On Sep 13, 2007, at 1:45 PM, Travis Love wrote:
> I'm trying to create a capture filter to help detect rogue DHCP
> servers with Wireshark. So far, what I've come up with is a capture
> and a viewing filter, each of which does half the work I need it
> to. The capture filter looks like:
>
> (port 67 or port 68) and !(ether host 00:04:23:XX:XX:XX) and !
> (ether host 00:04:23:XX:XX:YY)
>
> So it captures only DHCP packets that aren't to/from either of our
> DHCP servers. I then have to apply:
>
> frame[282:3] == 35:01:02 or frame[282:3] == 35:01:05 or frame[282:3]
> == 35:01:06
>
> as a viewing filter in order to see only NAK, ACK, and DHCP OFFER
> packets. Is there a way to put the viewing filter into the capture
> filter so my box's RAM doesn't fill up with packets I'm not
> interested in?
If you want to move a test from a display filter that refers to
frame[] to a capture filter that refers to frame[], for at least some
network types, you can do that, although, given that libpcap/WinPcap's
capture filter compiler isn't as fancy as Wireshark's display filter
compiler, it involves some extra work.
From the tcpdump man page:
expression
selects which packets will be dumped. If no
expression is
given, all packets on the net will be dumped.
Otherwise, only
packets for which expression is `true' will be dumped.
The expression consists of one or more primitives.
Primitives
usually consist of an id (name or number) preceded
by one or
more qualifiers. ...
...
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
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:
...
expr relop expr
True if the relation holds, where relop is one
of >, <,
>=, <=, =, !=, and expr is an arithmetic
expression com-
posed of integer constants (expressed in
standard C syn-
tax), the normal binary operators [+, -, *, /,
&, |, <<,
>>], a length operator, and special packet
data acces-
sors. 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 is one of ether, fddi, tr, wlan, ppp,
slip, link,
ip, arp, rarp, tcp, udp, icmp, ip6 or radio,
and indi-
cates 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, indi-
cated by the keyword len, gives the length of
the packet.
Fetches with a "size" value of 2 or 4 (the only valid values are 1, 2,
and 4; see previous comment about libpcap/WinPcap's limitations) are
big-endian.
That means that
frame[282:3] == 35:01:02 or frame[282:3] == 35:01:05 or frame[282:3]
== 35:01:06
should translate to
(link[282:2] == 0x3501 and link[284:1] == 0x02) or (link[282:2] ==
0x3501 and link[284:1] == 0x05) or (link[282:2] == 0x3501 and
link[284:1] == 0x06)
(simplification of that is left as an exercise for the reader). I did
a 2-byte comparison because that generates fewer BPF instructions; it
might be that a 4-byte comparison after masking out the 4th byte would
be even better.
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