While I can't speak to what Verizon is using, Both Exim and Postfix have the very same feature called "address verification". Its in use at a number of ISPs. My systems reject 1000's of messages every day because of verification failures. Roy Engehausen -----Original Message----- From: owner-nanog@merit.edu [mailto:owner-nanog@merit.edu]On Behalf Of Rich Kulawiec Sent: Friday, December 10, 2004 9:27 AM To: nanog@merit.edu Subject: Re: verizon.net and other email grief On Fri, Dec 10, 2004 at 02:43:21PM +0000, Simon Waters wrote:
The most obvious is none of the three UK ISPs I have ready access to can connect to port 25 on relay.verizon.net. (MX for all the verizon.net email addresses). We can ping it (I'm sure it isn't singular?), but we have no more luck delivering email than contacting verizon technical staff, logs suggests we are in day 3 of this. I'm now listening to hold music at International rates - ouch.
I think I can shine a little bit of light on what might be your Verizon problem. Summary: Verizon has put in place an exceedingly stupid "anti-spam" system which does not work, which facilitates DoS attacks, and which provides active assistance to spammers. Verizon has been told all of this, and it's been discussed on Spam-L. If there's been a response from Verizon, I haven't seen it: and AFAIK the practice continues. Anyone trying to deliver mail there might want to at least skim this to get an idea of the issues they may bump into. Please note that in places this is sketchy because it seems impossible to get Verizon to provide the information necessary to make it otherwise (or correct any errors). Details: When an incoming SMTP connection is made to one of Verizon's MX's, they allow it to proceed until the putative sender is specified, i.e. they wait for this part of the SMTP transaction: MAIL From:<blah@example.com> Then they pause the incoming connection. And then they start up an outbound SMTP connection from somewhere else on Verizon's network, back to one of the MX's for example.com. They then attempt to verify that "blah" is a valid, deliverable address there. Since most people have long since disabled SMTP VRFY, they actually construct a fake message and attempt delivery with RCPT. If delivery looks like it's going to succeed, they hang up this connection (which is rude), and un-pause the incoming one, and allow it to proceed. If delivery looks like it's going to fail, then they also hang up their outbound connection (still rude), un-pause the incoming one, and reject the traffic. This also means that if the MX they try to connect to is (a) busy (b) down (c) unaware of all the deliverable addresses (d) something else, that they'll refuse the incoming message. It also means that if the address that's trying to send mail to Verizon is something like "support@thuleracks.com", which is the address that the people at Thule Racks emit support traffic from, but which doesn't accept traffic, that Verizon will deny the message. (Yeah, this isn't very bright on Thule's part, either.) Whoops. This is bad for a whole bunch of reasons: two of the more obvious ones are (a) it's a pathetic "anti-spam" measure because ANY forged address ANYWHERE will do, and (b) it doesn't scale. Add to that (c) it abuses RCPT because apparently Verizon is unwilling to use VRFY and to accept the decision of many mail server operators to disable it. Oh, and (d) the behavior of their probe systems is nearly indistinguishable from that of spam-spewing zombies, which don't obey the SMTP protocol either. [ (b) is also how it lends itself to DoS attacks. Sure, Verizon could rate-limit the rate at which they make outbound connections, but then attacker X could impose significant delay on mail from domain Y just by forging a boatload of messages purporting to be from addresses in Y to Verizon. If Verizon rate-limits their outbound connections, then any real messages from Y will be stuck in the verification queue along with a kazillion forgeries. And beyond that: other people are foolishly adopting this "callback" nonsense as well. Slashdot carried a note the other day about a program _designed_ to do this. This allows attacker X to forge messages from domain Y to idiots I1, I2... In, for a very large "n", and then stand back as all of them simultaneously try to connect to the MX's for domain Y. General principle: any "anti-spam" measure that generates more junk SMTP traffic at a time when we're drowning in it is probably a bad idea. ] One thing that's not clear is whether or not Verizon caches any of this information. Doing so might help cut down on DoS attack methods that involve them, but of course it doesn't do anything about those which leverage everyone else who's doing "callbacks". And this is unfortunately, not the end of it. A lot of people, including me, are blocking particularly problematic spammer-controlled networks at (a) our border routers (b) our firewalls or (c) our mail servers. In other words, we not only won't accept mail from them, we won't even allow them to connect: we're blocking all IP traffic from them. This prevents them from spamming (at least directly from their own network space); it also prevents them from using their resources to build lists of deliverable addresses to sell to other spammers by poking at our mail servers. Since Verizon is doing this probing *from their network*, spammers can easily get around all of our blocking by getting Verizon to do it for them. They can thus use Verizon to build/check their lists...and there's no way for us to find out who's on the other side of these probes. Which means that Verizon is running a free, anonymizing, spam support service. Note that refusing Verizon's probes, either accidentally or deliberately, will of course cause Verizon to deny incoming mail. Which I suppose might be okay for some folks, it isn't workable here. So for now all we can do is explain to our users that it's causing problems and deal with it. But if you look in your MTA logs and find things like: Jul 15 07:24:51 <XXXXXXXXXXXXXXXXX@gsp.org>... User unknown Jul 15 07:24:51 lost input channel from sc014pub.verizon.net [206.46.170.58] to MTA after rcpt Jul 15 07:24:51 from=<antispam332078@west.verizon.net>, size=0, class=0, nrcpts=0, proto=SMTP, daemon=MTA, relay=sc014pub.verizon.net [206.46.170.58] then you can see what they're doing. ---Rsk