[Snort-users] Documentation suggestions regarding the unreliability flexresp.

Jeff Nathan jeff at ...950...
Mon Jul 28 12:45:18 EDT 2003

Hash: SHA1

When flexresp2 is ready, it will be accompanied by new documentation.

- -Jeff

- --On Friday, July 25, 2003 13:17 -0400 Matt Kettler <mkettler at ...4108...> 

> It seems to be a common misunderstanding that flexresp actually works
> well and is usable as a reliable alternative to a firewall.
> Certainly nobody that understands how flexresp works would be foolish
> enough to think of it as a firewall alternative, but the documentation
> that comes with snort fails to make it clear that flexresp can be
> bypassed 100% of the time by a skilled attacker, and that it may not even
> work relaibly against "routine" traffic.
> I'd suggest that all the documentation regarding flexresp be updated to
> have at least some mention of the fact that it is unreliable.
> docs/README.FLEXRESP is a VERY obvious target that should have a mention
> of this. I'd also suggest that the "react:block" in the web documentation
> have some mention of it.
> http://www.snort.org/docs/writing_rules/chap2.html#tth_sEc2.3.24
> Something along the lines of this would be appropriate:
> "It should be noted that the Flexresp mechanism is not a reliable one and
> should be treated as a "last resort" type option. If a skilled attacker
> is aware that flexresp is being used he can craft his packets to be able
> to evade flexresp with near 100% chance of success. Thus in the case of a
> skilled attacker flexresp will merely slow the attacker down by thwarting
> his "first try". This might give you some time you have to respond before
> he modifies his attack to get around it, but it will not stop a carefully
> crafted second try at the attack. Even in the case of an automated
> script, there is always a small chance that flexresp will fail to be able
> to close the connection before it is too late, so it cannot be relied
> upon as a sole defense against worms and scripts either.".
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