KB06051001 Reverse DNS
What is Reverse DNS?
Reverse DNS (also known as rDNS or Reverse MX A records (PTR) associates a given IP address with a specific domain, using the Pointer or PTR record.
Reverse DNS is used to authenticate an address with the domain.
For example, it might turn 220.127.116.11 (IP address) into example.mysite.com (hostname / domain name). rDNS does this by looking at the PTR record of the IP address in question and resolving it to a hostname based on the settings configured by a mail administrator - this is the domain name set in the Mailtraq Console at Options | Server
It is called reverse as it uses the IP address of the MX record in reverse order 18.104.22.168 -> 22.214.171.124
See example illustration below: MX 126.96.36.199 becomes 188.8.131.52.in-addr.arpa
It is used by many email servers to reduce spam by rejecting messages that do not have a valid Reverse DNS entry.
Do I need a reverse DNS entry?
The RFCs say you should have a reverse DNS for all your mail servers. It is recommended that you have them, as many mailservers will not accept mail from mailservers with no reverse DNS entry.
If your mail server does not have a PTR record set up to associate the IP address with the domain name, it will fail the rDNS test and may not be able to send email to other mail servers.
Note that although it is not technically required the rDNS record should directly reference your domain name. Anti-spam systems can check for the domain name to be in the (PTR) record.
184.108.40.206.in-addr.arpa - -> mail.example.com
How do I get a Reverse DNS entry?
This is not a setting inside Mailtraq: your ISP or Internet Service Provider must create the PTR record for you.
Contact Technical Support at your ISP and request they set up a PTR record to associate your domain name with your IP address.
You cannot do it yourself - it is your ISP that provides this service. This is not usually the same as the Domain Registrar (where you bought your domain name), but the company that provides you with Internet connectivity.
If you host your Mailtraq on an Amazon EC2 instance then Amazon must do this.
Request this from Amazon using this form...
Where can I find out more?
There are easy to follow articles about this topic at Wikipedia here
How can I check my rDNS setting?
Use one of the online services such as intoDNS...
Note that the reverse (PTR) record will be the MX Record IP Address in reverse order.
So, for the above example, the matching MX Record for the domain would point to: 220.127.116.11
If you have multiple IP Addresses be sure the ISP assigns the PTR record to the correct IP Address.
It is essential that the MX and rDNS addresses are aligned.
Changes to DNS records take up to 24-48 hours to update as mail services cache results and you have to wait for their caches to expire.