tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [ -adeflnNOpqRStuvxX ] [ -c count ]
[ -C file_size ] [ -F file ]
[ -i interface ] [ -m module ] [ -r file ]
[ -s snaplen ] [ -T type ] [ -w file ]
[ -E algo:secret ] [ 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
-b 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 pro
cessed by tcpdump.
Tcpdump will, if not run with the -c flag, continue cap
turing packets until it is interrupted by a SIGINT signal
(generated, for example, by typing your interrupt charac
ter, typically control-C) or a SIGTERM signal (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 pack
ets have been processed.
When tcpdump finishes capturing packets, it will report
counts of:
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 on other
OSes it counts only packets that were matched by
the filter expression and were processed by tcp
dump);
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, it will report those counts when it receives a SIG
INFO signal (generated, for example, by typing your ``sta
tus'' character, typically control-T) and will continue
you have special privileges:
Under SunOS 3.x or 4.x with NIT or BPF:
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 suffi
cient to allow tcpdump to capture in promiscuous
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.
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.
Under Ultrix and Digital UNIX:
Once the super-user has enabled promiscuous-mode
operation using pfconfig(8), any user may capture
network traffic with tcpdump.
Under BSD:
You must have read access to /dev/bpf*.
Reading a saved packet file doesn't require special privi
leges.
OPTIONS
-a Attempt to convert network and broadcast addresses
to names.
-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).
-dd Dump packet-matching code as a C program fragment.
-ddd Dump packet-matching code as decimal numbers (pre
ceded with a count).
-e Print the link-level header on each dump line.
-E Use algo:secret for decrypting IPsec ESP packets.
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 pre
sent if tcpdump was compiled with cryptography
enabled. secret the ascii text for ESP secret key.
We cannot take arbitrary binary value at this
moment. The option assumes RFC2406 ESP, not
RFC1827 ESP. The option is only for debugging pur
poses, and the use of this option with truly
`secret' key is discouraged. By presenting IPsec
secret key onto command line you make it visible to
others, via ps(1) and other occasions.
-f Print `foreign' internet addresses numerically
rather than symbolically (this option is intended
to get around serious brain damage in Sun's yp
server -- usually it hangs forever translating non-
local internet numbers).
-F Use file as input for the filter expression. An
additional expression given on the command line is
ignored.
-i Listen on interface. If unspecified, tcpdump
searches the system interface list for the lowest
numbered, configured up interface (excluding loop
back). Ties are broken by choosing the earliest
match.
On Linux systems with 2.2 or later kernels, an
interface argument of ``any'' can be used to cap
ture packets from all interfaces. Note that cap
tures on the ``any'' device will not be done in
promiscuous mode.
-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''.
-m Load SMI MIB module definitions from file module.
This option can be used several times to load sev
eral MIB modules into tcpdump.
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 infor
mation so output lines are shorter.
-R Assume ESP/AH packets to be based on old specifica
tion (RFC1825 to RFC1829). If specified, tcpdump
will not print replay prevention field. Since
there is no protocol version field in ESP/AH speci
fication, 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 mini
mum is actually 96). 68 bytes is adequate for IP,
ICMP, TCP and UDP but may truncate protocol infor
mation from name server and NFS packets (see
below). Packets truncated because of a limited
snapshot are indicated in the output with
``[|proto]'', where proto is the name of the proto
col level at which the truncation has occurred.
Note that taking larger snapshots both increases
the amount of time it takes to process packets and,
effectively, decreases the amount of packet buffer
ing. This may cause packets to be lost. You
should limit snaplen to the smallest number that
will capture the protocol information you're inter
ested in. Setting snaplen to 0 means use the
required length to catch whole packets.
-T Force packets selected by "expression" to be inter
cedure Call), rtp (Real-Time Applications proto
col), rtcp (Real-Time Applications control proto
col), snmp (Simple Network Management 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.
-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 verify
ing 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.
-X When printing hex, print ascii too. Thus if -x is
also set, the packet is printed in hex/ascii. This
is very handy for analysing new protocols. Even if
-x is not also set, some parts of some packets may
be printed in hex/ascii.
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 expres
sion is `true' will be dumped.
ber) 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
`null' link layers (i.e. point to point pro
tocols such as slip) the inbound and out
bound qualifiers can be used to specify a
desired direction.
proto qualifiers restrict the match to a particu
lar protocol. Possible protos are: ether,
fddi, tr, 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 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'.
[`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 Ether
net fields. FDDI headers also contain other
fields, but you cannot name them explicitly in a
filter expression.
Similarly, `tr' is an alias for `ether'; the previ
ous paragraph's statements about FDDI headers also
apply to Token Ring headers.]
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.
tives. E.g., `host foo and not port ftp and not
port ftp-data'. To save typing, identical quali
fier 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 desti
nation 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 desti
nation 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 neither the IP source
nor the IP destination 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.)
etc.). (An equivalent expression is
ether host ehost and not host host
which can be used with either names or num
bers 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 desti
nation 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 num
ber 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 traf
fic and udp/who traffic, 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.
True if either the source or destination
port of the packet is port. Any of the
above port expressions 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 identifiers 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 pro
tocol type protocol. Note that this primi
tive does not chase the protocol header
chain.
ip6 protochain protocol
True if the packet is IPv6 packet, and con
tains 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 example, authentica
tion header, routing header, or hop-by-hop
option header, between IPv6 header and TCP
header. The BPF code emitted by this primi
tive 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
True if the packet is an ethernet broadcast
packet. The ether keyword is optional.
ip broadcast
True if the packet is an IP broadcast
packet. It checks for both the all-zeroes
and all-ones broadcast conventions, and
looks up the local subnet mask.
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 proto
col. Protocol can be a number or one of the
names ip, ip6, arp, rarp, atalk, aarp, dec
net, 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') and Token Ring (e.g., `tr protocol
arp'), for most of those protocols, the pro
tocol identification comes from the 802.2
Logical Link Control (LLC) header, which is
usually layered on top of the FDDI or Token
Ring header.
When filtering for most protocol identifiers
on FDDI or Token Ring, 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, for which it checks
the DSAP (Destination Service Access Point)
and SSAP (Source Service Access Point)
fields of the LLC header, stp and netbeui,
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 proto
cols; the exceptions are iso, sap, and net
beui, for which it checks for an 802.3 frame
and then checks the LLC header as it does
for FDDI and Token Ring, atalk, where it
checks both for the Appletalk etype in an
Ethernet frame and for a SNAP-format packet
as it does for FDDI and Token Ring, aarp,
where it checks for the Appletalk ARP etype
in either an Ethernet frame or an 802.2 SNAP
frame with an OUI of 0x000000, and ipx,
where it checks for the IPX etype in an Eth
ernet 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
True if the DECNET destination address is
host.
decnet host host
True if either the DECNET source or destina
tion address is host.
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
encountered in expression changes the decod
ing 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 pro
tocol type protocol. Protocol can be a num
ber 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. Note
that tcpdump does an incomplete job of pars
ing these protocols.
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 accessors. To access data inside the
packet, use the following syntax:
proto [ expr : size ]
Proto is one of ether, fddi, tr, ip, arp,
rarp, tcp, udp, icmp or ip6, and indicates
the protocol layer for the index operation.
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 pro
tocol 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
multicast traffic. The expression `ip[0] &
0xf != 5' catches all IP packets with
options. The expression `ip[6:2] & 0x1fff =
0' catches only unfragmented datagrams and
frag zero of fragmented datagrams. This
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 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-redirect, icmp-echo,
icmp-routeradvert, icmp-routersolicit, icmp-
timxceed, icmp-paramprob, icmp-tstamp, icmp-
tstampreply, 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-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
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. Generally, if the
expression contains Shell metacharacters, it is
before being parsed.
EXAMPLES
To print all packets arriving at or departing from sun
down:
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
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 pack
ets) 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
gateway 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 follow
ing gives a brief description and examples of most of the
formats.
Link Level Headers
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 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
packet. Regardless of whether the '-e' option is speci
fied or not, the source routing information is printed for
source-routed packets.
(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 spe
cial 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 com
pressed TCP packet, with an implicit connection identi
fier; 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 argu
ments. The format is intended to be self explanatory.
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, ether
net 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 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) or R (RST) or a single `.' (no flags).
Data-seqno describes the portion of sequence space covered
by the data in this packet (see example below). Ack is
sequence number of the next data expected the other direc
tion 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 brack
ets (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 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 con
tained 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 pack
ets of the conversation, the difference between the cur
rent 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 fea
ture, 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 includ
ing 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 cap
ture 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 pre
sent but the IP datagram length is not long enough for the
options to actually be there, tcpdump reports it as ``[bad
hdr length]''.
There are 8 bits in the control bits section of the TCP
header:
&nbs