[sf-lug] email server problem

Bobbie Sellers bliss-sf4ever at dslextreme.com
Mon Oct 29 12:01:12 PDT 2012

On 10/29/2012 10:27 AM, Rick Moen wrote:
> Quoting Jim Stockford (jim at systemateka.com):
>>      I recently discovered a problem with the email
>> server system for Systemateka (mail.systemateka.com).
>> I've got details gleaned from various log files, but
>> so far I have not been able to find anything that
>> looks like a clue as to why the Postfix-Dovecot
>> system seems to refuse connections from other
>> internet email servers trying to pass email to
>> jim at systemateka.com
> Jim said offlist that, after the above query, friends helped him isolate
> the problem to a _firewall_ (an IP/port filter) built into his Comcast
> cable modem, that was unexpectedly blocking incoming SMTP connections.
> That sort of thing's a real puzzler when it happens, so I'm sympathetic.
> In retrospect, it's understandable that Comcast wants its overwhelmingly
> residential customers' computers to be unable by default to accept
> outbound SMTP sessions from arbitrary outside locations:  It's one of
> their antimalware things, and also Comcast typically tries to discourage
> customers generally from running Internet services.
> Anyway, it's useful to know how to manually simulate an SMTP delivery
> attempt.  On Saturday, when Jim said that he'd been unable to receive
> SMTP mail at mail.systemateka.com, I thought 'Well, what exactly happens
> when one tries?'  And there's no better way than observing an attempt in
> real time.
>    $ telnet mail.systemateka.com smtp
>    Trying
>    telnet: connect to address Operation timed out
>    telnet: Unable to connect to remote host
>    $
> The telnet utility is capable of opening an outbound connection on any
> service port, not just the telnet one.  By saying 'smtp' after the
> target hostname, I instructed the telnet client to connect to TCP port
> 25, the SMTP port.  (These numbers and names are associated in lookup
> file /etc/services.)
> The answer to 'what exactly happens' is... no connection whatsoever.
> I didn't reach the Postfix SMTP daemon that Jim said was running on the
> target host.  So, I advised Jim that he should check to see if either
> Postfix was failing to run as a daemon or firewalling was preventing
> connections.

     Is this not an interference with the Freedom of Speech, of Commerce
and of Association by Comcast?

     Thanks for the explication of the problem.

     Bobbie Sellers

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