changing packets with libipq

[Peter Hoare <Peter.Hoare@signal.qinetiq.com>]

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We are having problems in trying to use the userspace libipq to change
incoming packets to the kernel. We are picking up UDP packets off the
pre-routing queue, changing the packet header and checksums and then
accepting the new packet back through the ipq_set_verdict() call with
NF_ACCEPT.

This new packet is being delivered quite happily to an end-user
application on the local machine  but if the packet is to be forwarded
through this host to another machine it appears to be dropped by the
kernel. Is there a limit to what we are allowed to change in the packet
through libipq? (There are some hints in the documentation to do with
caching fields in the skb that may be the cause of our problem). Our
guess is that because we are significantly changing the header that the
new packet is no longer being recognised as related to the old one? 

Is what we are doing not possible using libipq? If not do we have to
write a kernel module to do it or is there any other way to do it in
userspace?

Thank you

Peter Hoare

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