Re: Blocking Port scans

[Chris Moody <chris () siliconhotrod com>]

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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