Re: Dropping syn+fin replies, but not really?

Eirik Øverby <ltning@xxxxxxxxxx> writes:
I have a FreeBSD based firewall (pfsense) and, behind it, a few dozen
FreeBSD servers. Now we're required to run external security scans
(nessus++) on some of the hosts, and they constantly come back with a
"high" or "medium" severity problem: The host replies to TCP packets
with SYN+FIN set.

Problem: Both the firewall (FreeBSD 6.2-based pfSense 1.2) and the
host in question (recent FreeBSD 7.2-PRERELEASE) have
net.inet.tcp.drop_synfin=1 - I would therefore expect this to be a
non- issue.

I added drop_synfin for one reason and one reason only: it prevented
nmap from reliably identifying a FreeBSD machine, and at the time, that
was sufficient to ward off the kind of script kiddies that would
regularly attack EFNet IRC servers. I don't think SYN+FIN packets were
ever a security issue, and I'm surprised Nessus thinks they are.
Perhaps someone read about drop_synfin and misunderstood its purpose?

Back to the issue at hand: you should use tcpdump to double-check
nessus's findings. Who knows, perhaps drop_synfin was broken in a
network stack reorganization.

Dag-Erling Smørgrav - des@xxxxxx
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