============================================================ NOTE: THIS DOCUMENT WAS DERIVED FROM THE TCPDUMP MAN PAGE. THIS DOCUMENT IS THE PROPERTY OF IT'S RESPECTIVE AUTHOR ============================================================ ** PCAP EXPRESSIONS ** So what is all this talk about pcap logic? Pcap logic aka pcap expressions. The expression consists of one or more primitives. Primitives usu­ ally consist of an id (name or number) preceded by one or more qualifiers. There are three different kinds of qualifier: type qualifiers say what kind of thing the id name or number refers to. Possible types are host, net and port. E.g., `host foo', `net 128.3', `port 20'. If there is no type qualifier, host is assumed. dir qualifiers specify a particular transfer direction to and/or from id. Possible directions are src, dst, src or dst and src and dst. E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If there is no dir qualifier, src or dst is assumed. For `null' link layers (i.e. point to point proto­ cols such as slip) the inbound and outbound qualifiers can be used to specify a desired direction. proto qualifiers restrict the match to a particular protocol. Possible protos are: ether, fddi, ip, arp, rarp, decnet, lat, sca, moprc, mopdl, tcp and udp. E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is no proto quali­ fier, all protocols consistent with the type are assumed. E.g., `src foo' means `(ip or arp or rarp) src foo' (except the latter is not legal syntax), `net bar' means `(ip or arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'. In addition to the above, there are some special `primitive' key­ words that don't follow the pattern: gateway, broadcast, less, greater and arithmetic expressions. All of these are described below. More complex filter expressions are built up by using the words and, or and not to combine primitives. E.g., `host foo and not port ftp and not port ftp-data'. To save typing, identical quali­ fier lists can be omitted. E.g., `tcp dst port ftp or ftp-data or domain' is exactly the same as `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'. Allowable primitives are: dst host host True if the IP destination field of the packet is host, which may be either an address or a name. src host host True if the IP source field of the packet is host. host host True if either the IP source or destination of the packet is host. Any of the above host expressions can be prepended with the keywords, ip, arp, or rarp as in: ip host host which is equivalent to: ether proto \ip and host host If host is a name with multiple IP addresses, each address will be checked for a match. ether dst ehost True if the ethernet destination address is ehost. Ehost may be either a name from /etc/ethers or a number (see ethers(3N) for numeric format). ether src ehost True if the ethernet source address is ehost. ether host ehost True if either the ethernet source or destination address is ehost. gateway host True if the packet used host as a gateway. I.e., the ether­ net 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 in both /etc/hosts and /etc/ethers. (An equivalent expression is ether host ehost and not host host which can be used with either names or numbers for host / ehost.) dst net net True if the IP destination address of the packet has a net­ work number of net. Net may be either a name from /etc/net­ works or a network number (see networks(4) for details). src net net True if the IP source address of the packet has a network number of net. net net True if either the IP source or destination address of the packet has a network number of net. net net mask mask True if the IP address matches net with the specific net­ mask. May be qualified with src or dst. net net/len True if the IP address matches net a netmask len bits wide. May be qualified with src or dst. dst port port True if the packet is ip/tcp or ip/udp and has a destination port value of port. The port 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 traf­ fic and udp/who traffic, and port domain will print both tcp/domain and udp/domain traffic). src port port True if the packet has a source port value of port. port port True if either the source or destination port of the packet is port. Any of the above port expressions can be prepended with the keywords, tcp or udp, as in: tcp src port port which matches only tcp packets whose source port is port. less length True if the packet has a length less than or equal to length. This is equivalent to: len <= length. greater length True if the packet has a length greater than or equal to length. This is equivalent to: len >= length. ip proto protocol True if the packet is an ip packet (see ip(4P)) of protocol type protocol. Protocol can be a number or one of the names icmp, igrp, udp, nd, 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. ether broadcast True if the packet is an ethernet broadcast packet. The ether keyword is optional. ip broadcast True if the packet is an IP broadcast packet. It checks for both the all-zeroes and all-ones broadcast conventions, and looks up the local subnet mask. 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 IP multicast packet. ether proto protocol True if the packet is of ether type protocol. Protocol can be a number or a name like ip, arp, or rarp. Note these identifiers are also keywords and must be escaped via back­ slash (\). [In the case of FDDI (e.g., `fddi protocol arp'), the protocol identification comes from the 802.2 Log­ ical Link Control (LLC) header, which is usually layered on top of the FDDI header. Tcpdump assumes, when filtering on the protocol identifier, that all FDDI packets include an LLC header, and that the LLC header is in so-called SNAP format.] ip, arp, rarp, decnet Abbreviations for: ether proto p where p is one of the above protocols. tcp, udp, icmp Abbreviations for: ip proto p where p is one of the above protocols. expr relop expr True if the relation holds, where relop is one of >, <, >=, <=, =, !=, and expr is an arithmetic expression composed of integer constants (expressed in standard C syntax), the nor­ mal binary operators [+, -, *, /, &, |], a length operator, and special packet data accessors. To access data inside the packet, use the following syntax: proto [ expr : size ] Proto is one of ether, fddi, ip, arp, rarp, tcp, udp, or icmp, and indicates the protocol layer for the index opera­ tion. 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 opera­ tor, indicated by the keyword len, gives the length of the packet. For example, `ether[0] & 1 != 0' catches all multicast traf­ fic. The expression `ip[0] & 0xf != 5' catches all IP pack­ ets with options. The expression `ip[6:2] & 0x1fff = 0' catches only unfragmented datagrams and frag zero of frag­ mented 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. Primitives may be combined using: A parenthesized group of primitives and operators (parenthe­ ses are special to the Shell and must be escaped). Negation (`!' or `not'). Concatenation (`&&' or `and'). Alternation (`||' or `or'). Negation has highest precedence. Alternation and concatenation have equal precedence and associate left to right. Note that explicit and tokens, not juxtaposition, are now required for con­ catenation. If an identifier is given without a keyword, the most recent key­ word is assumed. For example, not host vs and ace is short for not host vs and host ace which should not be confused with not ( host vs or ace ) Expression arguments can be passed to tcpdump as either a single argument or as multiple arguments, whichever is more convenient. Generally, if the expression contains Shell metacharacters, it is easier to pass it as a single, quoted argument. Multiple arguments are concatenated with spaces before being parsed. EXAMPLES To print all packets arriving at or departing from sundown: ipex host sundown To print traffic between helios and either hot or ace: ipex host helios and \( hot or ace \) To print all IP packets between ace and any host except helios: ipex ip host ace and not helios To print all traffic between local hosts and hosts at Berkeley: ipex net ucb-ether To print all ftp traffic through internet gateway snup: (note that the expression is quoted to prevent the shell from (mis-)interpreting the parentheses): ipex gateway snup and (port ftp or ftp-data) To print 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). ipex ip and not net localnet To print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a non-local host. ipex tcp[13] & 3 != 0 and not src and dst net localnet To print IP packets longer than 576 bytes sent through gateway snup: ipex gateway snup and ip[2:2] > 576 To print IP broadcast or multicast packets that were not sent via ethernet broadcast or multicast: ipex ether[0] & 1 = 0 and ip[16] >= 224 To print all ICMP packets that are not echo requests/replies (i.e., not ping packets): ipex icmp[0] != 8 and icmp[0] != 0