How To Configure Zimbra rDNS? | Zimbra

The rDNS or reverse DNS resolves the domain name associated with an IP as opposed to DNS which resolves the IP associated with a domain name. This can be used as an anti-spam technique by verifying the domain name with the help of rDNS to check if it is a dynamically assigned address that is unlikely to be used by a legitimate mail server so some anti-spam policies reject them assuming that emails from these addresses are more likely to spam. Emails which does not identify themselves with a PTR record in an rDNS record will be always rejected by some incoming mail servers. Configuring rDNS is one of the first and most crucial routines that admins do, so learning how to configure it would be essential.

How to Configure rDNS

To set up rDNS you need first to create the reverse zone then add a PTR record to it.

1. Create your inverse address
   • Start by dropping the final octet (last set of numbers) of your IP address, for example, 192.168.0.100 gives 192.168.0,
   • Reverse the remaining fragment of the IP address, for example, it gives you 0.168.192,
   • Append .in-addr.arpa to the end to get your inverse address, for example, it gives you 0.168.192.in-addr.arpa.
2. Create a reverse zone
   • Access your DNS provider, for example, GoDaddy, Network Solutions, etc,
   • Access your DNS Management or something like name server management,
   • Add a new DNS zone,
   • Insert your IP in the IP address field,
   • Insert created inverse address in the previous part,
   • Save the zone.
3. Add your PTR record to the DNS
   • Access your DNS provider, for example, GoDaddy, Network Solutions, etc.
   • Access your DNS Management or something like name server management.
   • Access the created reverse zone.
   • Add a new PTR entry.
   • Set the Record Type to PTR.
   • Insert your domain and IP in the Hostname field in this format yourIP.yourDomain.
   • Insert your IP in reverse in the Value field, for example, 100.0.168.192 is the reverse of 192.168.0.100.
   • Assign the Time to Live (TTL), for example, let’s use 3600s.
   • Save the entry.
4. Edit the SMTP Banner and match it with the external rDNS.

Fore Zimbra 8.0.X

zmlocalconfig -e postfix_smtpd_banner="mail.example.com"
zmcontrol restart

Fore Zimbra 8.5, and above

zmprov ms  `zmhostname` zimbraMtaSmtpdBanner mail.example.com
zmcontrol restart

How to Verify Your rDNS

The next step would be testing your rDNS to see if it works. To do so, please see How to Verify Your rDNS Record.