On Nov 12, 2013, at 7:57 PM, "Steve" <st...@iwwd.com> wrote:
> How can I get a working filter to capture the data I need?
Modify the code in libpcap's gencode.c so that the comment in gen_load():
/*
* The offset is relative to the beginning of
* the transport-layer header.
*
* Load the X register with the length of the IPv4 header
* (plus the offset of the link-layer header, if it's
* a variable-length header), in bytes.
*
* XXX - are there any cases where we want
* off_nl_nosnap?
* XXX - we should, if we're built with
* IPv6 support, generate code to load either
* IPv4, IPv6, or both, as appropriate.
*/
(in particular, the last XXX part) no longer applies.
Or write your own filter expression that looks at the link-layer header to
detect IPv6 packets, skips past the IPv6 header (you won't be able to handle
extension headers, as that involves looping, and the libpcap filter language
doesn't support that), and then loads the appropriate value from the TCP header
based on that.
There is no easy solution. Sorry.
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