#top tcpdump¶

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

Exit after receiving count packets.

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 1 and continuing upward. The units of file_size are millions of bytes (1,000,000 bytes, not 1,048,576 bytes).

Note that when used with -Z option (enabled by default), privileges are dropped before opening first savefile.

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

Dump packet-matching code as a C program fragment.

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

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() function.

Print the link-level header on each dump line.

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 permissions that tcpdump may have been given should already have been given up.

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

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

Use file as input for the filter expression. An additional expression given on the command line is ignored.

Listen on interface. If unspecified, tcpdump searches the system interface list for the lowest numbered, configured up interface (excluding loopback). 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 capture packets from all interfaces. Note that captures on the ''any'' device will not be done in promiscuous mode.

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

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''.

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

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

Use secret as a shared secret for validating the digests found in TCP segments with the TCP-MD5 option (RFC 2385), if present.

Don't convert host addresses to names. This can be used to avoid DNS lookups.

Don't convert protocol and port numbers etc. to names either.

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''.

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

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'.

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

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

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

Print absolute, rather than relative, TCP sequence numbers.

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

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

Don't print a timestamp on each dump line.

Print an unformatted timestamp on each dump line.

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

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

Print undecoded NFS handles.

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

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

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

When writing to a file with the -w option, report, every 10 seconds, the number of packets captured.

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

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

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 ''-''.

Used in conjunction with the -C option, this will limit the number of files created to the specified number, and begin overwriting files from the beginning, thus creating a 'rotating' buffer. In addition, it will name the files with enough leading 0s to support the maximum number of files, allowing them to sort correctly.

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

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

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

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

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

Drops privileges (if root) and changes user ID to user and the group ID to the primary group of user.

This behavior is enabled by default (-Z pcap), and can be disabled by -Z root.

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

dir

proto

['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 Ethernetlike source and destination addresses, and often contain Ethernet-like packet types, so you can filter on these FDDI fields just as with the analogous Ethernet fields. FDDI headers also contain other fields, but you cannot name them explicitly in a filter expression.

Similarly, 'tr' and 'wlan' are aliases for 'ether'; the previous paragraph's statements about FDDI headers also apply to Token Ring and 802.11 wireless LAN headers. For 802.11 headers, the destination address is the DA field and the source address is the SA field; the BSSID, RA, and TA fields aren't tested.]

In addition to the above, there are some special '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

src host host

host host

ether dst ehost

ether src ehost

ether host ehost

gateway host

dst net net

src net net

net net

net net mask netmask

net net/len

dst port port

src port port

port port

dst portrange port1-port2

src portrange port1-port2

portrange port1-port2

less length

greater length

ip proto protocol

ip6 proto protocol

ip6 protochain protocol

ip protochain protocol

ether broadcast

ip broadcast

ether multicast

ip multicast

ip6 multicast

ether proto protocol

decnet src host

decnet dst host

decnet host host

ifname interface

on interface

rnr num

rulenum num

reason code

rset name

ruleset name

srnr num

subrulenum num

action act

ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp, ipx, netbeui

lat, moprc, mopdl

vlan [vlan_id]
filters IPv4 protocols encapsulated in VLAN 300 encapsulated within any higher order VLAN.
</divindent30>

mpls [label_num]

pppoed

pppoes

tcp, udp, icmp

iso proto protocol

clnp, esis, isis

l1, l2, iih, lsp, snp, csnp, psnp

vpi n

vci n

lane

llc

oamf4s

oamf4e

oamf4

oam

metac

bcc

sc

ilmic

connectmsg

metaconnect

expr relop expr

Primitives may be combined using:


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,
is short for
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.