ISPs are asked to block yet another port

Paul Vixie vixie at
Mon Jun 23 17:56:50 UTC 2003

chris at UU.NET ("Christopher L. Morrow") writes:

> ISP's could block all ports and save everyone the hassle of having an
> Internet.... (I am just kidding of course)
> Two interesting points though:
> 1) Spammers adapt
> 2) default insecure OS installs cause problems

3) thoughtless reactionism at isp's does little good and sometimes some harm.

take for example port-25 blocking.  i've been getting relayprobed all
weekend by someone who gets around outbound at&t's tcp/25 SYN blocking
by sending their SYN's through a provider who shall remain nameless
(except that chris morrow happens to work there :-)) using at&t IP
source addresses.  i guess they multihomed their host and bind()'d the
outbound socket to one interface even while making sure the routing
used a different interface.  high rocket science?  NOT.

so if you're going to block tcp/25 SYNs on outbound, please make sure
you block SYN/ACK's on input too, or else you just give the spammers a
little more work to do instead of a lot more work to do.
Paul Vixie

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