The only way to effectively block SYN packets is to have Access-lists in
place for all the hosts you are protecting. If you don't allow any
traffic to a system, you stop the SYN.
Since a SYN flag is necessary to initiate a valid connection, there is
no way to mitigate against a SYN connection attempt shy of strict
filtering of who can contact what.
While an IPS inline could drop what it detects as a "scan", I'm
personally not as comfortable relying on a signature based system as I
am fine grained restricting traffic at layer3. If it's supposed to
talk, it's allowed...everything else is dropped. I find that this rules
out the vast majority of cases.
-Chris
BSK wrote:
Hello Everyone,
Just wanted some feedback from you people. I'm doing a
Firewall Assessment for a CISCO PIX firewall. The
firewall allows SYN, FIN, NULL and XMAS scans but
blocks ACK scans (largely means its a stateful
firewall).
Now what do we do to block the scans that are allowed.
I think it should be easy to block FIN, NULL and XMAS
scans but how do we block or limit or workaround a SYN
scan. 1 way that I think is probably blocking or
limiting the packets from the source (using IDS/IPS)
Looking ahead to some ideas, thoughts, hints.
thns bshan
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