PCAP-FILTER(7)

NAME
pcap-filter - packet filter syntax

DESCRIPTION

pcap_compile(3PCAP) 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(3PCAP), pcap_dispatch(3PCAP), pcap_next(3PCAP), or pcap_next_ex(3PCAP).

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, 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.

proto
    proto qualifiers restrict the match to a particular protocol. Possible protocols are: ether, fddi, tr, wlan, ip, ip6, arp, rarp, decnet, sctp, 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 `(ip6 or ip or arp or rarp) src foo', `net bar' means `(ip or arp or rarp) net bar' and `port 53' means `(tcp or udp or sctp) port 53' (note that these examples use invalid syntax to illustrate the principle).

[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 (or equivalently: `&&', `||' and `!' respectively) 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 hostnameaddr
    True if the IPv4/v6 destination field of the packet is hostnameaddr, which may be either an address or a name.

src host hostnameaddr
    True if the IPv4/v6 source field of the packet is hostnameaddr.

host hostnameaddr
    True if either the IPv4/v6 source or destination of the packet is hostnameaddr.

    Any of the above host expressions can be prepended with the keywords, ip, arp, rarp, or ip6 as in:
    ip host hostnameaddr
    which is equivalent to:
    ether proto \ip and host hostnameaddr
    If hostnameaddr is a name with multiple IPv4/v6 addresses, each address will be checked for a match.

ether dst ethernameaddr
    True if the Ethernet destination address is ethernameaddr. ethernameaddr 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 ethernameaddr
    True if the Ethernet source address is ethernameaddr.

ether host ethernameaddr
    True if either the Ethernet source or destination address is ethernameaddr.

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 ethernameaddr and not host hostnameaddr
    which can be used with either names or numbers for hostnameaddr / ethernameaddr.) This syntax does not work in IPv6-enabled configuration at this moment.

dst net netnameaddr
    True if the IPv4/v6 destination address of the packet has a network number of netnameaddr. 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 ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff, so IPv6 "network" matches are really always host matches, and a network match requires a netmask length.

src net netnameaddr
    True if the IPv4/v6 source address of the packet has a network number of netnameaddr.

net netnameaddr
    True if either the IPv4/v6 source or destination address of the packet has a network number of netnameaddr.

net netaddr mask netmask
    True if the IPv4 address matches netaddr with the specific netmask. May be qualified with src or dst. Note that this syntax is not valid for IPv6 netaddr.

net netaddr/len
    True if the IPv4/v6 address matches netaddr with a netmask len bits wide. May be qualified with src or dst.

dst port portnamenum
    True if the packet is IPv4/v6 TCP, UDP or SCTP and has a destination port value of portnamenum. The portnamenum 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 portnamenum
    True if the packet has a source port value of portnamenum.

port portnamenum
    True if either the source or destination port of the packet is portnamenum.

dst portrange portnamenum1-portnamenum2
    True if the packet is IPv4/v6 TCP, UDP or SCTP and has a destination port value between portnamenum1 and portnamenum2 (both inclusive). portnamenum1 and portnamenum2 are interpreted in the same fashion as the portnamenum parameter for port.

src portrange portnamenum1-portnamenum2
    True if the packet has a source port value between portnamenum1 and portnamenum2 (both inclusive).

portrange portnamenum1-portnamenum2
    True if either the source or destination port of the packet is between portnamenum1 and portnamenum2 (both inclusive).

    Any of the above port or port range expressions can be prepended with the keywords, tcp, udp or sctp, as in:
    tcp src port portnamenum
    which matches only TCP packets whose source port is portnamenum.

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 recognized by getprotobyname(3) (as in e.g. `getent(1) protocols'), typically from an entry in /etc/protocols, for example: ah, esp, eigrp (only in Linux, FreeBSD, NetBSD, DragonFly BSD, and macOS), icmp, igmp, igrp (only in OpenBSD), pim, sctp, tcp, udp or vrrp. Note that most of these example identifiers are also keywords and must be escaped via backslash (\). Note that this primitive does not chase the protocol header chain.

icmp
    Abbreviation for:
    ip proto 1

ip6 proto protocol
    True if the packet is an IPv6 packet of protocol type protocol. (See `ip proto' above for the meaning of protocol.) Note that the IPv6 variant of ICMP uses a different protocol number, named ipv6-icmp in AIX, FreeBSD, illumos, Linux, macOS, NetBSD, OpenBSD, Solaris and Windows. Note that this primitive does not chase the protocol header chain.

icmp6
    Abbreviation for:
    ip6 proto 58

proto protocol
    True if the packet is an IPv4 or IPv6 packet of protocol type protocol. (See `ip proto' above for the meaning of protocol.) Note that this primitive does not chase the protocol header chain.

ah, esp, pim, sctp, tcp, udp
    Abbreviations for:
    proto \protocol
    where protocol 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. (See `ip proto' above for the meaning of protocol.) 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 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. (See `ip proto' above for the meaning of protocol.)

protochain protocol
    True if the packet is an IPv4 or IPv6 packet of protocol type protocol. (See `ip proto' above for the meaning of 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 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 aarp, arp, atalk, decnet, ip, ip6, ipx, iso, lat, loopback, mopdl, moprc, netbeui, rarp, sca or stp. Note these identifiers (except loopback) are also keywords and must be escaped via backslash (\).

    [In the case of FDDI (e.g., `fddi proto \arp'), Token Ring (e.g., `tr proto \arp'), and IEEE 802.11 wireless LANs (e.g., `wlan proto \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, 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 and netbeui
        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, stp, and netbeui
        the filter checks for an 802.3 frame and then checks the LLC header as it does for FDDI, Token Ring, 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;

    aarp
        the filter checks for the AppleTalk ARP etype in either an Ethernet frame or an 802.2 SNAP frame with an OUI of 0x000000;

    ipx the filter 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.

ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp, ipx, netbeui
    Abbreviations for:
    ether proto \protocol
    where protocol is one of the above protocols.

lat, moprc, mopdl
    Abbreviations for:
    ether proto \protocol
    where protocol is one of the above protocols. Note that not all applications using pcap(3PCAP) currently know how to parse these protocols.

decnet src decnetaddr
    True if the DECnet source address is decnetaddr, 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 decnetaddr
    True if the DECnet destination address is decnetaddr.

decnet host decnetaddr
    True if either the DECnet source or destination address is decnetaddr.

llc True if the packet has an 802.2 LLC header. This includes:

    Ethernet packets with a length field rather than a type field that aren't raw NetWare-over-802.3 packets;

    IEEE 802.11 data packets;

    Token Ring packets (no check is done for LLC frames);

    FDDI packets (no check is done for LLC frames);

    LLC-encapsulated ATM packets, for SunATM on Solaris.

llc type
    True if the packet has an 802.2 LLC header and has the specified type. type can be one of:

    i   Information (I) PDUs

    s   Supervisory (S) PDUs

    u   Unnumbered (U) PDUs

    rr  Receiver Ready (RR) S PDUs

    rnr Receiver Not Ready (RNR) S PDUs

    rej Reject (REJ) S PDUs

    ui  Unnumbered Information (UI) U PDUs

    ua  Unnumbered Acknowledgment (UA) U PDUs

    disc
        Disconnect (DISC) U PDUs

    sabme
        Set Asynchronous Balanced Mode Extended (SABME) U PDUs

    test
        Test (TEST) U PDUs

    xid Exchange Identification (XID) U PDUs

    frmr
        Frame Reject (FRMR) U PDUs

inbound
    Packet was received by the host performing the capture rather than being sent by that host. This is only supported for certain link-layer types, such as SLIP and the ``cooked'' Linux capture mode used for the ``any'' device and for some other device types.

outbound
    Packet was sent by the host performing the capture rather than being received by that host. This is only supported for certain link-layer types, such as SLIP and the ``cooked'' Linux capture mode used for the ``any'' device and for some other device types.

ifindex interface_index
    True if the packet was logged via the specified interface (applies only to packets logged by the Linux "any" cooked v2 interface).

ifname interface
    True if the packet was logged as coming from the specified interface (applies only to packets logged by OpenBSD's or FreeBSD'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 or FreeBSD'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: match, bad-offset, fragment, short, normalize, and memory (applies only to packets logged by OpenBSD's or FreeBSD'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 OpenBSD's or FreeBSD's 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 OpenBSD's or FreeBSD's 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 OpenBSD's or FreeBSD's pf(4)).

wlan ra ehost
    True if the IEEE 802.11 RA is ehost. The RA field is used in all frames except for management frames.

wlan ta ehost
    True if the IEEE 802.11 TA is ehost. The TA field is used in all frames except for management frames and CTS (Clear To Send) and ACK (Acknowledgment) control frames.

wlan addr1 ehost
    True if the first IEEE 802.11 address is ehost.

wlan addr2 ehost
    True if the second IEEE 802.11 address, if present, is ehost. The second address field is used in all frames except for CTS (Clear To Send) and ACK (Acknowledgment) control frames.

wlan addr3 ehost
    True if the third IEEE 802.11 address, if present, is ehost. The third address field is used in management and data frames, but not in control frames.

wlan addr4 ehost
    True if the fourth IEEE 802.11 address, if present, is ehost. The fourth address field is only used for WDS (Wireless Distribution System) frames.

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 direction
    True if the IEEE 802.11 frame direction matches the specified direction. 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 the optional vlan_id is specified, only true if the packet has the specified vlan_id. Note that the first vlan keyword encountered in an expression changes the decoding offsets for the remainder of the expression on the assumption that the packet is a VLAN packet. The `vlan [vlan_id]` keyword may be used more than once, to filter on VLAN hierarchies. Each use of that keyword 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 protocol encapsulated in VLAN 300 encapsulated within any higher order VLAN.

mpls [label_num]
    True if the packet is an MPLS packet. If the optional label_num is specified, only true if the packet has the specified label_num. Note that the first mpls keyword encountered in an expression changes the decoding offsets for the remainder of the expression on the assumption that the packet is a MPLS-encapsulated IP packet. The `mpls [label_num]` keyword may be used more than once, to filter on MPLS hierarchies. Each use of that keyword 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 [session_id]
    True if the packet is a PPP-over-Ethernet Session packet (Ethernet type 0x8864). If the optional session_id is specified, only true if the packet has the specified session_id. Note that the first pppoes keyword encountered in an expression changes the decoding offsets for the remainder of the expression on the assumption that the packet is a PPPoE session packet.

    For example:
    pppoes 0x27 && ip
    filters IPv4 protocol encapsulated in PPPoE session id 0x27.

geneve [vni]
    True if the packet is a Geneve packet (UDP port 6081). If the optional vni is specified, only true if the packet has the specified vni. Note that when the geneve keyword is encountered in an expression, it changes the decoding offsets for the remainder of the expression on the assumption that the packet is a Geneve packet.

    For example:
    geneve 0xb && ip
    filters IPv4 protocol encapsulated in Geneve with VNI 0xb. This will match both IPv4 directly encapsulated in Geneve as well as IPv4 contained inside an Ethernet frame.

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.

clnp, esis, isis
    Abbreviations for:
    iso proto \protocol
    where protocol is one of the above protocols.

l1, l2, iih, lsp, snp, csnp, psnp
    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 an expression changes the tests done in the remainder of the 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.

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.

expr1 relop expr2
    True if the relation holds. Relop is one of {>, <, >=, <=, =, ==, !=} (where = means the same as ==). Each of expr1 and expr2 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 particular operating systems (for example: FreeBSD, Linux with 3.7 and later kernels, NetBSD); on all other systems (for example: AIX, illumos, Solaris, OpenBSD), 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.

    The length operator, indicated by the keyword len, gives the length of the packet.

    To access data inside the packet, use the following syntax:
    proto [ expr : size ]
    Proto is one of arp, atalk, carp, decnet, ether, fddi, icmp, icmp6, igmp, igrp, ip, ip6, lat, link, mopdl, moprc, pim, ppp, radio, rarp, sca, sctp, slip, tcp, tr, udp, vrrp or wlan, and indicates the protocol layer for the index operation. (ether, fddi, link, ppp, slip, tr and wlan 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. 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 (ICMPv6 type field), icmpcode (ICMP code field), icmp6code (ICMPv6 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 ICMPv6 type field values are available: icmp6-destinationunreach, icmp6-packettoobig, icmp6-timeexceeded, icmp6-parameterproblem, icmp6-echo, icmp6-echoreply, icmp6-multicastlistenerquery, icmp6-multicastlistenerreportv1, icmp6-multicastlistenerdone, icmp6-routersolicit, icmp6-routeradvert, icmp6-neighborsolicit, icmp6-neighboradvert, icmp6-redirect, icmp6-routerrenum, icmp6-nodeinformationquery, icmp6-nodeinformationresponse, icmp6-ineighbordiscoverysolicit, icmp6-ineighbordiscoveryadvert, icmp6-multicastlistenerreportv2, icmp6-homeagentdiscoveryrequest, icmp6-homeagentdiscoveryreply, icmp6-mobileprefixsolicit, icmp6-mobileprefixadvert, icmp6-certpathsolicit, icmp6-certpathadvert, icmp6-multicastrouteradvert, icmp6-multicastroutersolicit, icmp6-multicastrouterterm.

    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').

Negation has the highest precedence. Alternation and concatenation have equal precedence and associate left to right.

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)

EXAMPLES

To select all packets arriving at or departing from `sundown':
    host sundown

To select traffic between `helios' and either `hot' or `ace':
    host helios and (hot or ace)

To select all IPv4 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:
    net ucb-ether

To select all FTP traffic through Internet gateway `snup':
    gateway snup and (port ftp or ftp-data)

To select IPv4 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 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 the TCP packets with flags RST and ACK both set. (i.e. select only the RST and ACK flags in the flags field, and if the result is "RST and ACK both set", match)
    tcp[tcpflags] & (tcp-rst|tcp-ack) == (tcp-rst|tcp-ack)

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 IPv4 packets longer than 576 bytes sent through gateway `snup':
    gateway snup and ip[2:2] > 576

To select IPv4 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
    icmp6[icmp6type] != icmp6-echo and icmp6[icmp6type] != icmp6-echoreply

BACKWARD COMPATIBILITY
The ICMPv6 type code names, as well as the tcp-ece and tcp-cwr TCP flag names became available in libpcap 1.9.0.

The geneve keyword became available in libpcap 1.8.0.

The ifindex keyword became available in libpcap 1.10.0.

SEE ALSO
pcap(3PCAP)

BUGS
To report a security issue please send an e-mail to security@tcpdump.org.

To report bugs and other problems, contribute patches, request a feature, provide generic feedback etc please see the file CONTRIBUTING.md in the libpcap source tree root.

Filter expressions on fields other than those in Token Ring headers will not correctly handle source-routed Token Ring packets.

Filter expressions on fields other than those in 802.11 headers will not correctly handle 802.11 data packets with both To DS and From DS set.

`ip6 proto' should chase header chain, but at this moment it does not. `ip6 protochain' is supplied for this behavior. For example, to match IPv6 fragments: `ip6 protochain 44'

Arithmetic expression against transport layer headers, like tcp[0], does not work against IPv6 packets. It only looks at IPv4 packets.