[NCLUG] comcast blocking port 25?

DJ Eshelman djsbignews at gmail.com
Mon Mar 26 12:54:23 MDT 2007


indeed.  My big problem lately has been with Qwest DSL - the speed I get for
the price is hard to resist (6 Mbit down, 756Kbit up) and has been very
consistent... but when I signed up, I got the MSN package, which expects you
to send outgoing mail thru their http mail based servers.  This is a royal
pain, of course - and even if they had location-based SMTP servers, it'd
still be a problem for my laptop sending out.

(now, I'm kind of 'cheating' here because I happen to have this resource
available)

So I made a deal with my boss- I'd set up my CentOS server at our office and
allow our clients to use it for POP/IMAP/SMTP(auth) access.  Previously we'd
been using an old qMail setup that has been a real pain to get SMTPauth
working on.  Postfix with Dovecot was easier for me to work with.  So for a
little 'free work' and some squatters on my server, I've got a static IP
with reverse DNS on a T-1 line.

Now for the question:
Is it possible for mail from a dynamic IP to have a reverse DNS entry?  How
did you go about detecting if it was from a Dynamic IP or not?

I guess I shouldn't really be too concerned - the combination of SA-update
running daily and Reverse DNS is catching some 90% of UCEs coming to my
server anyway.  Another 5% or so are subject tagged as potentials.
My next project will be setting up a web-based quarantine system... but
that's down the road.

Life before having a dedicated SMTPauth server was a lot like life when I
had dialup :)  Sure, I could use webmail... but what's the fun of that?

-DJ

On 3/23/07, Bob Proulx <bob at proulx.com> wrote:
>
> DJ Eshelman wrote:
> > Most mail servers these days (mine included) are doing reverse DNS
> lookups,
> > which means if you are sending from your own machine on Comcast's
> network
> > you would not have a valid reverse DNS, and the mail server would either
> > reject your email or send the SA score thru the roof.
>
> I do something slightly different.  If the mail is being sent from a
> dynamic IP address then I reject it.  This cuts out a large amount of
> virus generated spam coming from the distributed spamming engines of
> infected MS machines.
>
> This almost never affects the random windoze user since they usually
> use comcast, aol, msn, yahoo, hotmail, gmail, etc. and of course all
> of those have static IP addresses.  So it is never a problem for them.
>
> Interestingly the only people that this tends to affect is a linux
> user who tries sending directly from a cable modem.  I find that
> ironic because they are more tech savvy but in this case a little bit
> of knowledge is a bad thing and they get tripped up.
>
> Mail relays should have static IP addresses.  (Okay, I have the flame
> proof underwear on, give me your best rebuttal.  Just don't expect me
> to receive your email.)  In order to send mail from a dynamic IP
> address the message needs to relay through a static IP such as the
> ISP's mail relay or perhaps through a VPN or other through another
> mail relay.  There are a lot of possibilities to make that happen.
> The random linux user on a cable modem shouldn't feel too cramped.
>
> Bob
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