tcpdump

Dump traffic on a network

Syntax:

tcpdump [-aAbdDefhHIJKlLnNOpqStuUvxX#] [ -B size ] [ -c count ]
    [ -C file_size ][ -E algo:secret ] [ -F file ]
    [ -g fibnum ] [ -G rotate_seconds ] [ -i interface ] 
    [ -j tstamptype ] [-m module] [ -M secret ] [ -Q direction ]
    [ -r file ] [ -s snaplen ] [ -T type ] [ -V file ]
    [ -w file ] [ -W filecount ] [ -y linktype ] [ -z command ]
    [ -Z user ] [ --time-stamp-precision=tstamp_precision ]
    [ --immediate-mode ] [ --version ] [ expression ]

Runs on:

QNX Neutrino

Options:

-a
Provide compatibility for old -a option (no effect).
-A
Print each packet (minus its link level header) in ASCII. Handy for capturing web pages.
-b
Print the AS number in BGP packets in ASDOT notation rather than ASPLAIN notation.
-B size
Set the OS capture buffer size to size, which is in KB.
-c count
Exit after receiving count packets.
-C file_size
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 one will have a name consisting of the string specified with the -w option, followed by a number that starts at 1 and continues upwards. 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. You can supply the interface name or the number to the -i option 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 modern Windows systems, where the interface name is a somewhat complex string.

-e
Print the link-level header on each dump line.
-E algo:secret,...
Use algo:secret for decrypting IPsec ESP packets that are addressed to addr and contain Security Parameter Index value spi. You can specify additional combinations, separating them with commas or newlines.
Note: Setting the secret for IPv4 ESP packets isn't currently supported.

The algorithm can be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or none. The default is des-cbc.

The secret is the ASCII text for the ESP secret key. If preceded by 0x, then a hexadecimal value is read.

The option assumes RFC 2406 ESP, not RFC 1827 ESP. The option is only for debugging purposes, and the use of this option with a true “secret” key is discouraged. By presenting the IPsec secret key onto command line, you make it visible to others, via ps and on other occasions.

In addition to the above syntax, you can use the syntax file_name to have tcpdump read the provided file. The file is opened on 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.

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 isn't 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 won't work correctly.

-F file
Use file as input for the filter expression. Any additional expression given on the command line is ignored.
-g fibnum
Capture the packets only from those interfaces listed in the FIB fibnum routing table. For instance, you can set -g to reduce the amount of packets captured to more easily check for loopbacks, because the lo0 interface exists in all routing tables.
-G rotate_seconds
If specified, rotates the dump file specified with the -w option every rotate_seconds seconds. Savefiles have the name specified by -w, which should include a time format as defined by strftime(). If no time format is specified, each new file overwrites the previous.

If used in conjunction with the -C option, file names take the form file<count>.

-h
Print the tcpdump and libpcap version strings, print a usage message, and exit.
-H
Attempt to detect 802.11s draft mesh headers.
-i interface
Listen on interface. If you don't specify this option, 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, you can use an interface argument of any to capture packets from all interfaces. Note that captures on the any device aren't done in promiscuous mode.

You can use an interface number as printed by that option as the interface argument.

-I
Put the interface in monitor mode.
-j tstamp_type
Set the timestamp type for the capture to tstamp_type.
-J
List the supported timestamp types for the interface and exit.
-K
Don't attempt to verify IP, TCP or UDP checksums. This is useful for interfaces that perform any of these checksum calculations in hardware; otherwise, all outgoing IP, TCP or UDP checksums are flagged as bad.
-l
(“el”) Make stdout line buffered. This is useful if you want to see the data while capturing it. For example, tcpdump -l | tee dat or tcpdump -l > dat & tail -f dat.
-L
List the known data link types for the interface, and then exit.
-m module
Load SMI MIB module definitions from the file module. You can use this option several times to load several MIB modules into tcpdump.
-M secret
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.) into names.
-N
Don't print domain name qualification of host names. For example, if you give this option, tcpdump prints nic instead of nic.ddn.mil.
-O
(“Oh”) Don't run the packet-matching code optimizer. This is useful only if you suspect there's 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, you can't use -p as an abbreviation for ether host {local-hw-addr} or ether broadcast.
-q
Be quiet; print less protocol information, so that output lines are shorter.
-Q direction
Choose the send or receive direction for which packets should be captured. The possible values for the direction argument are in, out and inout (case-sensitive).
-r file
Read packets from file (which was created with the -w option). If file is -, tcpdump uses standard input.
-S
Print absolute, rather than relative, TCP sequence numbers.
-s snaplen
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: 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. If you set snaplen to 0, tcpdump uses the required length to catch whole packets.
-T type
Force packets selected by expression to be interpreted as 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 Transport Protocol
  • rtcp — Real-time Control Protocol
  • snmp — Simple Network Management Protocol
  • tftp — Trivial File Transfer Protocol
  • vat — Visual Audio Tool
  • 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 (micro-second resolution) between the current and previous line on each dump line.
-tttt
Print a timestamp in default format preceded by the date on each dump line.
-ttttt
Print a delta (micro-second resolution) between the current and first line on each dump line.
-u
Print undecoded NFS handles.
-U
Make output saved via the -w option packet-buffered; as each packet is saved, write it to the output file, rather than writing it only when the output buffer fills.
-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. This option 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 hexadecimal as well.
-V file
Read a list of filenames from file. Standard input is used if file is -.
-w file
Write the raw packets to file rather than parsing and printing them. You can print them with the -r option. If file is -, tcpdump uses standard output.
-W filecount
When used with the -C option, the -W limits the number of files created to filecount, and begins overwriting files from the beginning, thus creating a “rotating” buffer. In addition, it names the files with enough leading zeroes to support the maximum number of files, allowing you to sort them correctly.

When used with the -G option, the -W option limits the number of rotated dump files that get created, exiting with a status of 0 when tcpdump reaches the limit. If you set the -C option as well, tcpdump uses cyclical files per timeslice.

-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 hexadecimal. The smaller of the entire packet or snaplen bytes is printed. Note that this is the entire link-layer packet, so for link layers that pad (e.g., Ethernet), the padding bytes are also 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 hexadecimal.
-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 hexadecimal and ASCII. This is very handy for analyzing 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 hexadecimal and ASCII.
-y linktype
Set the data link type to use while capturing packets to linktype.
-z command
Used in conjunction with the -C or -G options, this makes tcpdump run command on each savefile being closed after each rotation. For example, specifying -z gzip or -z bzip2 compresses each savefile using gzip or bzip2.
Note: The tcpdump utility runs the command in parallel with the capture, using the lowest priority so that this doesn't disturb the capture process.

If you want to use a command that itself takes options or different arguments, write a shell script that takes the savefile name as the only argument, arrange the options and arguments as required, and then execute the command that you want.

-Z user
Drop privileges (if root) and change the user ID to user and the group ID to the primary group of user.
-#
Print an optional packet number at the beginning of the line.
expression
Selects which packets will be dumped. For an explanation of the kinds of expressions you can provide, see below.

Description:

The tcpdump utility prints a description of the contents of packets on a network interface that match the boolean expression. You can also run it with the -w option, which causes it to save the packet data to a file for later analysis, and/or with the -r option, which causes it to read from a saved packet file rather than to read packets from a network interface. In all cases, tcpdump processes only those packets that match expression.

For information about the expression argument, see Expressions,” below.

The tcpdump utility, if not run with the -c option, continues capturing packets until it's interrupted by a SIGINT signal (generated, for example, by typing your interrupt character, typically ControlC) or a SIGTERM signal (typically generated with the kill command); if run with the -c option, tcpdump captures packets until it's interrupted by a SIGINT or SIGTERM signal, or the specified number of packets have been processed.

When tcpdump finishes capturing packets, it reports counts of:

You must be root in order to read packets from a network interface. Reading a saved packet file doesn't require special privileges; you just need read permission for the file.

Expressions

The expression on the command line selects which packets to dump. If no expression is given, all packets on the net will be dumped. Otherwise, only packets for which expression is true are dumped.

The expression consists of one or more primitives that usually consist of an ID (name or number) preceded by one or more qualifiers. The different kinds of qualifier are:

type
The kind of thing the ID name or number refers to. Possible types are host, net, port, and portrange. For example, host xyz, net 128.3, port 20, and portrange 6000-6008. If there's no type qualifier, tcpdump uses host.
dir
A particular transfer direction to and/or from the ID. Possible directions are src, dst, src or dst, and src and dst. For example, src xyz, 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, you can use the inbound and outbound qualifiers to specify a desired direction.

proto
Restrict the match to a particular protocol. Possible protocols are ether, fddi, tr, wlan, ip, ip6, arp, rarp, decnet, tcp, and udp. For example, ether src xyz, arp net 128.3, tcp port 21, and udp portrange 7000-7009.

If there's no proto qualifier, all protocols consistent with the type are assumed. For example, src xyz means (ip or arp or rarp) src xyz (except the latter isn't legal syntax), net abc means (ip or arp or rarp) net abc, and port 53 means (tcp or udp) port 53.

The fddi protocol 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 can't 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:

and arithmetic expressions. All of these are described below.

You can build more complex filter expressions by using the words and, or, and not to combine primitives. For example, host xyz and not port ftp and not port ftp-data. To save typing, you can omit identical qualifier lists. For example, tcp dst port ftp or ftp-data or domain is exactly the same as tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain.

Allowable primitives are:

dst host host
True if the IPv4/v6 destination field of the packet is host, which may be either an address or a name.
src host host
True if the IPv4/v6 source field of the packet is host.
host host
True if either the IPv4/v6 source or destination of the packet is host.

You can prepend any of the above host expressions 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 is checked for a match.

ether dst ehost
True if the Ethernet destination address is ehost. The 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. That is, the Ethernet source or destination address was host, but neither the IP source nor the IP destination was host. The 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 doesn't 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. The 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 netmask
True if the IPv4 address matches net with the specific netmask. May be qualified with src or dst. Note that this syntax isn't 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 you use a name, both the port number and protocol are checked. If you use a number or ambiguous name, 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. The 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.

You can prepend any of the above port or port range expressions 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. The protocol can be a number, or one of the names icmp, icmp6, igmp, igrp, pim, ah, esp, vrrp, udp, or tcp.
Note: The identifiers tcp, udp, and icmp are also keywords and must be escaped via backslash (\), which is \\ in the C shell. This primitive doesn't chase the protocol header chain.
ip6 proto protocol
True if the packet is an IPv6 packet of protocol type protocol. Note that this primitive doesn't chase the protocol header chain.
ip6 protochain protocol
True if the packet is IPv6 packet, and contains a protocol header with the 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 can't 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 isn't 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 doesn't 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. The 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:

  • isotcpdump checks the DSAP (Destination Service Access Point) and SSAP (Source Service Access Point) fields of the LLC header
  • stp and netbeuitcpdump checks the DSAP of the LLC header
  • atalktcpdump 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 netbeuitcpdump checks for an 802.3 frame, and then checks the LLC header as it does for FDDI, Token Ring, and 802.11
  • atalktcpdump 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
  • aarptcpdump checks for the AppleTalk ARP etype in either an Ethernet frame or an 802.2 SNAP frame with an OUI of 0x000000
  • ipxtcpdump 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 is in the form 10.123.
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).
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).
rulenum num
Synonymous 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).
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).
ruleset name
Synonymous 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).
subrulenum num
Synonymous 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).
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 doesn't currently know how to parse these protocols.

type wlan_type
True if the IEEE 802.11 frame type matches the specified wlan_type. Valid wlan_types are: mgt, ctl, and data.
type wlan_type subtype wlan_subtype
True if the IEEE 802.11 frame type matches the specified wlan_type, and frame subtype matches the specified wlan_subtype.

If the specified wlan_type is mgt, then valid wlan_subtypes are assoc-req, assoc-resp, reassoc-req, reassoc-resp, probe-req, probe-resp, beacon, atim, disassoc, auth, and deauth.

If the specified wlan_type is ctl, then valid wlan_subtypes are ps-poll, rts, cts, ack, cf-end, and cf-end-ack.

If the specified wlan_type is data, then valid wlan_subtypes are data, data-cf-ack, data-cf-poll, data-cf-ack-poll, null, cf-ack, cf-poll, cf-ack-poll, qos-data, qos-data-cf-ack, qos-data-cf-poll, qos-data-cf-ack-poll, qos, qos-cf-poll, and qos-cf-ack-poll.

subtype wlan_subtype
True if the IEEE 802.11 frame subtype matches the specified wlan_subtype, and frame has the type to which the specified wlan_subtype belongs.
vlan [vlan_id]
True if the packet is an IEEE 802.1Q VLAN packet. If [vlan_id] is specified, this is true only if the packet has the specified vlan_id.
Note: 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. You can use the vlan[vlan_id] expression 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, this is true only if the packet has the specified label_num.
Note: 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. You can use the mpls [label_num] expression 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: 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. The 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: 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 >, <, >=, <=, =, or !=, 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 greater than 0. To access data inside the packet, use the following syntax:
proto [expr : size ]

where 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: The tcp, udp, and other upper-layer protocol types apply only to IPv4, not IPv6 (this will be fixed in the future).

The byte offset, relative to the indicated protocol layer, is given by expr. The size is optional and indicates the number of bytes in the field of interest; it can be 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 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, and icmp-maskreply.

The following TCP flags field values are available: tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-ack, tcp-urg.

You can combine primitives by using:

Negation has highest precedence. Alternation and concatenation have equal precedence and associate from 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 shouldn't be confused with:

not ( host vs or ace )

You can pass expression arguments to tcpdump as either a single argument or as multiple arguments, whichever is more convenient. Generally, if the expression contains shell metacharacters, it's easier to pass it as a single, quoted argument. Multiple arguments are concatenated with spaces before being parsed.

Output format

The output of tcpdump is protocol-dependent. The following gives a brief description and examples of most of the formats.

Link-level headers

If you specify the -e option, 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 datagrams) 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 specified or not, the source routing information is printed for source-routed packets.

On 802.11 networks, the -e option causes tcpdump to print the frame-control fields, all of the addresses in the 802.11 header, and the packet length. As on FDDI networks, packets are assumed to contain an LLC packet.

Note: 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 information are printed out. The packet type is printed first. The three types are ip, utcp, and ctcp. No further link information is printed for ip packets. For TCP 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 isn't a special 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 compressed TCP packet, with an implicit connection identifier; the ack has changed by 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes of data and 6 bytes of compressed header:

O ctcp * A+6 S+49 I+6 3 (6)

ARP/RARP packets

ARP/RARP output shows the type of request and its arguments. The 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, Ethernet addresses are in uppercase, and Internet addresses are in lowercase).

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 hexadecimal 0806 (type ETHER_ARP) and the total length was 64 bytes.

TCP Packets

Note: The following description assumes familiarity with the TCP protocol described in RFC 793. If you aren't 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), R (RST), W (ECN CWR) or E (ECN-Echo), 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 direction 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 brackets (e.g., <mss 1024>).

The 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's the opening portion of an rlogin from host rtsg to host csam:

rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
rtsg.1023 > csam.login: . ack 1 win 4096
rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
csam.login > rtsg.1023: . ack 2 win 4096
rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1

The first line says that tcp port 1023 on rtsg sent a packet to port login on csam. The S indicates that the SYN flag was set. The packet sequence number was 768512 and it contained no data. (The notation is first:last(nbytes), which means “sequence numbers first up to but not including last which is nbytes bytes of user data.”) There was no piggy-backed ack, the available receive window was 4096 bytes and there was a max-segment-size option requesting an mss of 1024 bytes.

Csam replies with a similar packet except it includes a piggy-backed ack for rtsg's SYN. Rtsg then acks csam's SYN. The . means no flags were set. The packet contained no data so there is no data sequence number. Note that the ack sequence number is a small integer (1). The first time tcpdump sees a TCP “conversation”, it prints the sequence number from the packet. On subsequent packets of the conversation, the difference between the current packet's sequence number and this 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). The -S option overrides this feature, causing the original sequence numbers to be output.

On the sixth 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 seventh line, csam says it's received data sent by rtsg up to but not including 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 eighth and ninth lines, csam sends two bytes of urgent, pushed data to rtsg.

If the snapshot was small enough that tcpdump didn't capture the full TCP header, it interprets as much of the header as it can and then reports “[|tcp]” to indicate the remainder couldn't 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 doesn't interpret any further options (since it's impossible to tell where they start). If the header length indicates options are present but the IP datagram length isn't long enough for the options to actually be there, tcpdump reports it as “[bad hdr length]”.

Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)

There are 8 bits in the control bits section of the TCP header:

CWR | ECE | URG | ACK | PSH | RST | SYN | FIN

Let's assume that we want to watch packets used in establishing a TCP connection. Recall that TCP uses a 3-way handshake protocol when it initializes a new connection; the connection sequence with regard to the TCP control bits is:

  1. Caller sends SYN.
  2. Recipient responds with SYN, ACK.
  3. Caller sends ACK.

Now we're interested in capturing packets that have only the SYN bit set (Step 1). Note that we don't want packets from step 2 (SYN-ACK), just a plain initial SYN. What we need is a correct filter expression for tcpdump.

Recall the structure of a TCP header without options:

 0                            15                              31
-----------------------------------------------------------------
|          source port          |       destination port        |
-----------------------------------------------------------------
|                        sequence number                        |
-----------------------------------------------------------------
|                     acknowledgment number                     |
-----------------------------------------------------------------
|  HL   | rsvd  |C|E|U|A|P|R|S|F|        window size            |
-----------------------------------------------------------------
|         TCP checksum          |       urgent pointer          |
-----------------------------------------------------------------

A TCP header usually holds 20 octets of data, unless options are present. The first line of the graph contains octets 0 - 3, the second line shows octets 4 - 7 etc.

Starting to count with 0, the relevant TCP control bits are contained in octet 13:

 0             7|             15|             23|             31
----------------|---------------|---------------|----------------
|  HL   | rsvd  |C|E|U|A|P|R|S|F|        window size            |
----------------|---------------|---------------|----------------
|               |  13th octet   |               |               |

Let's have a closer look at the thirteenth octet:

                |               |
                |---------------|
                |C|E|U|A|P|R|S|F|
                |---------------|
                |7   5   3     0|

These are the TCP control bits we're interested in. We have numbered the bits in this octet from 0 to 7, right to left, so the PSH bit is bit number 3, while the URG bit is number 5.

Recall that we want to capture packets with only SYN set. Let's see what happens to octet 13 if a TCP datagram arrives with the SYN bit set in its header:

                |C|E|U|A|P|R|S|F|
                |---------------|
                |0 0 0 0 0 0 1 0|
                |---------------|
                |7 6 5 4 3 2 1 0|

Looking at the control bits section, we see that only bit number 1 (SYN) is set.

Assuming that octet number 13 is an 8-bit unsigned integer in network byte order, the binary value of this octet is 00000010, and its decimal representation is:

   7     6     5     4     3     2     1     0
0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2  =  2

We're almost done, because now we know that if only SYN is set, the value of the thirteenth octet in the TCP header, when interpreted as a 8-bit unsigned integer in network byte order, must be exactly 2.

This relationship can be expressed as:

tcp[13] == 2

We can use this expression as the filter for tcpdump in order to watch packets which have only SYN set:

tcpdump -i xl0 tcp[13] == 2

The expression says “let the thirteenth octet of a TCP datagram have the decimal value 2”, which is exactly what we want.

Now, let's assume that we need to capture SYN packets, but we don't care if ACK or any other TCP control bit is set at the same time. Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set arrives:

     |C|E|U|A|P|R|S|F|
     |---------------|
     |0 0 0 1 0 0 1 0|
     |---------------|
     |7 6 5 4 3 2 1 0|

Now bits 1 and 4 are set in the thirteenth octet. The binary value of octet 13 is 00010010, which translates to decimal:

   7     6     5     4     3     2     1     0
0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2   = 18

Now we can't just use 'tcp[13] == 18' in the tcpdump filter expression, because that would select only those packets that have SYN-ACK set, but not those with only SYN set. Remember that we don't care if ACK or any other control bit is set as long as SYN is set.

In order to achieve our goal, we need to logically AND the binary value of octet 13 with some other value to preserve the SYN bit. We know that we want SYN to be set in any case, so we'll logically AND the value in the thirteenth octet with the binary value of a SYN:

          00010010 SYN-ACK              00000010 SYN
     AND  00000010 (we want SYN)   AND  00000010 (we want SYN)
          --------                      --------
     =    00000010                 =    00000010

We see that this AND operation delivers the same result regardless whether ACK or another TCP control bit is set. The decimal representation of the AND value as well as the result of this operation is 2 (binary 00000010), so we know that for packets with SYN set the following relation must hold true:

( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )

This points us to the tcpdump filter expression

tcpdump -i xl0 'tcp[13] & 2 == 2'

Note that you should use single quotes or a backslash in the expression to hide the AND ('&') special character from the shell.

UDP Packets

UDP format is illustrated by this rwho packet:

actinide.who > broadcast.who: udp 84

This says that port who on host actinide sent a udp datagram to port who on host broadcast, the Internet broadcast address. The packet contained 84 bytes of user data.

Some UDP services are recognized (from the source or destination port number) and the higher level protocol information printed. In particular, Domain Name service requests (RFCs 1034 and 1035) and Sun RPC calls (RFC 1050) to NFS.

UDP Name Server Requests

Note: The following description assumes familiarity with the Domain Service protocol described in RFC 1035.

Name server requests are formatted as:

src > dst: id op? flags qtype qclass name (len)

h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)

Host h2opolo asked the domain server on helios for an address record (qtype=A) associated with the name ucbvax.berkeley.edu. The query ID was 3. The + indicates the recursion desired flag was set. The query length was 37 bytes, not including the UDP and IP protocol headers. The query operation was the normal one, Query, so the op field was omitted. If the op had been anything else, it would have been printed between the 3 and the +. Similarly, the qclass was the normal one, C_IN, and omitted. Any other qclass would have been printed immediately after the A.

A few anomalies are checked and may result in extra fields enclosed in square brackets: If a query contains an answer, authority records or additional records section, ancount, nscount, or arcount are printed as [na], [nn], or [nau], where n is the appropriate count. If any of the response bits are set (AA, RA or rcode) or any of the “must be zero” bits are set in bytes two and three, [b2&3=x] is printed, where x is the hexadecimal value of header bytes two and three.

UDP Name Server Responses

Name server responses are formatted as

src > dst: id op rcode flags a/n/au type class data (len)

helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)

In the first example, helios responds to query ID 3 from h2opolo with 3 answer records, 3 name server records and 7 additional records. The first answer record is type A (address) and its data is Internet address 128.32.137.3. The total size of the response was 273 bytes, excluding UDP and IP headers. The op (Query) and response code (NoError) were omitted, as was the class (C_IN) of the A record.

In the second example, helios responds to query 2 with a response code of non-existent domain (NXDomain) with no answers, one name server and no authority records. The * indicates that the authoritative answer bit was set. Since there were no answers, no type, class or data were printed.

Other flag characters that might appear are - (recursion available, RA, not set) and | (truncated message, TC, set). If the question section doesn't contain exactly one entry, [nq] is printed.

Note that name server requests and responses tend to be large and the default snaplen of 68 bytes may not capture enough of the packet to print. Use the -s flag to increase the snaplen if you need to seriously investigate name server traffic. For example, -s 128.

SMB/CIFS decoding

tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and NetBEUI SMB data is also done.

By default, a fairly minimal decode is done, with a much more detailed decode done if -v is used. Be warned that with -v a single SMB packet may take up a page or more, so only use -v if you really want all the details.

NFS Requests and Replies

Sun NFS (Network File System) requests and replies are printed as:

src.xid > dst.nfs: len op args
src.nfs > dst.xid: reply stat len op results

For example:

sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
wrl.nfs > sushi.6709: reply ok 40 readlink “../var”
sushi.201b > wrl.nfs:
        144 lookup fh 9,74/4096.6878 “xcolors”
wrl.nfs > sushi.201b:
        reply ok 128 lookup fh 9,74/4134.3150

In the first line, host sushi sends a transaction with ID 6709 to wrl (note that the number following the source host is a transaction ID, not the source port). The request was 112 bytes, excluding the UDP and IP headers. The operation was a readlink (read symbolic link) on file handle (fh) 21,24/10.731657119. (If you're lucky, as in this case, the file handle can be interpreted as a major, minor device number pair, followed by the inode number and generation number.) The wrl host replies ok with the contents of the link.

In the third line, sushi asks wrl to look up the name xcolors in directory file 9,74/4096.6878. Note that the data printed depends on the operation type. The format is intended to be self-explanatory if read in conjunction with an NFS protocol spec.

If you specify the -v (verbose) option, additional information is printed. For example:

sushi.1372a > wrl.nfs:
        148 read fh 21,11/12.195 8192 bytes @ 24576
wrl.nfs > sushi.1372a:
        reply ok 1472 read REG 100664 ids 417/0 sz 29388

(The -v option also prints the IP header TTL, ID, length, and fragmentation fields, which have been omitted from this example.) In the first line, sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte offset 24576. The wrl host replies ok; the packet shown on the second line is the first fragment of the reply, and hence is only 1472 bytes long (the other bytes will follow in subsequent fragments, but these fragments don't have NFS or even UDP headers and so might not be printed, depending on the filter expression used). Because the -v flag is given, some of the file attributes (which are returned in addition to the file data) are printed: the file type (“REG”, for regular file), the file mode (expressed as a 6-digit octal number; see the st_mode member in the stat structure), the uid and gid, and the file size.

If you specify more than one -v option, even more details are printed.

Note that NFS requests are very large and much of the detail won't be printed unless snaplen is increased. Try using -s 192 to watch NFS traffic.

NFS reply packets don't explicitly identify the RPC operation. Instead, tcpdump keeps track of “recent” requests, and matches them to the replies using the transaction ID. If a reply doesn't closely follow the corresponding request, it might not be parsable.

AFS Requests and Replies

Transarc AFS (Andrew File System) requests and replies are printed as:

src.sport > dst.dport: rx packet-type
src.sport > dst.dport: rx packet-type service call call-name args
src.sport > dst.dport: rx packet-type service reply call-name args

elvis.7001 > pike.afsfs:
        rx data fs call rename old fid 536876964/1/1 “.newsrc.new”
        new fid 536876964/1/1 “.newsrc”
pike.afsfs > elvis.7001: rx data fs reply rename

In the first line, host elvis sends a RX packet to pike. This was a RX data packet to the fs (fileserver) service, and is the start of an RPC call. The RPC call was a rename, with the old directory file ID of 536876964/1/1 and an old filename of .newsrc.new, and a new directory file ID of 536876964/1/1 and a new filename of .newsrc. The host pike responds with a RPC reply to the rename call (which was successful, because it was a data packet and not an abort packet).

In general, all AFS RPCs are decoded at least by RPC call name. Most AFS RPCs have at least some of the arguments decoded (generally only the “interesting” arguments, for some definition of interesting).

The format is intended to be self-describing, but it will probably not be useful to people who aren't familiar with the workings of AFS and RX.

If the -v (verbose) flag is given twice, acknowledgment packets and additional header information is printed, such as the RX call ID, call number, sequence number, serial number, and the RX packet flags.

If the -v flag is given twice, additional information is printed, such as the RX call ID, serial number, and the RX packet flags. The MTU negotiation information is also printed from RX ack packets.

If you specify the -v option three times, the security index and service ID are printed.

Error codes are printed for abort packets, with the exception of Ubik beacon packets (because abort packets are used to signify a yes vote for the Ubik protocol).

Note that AFS requests are very large and many of the arguments won't be printed unless snaplen is increased. Try using -s 256 to watch AFS traffic.

AFS reply packets don't explicitly identify the RPC operation. Instead, tcpdump keeps track of “recent” requests, and matches them to the replies using the call number and service ID. If a reply doesn't closely follow the corresponding request, it might not be parsable.

KIP AppleTalk (DDP in UDP)

AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated and dumped as DDP packets (i.e., all the UDP header information is discarded). The file /etc/atalk.names is used to translate AppleTalk net and node numbers to names. Lines in this file have the form:

number  name

1.254           ether
16.1            icsd-net
1.254.110       ace

The first two lines give the names of AppleTalk networks. The third line gives the name of a particular host. (A host is distinguished from a net by the third octet in the number; a net number must have two octets and a host number must have three octets.) The number and name should be separated by whitespace (blanks or tabs). The /etc/atalk.names file may contain blank lines or comment lines (lines starting with a #).

AppleTalk addresses are printed in the form:

net.host.port

144.1.209.2 > icsd-net.112.220
office.2 > icsd-net.112.220
jssmag.149.235 > icsd-net.2

(If the /etc/atalk.names doesn't exist or doesn't contain an entry for some AppleTalk host/net number, addresses are printed in numeric form.) In the first example, NBP (DDP port 2) on net 144.1 node 209 is sending to whatever is listening on port 220 of net icsd node 112. The second line is the same except the full name of the source node is known (office). The third line is a send from port 235 on net jssmag node 149 to broadcast on the icsd-net NBP port (note that the broadcast address (255) is indicated by a net name with no host number; for this reason it's a good idea to keep node names and net names distinct in /etc/atalk.names).

NBP (name binding protocol) and ATP (AppleTalk transaction protocol) packets have their contents interpreted. Other protocols just dump the protocol name (or number if no name is registered for the protocol) and packet size.

NBP packets are formatted like the following examples:

icsd-net.112.220 > jssmag.2: nbp-lkup 190: “=:LaserWriter@*”
jssmag.209.2 > icsd-net.112.220: nbp-reply 190: “RM1140:LaserWriter@*” 250
techpit.2 > icsd-net.112.220: nbp-reply 190: “techpit:LaserWriter@*” 186

The first line is a name-lookup request for laserwriters sent by net icsd host 112 and broadcast on net jssmag. The nbp ID for the lookup is 190. The second line shows a reply for this request (note that it has the same id) from host jssmag.209 saying that it has a laserwriter resource named RM1140 registered on port 250. The third line is another reply to the same request saying host techpit has laserwriter techpit registered on port 186.

ATP packet formatting is demonstrated by the following example:

jssmag.209.165 > helios.132: atp-req  12266<0-7> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002

The jssmag.209 host initiates transaction ID 12266 with host helios by requesting up to 8 packets (the <0-7>). The hexadecimal number at the end of the line is the value of the userdata field in the request.

The helios host responds with 8 512-byte packets. The :digit following the transaction ID gives the packet sequence number in the transaction and the number in parentheses is the amount of data in the packet, excluding the ATP header. The * on packet 7 indicates that the EOM bit was set.

The jssmag.209 host then requests that packets 3 and 5 be retransmitted; helios resends them, and then jssmag.209 releases the transaction. Finally, jssmag.209 initiates the next request. The * on the request indicates that XO (“exactly once”) wasn't set.

IP Fragmentation

Fragmented Internet datagrams are printed as:

(frag id:size@offset+)
(frag id:size@offset)

The first form indicates there are more fragments. The second indicates this is the last fragment.

Id is the fragment id. Size is the fragment size (in bytes) excluding the IP header. Offset is this fragment's offset (in bytes) in the original datagram.

The fragment information is output for each fragment. The first fragment contains the higher level protocol header and the frag info is printed after the protocol info. Fragments after the first contain no higher level protocol header and the frag info is printed after the source and destination addresses. For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't appear to handle 576 byte datagrams:

arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
arizona > rtsg: (frag 595a:204@328)
rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560

There are a couple of things to note here: First, addresses in the second line don't include port numbers. This is because the TCP protocol information is all in the first fragment and we have no idea what the port or sequence numbers are when we print the later fragments. Second, the tcp sequence information in the first line is printed as if there were 308 bytes of user data when, in fact, there are 512 bytes (308 in the first frag and 204 in the second). If you are looking for holes in the sequence space or trying to match up acks with packets, this can fool you.

A packet with the IP don't fragment flag is marked with a trailing (DF).

Timestamps

By default, all output lines are preceded by a timestamp. The timestamp is the current clock time, in the form hh:mm:ss.frac and is as accurate as the kernel's clock. The timestamp reflects the time io-pkt first saw the packet. No attempt is made to account for the time lag between when the Ethernet interface removed the packet from the wire and when io-pkt serviced the “new packet” interrupt.

Examples:

Print all packets arriving at or departing from sundown:

tcpdump host sundown

Print traffic between helios and either hot or ace:

tcpdump host helios and \( hot or ace \)

Print all IP packets between ace and any host except helios:

tcpdump ip host ace and not helios

Print all traffic between local hosts and hosts at Berkeley:

tcpdump net ucb-ether

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)'

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

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'

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)'

Print IP packets longer than 576 bytes sent through gateway snup:

tcpdump 'gateway snup and ip[2:2] > 576'

Print IP broadcast or multicast packets that weren't sent via Ethernet broadcast or multicast:

tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'

Print all ICMP packets that aren't echo requests/replies (i.e., not ping packets):

tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'

Exit status:

0
Successful completion.
1
An error occurred.

Contributing author:

The original authors are Van Jacobson, Craig Leres, and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of California, Berkeley, CA. The tcpdump utility is currently maintained by tcpdump.org.

IPv6/IPsec support is added by WIDE/KAME project. This program uses Eric Young's SSLeay library, under specific configuration.