PCAP Filter

Below document is a simplifed extract of the pcap-filter.7 man page. It is included here for the ease of use.

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The filter 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 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 dir qualifiers specify a particular transfer direction to and/or

from id. Possible directions are src, dst, src or dst, 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. The ra, ta, addr1, addr2, addr3, and addr4 qualifiers are only valid for IEEE 802.11 Wire- less LAN link layers.

proto proto qualifiers restrict the match to a particular protocol.

Possible protos are: ether, fddi, tr, wlan, ip, ip6, arp, rarp, decnet, tcp and udp. E.g., ether src foo, arp net 128.3, tcp port 21, udp portrange 7000-7009, wlan addr2 0:2:3:4:5:6. 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.

In addition to the above, there are some special primitive keywords that don't follow the pattern: broadcast, less, greater and arithmetic expressions. All of these are described below.

More complex filter expressions are built up by using the words and, or and not to combine primitives. E.g., host foo and not port ftp and not port ftp-data. To save typing, identical qualifier lists can be omitted. E.g., tcp dst port ftp or ftp-data or domain is exactly the same as tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain.

Allowable primitives are:

dst host host

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 10.2.3.4

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 numerical MAC address of the form xx:xx:xx:xx:xx:xx, xx.xx.xx.xx.xx.xx, xx-xx-xx-xx-xx- xx, xxxx.xxxx.xxxx, xxxxxxxxxxxx, or various mixes of :, ., and -, where each x is a hex digit (0-9, a-f, or A-F).

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.

dst net net

True if the IPv4/v6 destination address of the packet has a net- work 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 netmask

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 (). 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.

proto protocol

True if the packet is an IPv4 or IPv6 packet of protocol type protocol. Note that this primitive does not chase the protocol header chain.

tcp, udp, icmp

Abbreviations for proto p where p is one of the above protocols.

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, authentica- tion 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 the BPF optimizer code, and is not supported by filter engines in the kernel, so this can be somewhat slow, and may cause more packets to be dropped.

ip protochain protocol

Equivalent to ip6 protochain protocol, but this is for IPv4.

protochain protocol

True if the packet is an IPv4 or IPv6 packet of protocol type protocol. Note that this primitive chases the protocol header chain.

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 cap- ture is being done on the Linux "any" interface, which can cap- ture on more than one interface, this check will not work cor- rectly.

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, dec- net, sca, lat, mopdl, moprc, iso, stp, ipx, or netbeui. Note these identifiers are also keywords and must be escaped via backslash ().

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

Abbreviations for ether proto p where p is one of the above 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: filters on VLAN 200 encapsulated within VLAN 100 :: vlan 100 && vlan 200

filters IPv4 protocols encapsulated in VLAN 300 encapsulated within any higher order VLAN :: vlan && vlan 300 && ip

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.

The % and ^ operators are currently only supported for filtering in the kernel on Linux with 3.7 and later kernels; on all other systems, if those operators are used, filtering will be done in user mode, which will increase the overhead of capturing packets and may cause more packets to be dropped.

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, rela- tive 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 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), icmp6type (ICMP v6 type field) icmpcode (ICMP code field), icmp6code (ICMP v6 code field), and tcpflags (TCP flags field).

The following ICMP type field values are available: icmp-echore- ply, icmp-unreach, icmp-sourcequench, icmp-redirect, icmp-echo, icmp-routeradvert, icmp-routersolicit, icmp-timxceed, icmp- paramprob, icmp-tstamp, icmp-tstampreply, icmp-ireq, icmp-ire- qreply, icmp-maskreq, icmp-maskreply.

The following ICMPv6 type fields are available: icmp6-echo, icmp6-echoreply, icmp6-multicastlistenerquery, icmp6-multicas- tlistenerreportv1, icmp6-multicastlistenerdone, icmp6-routerso- licit, icmp6-routeradvert, icmp6-neighborsolicit, icmp6-neighbo- radvert, icmp6-redirect, icmp6-routerrenum, icmp6-nodeinforma- tionquery, icmp6-nodeinformationresponse, icmp6-ineighbordiscov- erysolicit, icmp6-ineighbordiscoveryadvert, icmp6-multicastlis- tenerreportv2, icmp6-homeagentdiscoveryrequest, icmp6-homeagent- discoveryreply, icmp6-mobileprefixsolicit, icmp6-mobileprefixad- vert, icmp6-certpathsolicit, icmp6-certpathadvert, icmp6-multi- castrouteradvert, icmp6-multicastroutersolicit, icmp6-multicas- trouterterm.

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

Primitives may be combined using:

A parenthesized group of primitives and operators.

Negation (! or not).

Concatenation (&& or and).

Alternation (|| or or).

Note

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.

Note

If an identifier is given without a keyword, the most recent keyword is assumed. For example not host 10.2.3.4 and 10.4.5.6 is short for not host 10.2.3.4 and host 10.4.5.6 which should not be confused with not ( host 10.2.3.4 or 10.4.5.6 )

Examples

To select traffic between 10.2.3.4 and either 10.4.5.6 or 10.7.8.9:

host 1.2.3.4 and \( 10.4.5.6 or 10.4.5.6 \)

To select all IP packets between 10.4.5.6 and any host except 1.2.3.4:

ip host 10.4.5.6 and not 1.2.3.4

To select the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a non-local host.

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

To select 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.)

tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)

To select IP broadcast or multicast packets that were not sent via Ethernet broadcast or multicast:

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

To select all ICMP packets that are not echo requests/replies (i.e., not ping packets):

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

Note

ip6 proto should chase header chain, but at this moment it does not.

ip6 protochain is supplied for this behavior.

Note

Arithmetic expression against transport layer headers, like tcp[0],

does not work against IPv6 packets. It only looks at IPv4 packets.