tcpdump filter for given bytes on given position

Question

I'm searching for a way to filter traffic for specific bytes.

What I have is a stream that I capture with wireshark or tcpdump. Now it logs way too much, so I want to drop every packet that has 0x23 at place 42 in the payload.

Is there an easy way to do this? I'm searching for gsmtap, but any example with any protocol should be fine.

Solution

tcpdump(1) uses libpcap(3), which uses a filter syntax documented in the pcap-filter(7) man page.

You might want to skip down to the expr relop expr section, and the square-bracket notation:

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 compar-
       isons are unsigned, so that, for example, 0x80000000 and 0xffffffff are >  0.
       To access data inside the packet, use the following syntax:
            proto [ expr : size ]
       Proto  is  one of ether, fddi, tr, wlan, ppp, slip, link, ip, arp, rarp, tcp,
       udp, icmp, ip6 or radio, and indicates the protocol layer for the index oper-
       ation.   (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 indi-
       cates 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.

So for example, if you wanted to filter out packets with 0x23 at location 42 of the payload of an Ethernet-II frame, that would be at offset 56 of the overall Ethernet frame (your offset of 42 plus an offset of 14 bytes to get past the Ethernet headers to the payload), so you could do something like this:

ether[56] != 0x23

I didn't fully read up on gsmtap so I'm not guaranteeing that the above filter is exactly what you need, but it should start you in the right direction.