PCAP-FILTER(7)

💖 pcap-filter(7) 💖

Miscellaneous Information Manual · the packet filter syntax, explained by your favorite unix girlie

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✨ NAME ✨

pcap-filter — packet filter syntax

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🎀 DESCRIPTION 🎀

okay bestie so here's the tea: pcap_compile(3PCAP) is the function that takes a string and compiles it into a filter program. that resulting filter program then gets applied to some stream of packets, and it decides which packets get handed over to pcap_loop(3PCAP), pcap_dispatch(3PCAP), pcap_next(3PCAP), or pcap_next_ex(3PCAP). it's giving bouncer at the club, but for packets. 💅

The filter expression is made of one or more primitives. A primitive is usually an id (a name, a number, or something a lil more complex, like a CIDR prefix) with one or more qualifiers in front of it. There are three whole kinds of qualifier, and yes we're going through all of them:

• proto — proto qualifiers restrict the match to a particular protocol. (⚠️ do NOT confuse this with the proto type qualifier down below, they are different girlies.) Possible protocols are: ether, link, wlan, ip, ip6, arp, tcp, udp, sctp, iso, isis, rarp, decnet, fddi, tr, ppp and slip. E.g. ether src foo, arp net 128.3, tcp port 21, ip proto ospf, ether proto 0x88CC, udp portrange 7000-7009, wlan addr2 0:2:3:4:5:6. If there's 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, proto tcp means (ip6 or ip) proto tcp, net bar means (ip6 or ip or arp or rarp) net bar, and port 53 means (tcp or udp or sctp) port 53 — and ngl, those examples use invalid syntax on purpose, just to illustrate the principle, don't come for me.
• dir — dir qualifiers specify a particular transfer direction to and/or from the 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's 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. no cap.
• type — type qualifiers say what kind of thing the id name or number actually refers to. Possible types are host, net, proto, port, portrange, protochain and gateway. E.g. host foo, net 128.3, port 20, portrange 6000-6008, proto 17. If there's no type qualifier, host is assumed.

So in primitives that follow this pattern, each qualifier kind can show up at most once, and if more than one kind is present, any proto qualifier has to be the first qualifier and any type qualifier has to be the last qualifier — for example, tcp dst port 80. iconic. Also, not all combinations of these qualifier kinds are valid syntax. Some just make zero sense in network protocol space, for example: ether port (the Ethernet header has no ports, babes), tcp net (the TCP header does not have layer 3 fields), dst proto (in a protocol header the same protocol applies to both the source and the destination), and so on. And some other combos aren't valid syntax because they're simply not implemented, even though hypothetically they could make sense, for example: iso net, aarp host, and so on.

💭 [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 statements about FDDI headers also apply to Token Ring and 802.11 wireless LAN headers. The same stands for the link, ppp and slip keywords.]

For IEEE 802.11 headers, the destination address is the DA field and the source address is the SA field. For both ARP and RARP headers, the destination address is the TPA (Target Protocol Address) field and the source address is the SPA (Sender Protocol Address) field. iykyk. 💫

On top of all that, there are also some special primitive keywords that don't follow the pattern at all (for example: broadcast, multicast, inbound, outbound, ifindex, llc, vlan, mpls, less, greater), plus packet data accessors and relations of two arithmetic expressions. All of these are described below, keep reading bestie. 🫶

More complex filter expressions get built up 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. And to save you some 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. we love efficiency. ✨

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🌈 PRIMITIVES 🌈

🦋 host hostnameaddr

True if the source or the destination ARP/IPv4/IPv6/RARP address of the packet is hostnameaddr. May be qualified with a specific protocol (arp, ip, ip6, rarp) and/or a different direction (src, dst, src and dst), and in the latter case the host keyword is optional. For example,

ip src hostnameaddr

for Ethernet-like link-layer types is equivalent to

ether proto \ip and ip src host hostnameaddr

hostnameaddr may be either an IPv4 host number or an IPv6 host number or a name.

An IPv4 host number can be written as a dotted quad (e.g., 192.168.10.20), dotted triple (e.g., 192.168.10 means the same as 192.168.10.0/24, so it's really a network match), dotted pair (e.g., 192.168 means 192.168.0.0/16, so it's really a network match as well), or a single number (e.g., [decimal] 168496141, [octal] 01202606015 and [hexadecimal] 0x0a0b0c0d all mean 10.11.12.13).

An IPv6 host number can be written as a complete 128-bit address (x:x:x:x:x:x:x:x) or using the zero compression (x::x) or the IPv4-mapped (x:x:x:x:x:x:d.d.d.d) notations as discussed in inet_pton(3). In this primitive this version of libpcap tolerates an optional /128 after an IPv6 host number — heads up, this behaviour is an implementation quirk and it may be removed in future, so don't get too attached. 💀

A host name must resolve using getaddrinfo(3), which typically means DNS or hosts(5), to at least one IPv4 address for arp host, ip host and rarp host, at least one IPv6 address for ip6 host, and at least one IPv4/IPv6 address for host. If the name resolves to more than one address, this primitive evaluates to true as soon as one of the addresses matches the packet. first match wins, periodt.

When not qualified with a protocol, this primitive implies all protocols that make sense for the address families of the specified/resolved addresses. When qualified with a protocol, this primitive ignores the address family that does not make sense for the protocol. For example, for a name that resolves only to an IPv6 address arp host returns an error, but host is valid and means IPv6; for a name that resolves only to an IPv4 address ip6 host returns an error, but host is valid and means ARP/IPv4/RARP; for a name that resolves to both IPv4 and IPv6 addresses ip host ignores the IPv6 addresses and ip6 host ignores the IPv4 addresses, and so on.

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🌸 ether host ethernameaddr

True if the source or the destination Ethernet/802.11/IPFC/ATM LANE/FDDI/Token Ring address of the packet is ethernameaddr. May be qualified with a different direction (src, dst, src and dst), in which case the host keyword is optional.

ethernameaddr may be either a name from ethers(5) or a numerical MAC-48 address. In the latter case a valid format is one of "xx:xx:xx:xx:xx:xx", "xx.xx.xx.xx.xx.xx" or "xx-xx-xx-xx-xx-xx", where every "xx" is a hexadecimal digit (0-9, a-f, or A-F) or a two-digit hexadecimal number — for example, "aa:00:04:00:1d:04" means the same as "AA-0-4-0-1D-4". Another valid format is one of "xxxx.xxxx.xxxx" or "xxxxxxxxxxxx", where each "x" is a hexadecimal digit — for example, "aa0004001d04". so many formats, pick your fighter. 💅

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👑 gateway host

True if the packet used host as a gateway. I.e., the source or the destination Ethernet-like address is host but neither the source nor the destination ARP/IPv4/RARP address is host. This primitive is valid only for the same link-layer types as the ether host primitive above, and also it cannot be applied to packets encapsulated in MPLS, VXLAN or Geneve. May be qualified with a specific protocol (arp, ip, rarp). For example,

ip gateway host

is equivalent to

ether host ethernameaddr and ip and not ip host hostnameaddr

which can be used with either names or numbers for hostnameaddr and ethernameaddr.

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.). both or nothing, bestie. 😤

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🍓 net netnameaddr

True if the source or the destination ARP/IPv4/IPv6/RARP address of the packet belongs to the specified network. May be qualified with a specific protocol (arp, ip, ip6, rarp) and/or a different direction (src, dst, src and dst), and in the latter case the net keyword remains mandatory (no skipping it like you can with host). netnameaddr may be either an IPv4 network number or an IPv6 network number or an IPv4 network name.

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 (/32) for a dotted quad (which means that it's really a host match), 255.255.255.0 (/24) for a dotted triple, 255.255.0.0 (/16) for a dotted pair, or 255.0.0.0 (/8) for a single number.

An IPv6 network number is an IPv6 address as discussed for the host primitive above; the implicit netmask is ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff (/128), so in this primitive IPv6 "network" matches are really always host matches. For an actual IPv6 network match, see the net netaddr/len primitive below. 🙈

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… and that's where part 1 ends, bestie — more coming, don't go anywhere 💖

okay so an IPv4 network name is a name that comes from the networks database — traditionally networks(5), but on OpenBSD it's hosts(5), and it could also be NIS, LDAP, whatever. 🙈

And here's the tea bestie: some operating systems literally do not implement a networks database at all, and in the ones that do, the implemented behaviour varies. 😭 POSIX doesn't specify a format for networks(5), so that line somenet 192.168 in there can translate to either 192.168.0.0/32 or 192.168.0.0/16 depending on the implementation. It's giving chaos, fr fr.

So in this sense, the only reliable address notation to be used with an IPv4 network name would be a dotted quad (/32) — and honestly, in that case it would be simpler to just use a host match with a hostname, as well as DNS. 💅 For these reasons, IPv4 network names should not be used in portable applications and this form of ID may be removed from this primitive in future. 👑

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🕸️ net netaddr mask netmask

True if the source or the destination ARP/IPv4/RARP address of the packet belongs to netaddr with the specified netmask. May be qualified with a specific protocol (arp, ip, rarp) and/or a different direction (src, dst, src and dst) — and in that latter case, the net keyword remains mandatory. In this implementation, this primitive does not support IPv6 networks. (no IPv6 here bestie, sorry 🫶)

Both netaddr and netmask use the IPv4 network number notation described above, except the "single number" form is not valid in this primitive. For example,

net 192.168 mask 255.255

is equivalent to

net 192.168.0.0 mask 255.255.0.0

The netmask can represent any 32-bit value, which is why the net netaddr/len primitive below is usually a better fit for use cases that require the value to be strictly one of the 33 CIDR masks (from /0 to /32). ✨

🌐 net netaddr/len

True if the source or the destination ARP/IPv4/IPv6/RARP address of the packet belongs to netaddr where the bit-length of the network mask equals len — in other words, the address belongs to the specified CIDR prefix. May be qualified with a specific protocol (arp, ip, ip6, rarp) and/or a different direction (src, dst, src and dst); in that latter case the net keyword is optional.

For IPv4, len is an integer between 0 and 32 (both inclusive) and netaddr is the same as the above. For IPv6, len is an integer between 0 and 128 (both inclusive) and netaddr is an IPv6 address as discussed for the host primitive above. For both IPv4 and IPv6 the maximum value of len is equivalent to a host match, and the 0 value (which implies an all-zeroes value of netaddr) matches any address. In that latter case this primitive reduces to matching the specified (or implied) protocols only. 🦋

🔌 port portnamenum

True if the source or the destination TCP/UDP/SCTP port of an IPv4/IPv6 packet is portnamenum. For IPv4 this also implies that the packet is the first fragment or is not fragmented; for IPv6 this also implies that the packet does not use any extension (e.g. Routing, Fragment, Destination Options) headers. May be qualified with a specific layer 4 protocol (tcp, udp, sctp) or a different direction (src, dst, src and dst) — in that latter case the port keyword remains mandatory. Cannot be qualified with a specific layer 3 protocol (IPv4/IPv6) in the same primitive, but can be trivially combined with other primitives to achieve the required effect, for example:

ip and tcp dst port 80

The portnamenum can be a number or a name used in /etc/services (see services(5)). 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). 🌸

📏 portrange portnamenum1-portnamenum2

This is a more generic form of the above: true if the port number in the packet is between portnamenum1 and portnamenum2 (both inclusive), everything else holds the same meaning. In this predicate portnamenum1 and portnamenum2 can be specified in either order. If the two values are equal, this primitive has the same effect as the port primitive above. 💖

🐜 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 of protocol type protocol. For this primitive it does not matter whether the IPv4 packet is fragmented or not. protocol can be a number or one of the names recognized by getprotobyname(3), for example: ah, esp, eigrp (only in Linux with glibc, FreeBSD, NetBSD, DragonFly BSD, MidnightBSD, macOS, and QNX), icmp, igmp, igrp (only in Haiku and 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.

Typically getprotobyname(3) parses the /etc/protocols file to translate protocol names to numbers, and the getent protocols command lists the protocols recognized by the function. This is not entirely so in AIX (which does not have the command), Haiku (which has the file at /system/data/network/protocols and does not have the command), on Linux with musl libc (which hard-codes the list of protocols) and on hosts that use a network database to resolve protocol names to numbers (see nsswitch.conf(5)). If a protocol name fails to translate to a number, this version of libpcap will treat the filter expression as invalid. 💀

🚗 carp, vrrp

Abbreviations for:

ip proto 112

🏓 icmp

Abbreviation for:

ip proto 1

📣 igmp

Abbreviation for:

ip proto 2

🦖 igrp

Abbreviation for:

ip proto igrpval

where igrpval is 88 on DragonFly BSD, FreeBSD, macOS, MidnightBSD, and QNX, and 9 on all other OSes. This abbreviation should not be used in portable applications and may be removed in future. ⚠️

🌈 ip6 proto protocol

True if the packet is an IPv6 packet of protocol type protocol. Besides IPv6 packets that have protocol in the Next Header field of the basic header, this also matches IPv6 packets that have 44 in the Next Header field of the basic header and protocol in the Next Header field of the immediately following Fragment extension header. (See ip proto above for the meaning of protocol.) Note that the IPv6 variant of ICMP uses a different protocol number, named ipv6-icmp. 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 destination Ethernet/802.11/IPFC/ARCnet/ATM LANE/FDDI/Token Ring address of the packet is the broadcast address (e.g. FF:FF:FF:FF:FF:FF for Ethernet). The ether keyword is optional.

📢 ip broadcast

True if the packet is an IPv4 packet with the host part of the destination address being either all-ones or all-zeroes. This primitive requires you to specify the netmask, which cannot be done in the filter expression — the only way to specify a netmask is via the netmask argument of the pcap_compile() function.

🚨 If a netmask has not been specified, an attempt to compile a filter expression with this primitive will return an error. no netmask, no vibes.

Note that this primitive ignores the network part of the destination address, thus it can match more packets than expected, especially if the interface has multiple IPv4 addresses with different netmasks. For example, if the interface has addresses 10.1.2.100/29 and 192.168.202.200/24 configured and the netmask argument corresponds to the first address, its value will be 0xFFFFFFF8 and the host mask value will be 0x00000007. This will match the expected two addresses in the first prefix (10.1.2.96 and 10.1.2.103), as well as 64 addresses in the second prefix (192.168.202.0, 192.168.202.7, 192.168.202.8, 192.168.202.15, 192.168.202.16 and so on), as well as any other IPv4 address with the lowest 3 bits being all-ones or all-zeroes (for example: 10.73.74.151, 192.168.50.63, 172.19.0.128) — in other words, 25% of the complete IPv4 address space. 😭

That is why in use cases that require more precision it would be better to match the required address(es) explicitly, for example:

ip dst host 10.1.2.96 or 10.1.2.103

• ether multicast — true if the destination Ethernet/802.11/IPFC/ARCnet/ATM LANE/FDDI/Token Ring address of the packet is a multicast address (like, ether[0] & 1 != 0 for Ethernet, bestie 💫). The ether keyword is optional, so it's giving low-maintenance.
• 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 these iconic names:
  • aarp — AppleTalk ARP Protocol
  • arp — ARP
  • atalk — AppleTalk
  • decnet — DECNet
  • ip — IPv4
  • ip6 — IPv6
  • ipx — Novell IPX
  • iso — OSI protocols
  • lat — DEC Local Area Transport (LAT)
  • lldp — Link-Layer Discovery Protocol
  • loopback — Loopback frames (DIX Ethernet Configuration Test Protocol)
  • mopdl — DEC DNA Dump/Load (MOP),
  • moprc — DEC DNA Remote Console (MOP)
  • netbeui — NetBIOS Frame Protocol
  • rarp — Reverse ARP
  • sca — DEC System Communication Architecture (SCA)
  • slow — IEEE Slow Protocols, such as LACP, OAM and OSSP
  • stp — IEEE Spanning Tree Protocol

Heads up bestie: these identifiers (except lldp, loopback and slow) are also keywords, so you gotta escape them with a backslash (\). No cap. 🚨

[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 EtherType.

In the case of Ethernet, the filter checks the EtherType 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 EtherType 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 EtherType in either an Ethernet frame or an 802.2 SNAP frame with an OUI of 0x000000;
• ipx — the filter checks for the IPX EtherType in an Ethernet frame, the IPX DSAP in the LLC header, the 802.3-with-no-LLC-header encapsulation of IPX, and the IPX EtherType in a SNAP frame.]

• ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp, ipx, netbeui — little abbreviations for:

ether proto \protocol

where protocol is one of the above protocols. ✨

• lat, mopdl, moprc, sca — also abbreviations for:

ether proto \protocol

where protocol is one of the above protocols, all of which originated at DEC, but are not the same as DECnet. (DEC walked so everyone else could run, honestly 💅)

• decnet host decnetaddr — true if the source or the destination DECnet address of the packet is decnetaddr. May be qualified with a different direction (src, dst, src and dst), in which case the host keyword is optional, it's very chill like that.

decnetaddr is an address of the form AREANUMBER.NODENUMBER, where the area number can be between 0 and 63 (both inclusive) and the node number can be between 0 and 1023 (both inclusive) and both numbers always use decimal base. For example:

decnet src 10.123

• 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
  • dm — Disconnected Mode (DM) 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 — true if the packet was received by the host performing the capture rather than sent by that host. This primitive is supported for certain link-layer types only, namely, DLT_SLIP, DLT_IPNET, DLT_LINUX_SLL, DLT_LINUX_SLL2, DLT_PFLOG, DLT_PPP_PPPD, a number of Juniper Networks private types and some types of Linux DSA tag. For reference, the ``any'' pseudo-interface uses DLT_LINUX_SLL or DLT_LINUX_SLL2 on Linux, and DLT_IPNET on Solaris. 🫶
• outbound — same as the above, but for the opposite direction (from the capturing host to the network). ⬅️➡️
• 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, for DLT_PFLOG only, if the packet was logged as coming from the specified interface.
• on interface — synonymous with the ifname primitive. Twin behavior, no notes. 👯
• rnr num — true, for DLT_PFLOG only, if the packet was logged as matching the specified PF rule number.
• rulenum num — synonymous with the rnr primitive.
• reason code — true, for DLT_PFLOG only, if the packet was logged with the specified PF reason code. Valid codes are: match, bad-offset, fragment, short, normalize, memory, bad-timestamp, congestion, ip-option, proto-cksum, state-mismatch, state-insert, state-limit, src-limit, synproxy, map-failed (on FreeBSD only), state-locked (on NetBSD only), translate (on OpenBSD only), no-route (on OpenBSD only) and dummynet (on macOS only). That's a whole lotta lore, bestie. 😮‍💨
• rset name — true, for DLT_PFLOG only, if the packet was logged as matching the specified PF ruleset name of an anchored ruleset.
• ruleset name — synonymous with the rset primitive.
• srnr num — true, for DLT_PFLOG only, if the packet was logged as matching the specified PF rule number of an anchored ruleset.
• subrulenum num — synonymous with the srnr primitive.
• action act — true, for DLT_PFLOG only, if PF took the specified action when the packet was logged. Valid actions are: pass (or accept), block (or drop) and, with later versions of pf(4), scrub, noscrub, nat, nonat, binat, nobinat, rdr, nordr, synproxy-drop, defer (on FreeBSD and OpenBSD only), match (on OpenBSD only), divert (on OpenBSD only), rt (on OpenBSD only), afrt (on OpenBSD only), dummynet (on macOS only), nodummynet (on macOS only), nat64 (on macOS only) and nonat64 (on macOS only). Phew, okay, read that list twice. 📝
• 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. 🦋
• wlan type wlan_type — true if the IEEE 802.11 frame type matches the specified wlan_type. Valid *wlan_type*s are: mgt, ctl and data. The wlan keyword is optional.
• wlan 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. The wlan keyword is optional.

If the specified wlan_type is mgt, then valid *wlan_subtype*s 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_subtype*s are: bar, ba, ps-poll, rts, cts, ack, cf-end and cf-end-ack.

If the specified wlan_type is data, then valid *wlan_subtype*s 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.

• wlan 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. The wlan keyword is optional.
• wlan dir direction — true when the IEEE 802.11 frame direction matches the direction you asked for. Valid directions are: nods, tods, fromds, dstods, or a numeric value. The wlan keyword is optional, bestie, you don't have to say it. ✨
• vlan [vlan_id] — true if the packet is an IEEE 802.1Q VLAN packet 💜. If you pass the optional vlan_id, it's only true when the packet has that exact vlan_id. Heads up though: the first vlan keyword it hits in an expression changes the decoding offsets for the rest of the expression, on the assumption that the packet is a VLAN packet. The vlan [vlan_id] keyword can be used more than once, so you can filter on VLAN hierarchies — each use of that keyword bumps 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. nesting era fr fr 🦋

• mpls [label_num] — true if the packet is an MPLS packet. With the optional label_num, it's only true when the packet has that specified label_num. Note that the first mpls keyword encountered in an expression changes the decoding offsets for the remainder of the expression, assuming the packet is an 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 (EtherType 0x8863). 🌸
• pppoes [session_id] — true if the packet is a PPP-over-Ethernet Session packet (EtherType 0x8864). If you give the optional session_id, it's only true when the packet has that session_id. Note that the first pppoes keyword encountered in an expression changes the decoding offsets for the remainder of the expression, assuming 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 you pass the optional vni, it's only true when the packet has that vni. Note that when the geneve keyword shows up in an expression, it changes the decoding offsets for the rest of the expression, assuming 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. versatile queen 👑

• vxlan [vni] — true if the packet is a VXLAN packet (UDP port 4789). With the optional vni, it's only true when the packet has that vni. Note that when the vxlan keyword is encountered in an expression, it changes the decoding offsets for the remainder of the expression, assuming the packet is a VXLAN packet. 🦋

For example:

vxlan 0x7 && ip6

filters IPv6 protocol encapsulated in VXLAN with VNI 0x7.

• 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. Also in this context es-is is an alias for esis and is-is is an alias for isis. aliases on aliases, it's giving ✨

• isis proto protocol — true if the packet is an IS-IS packet of protocol type protocol, which can be a number only. 🔥
• l1, l2, iih, lsp, snp, csnp, psnp — abbreviations for IS-IS PDU types.
• atmfield relop val — true if the packet is an ATM packet (for SunATM on Solaris) and the relation holds. atmfield is one of {vpi, vci}; relop is one of {>, <, >=, <=, =, ==, !=} (where = means the same as ==); val is an integer. vpi and vci stand for the virtual path identifier (VPI) and the virtual channel identifier (VCI) fields respectively. 💜
• atmfield val — abbreviation for:

atmfield == val

in the expression above.

• atmfield (val1 or ... or valN) — abbreviation for:

(atmfield == val1 or ... or atmfield == valN)

in the expression above.

• 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 assuming the packet is an LLC-encapsulated packet. 🎀

Also, the first lane keyword enables primitives that do not apply to ATM in general, such as link host and link multicast. unlocked 🔓

• oamf4sc — true if the packet is an ATM packet (for SunATM on Solaris) and is a segment OAM F4 flow cell (VPI=0 & VCI=3). ⭐
• oamf4ec — 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), 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), is on a meta signaling circuit, and is a Q.2931 Setup, Call Proceeding, Connect, Release, or Release Done message. 📞
• fisu — true if the packet is a Fill-In Signal Unit (FISU) MTP2 packet. 🔥
• lssu — true if the packet is a Link Status Signal Unit (LSSU) MTP2 packet. 🔥
• msu — true if the packet is a Message Signal Unit (MSU) MTP2 packet. 🔥
• mtpfield relop val — true if the relation holds. mtpfield is one of {sio, dpc, opc, sls}; relop is one of {>, <, >=, <=, =, ==, !=} (where = means the same as ==); val is an integer. sio stands for the Service Information Octet (SIO) field of the MTP2 MSU header. dpc, opc and sls stand for the Destination Point Code (DPC), Originating Point Code (OPC) and Signalling Link Selection (SLS) fields respectively of the MTP3 standard routing label. 💜
• mtpfield val — abbreviation for:

mtpfield == val

in the expression above.

• mtpfield (val1 or ... or valN) — abbreviation for:

(mtpfield == val1 or ... or mtpfield == valN)

in the expression above.

• hfisu, hlssu, hmsu, hsio, hdpc, hopc, hsls — same as fisu, lssu, msu, sio, dpc, opc and sls respectively, but only if the MTP2 link uses the extended sequence numbers encoding specified for high speed signalling links (HSL) in ITU-T Recommendation Q.703 Annex A. the h is for high speed, and also for hot 🥵
• link host mac8addr — true if the source or the destination ARCnet node ID (for DLT_ARCNET and DLT_ARCNET_LINUX only) or MS/TP station address (for DLT_BACNET_MS_TP only) is mac8addr. It may be qualified with a different direction (src, dst, src and dst), in which case the host keyword is optional. 🫶

mac8addr is a string of the form $xx or $x, where "x" is a hexadecimal digit. For example:

link host $2b

Also in ARCnet context broadcast and multicast are equivalent to link dst $0, and in MS/TP context broadcast is equivalent to link dst $ff. 🎀

⚠️ Note that this address syntax clashes with the parameter expansion syntax in POSIX-compatible shells and elsewhere, so depending on the use case the filter string may require the use of single quotes or a backslash. protect your $ bestie, the shell will absolutely eat it 😭

• byte idx op val — true if the value of the link layer byte number idx satisfies a condition with regard to val, which can be a number only. The condition is one of: "equals to" (if op is =), "less than" (if op is <), "greater than" (if op is >), "the result of bitwise AND is not zero" (if op is &), "the result of bitwise OR is not zero" (if op is |). 💅

🚨 The arithmetic expressions and packet data accessors below implement all of these and many other things much better, so this primitive will be removed in a future release and should not be used in applications that require forward compatibility. it's giving deprecated, don't get attached 💀

🧮 ARITHMETIC EXPRESSIONS 🧮

okay bestie so arithmetic expressions? they're the operands of a relational operator, in a relation that looks like this:

expr1 relop expr2

and the whole thing is true if and only if the relation actually holds. the relop (that's the relational operator, short king) is one of {>, <, >=, <=, =, ==, !=} — and yes, = means the exact same thing as ==, no cap. 💅

each of expr1 and expr2 is an arithmetic expression built from:

• integer constants (written in standard C syntax)
• the common arithmetic and bitwise binary operations {+, -, *, /, %, &, |, ^, <<, >>}
• a length accessor
• packet data accessors

and every single arithmetic expression, no matter how messy and complicated you make it, resolves to an integer value. periodt. ✨

lowkey important: all comparisons are unsigned, so like, both 0x80000000 and 0xffffffff are > 0. don't get caught out by that one bestie. 😭

also 💀 there's a genuinely cursed gotcha: 32-bit octal integer constants in the [010000000000 .. 037777777777] interval — which btw is literally 75% of all 32-bit integers — get interpreted as MAC-48 addresses when they're prepended with a single zero (so 012345670123 means 01:23:45:67:01:23). not me getting jump-scared by an octal literal. 🙈

to disambiguate the expression, you gotta either prepend that octal number with more zeroes (0012345670123), or write the same value as a decimal (1402433619) or a hexadecimal (0x53977053) number. slay. 🎀

---

the % and ^ situation

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, Hurd, illumos, Solaris, OpenBSD), if you use those operators, filtering will be done in user mode — which increases the overhead of capturing packets and may cause more packets to be dropped. 😭 it's giving "works but at what cost".

the length accessor

the length accessor is indicated by the keywords len or length, and it gives you the length of the packet. simple as that. 💖

---

📦 PACKET DATA ACCESSORS 📦

to use the packet data inside an arithmetic expression, you use this 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 it indicates the protocol layer for the index operation. btw ether, fddi, link, ppp, slip, tr and wlan all refer to the link layer, and radio refers to the "radio header" added to some 802.11 captures. iykyk. 🦋

real talk, read this bit: in the current implementation of packet data accessors, the tcp, udp, icmp, igmp, pim, vrrp, carp, sctp and igrp above do not match IPv6 packets (this may be fixed for some of these protocols in future). also the icmp6 above implies that the IPv6 packet does not have any extension (e.g. Fragment) headers. also the igrp above uses an OS-specific protocol number (as discussed for the igrp abbreviation earlier) and therefore should not be used in portable applications and may be removed in future. 💀

the byte offset, relative to the indicated protocol layer, is given by expr — which can be an integer constant or any other valid arithmetic expression.

Size is optional and indicates the number of bytes in the field of interest. it can be either one, two, or four, and it defaults to one. also it must be one of these valid integer constants only — it cannot be a more complex expression. no exceptions bestie. 🚫

---

examples that ate 💅

• ether[0] & 1 != 0 — catches all multicast traffic
• ip[0] & 0xf != 5 — catches all IPv4 packets with options
• ip[6:2] & 0x1fff = 0 — catches only unfragmented IPv4 datagrams and the first fragment of fragmented IPv4 datagrams

and that last check? it's implicitly applied to the tcp, udp, icmp, sctp, igmp, pim, igrp, vrrp and carp index operations. so for instance, tcp[0] always means the first byte of the TCP header, and never means the first byte of an intervening fragment. 🌟

---

🏷️ NAMED VALUES 🏷️

some offsets and field values can be expressed as names instead of numeric values. iconic honestly.

the following protocol header field offsets are available:

• icmptype — ICMP type field
• icmp6type — ICMPv6 type field
• icmpcode — ICMP code field
• icmp6code — ICMPv6 code field
• tcpflags — TCP flags field

---

ICMP type field values 🌈

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.

---

ICMPv6 type field values 🦋

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.

---

TCP flags field values 🎀

the following TCP flags field values are available:

tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-ack, tcp-urg, tcp-ece, tcp-cwr.

---

🔗 COMPOUND EXPRESSIONS 🔗

primitives and relations may be combined using:

• Parentheses.
• 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. 👑

---

the shortcut you gotta know 🫶

for primitives, 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 and host ace)

and that's the whole vibe check on that one bestie, parenthesize when in doubt 💖

🦋 PROTOCOL NAMES IN VARIOUS CONTEXTS 🦋

okay so this next bit is basically the guest list 💅 — it tells you which protocol names are allowed to show up in which contexts of the currently implemented syntax. no sneaking randos past the bouncer.

The name column contains a protocol name, which can be used as a keyword and/or as an ID for the proto case of the type qualifier kind. yes bestie, both jobs. versatile icon behavior.

For keywords:

• K/see — if the keyword is an alias, this column refers to the main keyword. it's giving "go ask their manager".
• K/abbr — tells whether the keyword can be used as an abbreviation (that is, if the keyword is the only contents of a primitive, it means a more complex expression).
• K/PDA — tells whether the keyword can be used in a packet data accessor.
• K/pqual — tells whether the keyword can be used as a case of the proto qualifier kind.

For IDs:

• ID/tqual — shows the valid context(s). that's it, that's the whole column.

---

• aarp — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: link proto \aarp
• ah — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: [ip|ip6] proto \ah
• arp — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: yes · ID/tqual: link proto \arp
• atalk — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: link proto \atalk
• carp — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: ip proto \carp
• clnp — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: iso proto \clnp
• csnp — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: —
• decnet — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: yes · ID/tqual: link proto \decnet
• esis — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: iso proto \esis
• es-is — K/see: esis · K/abbr: — · K/PDA: — · K/pqual: — · ID/tqual: —
• esp — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: [ip|ip6] proto \esp
• ether — K/see: link · K/abbr: — · K/PDA: — · K/pqual: — · ID/tqual: —
• fddi — K/see: link · K/abbr: — · K/PDA: — · K/pqual: — · ID/tqual: —
• icmp — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: ip proto \icmp
• icmp6 — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: —
• igmp — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: ip proto \igmp
• igrp — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: ip proto \igrp
• iih — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: —
• ip — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: yes · ID/tqual: link proto \ip
• ip6 — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: yes · ID/tqual: link proto \ip6
• ipx — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: link proto \ipx
• isis — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: yes · ID/tqual: iso proto \isis
• is-is — K/see: isis · K/abbr: — · K/PDA: — · K/pqual: — · ID/tqual: —
• iso — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: yes · ID/tqual: link proto \iso
• l1 — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: —
• l2 — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: —
• lat — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: link proto \lat
• link — K/see: — · K/abbr: no · K/PDA: yes · K/pqual: yes · ID/tqual: —
• lldp — K/see: — · K/abbr: — · K/PDA: — · K/pqual: — · ID/tqual: link proto lldp
• loopback — K/see: — · K/abbr: — · K/PDA: — · K/pqual: — · ID/tqual: link proto loopback
• lsp — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: —
• mopdl — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: link proto \mopdl
• moprc — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: link proto \moprc
• netbeui — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: link proto \netbeui
• pim — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: [ip|ip6] proto \pim
• ppp — K/see: link · K/abbr: — · K/PDA: — · K/pqual: — · ID/tqual: —
• psnp — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: —
• radio — K/see: — · K/abbr: no · K/PDA: yes · K/pqual: no · ID/tqual: —
• rarp — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: yes · ID/tqual: link proto \rarp
• sca — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: link proto \sca
• sctp — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: yes · ID/tqual: [ip|ip6] proto \sctp
• slip — K/see: link · K/abbr: — · K/PDA: — · K/pqual: — · ID/tqual: —
• slow — K/see: — · K/abbr: — · K/PDA: — · K/pqual: — · ID/tqual: link proto slow
• snp — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: —
• stp — K/see: — · K/abbr: yes · K/PDA: no · K/pqual: no · ID/tqual: link proto \stp
• tcp — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: yes · ID/tqual: [ip|ip6] proto \tcp
• tr — K/see: link · K/abbr: — · K/PDA: — · K/pqual: — · ID/tqual: —
• udp — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: yes · ID/tqual: [ip|ip6] proto \udp
• vrrp — K/see: — · K/abbr: yes · K/PDA: yes · K/pqual: no · ID/tqual: ip proto \vrrp
• wlan — K/see: link · K/abbr: — · K/PDA: — · K/pqual: — · ID/tqual: —

---

✨ EXAMPLES ✨

study time bestie, these are the ones you'll actually use 🫶

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 "when did this even drop" timeline, for when you're on an ancient libpcap and nothing works 😭

• The carp keyword became available in libpcap 1.2.1.
• The hfisu, hlssu, hmsu, hsio, hopc, hdpc and hsls keywords became available in libpcap 1.5.3.
• The modulo (%) and bitwise XOR (^) binary operators became available in libpcap 1.6.2.
• The geneve keyword became available in libpcap 1.8.0.
• 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 ifindex keyword became available in libpcap 1.10.0.
• The vxlan keyword became available in libpcap 1.11.0.
• The lldp and slow protocols became available for "ether proto" in libpcap 1.11.0.
• Before libpcap 1.11.0 the gateway primitive had a different definition, which did not produce the expected results, also its support was conditional and since libpcap 1.0.0 was disabled by default.

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🌸 SEE ALSO 🌸

• pcap(3PCAP) — go read that one next, bestie, it's the whole point 💅

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💀 BUGS 💀

okay so, the stuff that's lowkey broken. read it, don't skip it.

• 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.
• The sio and hsio keywords do not test whether the packet is an MSU packet. The dpc, opc, sls, hdpc, hopc and hsls keywords do not test whether the packet is an MTP3 packet.
• For ARP and RARP the current implementation assumes IPv4 over Ethernet and may incorrectly match packets that have a different combination of protocol and hardware.

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and that's the whole page, bestie!! you're officially a filter gremlin now 💖✨

this was PCAP-FILTER(7), from the libpcap source, dated 12 September 2026. mwah, go forth and filter responsibly 🎀🦋