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1. Introduction to TCPDUMP2. Platforms supported3. Where to download it4. Screenshots5. Some simple scripts
6. The manual page7. Support and Mailing Lists8. Add-on packages/tools1. Introduction to TCPDUMPAn advantage of TCPDUMP is that it can be run remotely through a SSH or TELNET session, and the machine running TCPDUMP does not have to be running x-windows. The application uses very little overhead, since it's a non-graphical interface.  got feedback?by state got feedback?by state got feedback?by state 3. Where to download it got feedback?by state RPM's: You can check the RPM database for your platform here: http://rpmfind.net/linux/rpm2html/search.php?query=tcpdump&submit=Search+... You may also download the source and compile it yourself from one of these locations...(I recommend a mirror, though SourceForge is extremely reliable): http://sourceforge.net/projects/tcpdump/ http://www.tcpdump.org/mirrors.html http://www.tcpdump.org/binaries.html
got feedback?by state 4. Screenshots got feedback?by state  got feedback?by state 5. Some simple scripts
1. I am not using the full path to TCPDUMP, which is usually located in `/usr/sbin/tcpdump'. 2. TCPDUMP required ROOT ACCESS or the program must have suid of root. 3. My public interface is `eth0', which is the interface that my examples use. If you wish to listen somewhere else, just replace it. To determine which interface you wish to use, first figure out what you want to see, then run `/sbin/ifconfig' and see what IP is assigned to what interface. 4. Running TCPDUMP in a work environment may not be acceptable. Check with the networking folks before you fire it off, and start reading other users' data.
got feedback?by state
Will show ALL traffic on interface eth0. got feedback?by state tcpdump -c 20 -i eth0 got feedback?by state
This filter will display the MAC address as well as the basic information. got feedback?by state
Sure can. Try this script. Keep in mind that your sniffer will need to be located where it can see all traffic on your network for this to be useful. got feedback?by state tcpdump -tnn -c 20000 -i eth0 | awk -F "." '{print $1"."$2"."$3"."$4}' | sort | uniq -c | sort -nr | awk ' $1 > 100 '
Depending on how busy your network is, you might want to lower the `-c 20000' (packet count) to fit your needs. This script will capture 20,000 packets and sort by top talkers. got feedback?by state
TCPDUMP(8) TCPDUMP(8)
NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [ -adeflnNOpqStvx ] [ -c count ] [ -F file ]
[ -i interface ] [ -r file ] [ -s snaplen ]
[ -T type ] [ -w file ] [ expression ]
DESCRIPTION
Tcpdump prints out the headers of packets on a network
interface that match the boolean expression.
Under SunOS with nit or bpf: To run tcpdump you must have
read access to /dev/nit or /dev/bpf*. Under Solaris with
dlpi: You must have read access to the network pseudo
device, e.g. /dev/le. Under HP-UX with dlpi: You must be
root or it must be installed setuid to root. Under IRIX
with snoop: You must be root or it must be installed
setuid to root. Under Linux: You must be root or it 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 run tcpdump.
Under BSD: You must have read access to /dev/bpf*.
OPTIONS
-a Attempt to convert network and broadcast addresses
to names.
-c Exit after receiving count packets.
-d Dump the compiled packet-matching code in a human
readable form to standard output and stop.
-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.
-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.
-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''.
-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 infor-
mation so output lines are shorter.
-r Read packets from file (which was created with the
-w option). Standard input is used if file is
``-''.
-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.
-T Force packets selected by "expression" to be inter-
preted the specified type. Currently known types
are rpc (Remote Procedure Call), rtp (Real-Time
Applications protocol), rtcp (Real-Time Applica-
tions control protocol), vat (Visual Audio Tool),
and wb (distributed White Board).
-S Print absolute, rather than relative, TCP sequence
numbers.
-t Don't print a timestamp on each dump line.
-tt Print an unformatted timestamp on each dump line.
-v (Slightly more) verbose output. For example, the
time to live and type of service information in an
IP packet is printed.
-vv Even more verbose output. For example, additional
fields are printed from NFS reply packets.
-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.
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.
The expression consists of one or more primitives.
Primitives usually consist of an id (name or num-
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, ip, arp, rarp, decnet, lat, sca,
moprc, mopdl, 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 syn-
tax), `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.]
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 primi-
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 IP destination field of the
packet is host, which may be either an
address or a name.
src host host
True if the IP source field of the packet is
host.
host host
True if either the IP source or destination
of the packet is host. Any of the above
host expressions can be prepended with the
keywords, ip, arp, or rarp 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 in both
/etc/hosts and /etc/ethers. (An equivalent
expression is
ether host ehost and not host host
which can be used with either names or num-
bers for host / ehost.)
dst net net
True if the IP 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 IP source address of the packet
has a network number of net.
net net
True if either the IP source or destination
address of the packet has a network number
of net.
net net mask mask
True if the IP address matches net with the
specific netmask. May be qualified with src
or dst.
net net/len
True if the IP address matches net a netmask
len bits wide. May be qualified with src or
dst.
dst port port
True if the packet is ip/tcp or ip/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).
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 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,
igrp, udp, nd, 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.
ether broadcast
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.
ether proto protocol
True if the packet is of ether type proto-
col. Protocol can be a number or a name
like ip, arp, or rarp. Note these identi-
fiers are also keywords and must be escaped
via backslash (\). [In the case of FDDI
(e.g., `fddi protocol arp'), the protocol
identification comes from the 802.2 Logical
Link Control (LLC) header, which is usually
layered on top of the FDDI header. Tcpdump
assumes, when filtering on the protocol
identifier, that all FDDI packets include an
LLC header, and that the LLC header is in
so-called SNAP format.]
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, arp, rarp, decnet
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.
tcp, udp, icmp
Abbreviations for:
ip proto p
where p is one of the above 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, ip, arp, rarp,
tcp, udp, or icmp, and indicates the proto-
col layer for the index operation. The byte
offset, relative to the indicated protocol
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
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.
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
easier to pass it as a single, quoted argument.
Multiple arguments are concatenated with spaces
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[13] & 3 != 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[0] != 8 and icmp[0] != 0"
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
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 Logical Link Control (LLC) packet; the
LLC header is printed if it is not an ISO datagram or a
so-called SNAP 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 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.
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, 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.,
got feedback?by state 7. Support and Mailing Listsor subscribe here: here got feedback?by state or subscribe here: here got feedback?by state Makes output from the tcpdump program easier to read and parse. Features: * Coloured console output * Directly accepts tcpdump options (including parsing from packet files) * Fully customisable output Requirements: * Linux operating system * Perl installed * tcpdump program installed * Privileges in order to run tcpdump Available here got feedback?by state | ||||||||||||
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