Port 25 filters - how many here deploy them bidirectionally?

Todd Vierling tv at duh.org
Mon Jan 10 16:08:05 UTC 2005

On Sun, 9 Jan 2005, Suresh Ramasubramanian wrote:

> Applying port 25 filters both ways (inbound and outbound to your
> dialup pool, instead of just outbound port 25 filtering) would help in
> such a situation.

It's good to clarify that this "bidirectional" filtering does not mean
filtering inbound to port 25 on the dialup box, but rather filtering inbound
*FROM* port 25 to the dialup box on any port (after all, you want to block
the 3WHS SYNACK and the subsequent in-stream ACKs).  This is a common, but
critical, mistake.

-- Todd Vierling <tv at duh.org> <tv at pobox.com>

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