tcpdump

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Dump traffic on a network.

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 ]
        [ -y datalinktype ]
        [ expression ]

Description

Tcpdump prints out a description of the contents 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, if run with the -c flag, will capture packets until 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 OSs 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 OSs it counts only packets that were matched by the filter expression regardless of whether tcpdump has read and processed them yet, and on other OSs 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).

The following options are available:

`-A`	Print each packet (minus its link level header) in ASCII. Handy for capturing web pages.
`-c`	Exit after receiving count packets.
`-C`	Before writing a raw packet to a savefile, check whether the file is currently larger than `file_size` and, if so, close the current savefile and open a new one. Savefiles after the first savefile will have the name specified with the `-w` flag, with a number after it, starting at 1 and continuing upward. The units of `file_size` are millions of bytes (1,000,000 bytes, not 1,048,576 bytes).
`-d`	Dump the compiled packet-matching code in a human 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 (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, where the interface name is a somewhat complex string. The -D flag will not be supported if tcpdump was built with an older version of libpcap that lacks the pcap_findalldevs() function.
`-e`	Print the link-level header on each dump line.
`-E`	Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that are addressed to addr and contain Security Parameter Index value spi. This combination may be repeated with comma or newline seperation. Note that setting the secret for IPv4 ESP packets is supported at this time. Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or none. The default is des-cbc. The ability to decrypt packets is only present if tcpdump was compiled with cryptography enabled. secret is the ASCII text for ESP secret key. If 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.
`-f`	Print 'foreign' IPv4 addresses numerically rather than symbolically (this option is intended to get around serious brain damage in Sun's NIS server -- usually it hangs forever translating non-local internet numbers). 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.
`-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 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.
`-l`	Make stdout line buffered. Useful if you want to see the data while capturing it. E.g., "tcpdump -l \| tee dat" or "tcpdump -l < dat & tail -f dat".
`-L`	List the known data link types for the interface and exit.
`-m`	Load SMI MIB module definitions from file module. This option can be used several times to load several MIB modules into tcpdump.
`-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 output lines are shorter.
`-R`	Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829). If specified, tcpdump will not print replay prevention field. Since there is no 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 "-".
`-S`	Print absolute, rather than relative, TCP sequence numbers.
`-s`	Snarf snaplen bytes of data from each packet rather than the default of 68 (with SunOS's NIT, the minimum is actually 96). 68 bytes is adequate for IP, ICMP, TCP and UDP but may truncate protocol information from name server and NFS packets (see below). Packets 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.
`-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 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).
`-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() function.
`-v`	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.
`-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 "-".
`-W`	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.
`-x`	When parsing and printing, in addition to printing the headers of each packet, print the data of each packet (minus its link level header) in hex. The smaller of the entire packet or snaplen bytes will be printed. Note that this is the entire link-layer packet, so for link layers that pad (e.g. Ethernet), the padding bytes will also be printed when the higher layer packet is shorter than the required padding.
`-xx`	When parsing and printing, in addition to printing the headers of each packet, print the data of each packet, including its link level header, in hex.
`-X`	When parsing and printing, in addition to printing the headers of each packet, print the data of each packet (minus its link level header) in hex and ASCII. This is very handy for analysing new protocols.
`-XX`	When parsing and printing, in addition to printing the headers of each packet, print the data of 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.

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 portrange. E.g., 'host foo', 'net 128.3', 'port 20', 'portrange 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 qualifier, src or dst is assumed. For some link layers, such as SLIP and the 'cooked' Linux capture mode used for the 'any' device and for some other device types, the inbound and outbound qualifiers can be used to specify a desired direction.
`proto`	qualifiers restrict the match to a particular protocol. Possible protos are: ether, fddi, tr, wlan, ip, ip6, arp, rarp, decnet, lat, sca, moprc, mopdl, iso, esis, isis, icmp, icmp6, 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'.

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

`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 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.) and by the machine's host-name-to-Ethernet-address resolution mechanism (/etc/ethers, etc.). (An equivalent expression is ether host ehost and not host host which can be used with either names or numbers for host / ehost.) This syntax does not work in IPv6-enabled configuration at this moment.
`dst net net`	True if the IPv4/v6 destination address of the packet has a network number of net. Net may be either a name from the networks database (/etc/networks, etc.) or a network number. An IPv4 network number can be written as a dotted quad (e.g., 192.168.1.0), dotted triple (e.g., 192.168.1), dotted pair (e.g, 172.16), or single number (e.g., 10); the netmask is 255.255.255.255 for a dotted quad (which means that it's really a host match), 255.255.255.0 for a dotted triple, 255.255.0.0 for a dotted pair, or 255.0.0.0 for a single number. An IPv6 network number must be written out fully; the netmask is ff:ff:ff:ff:ff:ff:ff:ff, so IPv6 'network' matches are really always host matches, and a network match requires a netmask length.
`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 netm`	ask 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 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.
`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: 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 IPv4 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 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 example, authentication header, routing header, or hop-by-hop option header, between IPv6 header and TCP header. The BPF code emitted by this primitive is complex and cannot be optimized by BPF optimizer code in tcpdump, so this can be somewhat slow.
`ip protochain protocol`	Equivalent to ip6 protochain protocol, but this is for IPv4.
`ether broadcast`	True if the packet is an Ethernet broadcast packet. The ether keyword is optional.
`ip broadcast`	True if the packet is an IPv4 broadcast packet. It checks for both the all-zeroes and all-ones broadcast conventions, and looks up the subnet mask on the interface on which the capture is being done. If the subnet mask of the interface on which the capture is being done is not available, either because the interface on which capture is being done has no netmask or because the capture is being done on the Linux 'any' interface, which can capture on more than one interface, this check will not work correctly.
`ether multicast`	True if the packet is an Ethernet multicast packet. The ether keyword is optional. This is shorthand for 'ether[0] & 1 != 0'.
`ip multicast`	True if the packet is an IPv4 multicast packet.
`ip6 multicast`	True if the packet is an IPv6 multicast packet.
`ether proto protocol`	True if the packet is of ether type protocol. Protocol can be a number or one of the names ip, ip6, arp, rarp, atalk, aarp, decnet, sca, lat, mopdl, moprc, iso, stp, ipx, or netbeui. Note these identifiers are also keywords and must be escaped via backslash (\). [In the case of FDDI (e.g., 'fddi protocol arp'), Token Ring (e.g., 'tr protocol arp'), and IEEE 802.11 wireless LANS (e.g., 'wlan protocol arp'), for most of those protocols, the protocol identification comes from the 802.2 Logical Link Control (LLC) header, which is usually layered on top of the FDDI, Token Ring, or 802.11 header. When filtering for most protocol identifiers on FDDI, Token Ring, or 802.11, tcpdump checks only 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`	True if the DECNET destination address is host.
`decnet host host`	True if either the DECNET source or destination address is host.
`ifname interface`	True if the packet was logged as coming from the specified interface (applies only to packets logged by OpenBSD's pf(4)).
`on interface`	Synonymous with the ifname modifier.
`rnr num`	True if the packet was logged as matching the specified PF rule number (applies only to packets logged by OpenBSD's pf(4)).
`rulenum num`	Synonomous with the rnr modifier.
`reason code`	True if the packet was logged with the specified PF reason code. The known codes are: match, bad-offset, fragment, short, normalize, and memory (applies only to packets logged by OpenBSD's pf(4)).
`rset name`	True if the packet was logged as matching the specified PF ruleset name of an anchored ruleset (applies only to packets logged by pf(4)).
`ruleset name`	Synonomous with the rset modifier.
`srnr num`	True if the packet was logged as matching the specified PF rule number of an anchored ruleset (applies only to packets logged by pf(4)).
`subrulenum num`	Synonomous with the srnr modifier.
`action act`	True if PF took the specified action when the packet was logged. Known actions are: pass and block (applies only to packets logged by OpenBSD's pf(4)).
`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 the packet is a VLAN packet. The vlan [vlan_id] expression may be used more than once, to filter on VLAN hierarchies. Each use of that expression increments the filter offsets by 4. For example: 'vlan 100 && vlan 200' filters on VLAN 200 encapsulated within VLAN 100, and 'vlan && vlan 300 && ip' filters IPv4 protocols encapsulated in VLAN 300 encapsulated within any higher order VLAN.
`mpls [label_num]`	True if the packet is an MPLS packet. If [label_num] is 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 keyword encountered in expression changes the decoding offsets 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 protocol. Protocol can be a number or one of the names clnp, esis, or isis.
`clnp, esis, isis`	Abbreviations for: 'iso proto p' where p is one of the above protocols.
`l1, l2, iih, lsp, snp, csnp, psnp`	Abbreviations for IS-IS PDU types.
`vpi n`	True if the packet is an ATM packet, for SunATM on Solaris, with a virtual path identifier of n.
`vci n`	True if the packet is an ATM packet, for SunATM on Solaris, with a virtual channel identifier of n.
`lane`	True if the packet is an ATM packet, for SunATM on Solaris, and is an ATM LANE packet. Note that the first lane keyword encountered in expression changes the tests done in the remainder of expression on the assumption that the packet is either a LANE emulated Ethernet packet or a LANE LE Control packet. If lane isn't specified, the tests are done under the assumption that the packet is an LLC-encapsulated packet.
`llc`	True if the packet is an ATM packet, for SunATM on Solaris, and is an LLC-encapsulated packet.
`oamf4s`	True if the packet is an ATM packet, for SunATM on Solaris, and is a segment OAM F4 flow cell (VPI=0 & VCI=3).
`oamf4e`	True if the packet is an ATM packet, for SunATM on Solaris, and is an end-to-end OAM F4 flow cell (VPI=0 & VCI=4).
`oamf4`	True if the packet is an ATM packet, for SunATM on Solaris, and is a segment or end-to-end OAM F4 flow cell (VPI=0 & (VCI=3 \| VCI=4)).
`oam`	True if the packet is an ATM packet, for SunATM on Solaris, and is a segment or end-to-end OAM F4 flow cell (VPI=0 & (VCI=3 \| VCI=4)).
`metac`	True if the packet is an ATM packet, for SunATM on Solaris, and is on a meta signaling circuit (VPI=0 & VCI=1).
`bcc`	True if the packet is an ATM packet, for SunATM on Solaris, and is on a broadcast signaling circuit (VPI=0 & VCI=2).
`sc`	True if the packet is an ATM packet, for SunATM on Solaris, and is on a signaling circuit (VPI=0 & VCI=5).
`ilmic`	True if the packet is an ATM packet, for SunATM on Solaris, and is on an ILMI circuit (VPI=0 & VCI=16).
`connectmsg`	True if the packet is an ATM packet, for SunATM on Solaris, and is on a signaling circuit and is a Q.2931 Setup, Call Proceeding, Connect, Connect Ack, Release, or Release Done message.
`metaconnect`	True if the packet is an ATM packet, for SunATM on Solaris, and is on a meta signaling circuit and is a Q.2931 Setup, Call Proceeding, Connect, Release, or Release Done message.
`expr relop expr`	True if the relation holds, where relop is one of >, <, , , =, !=, and expr is an arithmetic expression composed of integer constants (expressed in standard C syntax), the normal binary operators [+, -, *, /, &, \|, <<, >>], a length operator, and special packet data accessors. 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 ] Proto is one of ether, fddi, tr, wlan, ppp, slip, link, ip, arp, rarp, tcp, udp, icmp, ip6 or radio, and indicates the protocol layer for the index operation. (ether, fddi, wlan, tr, ppp, slip and link all refer to the link layer. radio refers to the 'radio header' added to some 802.11 captures.) 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 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 packe. 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 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-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 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 sundown:

tcpdump host sundown

To print traffic between helios and either hot or ace:

tcpdump host helios and ( hot or ace )

To print all IP packets between ace and any host except helios:

tcpdump ip host ace and not helios

To print all traffic between local hosts and hosts at 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 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

Authors

The original authors are:

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 http://www.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

Bugs

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 recommend that you use the latter.

When running tcpdump with the -v option on a network interface supporting checksum off-loading, IPpackets sourced from this machine will have many false 'bad cksum 0' errors.

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

Name server inverse queries are not dumped correctly: the (empty) question section is printed rather than real query in the answer section. Some believe that inverse queries are themselves a bug and prefer to fix the program generating them rather than tcpdump.

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

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

Filter expressions on fields other than those in 802.11 headers will set.

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

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

Requirements

Versions
INtime 4.0

Description

Examples

Authors

Bugs

Requirements

See Also