User Commands TCPDUMP(1)



NAME
tcpdump - dump traffic on a network

SYNOPSIS
tcpdump [ -AdDeflLnNOpqRStuUvxX ] [ -c count ]
[ -C file_size ] [ -F file ]
[ -i interface ] [ -m module ] [ -r file ]
[ -s snaplen ] [ -T type ] [ -w file ]
[ -E spi@ipaddr algo:secret,... ]
[ -y datalinktype ]
[ expression ]

DESCRIPTION
Tcpdump prints out the headers of packets on a network
interface that match the boolean expression. It can also be
run with the -w flag, which causes it to save the packet
data to a file for later analysis, and/or with the -r flag,
which causes it to read from a saved packet file rather than
to read packets from a network interface. In all cases,
only packets that match expression will be processed by
tcpdump.

Tcpdump will, if not run with the -c flag, continue captur-
ing packets until it is interrupted by a SIGINT signal (gen-
erated, for example, by typing your interrupt character,
typically control-C) or a SIGTERM signal (typically gen-
erated with the kill(1) command); if run with the -c flag,
it will capture packets until it is interrupted by a SIGINT
or SIGTERM signal or the specified number of packets have
been processed.

When tcpdump finishes capturing packets, it will report
counts of:

packets ``captured'' (this is the number of packets
that tcpdump has received and processed);

packets ``received by filter'' (the meaning of this
depends on the OS on which you're running tcpdump, and
possibly on the way the OS was configured - if a filter
was specified on the command line, on some OSes it
counts packets regardless of whether they were matched
by the filter expression and, even if they were matched
by the filter expression, regardless of whether tcpdump
has read and processed them yet, on other OSes it
counts only packets that were matched by the filter
expression regardless of whether tcpdump has read and
processed them yet, and on other OSes it counts only
packets that were matched by the filter expression and
were processed by tcpdump);

packets ``dropped by kernel'' (this is the number of



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packets that were dropped, due to a lack of buffer
space, by the packet capture mechanism in the OS on
which tcpdump is running, if the OS reports that infor-
mation to applications; if not, it will be reported as
0).

On platforms that support the SIGINFO signal, such as most
BSDs (including Mac OS X) and Digital/Tru64 UNIX, it will
report those counts when it receives a SIGINFO signal (gen-
erated, for example, by typing your ``status'' character,
typically control-T, although on some platforms, such as Mac
OS X, the ``status'' character is not set by default, so you
must set it with stty(1) in order to use it) and will con-
tinue capturing packets.

Reading packets from a network interface may require that
you have special privileges:

Under SunOS 3.x or 4.x with
You must have read access to /dev/nit or /dev/bpf*.

Under Solaris with DLPI:
You must have read/write access to the network pseudo
device, e.g. /dev/le. On at least some versions of
Solaris, however, this is not sufficient to allow
tcpdump to capture in promiscuous mode; on those ver-
sions of Solaris, you must be root, or tcpdump must be
installed setuid to root, in order to capture in prom-
iscuous mode. Note that, on many (perhaps all) inter-
faces, if you don't capture in promiscuous mode, you
will not see any outgoing packets, so a capture not
done in promiscuous mode may not be very useful.

Under HP-UX with DLPI:
You must be root or tcpdump must be installed setuid to
root.

Under IRIX with snoop:
You must be root or tcpdump must be installed setuid to
root.

Under Linux:
You must be root or tcpdump must be installed setuid to
root (unless your distribution has a kernel that sup-
ports capability bits such as CAP_NET_RAW and code to
allow those capability bits to be given to particular
accounts and to cause those bits to be set on a user's
initial processes when they log in, in which case you
must have CAP_NET_RAW in order to capture and
CAP_NET_ADMIN to enumerate network devices with, for
example, the -D flag).




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Under ULTRIX and Digital UNIX/Tru64 UNIX:
Any user may capture network traffic with tcpdump.
However, no user (not even the super-user) can capture
in promiscuous mode on an interface unless the super-
user has enabled promiscuous-mode operation on that
interface using pfconfig(8), and no user (not even the
super-user) can capture unicast traffic received by or
sent by the machine on an interface unless the super-
user has enabled copy-all-mode operation on that inter-
face using pfconfig, so useful packet capture on an
interface probably requires that either promiscuous-
mode or copy-all-mode operation, or both modes of
operation, be enabled on that interface.

Under BSD (this includes Mac OS
You must have read access to /dev/bpf*. On BSDs with a
devfs (this includes Mac OS X), this might involve more
than just having somebody with super-user access set-
ting the ownership or permissions on the BPF devices -
it might involve configuring devfs to set the ownership
or permissions every time the system is booted, if the
system even supports that; if it doesn't support that,
you might have to find some other way to make that hap-
pen at boot time.

Reading a saved packet file doesn't require special
privileges.

OPTIONS
-A Print each packet (minus its link level header) in
ASCII. Handy for capturing web pages.

-c Exit after receiving count packets.

-C Before writing a raw packet to a savefile, check
whether the file is currently larger than file_size
and, if so, close the current savefile and open a new
one. Savefiles after the first savefile will have the
name specified with the -w flag, with a number after
it, starting at 2 and continuing upward. The units of
file_size are millions of bytes (1,000,000 bytes, not
1,048,576 bytes).

-d Dump the compiled packet-matching code in a human read-
able form to standard output and stop.

-dd Dump packet-matching code as a C program fragment.

-ddd Dump packet-matching code as decimal numbers (preceded
with a count).

-D Print the list of the network interfaces available on



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the system and on which tcpdump can capture packets.
For each network interface, a number and an interface
name, possibly followed by a text description of the
interface, is printed. The interface name or the
number can be supplied to the -i flag to specify an
interface on which to capture.

This can be useful on systems that don't have a command
to list them (e.g., Windows systems, or UNIX systems
lacking ifconfig -a); the number can be useful on Win-
dows 2000 and later systems, where the interface name
is a somewhat complex string.

The -D flag will not be supported if tcpdump was built
with an older version of libpcap that lacks the
pcap_findalldevs() function.

-e Print the link-level header on each dump line.

-E Use spi@ipaddr algo:secret for decrypting IPsec ESP
packets that are addressed to addr and contain Security
Parameter Index value spi. This combination may be
repeated with comma or newline seperation.

Note that setting the secret for IPv4 ESP packets is
supported at this time.

Algorithms may be des-cbc, 3des-cbc, blowfish-cbc,
rc3-cbc, cast128-cbc, or none. The default is des-cbc.
The ability to decrypt packets is only present if
tcpdump was compiled with cryptography enabled.

secret is the ASCII text for ESP secret key. If pre-
ceeded by 0x, then a hex value will be read.

The option assumes RFC2406 ESP, not RFC1827 ESP. The
option is only for debugging purposes, and the use of
this option with a true `secret' key is discouraged.
By presenting IPsec secret key onto command line you
make it visible to others, via ps(1) and other occa-
sions.

In addition to the above syntax, the syntax file name
may be used to have tcpdump read the provided file in.
The file is opened upon receiving the first ESP packet,
so any special permissions that tcpdump may have been
given should already have been given up.

-f Print `foreign' IPv4 addresses numerically rather than
symbolically (this option is intended to get around
serious brain damage in Sun's NIS server - usually it
hangs forever translating non-local internet numbers).



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The test for `foreign' IPv4 addresses is done using the
IPv4 address and netmask of the interface on which cap-
ture is being done. If that address or netmask are not
available, available, either because the interface on
which capture is being done has no address or netmask
or because the capture is being done on the Linux "any"
interface, which can capture on more than one inter-
face, this option will not work correctly.

-F Use file as input for the filter expression. An addi-
tional expression given on the command line is ignored.

-i Listen on interface. If unspecified, tcpdump searches
the system interface list for the lowest numbered, con-
figured up interface (excluding loopback). Ties are
broken by choosing the earliest match.

On Linux systems with 2.2 or later kernels, an inter-
face argument of ``any'' can be used to capture packets
from all interfaces. Note that captures on the ``any''
device will not be done in promiscuous mode.

If the -D flag is supported, an interface number as
printed by that flag can be used as the interface argu-
ment.

-l Make stdout line buffered. Useful if you want to see
the data while capturing it. E.g.,
``tcpdump -l | tee dat'' or ``tcpdump -l >
dat & tail -f dat''.

-L List the known data link types for the interface and
exit.

-m Load SMI MIB module definitions from file module. This
option can be used several times to load several MIB
modules into tcpdump.

-n Don't convert addresses (i.e., host addresses, port
numbers, etc.) to names.

-N Don't print domain name qualification of host names.
E.g., if you give this flag then tcpdump will print
``nic'' instead of ``nic.ddn.mil''.

-O Do not run the packet-matching code optimizer. This is
useful only if you suspect a bug in the optimizer.

-p Don't put the interface into promiscuous mode. Note
that the interface might be in promiscuous mode for
some other reason; hence, `-p' cannot be used as an
abbreviation for `ether host {local-hw-addr} or ether



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broadcast'.

-q Quick (quiet?) output. Print less protocol information
so output lines are shorter.

-R Assume ESP/AH packets to be based on old specification
(RFC1825 to RFC1829). If specified, tcpdump will not
print replay prevention field. Since there is no pro-
tocol version field in ESP/AH specification, tcpdump
cannot deduce the version of ESP/AH protocol.

-r Read packets from file (which was created with the -w
option). Standard input is used if file is ``-''.

-S Print absolute, rather than relative, TCP sequence
numbers.

-s Snarf snaplen bytes of data from each packet rather
than the default of 68 (with SunOS's NIT, the minimum
is actually 96). 68 bytes is adequate for IP, ICMP,
TCP and UDP but may truncate protocol information from
name server and NFS packets (see below). Packets trun-
cated because of a limited snapshot are indicated in
the output with ``[|proto]'', where proto is the name
of the protocol level at which the truncation has
occurred. Note that taking larger snapshots both
increases the amount of time it takes to process pack-
ets and, effectively, decreases the amount of packet
buffering. This may cause packets to be lost. You
should limit snaplen to the smallest number that will
capture the protocol information you're interested in.
Setting snaplen to 0 means use the required length to
catch whole packets.

-T Force packets selected by "expression" to be inter-
preted the specified type. Currently known types are
aodv (Ad-hoc On-demand Distance Vector protocol), cnfp
(Cisco NetFlow protocol), rpc (Remote Procedure Call),
rtp (Real-Time Applications protocol), rtcp (Real-Time
Applications control protocol), snmp (Simple Network
Management Protocol), tftp (Trivial File Transfer Pro-
tocol), vat (Visual Audio Tool), and wb (distributed
White Board).

-t Don't print a timestamp on each dump line.

-tt Print an unformatted timestamp on each dump line.

-ttt Print a delta (in micro-seconds) between current and
previous line on each dump line.

-tttt



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Print a timestamp in default format proceeded by date
on each dump line.

-u Print undecoded NFS handles.

-U Make output saved via the -w option ``packet-
buffered''; i.e., as each packet is saved, it will be
written to the output file, rather than being written
only when the output buffer fills.

The -U flag will not be supported if tcpdump was built
with an older version of libpcap that lacks the
pcap_dump_flush() function.

-v (Slightly more) verbose output. For example, the time
to live, identification, total length and options in an
IP packet are printed. Also enables additional packet
integrity checks such as verifying the IP and ICMP
header checksum.

-vv Even more verbose output. For example, additional
fields are printed from NFS reply packets, and SMB
packets are fully decoded.

-vvv Even more verbose output. For example, telnet SB ...
SE options are printed in full. With -X Telnet options
are printed in hex as well.

-w Write the raw packets to file rather than parsing and
printing them out. They can later be printed with the
-r option. Standard output is used if file is ``-''.

-x Print each packet (minus its link level header) in hex.
The smaller of the entire packet or snaplen bytes will
be printed. Note that this is the entire link-layer
packet, so for link layers that pad (e.g. Ethernet),
the padding bytes will also be printed when the higher
layer packet is shorter than the required padding.

-xx Print each packet, including its link level header, in
hex.

-X Print each packet (minus its link level header) in hex
and ASCII. This is very handy for analysing new proto-
cols.

-XX Print each packet, including its link level header, in
hex and ASCII.

-y Set the data link type to use while capturing packets
to datalinktype.




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User Commands TCPDUMP(1)



expression
selects which packets will be dumped. If no expression
is given, all packets on the net will be dumped. Oth-
erwise, only packets for which expression is `true'
will be dumped.

The expression consists of one or more primitives.
Primitives usually consist of an id (name or number)
preceded by one or more qualifiers. There are three
different kinds of qualifier:

type qualifiers say what kind of thing the id name or
number refers to. Possible types are host, net
and port. E.g., `host foo', `net 128.3', `port
20'. If there is no type qualifier, host is
assumed.

dir qualifiers specify a particular transfer direction
to and/or from id. Possible directions are src,
dst, src or dst and src and dst. E.g., `src foo',
`dst net 128.3', `src or dst port ftp-data'. If
there is no dir qualifier, src or dst is assumed.
For some link layers, such as SLIP and the
``cooked'' Linux capture mode used for the ``any''
device and for some other device types, the
inbound and outbound qualifiers can be used to
specify a desired direction.

proto
qualifiers restrict the match to a particular pro-
tocol. Possible protos are: ether, fddi, tr,
wlan, ip, ip6, arp, rarp, decnet, tcp and udp.
E.g., `ether src foo', `arp net 128.3', `tcp port
21'. If there is no proto qualifier, all proto-
cols consistent with the type are assumed. E.g.,
`src foo' means `(ip or arp or rarp) src foo'
(except the latter is not legal syntax), `net bar'
means `(ip or arp or rarp) net bar' and `port 53'
means `(tcp or udp) port 53'.

[`fddi' is actually an alias for `ether'; the parser
treats them identically as meaning ``the data link
level used on the specified network interface.'' FDDI
headers contain Ethernet-like source and destination
addresses, and often contain Ethernet-like packet
types, so you can filter on these FDDI fields just as
with the analogous Ethernet fields. FDDI headers also
contain other fields, but you cannot name them expli-
citly in a filter expression.

Similarly, `tr' and `wlan' are aliases for `ether'; the
previous paragraph's statements about FDDI headers also



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apply to Token Ring and 802.11 wireless LAN headers.
For 802.11 headers, the destination address is the DA
field and the source address is the SA field; the
BSSID, RA, and TA fields aren't tested.]

In addition to the above, there are some special `prim-
itive' keywords that don't follow the pattern: gate-
way, broadcast, less, greater and arithmetic expres-
sions. All of these are described below.

More complex filter expressions are built up by using
the words and, or and not to combine primitives. E.g.,
`host foo and not port ftp and not port ftp-data'. To
save typing, identical qualifier lists can be omitted.
E.g., `tcp dst port ftp or ftp-data or domain' is
exactly the same as `tcp dst port ftp or tcp dst port
ftp-data or tcp dst port domain'.

Allowable primitives are:

dst host host
True if the IPv4/v6 destination field of the
packet is host, which may be either an address or
a name.

src host host
True if the IPv4/v6 source field of the packet is
host.

host host
True if either the IPv4/v6 source or destination
of the packet is host. Any of the above host
expressions can be prepended with the keywords,
ip, arp, rarp, or ip6 as in:
ip host host
which is equivalent to:
ether proto \ip and host host
If host is a name with multiple IP addresses, each
address will be checked for a match.

ether dst ehost
True if the ethernet destination address is ehost.
Ehost may be either a name from /etc/ethers or a
number (see ethers(3N) for numeric format).

ether src ehost
True if the ethernet source address is ehost.

ether host ehost
True if either the ethernet source or destination
address is ehost.




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User Commands TCPDUMP(1)



gateway host
True if the packet used host as a gateway. I.e.,
the ethernet source or destination address was
host but neither the IP source nor the IP destina-
tion was host. Host must be a name and must be
found both by the machine's host-name-to-IP-
address resolution mechanisms (host name file,
DNS, NIS, etc.) and by the machine's host-name-
to-Ethernet-address resolution mechanism
(/etc/ethers, etc.). (An equivalent expression is
ether host ehost and not host host
which can be used with either names or numbers for
host / ehost.) This syntax does not work in
IPv6-enabled configuration at this moment.

dst net net
True if the IPv4/v6 destination address of the
packet has a network number of net. Net may be
either a name from /etc/networks or a network
number (see networks(4) for details).

src net net
True if the IPv4/v6 source address of the packet
has a network number of net.

net net
True if either the IPv4/v6 source or destination
address of the packet has a network number of net.

net net mask netmask
True if the IP address matches net with the
specific netmask. May be qualified with src or
dst. Note that this syntax is not valid for IPv6
net.

net net/len
True if the IPv4/v6 address matches net with a
netmask len bits wide. May be qualified with src
or dst.

dst port port
True if the packet is ip/tcp, ip/udp, ip6/tcp or
ip6/udp and has a destination port value of port.
The port can be a number or a name used in
/etc/services (see tcp(4P) and udp(4P)). If a
name is used, both the port number and protocol
are checked. If a number or ambiguous name is
used, only the port number is checked (e.g., dst
port 513 will print both tcp/login traffic and
udp/who traffic, and port domain will print both
tcp/domain and udp/domain traffic).




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User Commands TCPDUMP(1)



src port port
True if the packet has a source port value of
port.

port port
True if either the source or destination port of
the packet is port. Any of the above port expres-
sions can be prepended with the keywords, tcp or
udp, as in:
tcp src port port
which matches only tcp packets whose source port
is port.

less length
True if the packet has a length less than or equal
to length. This is equivalent to:
len <= length.

greater length
True if the packet has a length greater than or
equal to length. This is equivalent to:
len >= length.

ip proto protocol
True if the packet is an IP packet (see ip(4P)) of
protocol type protocol. Protocol can be a number
or one of the names icmp, icmp6, igmp, igrp, pim,
ah, esp, vrrp, udp, or tcp. Note that the iden-
tifiers tcp, udp, and icmp are also keywords and
must be escaped via backslash (\), which is \\ in
the C-shell. Note that this primitive does not
chase the protocol header chain.

ip6 proto protocol
True if the packet is an IPv6 packet of protocol
type protocol. Note that this primitive does not
chase the protocol header chain.

ip6 protochain protocol
True if the packet is IPv6 packet, and contains
protocol header with type protocol in its protocol
header chain. For example,
ip6 protochain 6
matches any IPv6 packet with TCP protocol header
in the protocol header chain. The packet may con-
tain, for example, authentication header, routing
header, or hop-by-hop option header, between IPv6
header and TCP header. The BPF code emitted by
this primitive is complex and cannot be optimized
by BPF optimizer code in tcpdump, so this can be
somewhat slow.




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User Commands TCPDUMP(1)



ip protochain protocol
Equivalent to ip6 protochain protocol, but this is
for IPv4.

ether broadcast
True if the packet is an ethernet broadcast
packet. The ether keyword is optional.

ip broadcast
True if the packet is an IPv4 broadcast packet.
It checks for both the all-zeroes and all-ones
broadcast conventions, and looks up the subnet
mask on the interface on which the capture is
being done.

If the subnet mask of the interface on which the
capture is being done is not available, either
because the interface on which capture is being
done has no netmask or because the capture is
being done on the Linux "any" interface, which can
capture on more than one interface, this check
will not work correctly.

ether multicast
True if the packet is an ethernet multicast
packet. The ether keyword is optional. This is
shorthand for `ether[0] & 1 != 0'.

ip multicast
True if the packet is an IP multicast packet.

ip6 multicast
True if the packet is an IPv6 multicast packet.

ether proto protocol
True if the packet is of ether type protocol.
Protocol can be a number or one of the names ip,
ip6, arp, rarp, atalk, aarp, decnet, sca, lat,
mopdl, moprc, iso, stp, ipx, or netbeui. Note
these identifiers are also keywords and must be
escaped via backslash (\).

[In the case of FDDI (e.g., `fddi protocol arp'),
Token Ring (e.g., `tr protocol arp'), and IEEE
802.11 wireless LANS (e.g., `wlan protocol arp'),
for most of those protocols, the protocol identif-
ication comes from the 802.2 Logical Link Control
(LLC) header, which is usually layered on top of
the FDDI, Token Ring, or 802.11 header.

When filtering for most protocol identifiers on
FDDI, Token Ring, or 802.11, tcpdump checks only



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User Commands TCPDUMP(1)



the protocol ID field of an LLC header in so-
called SNAP format with an Organizational Unit
Identifier (OUI) of 0x000000, for encapsulated
Ethernet; it doesn't check whether the packet is
in SNAP format with an OUI of 0x000000. The
exceptions are:

iso tcpdump checks the DSAP (Destination Service
Access Point) and SSAP (Source Service Access
Point) fields of the LLC header;

stp and netbeui
tcpdump checks the DSAP of the LLC header;

atalk
tcpdump checks for a SNAP-format packet with
an OUI of 0x080007 and the AppleTalk etype.

In the case of Ethernet, tcpdump checks the Ether-
net type field for most of those protocols. The
exceptions are:

iso, sap, and netbeui
tcpdump checks for an 802.3 frame and then
checks the LLC header as it does for FDDI,
Token Ring, and 802.11;

atalk
tcpdump checks both for the AppleTalk etype
in an Ethernet frame and for a SNAP-format
packet as it does for FDDI, Token Ring, and
802.11;

aarp tcpdump checks for the AppleTalk ARP etype in
either an Ethernet frame or an 802.2 SNAP
frame with an OUI of 0x000000;

ipx tcpdump checks for the IPX etype in an Ether-
net frame, the IPX DSAP in the LLC header,
the 802.3-with-no-LLC-header encapsulation of
IPX, and the IPX etype in a SNAP frame.

decnet src host
True if the DECNET source address is 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 config-
ured to run DECNET.]

decnet dst host
True if the DECNET destination address is host.




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decnet host host
True if either the DECNET source or destination
address is host.

ifname interface
True if the packet was logged as coming from the
specified interface (applies only to packets
logged by OpenBSD's pf(4)).

on interface
Synonymous with the ifname modifier.

rnr num
True if the packet was logged as matching the
specified PF rule number (applies only to packets
logged by OpenBSD's pf(4)).

rulenum num
Synonomous with the rnr modifier.

reason code
True if the packet was logged with the specified
PF reason code. The known codes are: match,
bad-offset, fragment, short, normalize, and memory
(applies only to packets logged by OpenBSD's
pf(4)).

rset name
True if the packet was logged as matching the
specified PF ruleset name of an anchored ruleset
(applies only to packets logged by pf(4)).

ruleset name
Synonomous with the rset modifier.

srnr num
True if the packet was logged as matching the
specified PF rule number of an anchored ruleset
(applies only to packets logged by pf(4)).

subrulenum num
Synonomous with the srnr modifier.

action act
True if PF took the specified action when the
packet was logged. Known actions are: pass and
block (applies only to packets logged by OpenBSD's
pf(4)).

beui
ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp, ipx, net-
Abbreviations for:



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User Commands TCPDUMP(1)



ether proto p
where p is one of the above protocols.

lat, moprc, mopdl
Abbreviations for:
ether proto p
where p is one of the above protocols. Note that
tcpdump does not currently know how to parse these
protocols.

vlan [vlan_id]
True if the packet is an IEEE 802.1Q VLAN packet.
If [vlan_id] is specified, only true is the packet
has the specified vlan_id. Note that the first
vlan keyword encountered in expression changes the
decoding offsets for the remainder of expression
on the assumption that the packet is a VLAN
packet.

tcp, udp, icmp
Abbreviations for:
ip proto p or ip6 proto p
where p is one of the above protocols.

iso proto protocol
True if the packet is an OSI packet of protocol
type protocol. Protocol can be a number or one of
the names clnp, esis, or isis.

clnp, esis, isis
Abbreviations for:
iso proto p
where p is one of the above protocols.

l1, l2, iih, lsp, snp, csnp, psnp
Abbreviations for IS-IS PDU types.

vpi n
True if the packet is an ATM packet, for SunATM on
Solaris, with a virtual path identifier of n.

vci n
True if the packet is an ATM packet, for SunATM on
Solaris, with a virtual channel identifier of n.

lane True if the packet is an ATM packet, for SunATM on
Solaris, and is an ATM LANE packet. Note that the
first lane keyword encountered in expression
changes the tests done in the remainder of expres-
sion on the assumption that the packet is either a
LANE emulated Ethernet packet or a LANE LE Control
packet. If lane isn't specified, the tests are



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User Commands TCPDUMP(1)



done under the assumption that the packet is an
LLC-encapsulated packet.

llc True if the packet is an ATM packet, for SunATM on
Solaris, and is an LLC-encapsulated packet.

oamf4s
True if the packet is an ATM packet, for SunATM on
Solaris, and is a segment OAM F4 flow cell (VPI=0
& VCI=3).

oamf4e
True if the packet is an ATM packet, for SunATM on
Solaris, and is an end-to-end OAM F4 flow cell
(VPI=0 & VCI=4).

oamf4
True if the packet is an ATM packet, for SunATM on
Solaris, and is a segment or end-to-end OAM F4
flow cell (VPI=0 & (VCI=3 | VCI=4)).

oam True if the packet is an ATM packet, for SunATM on
Solaris, and is a segment or end-to-end OAM F4
flow cell (VPI=0 & (VCI=3 | VCI=4)).

metac
True if the packet is an ATM packet, for SunATM on
Solaris, and is on a meta signaling circuit (VPI=0
& VCI=1).

bcc True if the packet is an ATM packet, for SunATM on
Solaris, and is on a broadcast signaling circuit
(VPI=0 & VCI=2).

sc True if the packet is an ATM packet, for SunATM on
Solaris, and is on a signaling circuit (VPI=0 &
VCI=5).

ilmic
True if the packet is an ATM packet, for SunATM on
Solaris, and is on an ILMI circuit (VPI=0 &
VCI=16).

connectmsg
True if the packet is an ATM packet, for SunATM on
Solaris, and is on a signaling circuit and is a
Q.2931 Setup, Call Proceeding, Connect, Connect
Ack, Release, or Release Done message.

metaconnect
True if the packet is an ATM packet, for SunATM on
Solaris, and is on a meta signaling circuit and is



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User Commands TCPDUMP(1)



a Q.2931 Setup, Call Proceeding, Connect, Release,
or Release Done message.

expr relop expr
True if the relation holds, where relop is one of
>, <, >=, <=, =, !=, and expr is an arithmetic
expression composed of integer constants
(expressed in standard C syntax), the normal
binary operators [+, -, *, /, &, |, <<, >>], a
length operator, and special packet data acces-
sors. To access data inside the packet, use the
following syntax:
proto [ expr : size ]
Proto is one of ether, fddi, tr, wlan, ppp, slip,
link, ip, arp, rarp, tcp, udp, icmp or ip6, and
indicates the protocol layer for the index opera-
tion. (ether, fddi, wlan, tr, ppp, slip and link
all refer to the link layer.) Note that tcp, udp
and other upper-layer protocol types only apply to
IPv4, not IPv6 (this will be fixed in the future).
The byte offset, relative to the indicated proto-
col layer, is given by expr. Size 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 len, gives the length of the
packet.

For example, `ether[0] & 1 != 0' catches all mul-
ticast traffic. The expression `ip[0] & 0xf != 5'
catches all IP packets with options. The expres-
sion `ip[6:2] & 0x1fff = 0' catches only unfrag-
mented datagrams and frag zero of fragmented
datagrams. This check is implicitly applied to
the tcp and udp index operations. For instance,
tcp[0] always means the first byte of the TCP
header, and never means the first byte of an
intervening fragment.

Some offsets and field values may be expressed as
names rather than as numeric values. The follow-
ing protocol header field offsets are available:
icmptype (ICMP type field), icmpcode (ICMP code
field), and tcpflags (TCP flags field).

The following ICMP type field values are avail-
able: icmp-echoreply, icmp-unreach, icmp-
sourcequench, icmp-redirect, icmp-echo, icmp-
routeradvert, icmp-routersolicit, icmp-timxceed,
icmp-paramprob, icmp-tstamp, icmp-tstampreply,
icmp-ireq, icmp-ireqreply, icmp-maskreq, icmp-
maskreply.



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User Commands TCPDUMP(1)



The following TCP flags field values are avail-
able: tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-
ack, tcp-urg.

Primitives may be combined using:

A parenthesized group of primitives and operators
(parentheses are special to the Shell and must be
escaped).

Negation (`!' or `not').

Concatenation (`&&' or `and').

Alternation (`||' or `or').

Negation has highest precedence. Alternation and con-
catenation have equal precedence and associate left to
right. Note that explicit and tokens, not juxtaposi-
tion, are now required for concatenation.

If an identifier is given without a keyword, the most
recent keyword is assumed. For example,
not host vs and ace
is short for
not host vs and host ace
which should not be confused with
not ( host vs or ace )

Expression arguments can be passed to tcpdump as either
a single argument or as multiple arguments, whichever
is more convenient. Generally, if the expression con-
tains Shell metacharacters, it is easier to pass it as
a single, quoted argument. Multiple arguments are con-
catenated with spaces before being parsed.

EXAMPLES
To print all packets arriving at or departing from sundown:
tcpdump host sundown

To print traffic between helios and either hot or ace:
tcpdump host helios and \( hot or ace \)

To print all IP packets between ace and any host except
helios:
tcpdump ip host ace and not helios

To print all traffic between local hosts and hosts at Berke-
ley:
tcpdump net ucb-ether





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User Commands TCPDUMP(1)



To print all ftp traffic through internet gateway snup:
(note that the expression is quoted to prevent the shell
from (mis-)interpreting the parentheses):
tcpdump 'gateway snup and (port ftp or ftp-data)'

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).
tcpdump ip and not net localnet

To print the start and end packets (the SYN and FIN packets)
of each TCP conversation that involves a non-local host.
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'

To print IP packets longer than 576 bytes sent through gate-
way snup:
tcpdump 'gateway snup and ip[2:2] > 576'

To print IP broadcast or multicast packets that were not
sent via ethernet broadcast or multicast:
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'

To print all ICMP packets that are not echo requests/replies
(i.e., not ping packets):
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'

OUTPUT FORMAT
The output of tcpdump is protocol dependent. The following
gives a brief description and examples of most of the for-
mats.

Link Level Headers

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.

On FDDI networks, the '-e' option causes tcpdump 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, `async4'. Such packets are assumed to contain an
802.2 Logical Link Control (LLC) packet; the LLC header is
printed if it is not an ISO datagram or a so-called SNAP
packet.

On Token Ring networks, the '-e' option causes tcpdump to
print the `access control' and `frame control' fields, the
source and destination addresses, and the packet length. As
on FDDI networks, packets are assumed to contain an LLC



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User Commands TCPDUMP(1)



packet. Regardless of whether the '-e' option is specified
or not, the source routing information is printed for
source-routed packets.

On 802.11 networks, the '-e' option causes tcpdump to print
the `frame control' fields, all of the addresses in the
802.11 header, and the packet length. As on FDDI networks,
packets are assumed to contain an LLC packet.

(N.B.: The following description assumes familiarity with
the SLIP compression algorithm described in RFC-1144.)

On SLIP links, a direction indicator (``I'' for inbound,
``O'' for outbound), packet type, and compression informa-
tion are printed out. The packet type is printed first.
The three types are ip, utcp, and ctcp. No further link
information is printed for ip 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 *S+n and *SA+n, where n
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.

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:
O ctcp * A+6 S+49 I+6 3 (6)

ARP/RARP Packets

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
rtsg to host csam:
arp who-has csam tell rtsg
arp reply csam is-at CSAM
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).

This would look less redundant if we had done tcpdump -n:
arp 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




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User Commands TCPDUMP(1)



If we had done tcpdump -e, the fact that the first packet is
broadcast and the second is point-to-point would be visible:
RTSG Broadcast 0806 64: arp who-has csam tell rtsg
CSAM RTSG 0806 64: arp reply csam is-at CSAM
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.

TCP Packets

(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.)

The general format of a tcp protocol line is:
src > dst: flags data-seqno ack window urgent options
Src and dst are the source and destination IP addresses and
ports. Flags are some combination of S (SYN), F (FIN), P
(PUSH), R (RST), W (ECN CWR) or E (ECN-Echo), or a single
`.' (no flags). Data-seqno describes the portion of
sequence space covered by the data in this packet (see exam-
ple below). Ack is sequence number of the next data
expected the other direction on this connection. Window is
the number of bytes of receive buffer space available the
other direction on this connection. Urg indicates there is
`urgent' data in the packet. Options are tcp options
enclosed in angle brackets (e.g., <mss 1024>).

Src, dst and flags are always present. The other fields
depend on the contents of the packet's tcp protocol header
and are output only if appropriate.

Here is the opening portion of an rlogin from host rtsg to
host csam.
rtsg.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
The first line says that tcp port 1023 on rtsg sent a packet
to port login on csam. The S indicates that the SYN flag
was set. The packet sequence number was 768512 and it con-
tained no data. (The notation is `first:last(nbytes)' which
means `sequence numbers first up to but not including last
which is nbytes bytes of user data'.) There was no piggy-
backed ack, the available receive window was 4096 bytes and



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User Commands TCPDUMP(1)



there was a max-segment-size option requesting an mss of
1024 bytes.

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
tcpdump 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.

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.

If the snapshot was small enough that tcpdump didn't capture
the full TCP header, it interprets as much of the header as
it can and then reports ``[|tcp]'' 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 ``[bad opt]''
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 ``[bad hdr length]''.

Capturing TCP packets with particular flag

There are 8 bits in the control bits section of the TCP
header:

CWR | ECE | URG |

Let's assume that we want to watch packets used in estab-
lishing a TCP connection. Recall that TCP uses a 3-way
handshake protocol when it initializes a new connection; the
connection sequence with regard to the TCP control bits is




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User Commands TCPDUMP(1)



1) Caller sends SYN
2) Recipient responds with SYN, ACK
3) Caller sends ACK

Now we're interested in capturing packets that have only the
SYN bit set (Step 1). Note that we don't want packets from
step 2 (SYN-ACK), just a plain initial SYN. What we need is
a correct filter expression for tcpdump.

Recall the structure of a TCP header without options:

0 15 31
-----------------------------------------------------------------
| source port | destination port |
-----------------------------------------------------------------
| sequence number |
-----------------------------------------------------------------
| acknowledgment number |
-----------------------------------------------------------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
-----------------------------------------------------------------
| TCP checksum | urgent pointer |
-----------------------------------------------------------------

A TCP header usually holds 20 octets of data, unless options
are present. The first line of the graph contains octets 0
- 3, the second line shows octets 4 - 7 etc.

Starting to count with 0, the relevant TCP control bits are
contained in octet 13:

0 7| 15| 23| 31
----------------|---------------|---------------|----------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
----------------|---------------|---------------|----------------
| | 13th octet | | |

Let's have a closer look at octet no. 13:

| |
|---------------|
|C|E|U|A|P|R|S|F|
|---------------|
|7 5 3 0|

These are the TCP control bits we are interested in. We
have numbered the bits in this octet from 0 to 7, right to
left, so the PSH bit is bit number 3, while the URG bit is
number 5.

Recall that we want to capture packets with only SYN set.
Let's see what happens to octet 13 if a TCP datagram arrives



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User Commands TCPDUMP(1)



with the SYN bit set in its header:

|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 0 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|

Looking at the control bits section we see that only bit
number 1 (SYN) is set.

Assuming that octet number 13 is an 8-bit unsigned integer
in network byte order, the binary value of this octet is

00000010

and its decimal representation is

7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2

We're almost done, because now we know that if only SYN is
set, the value of the 13th octet in the TCP header, when
interpreted as a 8-bit unsigned integer in network byte
order, must be exactly 2.

This relationship can be expressed as
tcp[13] == 2

We can use this expression as the filter for tcpdump in
order to watch packets which have only SYN set:
tcpdump -i xl0 tcp[13] == 2

The expression says "let the 13th octet of a TCP datagram
have the decimal value 2", which is exactly what we want.

Now, let's assume that we need to capture SYN packets, but
we don't care if ACK or any other TCP control bit is set at
the same time. Let's see what happens to octet 13 when a
TCP datagram with SYN-ACK set arrives:

|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 1 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|

Now bits 1 and 4 are set in the 13th octet. The binary
value of octet 13 is

00010010




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User Commands TCPDUMP(1)



which translates to decimal

7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18

Now we can't just use 'tcp[13] == 18' in the tcpdump filter
expression, because that would select only those packets
that have SYN-ACK set, but not those with only SYN set.
Remember that we don't care if ACK or any other control bit
is set as long as SYN is set.

In order to achieve our goal, we need to logically AND the
binary value of octet 13 with some other value to preserve
the SYN bit. We know that we want SYN to be set in any
case, so we'll logically AND the value in the 13th octet
with the binary value of a SYN:

00010010 SYN-ACK 00000010 SYN
AND 00000010 (we want SYN) AND 00000010 (we want SYN)
-------- --------
= 00000010 = 00000010

We see that this AND operation delivers the same result
regardless whether ACK or another TCP control bit is set.
The decimal representation of the AND value as well as the
result of this operation is 2 (binary 00000010), so we know
that for packets with SYN set the following relation must
hold true:

( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )

This points us to the tcpdump filter expression
tcpdump -i xl0 'tcp[13] & 2 == 2'

Note that you should use single quotes or a backslash in the
expression to hide the AND ('&') special character from the
shell.

UDP Packets

UDP format is illustrated by this rwho packet:
actinide.who > broadcast.who: udp 84
This says that port who on host actinide sent a udp datagram
to port who on host broadcast, the Internet broadcast
address. The packet contained 84 bytes of user data.

Some UDP services are recognized (from the source or desti-
nation port number) and the higher level protocol informa-
tion printed. In particular, Domain Name service requests
(RFC-1034/1035) and Sun RPC calls (RFC-1050) to NFS.

UDP Name Server Requests



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User Commands TCPDUMP(1)



(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.)

Name server requests are formatted as
src > dst: id op? flags qtype qclass name (len)
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
Host h2opolo asked the domain server on helios for an
address record (qtype=A) associated with the name
ucbvax.berkeley.edu. The query id was `3'. The `+' indi-
cates the recursion desired flag was set. The query length
was 37 bytes, not including the UDP and IP protocol headers.
The query operation was the normal one, Query, so the op
field was omitted. If the op had been anything else, it
would have been printed between the `3' and the `+'. Simi-
larly, the qclass was the normal one, C_IN, and omitted.
Any other qclass would have been printed immediately after
the `A'.

A few anomalies are checked and may result in extra fields
enclosed in square brackets: If a query contains an answer,
authority records or additional records section, ancount,
nscount, or arcount are printed as `[na]', `[nn]' or
`[nau]' where n 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=x]' is
printed, where x is the hex value of header bytes two and
three.

UDP Name Server Responses

Name server responses are formatted as
src > dst: id op rcode flags a/n/au type class data (len)
helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query id 3 from
h2opolo with 3 answer records, 3 name server records and 7
additional 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.

In the second example, helios 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 authoritative answer bit was set. Since
there were no answers, no type, class or data were printed.

Other flag characters that might appear are `-' (recursion
available, RA, not set) and `|' (truncated message, TC,



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User Commands TCPDUMP(1)



set). If the `question' section doesn't contain exactly one
entry, `[nq]' is printed.

Note that name server requests and responses tend to be
large and the default snaplen of 68 bytes may not capture
enough of the packet to print. Use the -s flag to increase
the snaplen if you need to seriously investigate name server
traffic. `-s 128' has worked well for me.


SMB/CIFS decoding

tcpdump now includes fairly extensive SMB/CIFS/NBT decoding
for data on UDP/137, UDP/138 and TCP/139. Some primitive
decoding of IPX and NetBEUI SMB data is also done.

By default a fairly minimal decode is done, with a much more
detailed decode done if -v is used. Be warned that with -v
a single SMB packet may take up a page or more, so only use
-v if you really want all the gory details.

If you are decoding SMB sessions containing unicode strings
then you may wish to set the environment variable
USE_UNICODE to 1. A patch to auto-detect unicode strings
would be welcome.

For information on SMB packet formats and what all te fields
mean see www.cifs.org or the pub/samba/specs/ directory on
your favorite samba.org mirror site. The SMB patches were
written by Andrew Tridgell (tridge@samba.org).


NFS Requests and Replies

Sun NFS (Network File System) requests and replies are
printed as:
src.xid > dst.nfs: len op args
src.nfs > dst.xid: reply stat len op results

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

In the first line, host sushi sends a transaction with id
6709 to wrl (note that the number following the src host is
a transaction id, not the source port). The request was 112
bytes, excluding the UDP and IP headers. The operation was
a readlink (read symbolic link) on file handle (fh)
21,24/10.731657119. (If one is lucky, as in this case, the



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User Commands TCPDUMP(1)



file handle can be interpreted as a major,minor device
number pair, followed by the inode number and generation
number.) Wrl replies `ok' with the contents of the link.

In the third line, sushi asks wrl to lookup the name
`xcolors' 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.

If the -v (verbose) flag is given, additional information is
printed. For example:

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

(-v also prints the IP header TTL, ID, length, and fragmen-
tation fields, which have been omitted from this example.)
In the first line, sushi asks wrl to read 8192 bytes from
file 21,11/12.195, at byte offset 24576. Wrl 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 frag-
ments 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.

If the -v flag is given more than once, even more details
are printed.

Note that NFS requests are very large and much of the detail
won't be printed unless snaplen is increased. Try using `-s
192' to watch NFS traffic.

NFS reply packets do not explicitly identify the RPC opera-
tion. Instead, tcpdump 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.

AFS Requests and Replies

Transarc AFS (Andrew File System) requests and replies are
printed as:

src.sport > dst.dport: rx packet-type
src.sport > dst.dport: rx packet-type service call call-name args



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src.sport > dst.dport: rx packet-type service reply call-name args

elvis.7001 > pike.afsfs:
rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
new fid 536876964/1/1 ".newsrc"
pike.afsfs > elvis.7001: rx data fs reply rename

In the first line, host elvis sends a RX packet to pike.
This was a RX data packet to the fs (fileserver) service,
and is the start of an RPC call. The RPC call was a rename,
with the old directory file id of 536876964/1/1 and an old
filename of `.newsrc.new', and a new directory file id of
536876964/1/1 and a new filename of `.newsrc'. The host
pike responds with a RPC reply to the rename call (which was
successful, because it was a data packet and not an abort
packet).

In general, all AFS RPCs are decoded at least by RPC call
name. Most AFS RPCs have at least some of the arguments
decoded (generally only the `interesting' arguments, for
some definition of interesting).

The format is intended to be self-describing, but it will
probably not be useful to people who are not familiar with
the workings of AFS and RX.

If the -v (verbose) flag is given twice, acknowledgement
packets and additional header information is printed, such
as the the RX call ID, call number, sequence number, serial
number, and the RX packet flags.

If the -v flag is given twice, additional information is
printed, such as the the RX call ID, serial number, and the
RX packet flags. The MTU negotiation information is also
printed from RX ack packets.

If the -v flag is given three times, the security index and
service id are printed.

Error codes are printed for abort packets, with the excep-
tion of Ubik beacon packets (because abort packets are used
to signify a yes vote for the Ubik protocol).

Note that AFS requests are very large and many of the argu-
ments won't be printed unless snaplen is increased. Try
using `-s 256' to watch AFS traffic.

AFS reply packets do not explicitly identify the RPC opera-
tion. Instead, tcpdump keeps track of ``recent'' requests,
and matches them to the replies using the call number and
service ID. If a reply does not closely follow the
corresponding request, it might not be parsable.



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KIP AppleTalk (DDP in UDP)

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 /etc/atalk.names
is used to translate AppleTalk net and node numbers to
names. Lines in this file have the form
number name

1.254 ether
16.1 icsd-net
1.254.110 ace
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 must have two octets and a host number must
have three octets.) The number and name should be separated
by whitespace (blanks or tabs). The /etc/atalk.names file
may contain blank lines or comment lines (lines starting
with a `#').

AppleTalk addresses are printed in the form
net.host.port

144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2
(If the /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).

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.

NBP packets are formatted like the following examples:
icsd-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
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



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User Commands TCPDUMP(1)



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 laser-
writer "techpit" registered on port 186.

ATP packet formatting is demonstrated by the following exam-
ple:
jssmag.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
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.

Helios responds with 8 512-byte packets. The `:digit' fol-
lowing 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.

Jssmag.209 then requests that packets 3 & 5 be retransmit-
ted. Helios resends them then jssmag.209 releases the tran-
saction. Finally, jssmag.209 initiates the next request.
The `*' on the request indicates that XO (`exactly once')
was not set.


IP Fragmentation

Fragmented Internet datagrams are printed as
(frag id:size@offset+)
(frag id:size@offset)
(The first form indicates there are more fragments. The
second indicates this is the last fragment.)

Id is the fragment id. Size is the fragment size (in bytes)
excluding the IP header. Offset is this fragment's offset
(in bytes) in the original datagram.





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User Commands TCPDUMP(1)



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:
arizona.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
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.

A packet with the IP don't fragment flag is marked with a
trailing (DF).

Timestamps

By default, all output lines are preceded by a timestamp.
The timestamp is the current clock time in the form
hh:mm:ss.frac
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.

SEE ALSO
stty(1), pcap(3), bpf(4), nit(4P), pfconfig(8)

AUTHORS
The original authors are:

Van Jacobson, Craig Leres and Steven McCanne, all of the
Lawrence Berkeley National Laboratory, University of Cali-
fornia, Berkeley, CA.

It is currently being maintained by tcpdump.org.

The current version is available via http:

http://www.tcpdump.org/




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User Commands TCPDUMP(1)



The original distribution is available via anonymous ftp:

ftp://ftp.ee.lbl.gov/tcpdump.tar.Z

IPv6/IPsec support is added by WIDE/KAME project. This pro-
gram uses Eric Young's SSLeay library, under specific confi-
guration.

BUGS
Please send problems, bugs, questions, desirable enhance-
ments, etc. to:

tcpdump-workers@tcpdump.org

Please send source code contributions, etc. to:

patches@tcpdump.org

NIT doesn't let you watch your own outbound traffic, BPF
will. We recommend that you use the latter.

On Linux systems with 2.0[.x] kernels:

packets on the loopback device will be seen twice;

packet filtering cannot be done in the kernel, so that
all packets must be copied from the kernel in order to
be filtered in user mode;

all of a packet, not just the part that's within the
snapshot length, will be copied from the kernel (the
2.0[.x] packet capture mechanism, if asked to copy only
part of a packet to userland, will not report the true
length of the packet; this would cause most IP packets
to get an error from tcpdump);

capturing on some PPP devices won't work correctly.

We recommend that you upgrade to a 2.2 or later kernel.

Some attempt should be made to reassemble IP fragments or,
at least to compute the right length for the higher level
protocol.

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 generat-
ing them rather than tcpdump.

A packet trace that crosses a daylight savings time change
will give skewed time stamps (the time change is ignored).



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User Commands TCPDUMP(1)



Filter expressions on fields other than those in Token Ring
headers will not correctly handle source-routed Token Ring
packets.

Filter expressions on fields other than those in 802.11
headers will not correctly handle 802.11 data packets with
both To DS and From DS set.

ip6 proto should chase header chain, but at this moment it
does not. ip6 protochain is supplied for this behavior.

Arithmetic expression against transport layer headers, like
tcp[0], does not work against IPv6 packets. It only looks
at IPv4 packets.