Pcap Filter Syntax Reference

Qosium uses Pcap syntax for defining manual packet filters. To master at least the very basic filtering cases is very useful when working with Qosium. This section provides information about general Pcap filter syntax that is often relevant to Qosium.

1. Pcap Filter Syntax #

While some information is given next, a full syntax reference is found here.

1.1. Primitives #

Important Pcap filter primitives are the following:

1.1.1. Host #

Syntax Condition
dst host <host> IPv4/v6 destination field of the packet is host, which may be either an address or a name
src host <host> IPv4/v6 source field of the packet is host
host <host> IPv4/v6 source or destination of the packet is host

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

1.1.2. Ether #

Syntax Condition
ether dst <ehost> Ethernet destination address is ehost, which may be either a name from /etc/ethers or a number
ether src <ehost> Ethernet source address is ehost
ether host <ehost> Ethernet source or destination address is ehost

True if the packet used the host as a gateway. I.e., the Ethernet source or destination address was host, but neither the IP source nor the IP destination was the host. The host must be a name and must be found both by the machine’s host-name-to-IP-address resolution mechanisms (hostname 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 an IPv6-enabled configuration at this moment.

gateway <host>

1.1.3. Net #

Syntax Condition
dst net <net> IPv4/v6 destination address of the packet has a network number of net
src net <net> IPv4/v6 source address of the packet has a network number of net
net <net> IPv4/v6 source or destination address of the packet has a network number of net
net <net>mask <netmask> IPv4 address matches net with the specific netmask. May be qualified with src or dst. Notice that this syntax is not valid for IPv6 net
net <net>/<len> IPv4/v6 address matches netwith a netmask len bits wide. May be qualified with src or dst

Net may be either a name from the network’s 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 a 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.

1.1.4. Port #

Syntax Condition
dst port <port> Packet has a destination port value of port
src port <port> Packet has a source port value of port
port <port> Either the source or destination port of the packet is port
dst portrange <port1>-<port2> Packet has a destination port value between port1 and port2
src portrange <port1>-<port2> Packet has a source port value between port1 and port2
portrange <port1>-<port2> Packet has a source or a destination port value between port1 and port2

True if the packet is IPv4/IPv6 TCP, IPv4/IPv6 UDP, or IPv4/IPv6 SCTP, in some systems, and has a destination port value of port. The port can be a number or a name used in /etc/services. 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).

Any of the above port or port range expressions can be prepended with the keywords tcp orudp. For example,tcp src port matches only TCP packets whose source port is`.

1.1.5. Packet Size #

Syntax Condition
less <length> Packet has a length less than or equal to <length>. This is equivalent to len <= <length>
greater <length> Packet has a length greater than or equal to <length>. This is equivalent to len >= <length>

1.1.6. Protocol #

Syntax Condition
tcp Short for proto tcp
udp Short for proto udp
icmp Short for proto icmp
ip proto <protocol> Packet is an IPv4 packet of protocol type <protocol>
ip protochain <protocol> Packet is IPv4 packet, and contains protocol header with type <protocol> in its protocol header chain
ip6 proto <protocol> Packet is an IPv6 packet of protocol type <protocol>
ip6 protochain <protocol> Packet is IPv6 packet, and contains protocol header with type <protocol> in its protocol header chain
ether proto <protocol> Packet is of ether type <protocol>, where protocol can be ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp, ipx, netbeui, lat, moprc, mopdl. Note that not all applications using currently know how to parse these protocols
iso proto <protocol> Packet is an OSI packet of protocol <protocol>. Protocol can be a number or one of the names clnp, esis, or isis. Abbreviations for IS-IS PDU types are: l1, l2, iih, lsp, snp, csnp, psnp

The protocol can be a number or, e.g., one of the names icmp, icmp6, igmp, igrp, pim, ah, esp, vrrp, udp, or tcp. Note that the identifiers tcp, udp, and icmp are also keywords and must be escaped via backslash (), which is \ in the C-shell. Note that this primitive does not chase the protocol header chain.

ip6 protochain 6 matches any IPv6 packet with TCP protocol header in the protocol header chain. The packet may contain, for example, an 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, which can be somewhat slow.

Ethernet protocol can be a number or one of the names ip, ip6, arp, rarp, atalk, aarp, decnet, sca, lat, mopdl, moprc, iso, stp, ipx, 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 headers.

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
  • talk - 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

1.1.7. Broadcast #

Syntax Condition
ether broadcast Packet is an Ethernet broadcast packet. The ether keyword is optional
ip broadcast Packet is an IPv4 broadcast packet
ether multicast Packet is an Ethernet multicast packet. The ether keyword is optional. This is shorthand for ether[0] & 1 != 0
ip multicast Packet is an IPv4 multicast packet
ip6 multicast Packet is an IPv6 multicast packet

ip broadcast checks for both the all-zeros 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.

1.1.8. WLAN #

Syntax Condition
wlan addr1 <ehost> First IEEE 802.11 address is <ehost>
wlan addr2 <ehost> 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> 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> Fourth IEEE 802.11 address, if present, is <ehost>. The fourth address field is only used for WDS (Wireless Distribution System) frames
dir <dir IEEE 802.11 frame direction matches the specified dir. Valid directions are nods, tods, fromds, dstods, or a snumeric value
type <wlan_type> IEEE 802.11 frame type matches the specified <wlan_type>. Valid WLAN types are mgt, ctl and data
subtype <wlan_subtype> IEEE 802.11 frame subtype matches the specified <wlan_subtype> and frame has the type to which the specified WLAN subtype belongs
type <wlan_type> subtype <wlan_subtype> 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

1.1.9. VLAN #

Virtual LAN (VLAN) IEEE 802.1Q tagged packets can be filtered with the vlan keyword. Filter vlan <vlan_id> yields packets that have the corresponding VLAN ID. Note that the first vlan keyword encountered in 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> 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, to filter on VLAN 200 encapsulated within VLAN 100:

vlan 100 && vlan 200

To filter IPv4 protocols encapsulated in VLAN 300 encapsulated within any higher order VLAN:

vlan && vlan 300 && ip

1.1.10. Multiprotocol Label Switching (MPLS) #

Syntax Condition
mpls <label_num> Packet is an MPLS packet. If <label_num> is specified, the packet must have the corresponding label_num. Note that the first mpls keyword encountered in expression changes the decoding offsets for the remainder of the expression on the assumption that the packet is an MPLS-encapsulated IP packet. This 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, filter packets with an outer label of 100000 and an inner label of 1024:

mpls 100000 && mpls 1024

Filter packets to or from 192.9.200.1 with an inner label of 1024 and any outer label:

mpls && mpls 1024 && host 192.9.200.1

1.1.11. Point-to-Point Protocol over Ethernet (PPPoE) #

Syntax Condition
pppoed Packet is a PPP-over-Ethernet Discovery packet (Ethernet type 0x8863)
pppoes 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, filter IPv4 protocols encapsulated in PPPoE:

pppoes && ip

1.2. Relation Expression #

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 can be 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 &-sign means bit-wise masking, so the above expression basically checks the first byte’s last bit 1. The expression ip[0] & 0xf != 5 catches all IPv4 packets with options. The expression ip[6:2] & 0x1fff = 0 catches only unfragmented IPv4 datagrams and frag zero of fragmented IPv4 datagrams. This check is implicitly applied to the tcp and udp index operations. For instance, tcp[0] always means the first byte of the TCP header and never means the first byte of an intervening fragment.

Some offsets and field values may be expressed as names rather than as numeric values. The following protocol header field offsets are available: icmptype (ICMP type field), icmpcode (ICMP code field), and tcpflags (TCP flags field).

The following ICMP type field values are available: icmp-echoreply, icmp-unreach, icmp-sourcequench, icmp-redirect, icmp-echo, icmp-routeradvert, icmp-routersolicit, icmp-timxceed, icmp-paramprob, icmp-tstamp, icmp-tstampreply, icmp-ireq, icmp-ire-qreply, icmp-maskreq, icmp-maskreply.

The following TCP flags field values are available: tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-ack, tcp-urg.

1.3. 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 the 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 ).

2. Examples #

2.1. Basic and Common Scenarios #

Next, some elementary examples of the use of Pcap syntax are given:

To get all traffic seen in the interface, enter an empty filter.

To get all IPv4 based traffic is:

ip

To get all traffic that involves my host (called myhost):

host myhost

To select all IPv4 traffic between 192.168.1.1 and 192.168.1.7:

ip host 192.168.1.1 and host 192.168.1.7

To select all IPv4 traffic between 192.168.1.1 and any host except 192.168.1.7:

ip host 192.168.1.1 and not host 192.168.1.7

To select all TCP traffic, including port 80 between 192.168.1.1 and 192.168.1.7:

ip host 192.168.1.1 and host 192.168.1.7 and tcp and port 80

To select all UDP traffic with even source ports between 192.168.1.1 and 192.168.1.7 or 192.168.1.6

ip and udp and (host 192.168.1.1 and (host 192.168.1.7) or (host 192.168.1.6)) and (udp[0:2] & 1 = 0)

2.2. PROFINET over Ethernet #

When running the PROFINET protocol in the real-time mode, it runs directly over Ethernet without IP. Currently, there is no automatic filtering for Ethernet packets, so one has to perform the filtering manually.

Assume that there are two hosts, 01:02:03:04:05:06 and 07:08:09:0a:0b:0c, which communicate with PROFINET. If there is no other traffic traveling between the hosts, traffic between these hosts can be measured with a simple filter:

ether host 01:02:03:04:05:06 and ether host 07:08:09:0a:0b:0c

This will include all Ethernet traffic between the hosts. Thus, if there are, e.g., IP streams above Ethernet, those will also be included. When limiting the focus purely on PROFINET, this is done by a filter:

ether proto 0x8892

The reason for this kind of syntax is that PROFINET’s protocol ID, or Type in Ethernet, is 8892 in hex. Pcap does not support PROFINET protocol directly. The filter above can be enough if one is sure that PROFINET communications take place only between the selected hosts. However, if there are more PROFINET streams, from the hosts to other hosts, the hosts’ Ethernet addresses need to be included:

(ether host 01:02:03:04:05:06 and ether host 07:08:09:0a:0b:0c) and (ether proto 0x8892)

When using PROFINET, Virtual LANs are often used to improve QoS. This needs to be taken into account in the filter level as well, as discussed earlier. Thus, the final filter of this example will be in the form:

(ether host 01:02:03:04:05:06 and ether host 07:08:09:0a:0b:0c) and
((ether proto 0x8892) or (vlan and ether proto 0x8892))

2.3. Odd and Even Port Numbers #

Capturing all UDP packets with an even-numbered port becomes topical when measuring RTP streams without RTCP-messages. Typically, RTP uses even-numbered ports and RTCP odd-numbered ones.

We cannot use the port statement since it cannot be manipulated in the way we need. Instead, we need to dig the protocol’s port fields. By masking the least significant bit, we can reach our goal. A filter udp[0:2] & 1 = 0 will take the first two bytes bits into checking, i.e., the source port. It masks that with 1, so when this is 0, the port must be even-numbered. Thus, the above filter takes all packets whose source port has an even-numbered value. Similarly, a filter udp[2:2] & 1 = 0 includes all packets whose destination port is even-numbered. Then again, the filter udp[2:2] & 1 = 1 includes all packets whose destination port is odd-numbered.

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