Small blog about system administration.

Understanding DNS MX records


What is an MX Record

MX stands for Mail Exchange Records.
MX records are used in DNS records(or Zone files) to specify how email should be routed.

Lets take an example of say liz@mydomain.com.

This is how a typical DNS record(for mydomain.com) looks like.

;
; Zone file for mydomain.com

@ 14400 IN SOA ns.mynameserver.com. root.ns.mynameserver.com. (
   109157199
   86000
   7200
   3600000
   600 )
mydomain.com. 14400 IN NS ns.mynameserver.com.
mydomain.com. 14400 IN NS ns2.mynameserver.com.
mydomain.com. 14400 IN NS ns3.mynameserver.com.

; A Record
mydomain.com. 14400 IN A 216.34.94.184

localhost.mydomain.com. 14400 IN A 127.0.0.1

; MX record
mydomain.com. 14400 IN MX 0 mydomain.com. 

mail 14400 IN CNAME mydomain.com.
www 14400 IN CNAME mydomain.com.
ftp 14400 IN CNAME mydomain.com.


Notice the line with the “MX” in it. 
This is called the MX 

record.mydomain.com. 14400 IN MX 0 mydomain.com.

The MX record shows that all emails @ mydomain.com should be routed to the mail server at mydomain.com. The DNS record shows that mydomain.com is located at 216.34.94.184. 

This means that email meant for liz@mydomain.com will be routed to the email server at 216.34.94.184. This finishes the task of the MX record. 
The email server on that server(say sendmail) then takes over, collects the email and then proceeds to distribute it to the user “liz”.

It is important that there be a dot(“.”) after the domain name in the MX record. If the dot is absent, it routes to “mydomain.com.mydomain.com”. The number 0, indicates Preferance number. Mail is always routed to the server which has the lowest Preferance number. If there is only one mail server, it is safe to mark it 0.

Multiple mail servers


Multiple email servers are useful for the sake of redundancy. If the Highest Priority email server (one with the lowest Preference number) is down, then the email is routed to the Server with the second highest Preference number.

For example

mydomain.com. 14400 IN A 216.34.94.184
server2.mydomain.com. 14400 IN A 216.34.94.185
mydomain.com. 14400 IN MX 0 mydomain.com.
mydomain.com. 14400 IN MX 30 server2.mydomain.com.

You can have unlimited MX entries for Fallback.

If all the MX records are equal Preference numbers, the client simply attempts all equal Preference servers in random order, and then goes to MX record with the next highest Preference number.

Pointing MX records to an IP

Its not possible to have an MX record pointing directly to an IP. 

For example 

‘mydomain.com. 14400 IN MX 0 216.34.94.184“ 

is wrong. 
Define an “A Record” first and then have the MX record pointing to it.

server2.mydomain.com. 14400 IN A 216.34.94.185
mydomain.com. 14400 IN MX 30 server2.mydomain.com.

MX records for Subdomains


A Subdomain is something like this “Subdomain.mydomain.com”. 
Assume you want to send an email to liz@subdomain.mydomain.com and to capture that on another server.


mydomain.com. 14400 IN A 216.34.94.184
server2.mydomain.com. 14400 IN A 216.34.94.185
mydomain.com. 14400 IN MX 30 mydomain.com.
subdomain.mydomain.com. 14400 IN MX 30 server2.mydomain.com.

In this configuration, liz@subdomain.mydomain.com would go to 216.34.94.185 and liz@mydomain.com would go to 216.34.94.184.

Testing the MX record


Once you setup your MX record, always test it to see if it is setup correctly. 
You can do with tools like nslookup.

[root@localhost sangeetha]# nslookup
> set q=mx
> yahoo.com
Server: 192.168.1.1 Address: 192.168.1.1#53
Non-authoritative answer:
yahoo.com mail exchanger = 1 mx1.mail.yahoo.com.
yahoo.com mail exchanger = 1 mx2.mail.yahoo.com.
yahoo.com mail exchanger = 1 mx3.mail.yahoo.com.
yahoo.com mail exchanger = 5 mx4.mail.yahoo.com.
Authoritative answers can be found from:
yahoo.com nameserver = ns2.yahoo.com.
yahoo.com nameserver = ns3.yahoo.com.
yahoo.com nameserver = ns4.yahoo.com.
yahoo.com nameserver = ns5.yahoo.com.
yahoo.com nameserver = ns1.yahoo.com.
mx1.mail.yahoo.com internet address = 4.79.181.14
mx1.mail.yahoo.com internet address = 4.79.181.15
mx1.mail.yahoo.com internet address = 67.28.113.10
mx1.mail.yahoo.com internet address = 67.28.113.11
ns1.yahoo.com internet address = 66.218.71.63
ns2.yahoo.com internet address = 66.163.169.170
ns3.yahoo.com internet address = 217.12.4.104
ns4.yahoo.com internet address = 63.250.206.138
ns5.yahoo.com internet address = 216.109.116.17
>

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