Berkeley Packet Filter (BPF) syntax

Qualifiers

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 hostnet , 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 _qualifiers specify a particular transfer direction to and/or from id.

    Possible directions are srcdstsrc 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 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: etherfdditrwlanipip6arprarpdecnettcp and udp.

    E.g., “ether src foo”, “arp net 128.3”, “tcp port 21”, “udp portrange 7000-7009”.

    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: gatewaybroadcastlessgreater and arithmetic expressions. All of these are described below.

More complex filter expressions are built up by using the words andor 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”.

Primitives

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, iparprarp, 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(5) 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(7) and udp(7)). 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 icmpicmp6igmpigrppimahespvrrpudp, or tcp. Note that the identifiers tcpudp, 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 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 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, either because the interface on which capture is being done has no netmask or because the capture is being done on the Linux “any” interface, which can capture on more than one interface, 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 ipip6arprarpatalkaarpdecnetscalatmopdlmoprcisostpipx, or netbeui. Note 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 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

_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:

isostp, 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 (applies only to packets logged by OpenBSD”s 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 OpenBSD”s 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: matchbad-offsetfragmentshortnormalize, and memory (applies only to packets logged by OpenBSD”s 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 (applies only to packets logged by OpenBSD”s pf(4)).ipip6arprarpatalkaarpdecnetisostpipx, _netbeui _Abbreviations for:

ether proto p

where _p_ is one of the above protocols.

latmoprcmopdl

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 _[vlanid]

True if the packet is an IEEE 802.1Q VLAN packet. If _[vlan_id]_ is specified, only true if the packet has the specified _vlanid. 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. Thevlan [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.

mpls _[labelnum]

True if the packet is an MPLS packet. If _[label_num]_ is specified, only true is the packet has the specified _labelnum. Note that the first mpls keyword encountered in _expression_ changes the decoding offsets for the remainder of _expression on the assumption that the packet is a MPLS-encapsulated IP packet. The mpls [label_num]_ expression may be used more than once, to filter on MPLS hierarchies. Each use of that expression increments the filter offsets by 4.For example:

mpls 100000 && mpls 1024

filters packets with an outer label of 100000 and an inner label of 1024, and

mpls && mpls 1024 && host 192.9.200.1

filters packets to or from 192.9.200.1 with an inner label of 1024 and any outer label.

pppoed

****True if the packet is a PPP-over-Ethernet Discovery packet (Ethernet type 0x8863).

pppoes

True if the packet is a PPP-over-Ethernet Session packet (Ethernet type 0x8864). Note that the first pppoes keyword encountered in _expression_ changes the decoding offsets for the remainder of _expression_ on the assumption that the packet is a PPPoE session packet.For example:

pppoes && ip

filters IPv4 protocols encapsulated in PPPoE.

tcpudpicmp

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 clnpesis, or isis.clnpesisisisAbbreviations for:

iso proto p

where _p_ is one of the above protocols.

l1l2iihlspsnpcsnppsnp

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. 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 andudp 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-echoreplyicmp-unreachicmp-sourcequenchicmp-redirecticmp-echoicmp-routeradverticmp-routersoliciticmp-timxceedicmp-paramprobicmp-tstampicmp-tstampreplyicmp-ireqicmp-ireqreplyicmp-maskreqicmp-maskreply.

The following TCP flags field values are available: tcp-fintcp-syntcp-rsttcp-pushtcp-acktcp-urg.

Combining Primitives

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 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. 

Examples

To capture all packets arriving at or departing from sundown:

host sundown

To capture traffic between helios and either hot or ace:

host helios and \( hot or ace \)

To capture all IP packets between ace and any host except helios:

ip host ace and not helios

To capture all traffic between local hosts and hosts at Berkeley:

net ucb-ether

To capture all ftp traffic through internet gateway snup: (note that the expression is quoted to prevent the shell from (mis-)interpreting the parentheses):

gateway snup and (port ftp or ftp-data)

To capture 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).

ip and not net localnet

To capture 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 capture 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 capture IP packets longer than 576 bytes sent through gateway snup:

gateway snup and ip[2:2] > 576

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

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

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

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

This was taken from the man page of tcpdump via this page.



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