tcpdump Command

Purpose

Dumps traffic on a network

Syntax

tcpdump [ -a ] [ -A ] [ -B buffer_size ] [ -d ] [ -D ] [ -e ] [ -f ] [ -l ] [ -K ] [ -L ] [ -M secret ] [ -r file ]][ -n ] [ -N ] [ -O ] [ -p ][ -q ] [ -Q [ -V ] ] [ -R ] [ -S ] [ -t ] [ -T ][ -u ] [ -U ] [ -v ] [ -x ] [ -X ] [ -c count ][ -C file_size ] [ -F file ] [ -G rotate_seconds ] [ -i  interface ] [ -s snaplen ] [ -w file ][ -E addr ] [ -y datalinktype ] [-z command ] [-Z user ] [ expression ]

Description

The tcpdump command prints the headers of packets on a network interface that match the boolean expression. You can run the command with the -w flag to save the packet data in a file for further analysis. You can also run the command with the -r flag to read data from a saved packet file instead reading the packets from a network interface. In all cases, only packets that match expression is processed by the tcpdump command.

If it is not run with the -c flag, tcpdump continues capturing packets until it is interrupted by a SIGINT signal (typically control-C) or a SIGTERM signal (typically the kill(1) command). If tcpdump is run with the -c flag, it captures the packets until it is interrupted by a SIGINT or SIGTERM signal or the specified number of packets have been processed.

Allowable Primitives

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 is 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. For example, 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, and so on). 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.

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 IP address matches net with the specific netmask. This might be qualified with src or dst. 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 orip6/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 (For example, dst port 513 prints both tcp/login traffic and udp/who traffic, and port domain prints both tcp/domain and udp/domain traffic).

src port port

True if the packet has a source port value of port.

port port

True if either the source or destination port of the packet is port. Any of the above port expressions can be prepended with the keywords, tcp or udp, as in: tcp src port port which matches only tcp packets whose source port is port.

less length

True if the packet has a length less than or equal to length. This is equivalent to len <= length.

greater length

True if the packet has a length greater than or equal to length. This is equivalent to: len >= length.

ip proto protocol

True if the packet is an IP packet 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 Berkeley Packet Filter (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 used 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, for example, because the interface on which capture is being done has no netmask this check does 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 IP multicast packet.

ip6 multicast

True if the packet is an IPv6 multicast packet.

ether proto protocol

True if the packet is of ether type 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 that 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 UnitIdentifier (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, sap, 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.

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)).

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))

netbeui

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

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 the packets that have the specified vlan_id are true. 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.

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. 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 or ip6, and indicates the protocol layer for the index operation. (ether, fddi, wlan, tr, ppp, slip and link all refer to the link layer.) 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 packet.

For example, ether[0] & 1 != 0 catches all multicast traffic. The expression ip[0] & 0xf !=5 catches all IP packets with options. The expression ip[6:2] & 0x1fff = 0 catches only unfragmented datagrams and frag zero of fragmented datagrams. This check is implicitly applied to the tcp and udp index operations. For instance, tcp[0] always means the first byte of the TCP header, and never means the first byte of an intervening fragment.

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.

Combining Primitives

          Negation (`!' or `not').

          Concatenation (`&&' or `and').

          Alternation (`||' or `or').

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.

Flags

tcpdump -l | tee dat
  or  
tcpdump -l   >   dat & tail -f dat

Parameters

expressions

Selects the packets that are to be dumped. If an expression is provided, only the packets for which the expressions is true are dumped; otherwise, all the packets on the net are 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 type of primitive the id name or number refers to. Possible types are host, net and port. For example, `host foo', `net 128.3', `port 20'. If there is no type qualifier, host is assumed.
• dir qualifiers specify a particular transfer direction to and/or from id. Possible directions are src, dst, src or dst and src and dst. If there is no dir qualifier, src or dst is assumed. For some link layers, such as SLIP 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 fddi, tr, wlan, ip, ip6, arp, rarp, decnet, tcp and udp. If there is no proto qualifier, all protocols consistent with the type are assumed.

fddi is an alias for ether. The parser treats it 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 they cannot be named in a filter expression.

Like fddi, 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 by using the words and, or, and not to combine primitives.

Environment Variables

ksh$ LIBPATH=/opt/freeware/lib tcpdump -E"algo:secret"

Exit Status

Security

Reading packets from a network interface requires read access to /dev/bpf*, which is typically root-only. Reading packets from a file does not require any special privileges except file read permission.

Examples

1. To print all packets arriving at or departing from sundown, enter:

   tcpdump host sundown

2. To print traffic between helios and either hot or ace, enter:

   tcpdump host helios and \( hot or ace \)

3. To print all IP packets between ace and any host except helios, enter:

   tcpdump ip host ace and not helios

4. To print all traffic between local hosts and hosts at Berkeley, enter:

   tcpdump net ucb-ether

5. To print all ftp traffic through internet gateway snup, enter:

   tcpdump 'gateway snup and (port ftp or ftp-data)'

   Note: The expression is quoted to prevent the shell from mis-interpreting the parentheses.
6. To print traffic neither sourced from nor destined for local hosts (if you gateway to one other net, this should never make it onto your local net), enter:

   tcpdump ip and not net localnet

7. To print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a non-local host, enter:

   tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and d dst net localnet'

8. To print IP packets longer than 576 bytes sent through gateway snup, enter:

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

9. To print IP broadcast or multicast packets that were not sent via ethernet broadcast or multicast, enter:

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

10. To print all ICMP packets that are not echo requests/replies (for instance, not ping packets), enter:

    tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-e choreply'

Standard Error

All errors and warnings are sent to stderr.

Limitations

• A packet trace that crosses a Daylight Saving Time change gives skewed time stamps (the time change is ignored).
• Filter expressions on fields other than those in Token Ring headers handles the source-routed Token Ring packets incorrectly.
• Filter expressions on fields other than those in 802.11 headers handles the 802.11 data packets with both To DS and From DS set incorrectly.
• 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.

Packet tracing does not work in WPAR environment because the underlying BPF driver is not WPAR aware.

Files