Are separate dns records necessary for web and mail on a single server

domain-name-systememail-serverhostnamemx-record

When setting up a web/mail server on a single server or VPS with one IP address almost every guide online follows the same structure for DNS:

example.com.               IN A    192.0.2.0  
hostname.example.com.      IN A    192.0.2.0  
mail.example.com.          IN A    192.0.2.0  
example.com.               IN MX   10 mail.example.com.  
0.2.0.192.in-addr.arpa.  IN PTR  hostname.example.com

I used this several times and it has always worked for me. But I was wondering if there's a reason why the mail server points to a different name? Would it be possible to point the MX record to the hostname and simply use hostname.example.com for SMTP and POP3?

Best Answer

Yes, it would be possible, but you will lose some important advantages if you choose to do so:

If you point all services to the same DNS name, you can't put them onto separate servers any more without reconfiguring any client that refers to them.

As an example: With different names, when the load on the server grows too much, you can simply offload the mail services to another server without affecting the clients. All you have to do is to adapt your DNS records.

Related Solutions

DNS and PTR for SMTP: shared IPs and subdomains

You can have as many A RRs as you want point to 1.2.3.4.
You must have one PTR point back to a name.
Therefore all you need is to make sure that the PTR points to the name you want and that this name points back to 1.2.3.4.

This means that you can have:

example.com. IN A 1.2.3.4

example.com. IN MX 10 mail.example.com.

server.example.com. IN A 1.2.3.4

www.example.com. IN A 1.2.3.4

mail.example.com. IN A 1.2.3.4

and:

4.3.2.1.in-addr.arpa. IN PTR server.example.com.

Why is this the requirement? Because when your SMTP server connects to a remote SMTP server to deliver mail the SMTP server knows only the address of your server and not the name. With the address at hand (1.2.3.4) it queries the DNS and gets the PTR response (server.example.com). The remote server now will ask the DNS again "what is the address of server.example.com" and it expects that answer to be 1.2.3.4.

What you send in the HELO string should not frighten you. You can read about the purpose of the SMTP HELO and find about the exceptions where it is allowed to block based on what is given in the HELO string.

Setting up DNS records for a shared host

Addressing 1 and 2: In most cases, I would recommend doing a single "A" record for the actual IP associated with the box and CNAME everything you need to that. If you are going to have a lot of somethings.example.com, I would make use of a wildcard entry

example.com     A      192.168.0.1
*.example.com   CNAME  example.com

The benefit being that you don't have to do a lot of DNS maintenance as whatever .example.com you use will already match. You just let apache figure out what to do with it based on the name given. This would result in 2 look-ups, and I would only do many CNAMEs to one A, and not try doing CNAME->CNAME. Additionally MX records must only point to an A record name.

If you ever need to break a vhost off to its own IP addres, you can then just add an "A" record for that one host and the most-specific answer will win.

example.com     A      192.168.0.1
*.example.com   CNAME  example.com
new.example.com A      192.168.0.2

Best Answer

Related Solutions

DNS and PTR for SMTP: shared IPs and subdomains

Setting up DNS records for a shared host

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