selects which packets will be dumped. If no expression
is given, all packets on the net will be dumped. Otherwise,
only packets for which expression is `true' will be dumped.
The expression consists of one or more
primitives.
Primitives usually consist of an
id
(name or number) preceded by one or more qualifiers. There are three
different kinds of qualifier:
- type
-
qualifiers say what kind of thing the id name or number refers to.
Possible types are
host,
net
and
port.
E.g., `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.
E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If
there is no dir qualifier,
src or dst
is assumed.
For `null' link layers (i.e. point to point protocols such as slip) the
inbound
and
outbound
qualifiers can be used to specify a desired direction.
- proto
-
qualifiers restrict the match to a particular protocol. Possible
protos are:
ether,
fddi,
ip,
arp,
rarp,
decnet,
lat,
sca,
moprc,
mopdl,
tcp
and
udp.
E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is
no proto qualifier, all protocols consistent with the type are
assumed. E.g., `src foo' means `(ip or arp or rarp) src foo'
(except the latter is not legal syntax), `net bar' means `(ip or
arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'.
[`fddi' is actually an alias for `ether'; the parser treats them
identically as meaning ``the data link level used on the specified
network interface.'' FDDI headers contain Ethernet-like source
and destination addresses, and often contain Ethernet-like packet
types, so you can filter on these FDDI fields just as with the
analogous Ethernet fields. FDDI headers also contain other fields,
but you cannot name them explicitly in a filter expression.]
In addition to the above, there are some special `primitive' keywords
that don't follow the pattern:
gateway,
broadcast,
less,
greater
and arithmetic expressions. All of these are described below.
More complex filter expressions are built up by using the words
and,
or
and
not
to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'.
To save typing, identical qualifier lists can be omitted. E.g.,
`tcp dst port ftp or ftp-data or domain' is exactly the same as
`tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
Allowable primitives are:
- dst host host
-
True if the IP destination field of the packet is host,
which may be either an address or a name.
- src host host
-
True if the IP source field of the packet is host.
- host host
-
True if either the IP source or destination of the packet is host.
Any of the above host expressions can be prepended with the keywords,
ip, arp, or rarp as in:
ip host host
which is equivalent to:
ether proto \ip and host host
If host is a name with multiple IP addresses, each address will
be checked for a match.
- ether dst ehost
-
True if the ethernet destination address is ehost. Ehost
may be either a name from /etc/ethers or a number (see
ethers(3N)
for numeric format).
- ether src ehost
-
True if the ethernet source address is ehost.
- ether host ehost
-
True if either the ethernet source or destination address is ehost.
- gateway host
-
True if the packet used host as a gateway. I.e., the ethernet
source or destination address was host but neither the IP source
nor the IP destination was host. Host must be a name and
must be found in both /etc/hosts and /etc/ethers. (An equivalent
expression is
ether host ehost and not host host
which can be used with either names or numbers for host / ehost.)
- dst net net
-
True if the IP destination address of the packet has a network
number of net. Net may be either a name from /etc/networks
or a network number (see networks(4) for details).
- src net net
-
True if the IP source address of the packet has a network
number of net.
- net net
-
True if either the IP source or destination address of the packet has a network
number of net.
- net net mask mask
-
True if the IP address matches net with the specific netmask.
May be qualified with src or dst.
- net net/len
-
True if the IP address matches net a netmask len bits wide.
May be qualified with src or dst.
- dst port port
-
True if the packet is ip/tcp or ip/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.
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 (see
ip(4P))
of protocol type protocol.
Protocol can be a number or one of the names
icmp, igrp, udp, nd, 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.
- 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[0] & 1 != 0'.
- ip multicast
-
True if the packet is an IP multicast packet.
- ether proto protocol
-
True if the packet is of ether type protocol.
Protocol can be a number or a name like
ip, arp, or rarp.
Note these identifiers are also keywords
and must be escaped via backslash (\).
[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.
Tcpdump 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.]
- 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.
- ip, arp, rarp, decnet
-
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
Snort does not currently know how to parse these protocols.
- tcp, udp, icmp
-
Abbreviations for:
ip proto p
where p is one of the above protocols.
- 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,
ip, arp, rarp, tcp, udp, or icmp, and
indicates the protocol layer for the index operation.
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.
Primitives may be combined using:
-
A parenthesized group of primitives and operators
(parentheses are special to the Shell and must be escaped).
-
Negation (`!' or `not').
-
Concatenation (`&&' or `and').
-
Alternation (`||' or `or').
Negation has highest precedence.
Alternation and concatenation have equal precedence and associate
left to right. Note that explicit and tokens, not juxtaposition,
are now required for concatenation.
If an identifier is given without a keyword, the most recent keyword
is assumed.
For example,
not host vs and ace
is short for
not host vs and host ace
which should not be confused with
not ( host vs or ace )
Expression arguments can be passed to Snort 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.