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Defending Your Site Against Spam, Part 2
Pages: 1, 2

Incoming SMTP Mail

Now that we have some of that light background on the philosophy of qmail, let's see how it expresses itself through an incoming email.



qmail, like most email systems, is all about moving email into and out of various queues for delivery. qmail has one queue where it receives all incoming email to be analyzed for delivery. Email is injected into that queue via a program called qmail-queue. True to the DJB philosophy, that program insures that email properly gets into the incoming queue safely and securely. It will not respond with a success code unless every step went properly. It is the responsibility of other small programs to receive mail via SMTP, QMQP, or the command line, and then properly feed the mail to qmail-queue. For our case, we're only concerned with the SMTP handler, since that is how systems receive mail from the internet. The specific qmail program that handles an SMTP transaction is qmail-smtpd.

Since qmail programs are designed to do the minimum necessary, the qmail-smtpd program just handles the SMTP conversation on a socket. To be clear, it will not setup the socket for listening or wait for connections. It relies on some other program to do those things. This is an interesting design choice because it allows us to insert other programs to perform checks in front of qmail-smtpd.

In the past, most servers on Unix would use the inetddaemon to setup and listen on sockets. qmail, being a program from that era, was probably just conforming to that norm. These days, it is rare for a server to use an inetd since it is poor at handling lots of connections or a hostile Internet. The interesting thing here is that the older design style allows us to add new filtering capabilities to qmail without having to change any of the existing qmail code. So in order to solve the inetd deficiencies and to keep the system extensible, DJB wrote another small program called tcpserver. Its sole purpose is to perform socket setup, filtering, and listening. It has several parameters for setting TCP options, checking a simple list of IP addresses to block, or performing anti-spoofing checks. When a connection does arrive, tcpserver sets some environment variables and starts the program specified on it's command line. The main environment variable we are concerned with is TCPREMOTEIP,which contains a string representation of the remote host's IP address.

Normally, in a qmail installation, tcpserver will run qmail-smtpd as its next program in the pipeline. However, we want to filter our incoming SMTP connections before we accept an email. Therefore we need something to query our Trustic account via IP4R before we accept an email via TCP. As luck would have it, DJB wrote a program to do just this. That program is called rblsmtpd.

The rblsmptd program, as you'd expect, does one basic thing. It checks the incoming connection using IP4R. If all goes well, it runs a program specified on its command line. This is that same basic pipeline API of handing off the socket. In order for rblsmtpd to do its job, it checks the TCPREMOTEIP environment variable, making an IP4R query against some system specified on the command line. Depending on the outcome of that and the influence of a few command line arguments, it will either end the SMTP transaction or run the next program. In our case the next program to run will be the qmail-smtpd.

rblsmtpd has some important command line arguments that have important semantics, so I'll cover them in detail here. (These descriptions are straight from the documentation). They handle how your system deals with an IP4R request success or failure. That success or failure in turn determines how and when mail will be rejected. Obviously, this is important.

Command line options for rblsmtpd:

  • -r base: use base as an IP4R/RBL source. An IP address a.b.c.d is listed by that source if d.c.b.a.base has a TXT record. rblsmtpd uses the contents of the TXT record as an error message for the client.
  • -B: (default) use a 451 error code for IP addresses listed in the RBL.
  • -b: use a 553 error code for IP addresses listed in the RBL.
  • -C: (default) handle RBL lookups in a "fail-open" mode. If an RBL lookup fails temporarily, assume that the address is not listed.
  • -c: handle RBL lookups in a "fail-closed" mode. If an RBL lookup fails temporarily, assume that the address is listed (but use a 451 error code even with -b.

For my system, I chose the following change to my tcpserver's startup line:

rblsmtpd -b -c -r 1234567.query.trustic.com \
	/var/qmail/bin/qmail-smtpd

Primer on Mail Bounces

In order to understand the effects of these, let met give a quick primer on mail rejection or bouncing. There are really only two forms of mail bounces, in layman terms, "Hard Bounce" and "Soft Bounce". A Hard Bounce causes the sending mail system to give up immediately on delivering an email, generating an error email or "bounce" to be delivered to the sender. A Soft Bounce causes the sending mail system to give up temporarily on delivering an email. The sending mail server may then try again after an hour or some other specified timeout. If the mail continues to get a "Soft Bounce" code from the receiving mail server for some specified time (between two days and a week), the sending mail server will give up and deliver a bounce message. In both cases, the sending mail server takes responsibility for generating or delivering the bounce.

In the above configuration, I chose the -b and -c command line parameters. With -b, I'm choosing to cause a Hard Bounce to any email from an untrusted host. This is important because I want senders to know immediately if there is a problem. With the -c parameter, I'm choosing to Soft Bounce if Trustic should happen to have any downtime. I wouldn't want to allow spam into my system just because of a small outage and I don't mind delaying that email. Besides, I monitor my systems so I could change this if it ever happened for a long time. If someone were blocked, they would know immediately that the email delivery failed.

What Would a Blocked Sender See?

Finally, Trustic provides the optional TXT record that the IP4R protocol specifies (see previous article on the IP4R protocol). The rblsmtpd program will include that message in the SMTP error code. The sending mail servers will then copy that SMTP error into the bounce message to the sender. Trustic puts a URL into the message so the sender can go to a site and read the details on why the block occurred.

Here is an example from my logs:

553 Message rejected, please see
	http://www.trustic.com/help/bounce?ip=200.163.45.155

As always, test your setup once you have it set up. You wouldn't want to lose mail for half a day because you didn't test something.

Flipping the Switch

After I set the system up, not much happened. I sent recommendations and only some of them blocked spam. The service was too new to be effective. However, after about fifty users joined, things started to happen. Soon I was blocking about 8 to 10 spam a day without making those recommendations. After a few weeks, the real test came. I got hit with 1500 email attempts to my server. The attack wasn't as severe as the previous one, but it could have caused the same problems. This time, however, it produced zero load on my system. I was dropping all of the requests. I didn't even know it was occurring until I checked my logs for that day. Success!

After that, another interesting thing happened. After a while, my recommendations were having a good effect on the other mail systems using Trustic. I could see in the reports that some of my recommendations were blocking hundreds of spam emails from being delivered to other systems. It felt really good to give back to the others that helped block spam for me.

Conclusion

I hope these two articles have been achieved their goal of providing some good coverage of network level spam defenses. From my own recent experiences, I have seen how the use of some simple, existing protocols and a trust network could become a serious deterrent to the new spam attacks. In the future, these defenses may protect us from more than just spam. I look forward to seeing more people joining in and applying these systems to their own networks.

Resources

Dru Nelson has been on the Internet since 1988. After starting an ISP in Florida, he moved to the San Francisco Bay area and has been involved with large Internet infrastructure at companies like Four11 (Yahoo Mail), eGroups (Yahoo Groups), and Plaxo. He is now the CTO and co-founder of BrightRoll.com.


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