As part of the agent configuration group parameters you can specify a Berkeley Packet Filter (BPF) expression. In the agent configuration it's passed as FilterExpression=<expression>

Packet filters are normally used to tell agents to only examine packets to and from specific hosts.

If no filter is applied, no packets will be rejected by the filtering mechanism. Bear in mind that packets may also be filtered out by other settings, such as the agent configuration group parameters List of Excluded local TCP ports and List of Excluded local UDP ports.

Packet Filter Syntax

The BPF expression specifies which packets should be analyzed by agents that belong to the agent configuration group that uses the packet filter.

Keyword	Description
Dir	dir qualifiers specify a particular transfer direction to and/or from ID. Possible directions are: src, dst src or dst src and dst Example: `src foo', `dst net 128.3', `src or dst port ftp-data'. If there is no dir qualifier, src or dst is assumed.
proto	proto qualifiers restrict the match to a particular protocol. Possible protos are: \| ether\| \| fddi\| \| tr\| \| ip\| \| ip6\| \| arp\| \| rarp\| \| decnet\| \| lat\| \| sca\| \| moprc\| \| mopdl\| \| iso\| \| esis\| \| isis\| \| icmp\| \| icmp6\| \| tcp \| \| udp\| \| \| Example: `ether src foo', `arp net 128.3', `tcp port 21'. If there is no proto qualifier, all protocols consistent with the type are assumed. For example, `src foo' means `(ip or arp or rarp) src foo' (except the latter isn't legal syntax), `net bar' means `(ip or arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'. `fddi' is actually an alias for `ether'; the parser treats them identically as meaning the data link level used on the specified network interface. FDDI headers contain Ethernet-like source and destination addresses, and often contain Ethernet-like packet types, so you can filter on these FDDI fields just as with the analogous Ethernet fields. FDDI headers also contain other fields, but you can't name them explicitly in a filter expression. Similarly, `tr' is an alias for `ether'.

Keyword

Description

Dir

dir qualifiers specify a particular transfer direction to and/or from ID. Possible directions are:

src, dst
src or dst
src and dst

Example:
`src foo', `dst net 128.3', `src or dst port ftp-data'.
If there is no dir qualifier, src or dst is assumed.

proto

proto qualifiers restrict the match to a particular protocol. Possible protos are:

| ether| | fddi| | tr| | ip| | ip6| | arp| | rarp| | decnet| | lat| | sca| | moprc| | mopdl| | iso| | esis| | isis| | icmp| | icmp6| | tcp | | udp| | |

Example:

`ether src foo', `arp net 128.3', `tcp port 21'.

If there is no proto qualifier, all protocols consistent with the type are assumed. For example, `src foo' means `(ip or arp or rarp) src foo' (except the latter isn't legal syntax), `net bar' means

`(ip or arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.

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

Similarly, `tr' is an alias for `ether'.

Primitives

There are also some special `primitive' keywords that don't follow the pattern:
*gateway, broadcast, less, greater and arithmetic expressions.
All of these are described in the following.
More complex filter expressions are built up by using the words and, or and not to combine primitives. Example:

 host foo and not port ftp and not port ftp-data

To save typing, identical qualifier lists can be omitted. 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

True if either the IPv4/v6 source or destination of the packet is host. Any of the host expressions can be prepended with the keywords, ip, arp, rarp, or ip6 as in:

 ip host host

which is equivalent to:

 ether proto \ip and host host

If host is a name with multiple IP addresses, each address will be checked for a match.
Abbreviations for:

 iso proto p

where p is one of the protocols. Note that the agent does an incomplete job of parsing these protocols.

Primitive	Description
ether dst ehost	True if the Ethernet destination address is ehost. Ehost may be either a name or in 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.
dst net net	True if the IPv4/v6 destination address of the packet has a network number of net. Net may be either a name from /etc/networks or a network number.
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.
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 is a number.
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 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, udp, or tcp. Note that this primitive doesn't chase 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 protocol header chain. May 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 IP broadcast packet. It checks for both the all-zeroes and all-ones broadcast conventions, and looks up the local subnet mask.
ether multicast	True if the packet is an Ethernet multicast packet. The ether keyword is optional. This is shorthand for `ether[manu: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, dec-net, sca, lat, mopdl, moprc, or iso. Note these identifiers are also keywords and must be escaped via backslash( ). [manu:In the case of FDDI (e.g., `fddi protocol arp'), the protocol identification comes from the 802.2 Logical Link Control (LLC) header, which is usually layered on top of the FDDI header. The agent assumes, when filtering on the protocol identifier, that all FDDI packets include an LLC header, and that the LLC header is in so-called SNAP format. The same applies to Token Ring.]
lat, moprc, mopdl	Abbreviations for: ether proto p where p is one of the protocols. Note that tcpdump doesn't currently know how to parse these protocols.
vlan[manu:vlan_id]	True if the packet is an IEEE 802.1Q VLAN packet. If [manu:vlan_id] is specified, only true is the packet has the specified vlan_id. Note that the first vlan keywor 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 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.
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 [manu:+, -, *, /, &, \|], 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, ip, arp, rarp, tcp, udp, icmp or p6, and indicates the protocol layer for the index operation. Note that tcp, udp and other upper layer protocol types only apply to IPv4, not IPv6. The byte offset, relative to the indicated pro udp index operations. For instance, tcp[manu:0] always means the first byte of the TCP header, and never means the first byte of an intervening fragment.

Combination of Primitives

Primitives may be combined using:

A parenthesized group of primitives and operators.
Negation (`!' or `not').
Concatenation (`&&' or `and').
Alternation (`||' or `or').

Negation has highest precedence. Alternation and concatenation have equal precedence and associate left to right. Note that explicit and tokens, not juxtaposition, are now required for concatenation. If an identifier is given without a keyword, the most recent keyword is assumed. For example, not host vs and howdy is short for not host vs and host howdy which should not be confused with not ( host vs or howdy ).

Examples:

To process IP packets to or from the network 192.168.1.0/24 when the agent itself is in another network

 net 192.168.1.0/24

To process IP packets to or from the network 192.168.1.0/24 when the agent is in the same network

src net 192.168.1.0/24 and dst net 192.168.1.0/24

To process all packets arriving at or departing from helios:

host helios

To process traffic between helios and either hotrod or howdy:

host helios and ( hotrod or howdy )

To process all IP packets between howdy and any host except helios:

ip host howdy and not helios

To process all traffic between local hosts and hosts at Berkeley: host.

tcp[manu:13] & 3 != 0 and not src and dst net localnet

To process IP packets longer than 576 bytes sent through gateway geezer:

 gateway geezer and ip[2:2] > 576

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