.195,
|
|
at byte offset 24576. \fIWrl\fP replies `ok'; the packet shown on the
|
|
second line is the first fragment of the reply, and hence is only 1472
|
|
bytes long (the other bytes will follow in subsequent fragments, but
|
|
these fragments do not have NFS or even UDP headers and so might not be
|
|
printed, depending on the filter expression used). Because the \-v flag
|
|
is given, some of the file attributes (which are returned in addition
|
|
to the file data) are printed: the file type (``REG'', for regular file),
|
|
the file mode (in octal), the uid and gid, and the file size.
|
|
.LP
|
|
If the \-v flag is given more than once, even more details are printed.
|
|
.LP
|
|
Note that NFS requests are very large and much of the detail won't be printed
|
|
unless \fIsnaplen\fP is increased. Try using `\fB\-s 192\fP' to watch
|
|
NFS traffic.
|
|
.LP
|
|
NFS reply packets do not explicitly identify the RPC operation. Instead,
|
|
\fItcpdump\fP keeps track of ``recent'' requests, and matches them to the
|
|
replies using the transaction ID. If a reply does not closely follow the
|
|
corresponding request, it might not be parsable.
|
|
.HD
|
|
KIP Appletalk (DDP in UDP)
|
|
.LP
|
|
Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
|
|
and dumped as DDP packets (i.e., all the UDP header information is
|
|
discarded). The file
|
|
.I /etc/atalk.names
|
|
is used to translate appletalk net and node numbers to names.
|
|
Lines in this file have the form
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\fInumber name\fP
|
|
|
|
\f(CW1.254 ether
|
|
16.1 icsd-net
|
|
1.254.110 ace\fP
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
The first two lines give the names of appletalk networks. The third
|
|
line gives the name of a particular host (a host is distinguished
|
|
from a net by the 3rd octet in the number \-
|
|
a net number \fImust\fP have two octets and a host number \fImust\fP
|
|
have three octets.) The number and name should be separated by
|
|
whitespace (blanks or tabs).
|
|
The
|
|
.I /etc/atalk.names
|
|
file may contain blank lines or comment lines (lines starting with
|
|
a `#').
|
|
.LP
|
|
Appletalk addresses are printed in the form
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\fInet.host.port\fP
|
|
|
|
\f(CW144.1.209.2 > icsd-net.112.220
|
|
office.2 > icsd-net.112.220
|
|
jssmag.149.235 > icsd-net.2\fP
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
(If the
|
|
.I /etc/atalk.names
|
|
doesn't exist or doesn't contain an entry for some appletalk
|
|
host/net number, addresses are printed in numeric form.)
|
|
In the first example, NBP (DDP port 2) on net 144.1 node 209
|
|
is sending to whatever is listening on port 220 of net icsd node 112.
|
|
The second line is the same except the full name of the source node
|
|
is known (`office'). The third line is a send from port 235 on
|
|
net jssmag node 149 to broadcast on the icsd-net NBP port (note that
|
|
the broadcast address (255) is indicated by a net name with no host
|
|
number \- for this reason it's a good idea to keep node names and
|
|
net names distinct in /etc/atalk.names).
|
|
.LP
|
|
NBP (name binding protocol) and ATP (Appletalk transaction protocol)
|
|
packets have their contents interpreted. Other protocols just dump
|
|
the protocol name (or number if no name is registered for the
|
|
protocol) and packet size.
|
|
|
|
\fBNBP packets\fP are formatted like the following examples:
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\s-2\f(CWicsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
|
|
jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
|
|
techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186\fP\s+2
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
The first line is a name lookup request for laserwriters sent by net icsd host
|
|
112 and broadcast on net jssmag. The nbp id for the lookup is 190.
|
|
The second line shows a reply for this request (note that it has the
|
|
same id) from host jssmag.209 saying that it has a laserwriter
|
|
resource named "RM1140" registered on port 250. The third line is
|
|
another reply to the same request saying host techpit has laserwriter
|
|
"techpit" registered on port 186.
|
|
|
|
\fBATP packet\fP formatting is demonstrated by the following example:
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\s-2\f(CWjssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001
|
|
helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
|
|
helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
|
|
helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
|
|
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
|
|
helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
|
|
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
|
|
helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
|
|
helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
|
|
jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001
|
|
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
|
|
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
|
|
jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001
|
|
jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002\fP\s+2
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
Jssmag.209 initiates transaction id 12266 with host helios by requesting
|
|
up to 8 packets (the `<0-7>'). The hex number at the end of the line
|
|
is the value of the `userdata' field in the request.
|
|
.LP
|
|
Helios responds with 8 512-byte packets. The `:digit' following the
|
|
transaction id gives the packet sequence number in the transaction
|
|
and the number in parens is the amount of data in the packet,
|
|
excluding the atp header. The `*' on packet 7 indicates that the
|
|
EOM bit was set.
|
|
.LP
|
|
Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
|
|
resends them then jssmag.209 releases the transaction. Finally,
|
|
jssmag.209 initiates the next request. The `*' on the request
|
|
indicates that XO (`exactly once') was \fInot\fP set.
|
|
|
|
.HD
|
|
IP Fragmentation
|
|
.LP
|
|
Fragmented Internet datagrams are printed as
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB+)\fR
|
|
\fB(frag \fIid\fB:\fIsize\fB@\fIoffset\fB)\fR
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
(The first form indicates there are more fragments. The second
|
|
indicates this is the last fragment.)
|
|
.LP
|
|
\fIId\fP is the fragment id. \fISize\fP is the fragment
|
|
size (in bytes) excluding the IP header. \fIOffset\fP is this
|
|
fragment's offset (in bytes) in the original datagram.
|
|
.LP
|
|
The fragment information is output for each fragment. The first
|
|
fragment contains the higher level protocol header and the frag
|
|
info is printed after the protocol info. Fragments
|
|
after the first contain no higher level protocol header and the
|
|
frag info is printed after the source and destination addresses.
|
|
For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa
|
|
over a CSNET connection that doesn't appear to handle 576 byte datagrams:
|
|
.RS
|
|
.nf
|
|
.sp .5
|
|
\s-2\f(CWarizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
|
|
arizona > rtsg: (frag 595a:204@328)
|
|
rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560\fP\s+2
|
|
.sp .5
|
|
.fi
|
|
.RE
|
|
There are a couple of things to note here: First, addresses in the
|
|
2nd line don't include port numbers. This is because the TCP
|
|
protocol information is all in the first fragment and we have no idea
|
|
what the port or sequence numbers are when we print the later fragments.
|
|
Second, the tcp sequence information in the first line is printed as if there
|
|
were 308 bytes of user data when, in fact, there are 512 bytes (308 in
|
|
the first frag and 204 in the second). If you are looking for holes
|
|
in the sequence space or trying to match up acks
|
|
with packets, this can fool you.
|
|
.LP
|
|
A packet with the IP \fIdon't fragment\fP flag is marked with a
|
|
trailing \fB(DF)\fP.
|
|
.HD
|
|
Timestamps
|
|
.LP
|
|
By default, all output lines are preceded by a timestamp. The timestamp
|
|
is the current clock time in the form
|
|
.RS
|
|
.nf
|
|
\fIhh:mm:ss.frac\fP
|
|
.fi
|
|
.RE
|
|
and is as accurate as the kernel's clock.
|
|
The timestamp reflects the time the kernel first saw the packet. No attempt
|
|
is made to account for the time lag between when the
|
|
ethernet interface removed the packet from the wire and when the kernel
|
|
serviced the `new packet' interrupt.
|
|
.SH "SEE ALSO"
|
|
bpf(4), pcap(3)
|
|
.SH AUTHORS
|
|
Van Jacobson,
|
|
Craig Leres and
|
|
Steven McCanne, all of the
|
|
Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
|
|
.LP
|
|
The current version is available via anonymous ftp:
|
|
.LP
|
|
.RS
|
|
.I ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
|
|
.RE
|
|
.SH BUGS
|
|
Please send bug reports to tcpdump@ee.lbl.gov.
|
|
.LP
|
|
NIT doesn't let you watch your own outbound traffic, BPF will.
|
|
We recommend that you use the latter.
|
|
.LP
|
|
Some attempt should be made to reassemble IP fragments or, at least
|
|
to compute the right length for the higher level protocol.
|
|
.LP
|
|
Name server inverse queries are not dumped correctly: The (empty)
|
|
question section is printed rather than real query in the answer
|
|
section. Some believe that inverse queries are themselves a bug and
|
|
prefer to fix the program generating them rather than tcpdump.
|
|
.LP
|
|
Apple Ethertalk DDP packets could be dumped as easily as KIP DDP
|
|
packets but aren't.
|
|
Even if we were inclined to do anything to promote the use of
|
|
Ethertalk (we aren't), LBL doesn't allow Ethertalk on any of its
|
|
networks so we'd would have no way of testing this code.
|
|
.LP
|
|
A packet trace that crosses a daylight savings time change will give
|
|
skewed time stamps (the time change is ignored).
|
|
.LP
|
|
Filters expressions that manipulate FDDI headers assume that all FDDI
|
|
packets are encapsulated Ethernet packets. This is true for IP, ARP,
|
|
and DECNET Phase IV, but is not true for protocols such as ISO CLNS.
|
|
Therefore, the filter may inadvertently accept certain packets that
|
|
do not properly match the filter expression.
|