SMTP relaying policies for Commercial ISP customers...?

Fri Feb 13 17:54:42 UTC 2004

Andy,
These are exactly my concerns, and exactly what I feel I'm going to hear from the staff and the customers.  I am going to go back and make sure there isn't a "better" solution.  Thanks for the input.

The issue we have as a dynamic IP broadband provider is that it's a royal pain to shutdown a user - especially in regards to just mail.  Lets say we have a spammer and a script detects it. We then have to track him back to the MAC address of the modem, lookup that MAC in the customer DB, shutdown his access and then reset the modem.  And at the end, he loses all access, not just mail.  With AUTH we can just stop mail access.  Yeah, sure we could try to push some access list to the modem itself, blocking mail, but those modems are so flaky to start, it'll never work reliably.  Can't just block the IP on the mail server because the user will or could just get a new IP, and then you are blocking a legit user.

I'm still not sure if the norm is for providers to let t1+ customers relay.  I have multiple OC3's and 12's from AT&T, MCI,...  Will they let me relay off their servers without SMTPAUTH?  Probably not.  

As always, comments welcome.

--
Daniel Ellis, CTO, PenTeleData
(610)826-9293

     "The only way to predict the future is to invent it."
                                                  --Alan Kay

> -----Original Message-----
> From: Andy Dills [mailto:andy at xecu.net]
> Sent: Friday, February 13, 2004 12:35 PM
> To: Dan Ellis
> Cc: nanog at merit.edu
> Subject: Re: SMTP relaying policies for Commercial ISP customers...?
> 
> 
> On Fri, 13 Feb 2004, Dan Ellis wrote:
> 
> > 1)       Residential Policy:  Enable SMTPAUTH and disallow relaying
> > unless the customer has a valid username/password.  If you're not paying
> > for a mailbox, you don't get to relay outbound.  This should not break
> > anything except those residential accounts that *should* be commercial
> > anyway.
> >
> > 2)       Broadband commercial: This is the difficult one.  These are the
> > customers that aren't big enough to rightfully run their own mailserver,
> > but they are big enough to have roaming users on their networks (coffee
> > shops, branch offices, hotels, SOHO....).  They expect relaying service
> > for either their mailserver or for all their various PC's.  At the same
> > time, they don't have many, if any mailboxes through the ISP.  My
> > thought is that they should ONLY be allowed to relay via SMTPAUTH by
> > using a residential mailbox login/pass OR they need to purchase a
> > commercial relay service (expensive because of the openness of it) for
> > their IP space.
> >
> > 3)       T1+ : These customers should not be allowed to relay unless
> > they purchase (expensive) relay services for their IP space.  Of course,
> > they can always use a residential mailbox, but will have to use SMTPAUTH
> > for it and will be restrained by the same policies residential mailboxes
> > have (low tolerance tarpitting,...).
> 
> While the amount of effort you put into this so far is commendable, I
> really think you're barking up the wrong tree.
> 
> At the end of the day, what have you done, besides annoy your customers
> and increase the load on your support staff?
> 
> I don't really see what you're suggesting being anything other than a huge
> effort, solving the wrong problem.
> 
> For any responsible ISP, the problem is the spam coming into your
> mailservers, not leaving. As long as you quickly castrate the people who
> do relay spam through you, you're not going to have an egress spam
> problem.
> 
> Since you seem to have countless hours to invest in this problem, you'd be
> better off writing a log parser to identify WHEN somebody is relaying spam
> through you, so you can react.
> 
> Something else I've seen implemented is rate limiting. Keep track of the
> number of messages sent by an IP over a variable amount of time and
> implement thresholds.
> 
> 
> I'd love to hear some of the conversations you have with your leased line
> customers, when you tell them they have to pay for "(expensive) relay
> services" to send mail through your mail server. How many times will they
> laugh before hanging up on you? :)
> 
> That's like the IRS trying to charge you for the forms...
> 
> And I'd also like to see the looks on your technical support staff's faces
> when you tell them they need to assist your ENTIRE USER BASE in switching
> to authenticated SMTP :)
> 
> And then you have to deal with the customers who have MTAs that don't
> support authenticated SMTP...and on and on.
> 
> Whenever the solution is more expensive than the problem, you need to go
> back to the drawing board.
> 
> Andy
> 
> ---
> Andy Dills
> Xecunet, Inc.
> www.xecu.net
> 301-682-9972
> ---