XFOR: DNS MX Records and CNAMEs

MORE INFORMATION

DNS is used to identify computers on the Internet. The Internet Mail Connector (IMC) uses DNS to resolve Internet Protocol (IP) addresses when sending mail. A sending Simple Mail Transfer Protocol (SMTP) server also uses DNS to determine which host on the destination network is appropriate to receive mail. To determine mail hosts, the sending server checks for an MX record. Next, the sending server resolves the MX record to an IP address by checking for an address record (A record). If an A record is found, the address is fully canonicalized and mail can be delivered.

However, if an alias record (CNAME) is used for the hostname listed in the MX record, the sending host might re-write the envelope and redirect the RCPT command to the alias hostname and not the original recipient. This might cause the destination SMTP host to reject the message.

Example:

company.com.      MX 10       mail.company.com.
mail.company.com.    IN CNAME    server.company.com.
				

When you address mail to "admin@company.com" with the above configuration, the sending host might detect the fact that the "mail.company.com" is an alias and rewrite the RCPT-TO command to "server.company.com". Thus, the mail envelope written during SMTP mail transmission might be changed to "admin@server.company.com". If the mail system isn't configured to accept mail for "server.company.com" the message may be returned as undeliverable. This issue can be difficult to detect since the body of the message with the TO: line is left unchanged.

Desired Configuration:

company.com.      MX 10       mail.company.com.
mail.company.com.    IN A     127.127.127.127
				

In the above example, the MX record resolves directly to an IP address. This causes the sending host to realize that the resolved address is canonical and the final destination. Alias records (CNAME) aren't needed because the connection can be redirected to the desired CNAME's IP address directly instead of using an alias record.

RFC 1123 explicitly states that SMTP mail should be addressed to canonical name hosts. To be canonical, the DNS entry must be an A record or an MX record. CNAME records are not canonical and should not be mixed with MX records.

Note that most SMTP servers do not rewrite the message envelope when resolving through aliases. Usually this configuration issue can be detected if there are complaints that a user receives the majority of their SMTP mail but mail from a particular host is rejected even though the addressing is the same.