How to add data to connection tracker

[Nils Rennebarth <Nils.Rennebarth@web.de>]

Hi,

I am developing a netfilter target extension synproxy, that will work similar to the openbsd pf synproxy,
i.e. it will (if a synflood to the destination address is detected) block the syn packet and answer with a
syn cookie. If a correct ACK to the cookie is found it will send the syn packet to the actual server, intercepts the
reply and then pass packets in both directions, only translating sequence numbers. The extension could
then be used on a firewall to protect systems behind it from synflood attacks.

I need to store some additional data to a connection in the connection tracker. Although infrastructure to do that
appears to be in place, I could not find an obvious way to do that. I *did* read the kernel source and already know
how to write and register a new netfilter extension.

Btw, the netfilter hacking howto appears to be thoroughly outdated. I managed to find http://jengelh.medozas.de/documents/Netfilter_Modules.pdf which helped a lot, but not in this particular area.
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