pcap-filter - packet filter syntax
pcap_compile() is used to compile a string into a filter
program. The resulting filter program can then be applied to some stream of
packets to determine which packets will be supplied to pcap_loop(),
pcap_dispatch(), pcap_next(), or pcap_next_ex().
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
- 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, src and dst, ra, ta, addr1,
addr2, addr3, and addr4. 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 Wireless LAN link layers. For some link layers, such as SLIP
and the ``cooked'' Linux capture mode used for the ``any'' device 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: ether, fddi, tr, wlan, ip,
ip6, arp, rarp, decnet, tcp and
udp. E.g., `ether src foo', `arp net 128.3', `tcp port 21' `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'.
[`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.
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: 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 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 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 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 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. 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(4) and udp(4)). 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.
- 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 (\), 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.
- 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 this check will 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 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, decnet, sca, lat or
stp. Note these identifiers are also keywords and must be escaped
via backslash (\).
- [In the case of FDDI (e.g., `fddi 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 or
802.11 header.
- When filtering for most protocol identifiers on FDDI or 802.11, the filter
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:
- iso
- the filter checks the DSAP (Destination Service Access Point) and SSAP
(Source Service Access Point) fields of the LLC header;
- stp
- the filter checks the DSAP of the LLC header;
- atalk
- the filter checks for a SNAP-format packet with an OUI of 0x080007 and the
AppleTalk etype.
- In the case of Ethernet, the filter checks the Ethernet type field for
most of those protocols. The exceptions are:
- iso and
stp
- the filter checks for an 802.3 frame and then checks the LLC header as it
does for FDDI and 802.11;
- atalk
- the filter 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;
- 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
(applies only to packets logged by pf(4)).
- 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 pf(4)).
- 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
pf(4)).
- 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(4)).
- 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(4)).
- 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 and, with later versions of
pf(4)), nat, rdr, binat and scrub
(applies only to packets logged by pf(4)).
- ip, ip6,
arp, rarp, atalk, decnet, iso,
stp
- 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 not all applications
using pcap(3) 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.
- dir
dir
- True if the IEEE 802.11 frame direction matches the specified dir.
Valid directions are: nods, tods, fromds,
dstods, or a numeric value.
- 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:
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.
- tcp, udp,
icmp
- Abbreviations for:
ip proto p or ip6 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. Note that all comparisons are unsigned,
so that, for example, 0x80000000 and 0xffffffff are > 0. 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, 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 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, icmp-maskreply.
The following TCP flags field values are available:
tcp-fin, tcp-syn, tcp-rst, tcp-push,
tcp-ack, tcp-urg.
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 )
To select all packets arriving at or departing from
sundown:
To select traffic between helios and either hot or
ace:
host helios and \( hot or ace \)
To select all IP packets between ace and any host except
helios:
ip host ace and not helios
To select all traffic between local hosts and hosts at
Berkeley:
To select all ftp traffic through internet gateway
snup:
gateway snup and (port ftp or ftp-data)
To select 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).
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 packets longer than 576 bytes sent through gateway
snup:
gateway snup and ip[2:2] > 576
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
The original authors are:
Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence
Berkeley National Laboratory, University of California, Berkeley, CA.