DNS doesn't end at MX you know...

Ok, so you understand MX records, you're ready to take on the world, right?

Not quite yet. Patience young grasshopper.

So, you're sending email to user@domain.com, you queried dns1.domain.com for the MX records and now you've got these MX records:

Host Priority Record
domain.com 10 mail1.domain.com
domain.com 20 mail2.domain.com
domain.com 30 mail3.domain.com

Ummm, so, computers don't speak English, they don't know what a "mail1.domain.com" is. They want digits that can turn into nice 1s and 0s that they can compute . What's a poor mail server to do with this stuff?
While we're at it, how did we even know to talk to dns1.domain.com to get these records?
Also, once we knew to talk to dns1.domain.com, we're back to the first question of how does the mail server know WTH a "dns1.domain.com" is, and where are the 1s and 0s it needs?

I is so confus :-(

Don't worry, there's more to an email's life than just MX records. There's lots of types of records including type A, type SOA, and type CNAME.

I. Type A records

Type A records ("A" for "Address") turn "mail1.domain.com" into a nice pretty number (the "IP Address" or "Internet Protocol address") for our mail server (and all other computers) to compute. The A record might look like this:

Mail1.domain.com IN A 10.0.100.28

Now we can see that the first MX record for domain.com has a name of "mail1.domain.com", but that name actually resides at 10.0.100.28. This number means nothing to most of us silly humans, but to a computer it's a nice string of digits that it can compute and then connect to.

TCP/IP and all of it's routing mechanisms are beyond the scope of this article, so you'll just have to make do with "once you have the IP you can connect to the server". :-P pbbt!


II. SOA records

OK, so this is a little backwards since you need the SOA records before you can get the MX records or A records, but you should already understand MX records, and you need to understand A records before you can get SOA records.

SOA records are the "Start Of Authority" records. There are literally millions of DNS servers in the world. How do you know which ones are responsible for the records you're querying? Well, that's where SOA records come into play. SOA records are given by the "root" name servers to offload queries to other DNS servers, and to provide a distributed architecture that is easier to maintain and provides more fault tolerance.

Root name servers are the biggest and baddest name servers in the world. There are two sets of them for your .com, .net, and the like, one at root-servers.net and one at gtld-servers.net. If queried, they will either reference the other one (like a father who says "go ask your mother", or vice-versa) or they will give the DNS server registered with them as authoritative.

You can play with this yourself using nslookup on Windows or dig on *nix.

Once the root servers give the name of the DNS server, that server's A record must be resolved then it can be queried for the MX, A, CNAME, or any other type of records you wish.

III. CNAME records

CNAME records are basically aliases to A records. For instance, "www.domain.com" might be the same server as "ftp.domain.com", so you could make ftp.domain.com a CNAME for www.domain.com, then an additional query would happen for the A record "www.domain.com" which when resolved would then allow transmission.

Typically, CNAMEs should not be in MX records. There's a ton of debate over whether they're allowed in the MX at all according to RFCs. I'm not going to enter that debate, but I will say that there are enough mail servers on the planet that don't respect CNAMEs in the MX that you should do everything in your power not to put them there. At the very least it adds an additional query to your DNS server (to get the IP of the A record in the CNAME), so for simplicity's sake I'd recommend against it as well.

One time a CNAME might be used is if you have a server that accepts email that does not have MX records. Originally, user@domain.com was actually literally user@some.server.com. You could even address email to an IP address like "user@10.0.100.28" for example and it would work. Most mail servers will fall back to this convention if MX records cannot be resolved, so you could for instance have a CNAME errors.domain.com pointing to "mail1.domain.com", then you could email "www-server@errors.domain.com", "email-server@errors.domain.com", etc which would then actually send the email to mail1.domain.com.

NOTE: this does not alias other record types, only A records. For instance, setting domain2.com as a CNAME for domain1.com DOES NOT make domain2.com's MX records the same as domain1.com's. CNAME records are not copies of the zone, they are address aliases and will only route traffic to the A record they point to. Additionally, it's technically against RFCs to have both MX and CNAMEs for the same zone. I.e. if you have www.domain.com as a CNAME, you can't also have MX for www.domain.com.



So, there you have it. MX record resolution requires SOA lookups, which require their own A record resolution. MX records include A records typically, which require their own resolution by the SOA. CNAMEs shouldn't be used in the MX, and can't have their own MX, but they can still be used to route email in a pinch.

Try using nslookup or dig to do your own DNS recursion. Once you realize how many DNS queries each email requires, be sure to give your DNS server a big pat on the back for so many jobs well done.


-TEA