rDNS: Monitoring the Flip Side of DNS

DNS monitoring is an important part of keeping your services available. DNS is what allows your browser to turn the name ‘nodeping.com’ into the IP address 192.95.37.22. Without proper functioning DNS, your website, email, and other services would be unreachable.

One often neglected part of DNS monitoring is the rDNS, or reverse DNS, entries. As the name suggests, rDNS is the reverse of DNS. It maps an IP address to a hostname using a special PTR DNS record type. In essence, it associates an IP address to a specific hostname or domain.

PTR records are used by all kinds of utilities and services like the humble ‘ping’ and ‘traceroute’ as well as more complex FCrDNS-enabled services. A forward-confirmed reverse DNS (FCrDNS) verification uses the PTR record to associate a domain owner with an IP address. It’s not a rock-solid form of validating ownership but is usually considered enough to be used for whitelisting servers for SMTP services because spammers usually can’t fake an rDNS record when they forge domains.

If you’re sending email from a server, you should have a proper PTR record in place that includes the domain for the ‘from’ address. This will help ensure your email from that server will not get sent to the spam bucket. A PTR, or ‘pointer’, record is usually configured by whomever owns the IP address so you often have to put in a ticket with your colocation or service provider to set or change rDNS entries.

NodePing is one of very few server monitoring services that can monitor rDNS entries. To set up a rDNS check, select ‘DNS’ from the check type and ‘PTR’ from the record type dropdown. It’s important to note that your PTR record will be in what is commonly called ‘arpa’ format. An example of the ‘arpa’ format for the IP 192.95.37.22 is ‘22.37.95.192.in-addr.arpa’ – please note how the octets are in reverse order, not the numbers. Set the ‘arpa’ address for your IP address in the ‘Query’ text field. You’ll also want to set the ‘Expected Response:’ field to the hostname for that IP, example: ‘api.nodeping.com’.

For more information about rDNS/PTR monitoring or our DNS check capabilities in general, check out our DNS check documentation.