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IPTABLE: How to ignore forwarded traffic?

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Consider the following example: you have a router between two networks, and you want to cut off the
router from the outside world using some iptables rules. However, all traffic that is forwarded by the router between the two networks basically is to be ignored by iptables (i.e., the router does not play firewall for any of the two networks).

Currently, if conntrack is loaded on the router, then conntrack -L on the router lists all the connections, not only those to and from the router, but also all connections between the two. Certainly, it takes some CPU cycles for the router to keep track of all the connections. Also, the number of connections that conntrack can take of is limited.

So is there a way to let Linux "bypass" conntrack and maybe other netfilter stuff when it comes to forwarded packets?

posted Apr 21, 2014 by Deepak Dasgupta

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You can disable conntrack at all by removing of the module. Also you can disable conntrack only for specifyed connections with CT target (--notrack option).
> You can disable conntrack at all by removing of the module.

Assume, the firewall protecting the router is stateful (i.e., it uses conntrack).

> Also you can disable conntrack only for specifyed connections with CT target (--notrack option).

I know have the following three rules:

iptables -t raw -A PREROUTING -d -j CT
iptables -t raw -A PREROUTING -d -j CT
iptables -t raw -A PREROUTING -j CT --notrack

So any traffic directed at the router or coming from the router should be conntracked while all other traffic is not. Note, that I don't have any rules in the OUTPUT chain of the raw table, as it seems to me that the default (connections are conntracked) is fine.

Do the above rules seem OK to you?

1 Answer

+1 vote

Seems like OK, but you should add iptables -t raw -A OUTPUT -j CT --notrack to prevent of tracking of the local originated traffic.

answer Apr 22, 2014 by anonymous
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I tried to use numeric gid, it failed too..

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+1 vote

According to your experience what would be the best strategy to intercept traffic from one machine to another and process some (not all) request in a transparent way.

I explain, i have two machines:

192.168.1.1/24  192.168.1.2/24

All I want to do is to intercept traffic from a specific port(s), i.e. 4000/tcp and process it in a 'machine in the middle'.

192.168.1.1/24  machine-in-the-middle  192.168.1.2/24

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