ISP port blocking practice

Kevin Oberman oberman at
Sun Sep 5 06:28:42 UTC 2010

> From: "Dobbins, Roland" <rdobbins at>
> Date: Fri, 3 Sep 2010 21:07:49 +0000
> On Sep 3, 2010, at 8:02 PM, Patrick W. Gilmore wrote:
> > Could you point to more than one instance?  I've not yet found one.
> I've yet to run across this, either, FWIW, except on extremely
> restrictive special-purpose endpoint networks.  Doesn't mean that it
> doesn't happen, but it doesn't seem to be nearly as prevalent as
> TCP/25 blockage on general SP access networks.

Worst case I have seen was the visitors network at EBC at one of the
nation's largest telephone and Internet transit providers. They seemed
to block ALL outgoing ports except 80, 443, and 22. No VPN. No
submission port. No IMAP. (Didn't try POP3.) I tunneled mail over ssh,
but I can imagine that a lot of corporate types who meet there are
rather annoyed that they can't access mail.
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751

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