Reset conntrack...

Carl-Daniel Hailfinger c-d.hailfinger.kernel.2004 at
Fri Dec 3 22:07:55 CET 2004

Sven Anders schrieb:
> Patrick Schaaf wrote:
> |>Is it possible to reset the conntrack list or set any entry to the state
> |>NEW to force a recheck against new filter rules?
> |>~  If I set the (new) filtering rules with the target DROP, I want old
> |>~  (existing) connections to be dropped immediatly.
> |
> |
> | Consider using REJECT. This has two advantages: it gives the end
> | systems you are now blocking a chance at state cleanup (instead of
> | needlessly wasting memory and CPU resources on a connection that you
> | now elect to forbit). But, the greater advantage: the packets that
> | the end systems exchange in response to the connection teardown,
> | are JUST what you need to get rid of their conntracks.
> This is not exactly what I'm meant...
> Consider the following scenario:
> ~ 1. Set firewall rules
> ~     with:
> ~      a) ACCEPT on all --state RELATED,ESTABLISHED
> ~      b) with ACCEPT on ports 22 and 80
> ~ 2. Remote client creates connection through the firewall
> ~     on the port 80 (CONNTRACK state is: NEW)
> ~     It will be allowed due to the ACCEPT policy...
> ~ 3. Server answers and connection CONNTRACK state will be changed
> ~ 4. Set new firewall rules:
> ~      Changed b) to only allow port 22
> ~ 5. The connection to port 80 will continue to exists, because
> ~    it CONNTRACK state did not change and we have rule a)...
> Possible solutions:
> ~ 1) Recheck all CONNTRACK entries against the new firewall rules.
> ~ 2) Set all CONNTRACK entries with states RELATED or ESTABLISHED to
> ~     NEW, to force the recheck.

Why so complicated? Insert a rule which rejects connections to port 80
before rule a). That way any packet coming to you on port 80 will cause
a reset, tearing down the connection and the problem is solved. The only
disadvantage is that your machine may continue sending data until it
requires an ack of the remote side to continue. That ack will then cause
teardown, too. If that is not good enough, consider adding a reject
rule for source port 80 in your OUTPUT table.
This will cause any connection which is now forbidden to be teared down
once either side wants to send any data.

> Is there any way to accomplish this?

Yes. (Well, it forces you to add an explicit REJECT rule for every
ACCEPT rule you remove, but that is less complicated than patching the


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