Finding asymmetric path

William Herrin herrin-nanog at
Sat Nov 28 22:28:01 UTC 2009

On Sat, Nov 28, 2009 at 2:14 PM, ML <ml at> wrote:
> Brielle is correct.  The customer in question is spamming networks and we
> are having trouble filtering them because another provider allows them to
> source traffic however they please.

What trouble? SMTP requires two-way traffic with a static port number
that nothing else uses. If for some reason you don't want to simply
terminate their account altogether, block packets outbound to your
customer sourced from TCP port 25 but not from your SMTP smarthosts.

Seriously though, if you can prove they're spamming (regardless of
whether the packets pass through your network) save yourself some
grief and just terminate the account.

Bill Herrin

William D. Herrin ................ herrin at  bill at
3005 Crane Dr. ...................... Web: <>
Falls Church, VA 22042-3004

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