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Re: packets with SYN and FIN set not discarded! what does"scrub" actually do ?



--As off Saturday, January 24, 2004 6:42 PM +0100, Per-Olov Sjöholm is alleged to have said:

Hi !

A friend yesterday scanned my firewall with nessus. One thing he
found was that nessus said:
"The remote host does not discard TCP SYN packet which have the FIN
flag set. Depending on the kind of firewall you are using, an
attacker may use this flaw to bypass its rules."

I do however use:
block log all
scrub in on $INTERNET_INT all fragment reassemble
And on all incoming TCP "permit" rules I use "S/SA" as the flag
combination.

The 'S/SA' is what is confusing you here. The syntax for that is: 'accepted/watch'. So pf here is only checking to see if the packets have the S or A flags set, and only accepting those that have the S flag (and not the A flag). All other flags are ignored. If you want to block packets with SF set, you need to put that in the 'watch' section: 'S/SAF'


Exactly which flags you should watch is a subject of much debate. A general consensus at one time was you should say at least 'S/SAFR', but there were various opinions about what else might be a good idea.

Scrub doesn't touch the flags.

Daniel T. Staal

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