SYNOPSIS
tcpdump [ -AdDeflLnNOpqRStuUvxX ] [ -c count ]
[ -C file_size ] [ -F file ]
[ -i interface ] [ -m module ] [ -M secret ]
[ -r file ] [ -s snaplen ] [ -T type ] [ -w file ]
[ -W filecount ]
[ -E spi@ipaddr algo:secret,... ]
[ -y datalinktype ] [ -Z user ]
[ 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 capturing packets
until it is interrupted by a SIGINT signal (generated, for example, by
typing your interrupt character, typically control-C) or a SIGTERM sig-
nal (typically generated 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 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 information 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 (generated, for example, by
typing your ``status'' character, typically control-T, although on some
ous mode; on those versions of Solaris, you must be root, or
tcpdump must be installed setuid to root, in order to capture in
promiscuous mode. Note that, on many (perhaps all) interfaces,
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 supports 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).
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 interface using
pfconfig, so useful packet capture on an interface probably
requires that either promiscuous-mode or copy-all-mode opera-
tion, or both modes of operation, be enabled on that interface.
Under BSD (this includes Mac OS X):
You must have read access to /dev/bpf*. On BSDs with a devfs
(this includes Mac OS X), this might involve more than just hav-
ing somebody with super-user access setting 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 sup-
port that, you might have to find some other way to make that
happen 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.
-ddd Dump packet-matching code as decimal numbers (preceded with a
count).
-D Print the list of the network interfaces available on 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 Windows 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() func-
tion.
-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 preceeded 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 occasions.
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 per-
missions that tcpdump may have been given should already have
been given up.
-f Print `foreign' IPv4 addresses numerically rather than symboli-
cally (this option is intended to get around serious brain dam-
age in Sun's NIS server -- usually it hangs forever translating
tem interface list for the lowest numbered, configured up inter-
face (excluding loopback). Ties are broken by choosing the ear-
liest match.
On Linux systems with 2.2 or later kernels, an interface argu-
ment of ``any'' can be used to capture packets from all inter-
faces. 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 argument.
-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 tcp-
dump.
-M Use secret as a shared secret for validating the digests found
in TCP segments with the TCP-MD5 option (RFC 2385), if present.
-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 broadcast'.
-q Quick (quiet?) output. Print less protocol information so out-
put lines are shorter.
-R Assume ESP/AH packets to be based on old specification (RFC1825
to RFC1829). If specified, tcpdump will not print replay pre-
vention field. Since there is no protocol 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 ``-''.
ture the protocol information you're interested in. Setting
snaplen to 0 means use the required length to catch whole pack-
ets.
-T Force packets selected by "expression" to be interpreted 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 proto-
col), rtcp (Real-Time Applications control protocol), snmp (Sim-
ple Network Management Protocol), tftp (Trivial File Transfer
Protocol), 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 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() func-
tion.
-v When parsing and printing, produce (slightly more) verbose out-
put. 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.
When writing to a file with the -w option, report, every 10 sec-
onds, the number of packets captured.
-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. Stan-
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 protocols.
-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.
-Z Drops privileges (if root) and changes user ID to user and the
group ID to the primary group of user.
This behavior can also be enabled by default at compile time.
expression
selects which packets will be dumped. If no expression is
given, all packets on the net will be dumped. Otherwise, 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 , port and por-
trange. E.g., `host foo', `net 128.3', `port 20', `por-
trange 6000-6008'. 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 quali-
fier, 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 protocol.
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', `udp portrange 7000-7009'. If
there is no proto qualifier, all protocols 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'.
the SA field; the BSSID, RA, and TA fields aren't tested.]
In addition to the above, there are some special `primitive'
keywords that don't follow the pattern: gateway, broadcast,
less, greater and arithmetic expressions. 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.
gateway host
True if the packet used host as a gateway. I.e., the
Ethernet source or destination address was host but nei-
ther the IP source nor the IP destination was host. Host
must be a name and must be found both by the machine's
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 IPv4 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 traf-
fic, and port domain will print both tcp/domain and
udp/domain traffic).
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.
dst portrange port1-port2
True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp
and has a destination port value between port1 and port2.
port1 and port2 are interpreted in the same fashion as
the port parameter for port.
src portrange port1-port2
True if the packet has a source port value between port1
and port2.
portrange port1-port2
True if either the source or destination port of the
packet is between port1 and port2.
Any of the above port or port range expressions can be
prepended with the keywords, tcp or udp, as in:
True if the packet is an IPv4 packet (see ip(4P)) of pro-
tocol 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 identifiers tcp, udp, and
icmp are also keywords and must be escaped via backslash
(\), which is \\ in the C-shell. Note that this primi-
tive 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 contain, for exam-
ple, 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.
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 inter-
face 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 inter-
face 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'.
Ring (e.g., `tr protocol arp'), and IEEE 802.11 wireless
LANS (e.g., `wlan protocol arp'), for most of those pro-
tocols, the protocol identification comes from the 802.2
Logical Link Control (LLC) header, which is usually lay-
ered 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 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 Ethernet type
field for most of those protocols. The exceptions are:
iso, stp, 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 Ethernet
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 configured to run DECNET.]
decnet dst host
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 rea-
son code. The known codes are: match, bad-offset, frag-
ment, short, normalize, and memory (applies only to pack-
ets 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)).
ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp, ipx, netbeui
Abbreviations for:
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 if 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
specified, only true is the packet has the specified
label_num. Note that the first mpls keyword encountered
in expression changes the decoding offsets for the
remainder of expression on the assumption that the packet
is a MPLS-encapsulated IP packet. The mpls [label_num]
expression may be used more than once, to filter on MPLS
hierarchies. Each use of that expression increments the
filter offsets by 4.
For example:
mpls 100000 && mpls 1024
filters packets with an outer label of 100000 and an
inner label of 1024, and
mpls && mpls 1024 && host 192.9.200.1
filters packets to or from 192.9.200.1 with an inner
label of 1024 and any outer label.
pppoed True if the packet is a PPP-over-Ethernet Discovery
packet (Ethernet type 0x8863).
pppoes True if the packet is a PPP-over-Ethernet Session packet
(Ethernet type 0x8864). Note that the first pppoes key-
word encountered in expression changes the decoding off-
sets for the remainder of expression on the assumption
that the packet is a PPPoE session packet.
For example:
pppoes && ip
filters IPv4 protocols encapsulated in PPPoE.
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 pro-
tocol. 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
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 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 com-
posed of integer constants (expressed in standard C syn-
tax), the normal binary operators [+, -, *, /, &, |, <<,
>>], a length operator, and special packet data acces-
sors. Note that all comparisons are unsigned, so that,
for example, 0x80000000 and 0xffffffff are > 0. To
access data inside the packet, use the following syntax:
proto [ expr : size ]
For example, `ether[0] & 1 != 0' catches all multicast
traffic. The expression `ip[0] & 0xf != 5' catches all
IPv4 packets with options. The expression `ip[6:2] &
0x1fff = 0' catches only unfragmented IPv4 datagrams and
frag zero of fragmented IPv4 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 inter-
vening fragment.
Some offsets and field values may be expressed as names
rather than as numeric values. The following 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 available: icmp-
echoreply, icmp-unreach, icmp-sourcequench, icmp-redi-
rect, icmp-echo, icmp-routeradvert, icmp-routersolicit,
icmp-timxceed, icmp-paramprob, icmp-tstamp, icmp-tstam-
preply, icmp-ireq, icmp-ireqreply, icmp-maskreq, icmp-
maskreply.
The following TCP flags field values are available: tcp-
fin, tcp-syn, tcp-rst, tcp-push, tcp-ack, tcp-urg.
Primitives may be combined using:
A parenthesized group of primitives and operators (paren-
theses are special to the Shell and must be escaped).
Negation (`!' or `not').
Concatenation (`&&' or `and').
Alternation (`||' or `or').
Negation has highest precedence. Alternation and concatenation
have equal precedence and associate left to right. Note that
explicit and tokens, not juxtaposition, 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.
To print all traffic between local hosts and hosts at Berkeley:
tcpdump net ucb-ether
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 all IPv4 HTTP packets to and from port 80, i.e. print only
packets that contain data, not, for example, SYN and FIN packets and
ACK-only packets. (IPv6 is left as an exercise for the reader.)
tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
To print IP packets longer than 576 bytes sent through gateway snup:
tcpdump 'gateway snup and ip[2:2] > 576'
To print IP broadcast or multicast packets that were not sent via Eth-
ernet 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 formats.
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 data-
grams) are `async' packets, with a priority value between 0 and 7; for
example, `async4'. Such packets are assumed to contain an 802.2 Logi-
cal Link Control (LLC) packet; the LLC header is printed if it is not
an ISO datagram or a so-called SNAP packet.
On SLIP links, a direction indicator (``I'' for inbound, ``O'' for out-
bound), packet type, and compression information 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 pack-
ets, 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 num-
ber), 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 for-
mat 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 Ether-
net 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
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 proto-
col 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:
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 contained 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 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 ini-
tial 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 sit-
ting 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]''.
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 with the SYN bit set
in its header:
|C|E|U|A|P|R|S|F|
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 dec-
imal 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
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)
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 con-
tained 84 bytes of user data.
Some UDP services are recognized (from the source or destination port
number) and the higher level protocol information printed. In particu-
lar, Domain Name service requests (RFC-1034/1035) and Sun RPC calls
(RFC-1050) to NFS.
UDP Name Server Requests
(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 `+' indicates the recursion desired flag was set. The
query length was 37 bytes, not including the UDP and IP protocol head-
ers. 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 `+'. Similarly, 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
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 seri-
ously 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 Net-
BEUI 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.
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
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:
(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 operation.
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
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 success-
ful, 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 exception of Ubik
AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
and dumped as DDP packets (i.e., all the UDP header information is dis-
carded). 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 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.
jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
Jssmag.209 initiates transaction id 12266 with host helios by request-
ing 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' 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.
Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
resends them then jssmag.209 releases the transaction. Finally, jss-
mag.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 indi-
cates 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.
The fragment information is output for each fragment. The first frag-
ment 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. Sec-
ond, 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
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence
Berkeley National Laboratory, University of California, Berkeley, CA.
It is currently being maintained by tcpdump.org.
The current version is available via http:
http://www.tcpdump.org/
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 program uses
Eric Young's SSLeay library, under specific configuration.
BUGS
Please send problems, bugs, questions, desirable enhancements, 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 rec-
ommend 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 pack-
ets 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 cap-
ture mechanism, if asked to copy only part of a packet to user-
land, 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) ques-
tion section is printed rather than real query in the answer section.
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.
18 April 2005 TCPDUMP(1)
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