Reset conntrack...

Sven Anders anders at
Wed Dec 8 12:20:25 CET 2004

Hash: SHA1

Carl-Daniel Hailfinger wrote:
| 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.

The rules will be set by an (normal) user. So I cannot assume the user to
understand why he should insert this rule too...
Automatically generating this rule is complicated too, because this
rule could conflict with some other rule...

|>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
| kernel.)

~ Sven

- --
~ Sven Anders <anders at>

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