As IT professionals, we are more than often prone to fall to the perils of magical thinking. (I'm sure this is a side-effect of being human, which is a pesky and bothersome condition I will have to do something about one of these days.) Magical thinking in this context is when we have not internalized the intricacies of a problem and instead rely on formulas rather than true understanding to come up with solutions.
At one ISP I used to work at, we had a glorious reclaimed piece of technology, an Auspex NS-5500 file server. Every now and then on reboot, this old beast of a machine would fail to boot up; the cure was to open the cover over the drive cage and give it a good swift whack. We all assumed that this was because one of the drive connectors was a bit loose, but when our "magic" fix failed to work one night I discovered that it was in fact because one of the screws holding things in place was missing, allowing the drive bay to sag just a tiny bit. It was this tiny bit of sag that put just enough stress on the connector for drive 0. Had we actually opened the case up earlier, we'd have been able to solve the problem -- and prevent a year of whacking the server.
All too often, I see magical thinking in the field of security. Case in point: I recently heard about a gentleman who has a client that is requesting ETRN support be added back to Exchange 2007, either natively or through an add-on. They want to deploy the Edge role in their DMZ, have it queue up mail for the internal organization, and then have their Hub Transports (in the internal protected network) initiate a connection out to de-queue the messages using the ETRN SMTP extension. The reason they want this is that they've done due diligence and read some very thorough documents about computer network zones and have come to the conclusion that all network connections must be initiated from the most secure network. This, they say, removes the threat of malware taking over the Edge server in the DMZ and allowing an attacker to use it as a launching point to the protected network.
Now, the recommendation for connections to be initiated from a more secure network to a less secure network is a good general baseline to follow when it makes sense. However, it is not realistic in all cases (if we followed this to the letter, nobody would be able to receive e-mail from external senders except through random polling of Internet SMTP hosts, which is not at all scalable). This is doubly true if you don't understand how the underlying protocols work. Case in point: ETRN, defined by RFC 1985, "SMTP Service Extension for Remote Message Queue Starting". Quoting from section 3, "The Remote Queue Processing Declaration service extension" (emphasis added):
To save money, many small companies want to only maintain transient connections to their service providers. In addition, there are some situations where the client sites depend on their mail arriving quickly, so forcing the queues on the server belonging to their service provider may be more desirable than waiting for the retry timeout to occur.
Both of these situations could currently be fixed using the TURN command defined in [1], if it were not for a large security loophole in the TURN command. As it stands, the TURN command will reverse the direction of the SMTP connection and assume that the remote host is being honest about what its name is. The security loophole is that there is no documented stipulation for checking the authenticity of the remote host name, as given in the HELO or EHLO command. As such, most SMTP and ESMTP implementations do not implement the TURN command to avoid this security loophole.
This has been addressed in the design of the ETRN command. This extended turn command was written with the points in the first paragraph in mind, yet paying attention to the problems that currently exist with the TURN command. The security loophole is avoided by asking the server to start a new connection aimed at the specified client.
See the problem? ETRN was not designed to solve a security problem; it was designed to solve a financial problem back in days when always-on bandwidth was a lot more expensive and most ISPs metered traffic. It masquerades as solving a security problem only because it's designed to avoid a loophole in an insecure and exploitable feature. As a result, ETRN won't solve the problem these people want it to solve; all it does is tell the system in the DMZ to initiate a new connection to the Hub Transport servers. It doesn't reuse the existing connection initiated by the Hub Transport servers. They can't use a firewall rule to block outgoing access from the Edge to the Hub Transport and be safe, because they'll cut off all incoming traffic.
However, let us for a moment assume that it did work the way they wanted it to: my Hub Transport initiates an outbound SMTP session to the Edge. In this session, HT is the SMTP client, ET is the SMTP server. As soon as HT issues the ETRN command, they still have to swap roles -- HT is now using the SMTP server code paths, while the ET is using the SMTP client code paths. Any theoretical vulnerabilities that are in the HT SMTP implementation are still going to be there, still exposed to the message traffic about to be sent down the connection, still open to exploitation.
This is the magical thinking: firewalls and a DMZ will protect my traffic. This is not true; firewalls and networks zones are two components of a complete security plan. Neither firewalls nor network zones can protect legitimate traffic, nor are they designed to; they are designed to allow you to designate which traffic is legitimate. If you want to secure that traffic, you need to turn to other measures.