Reverse DNS (PTR)¶

General concept¶

A PTR (or “reverse DNS”) record is a DNS record that returns a name from a question containing an IP-address. DNS would normally do the opposite, that is return an IP-address from a domain name. Thus, when having the IP-address but not knowing the name, reverse DNS comes into play.

The most common use-case for reverse DNS is when setting up an e-mail relay. This is because e-mail servers normally use their domain name when communicating with each other via the so called “HELO” (or “EHLO”) message. For anti spam purposes, its been common praxis on the internet to require that the HELO message (which might be for instance “smtp.binero.com”) comes from an IP-address that in turn has the same PTR record. By comparing HELO with the PTR and with a forward lookup (so basically asking what IP-address smpt.binero.com points to), its less likely that the e-mail is SPAM.

Aside from this, when doing traceroutes (or other diagnostics involving IP-addresses and routing) its good practice to add PTRs so as to quickly be able to understand to which domain an IP-address belongs.

Using PTR in Binero cloud¶

PTR is (via the platform) only available on Floating IPs (not on Directly attached IPs). While you would be able to setup a local DNS server on your project to service PTR records for your various internal IPs (as well as directly attached IPs), those would not be publicly available on the internet (since they are on private IP-space that many people use).

TTL¶

When adding a PTR record, you will also need to select a TTL (short for “Time To Live”). This is a value that regulates how long a DNS record should be cached. What this means is that when a PTR record is looked up by a resolver (which is the server that the end-user would use to lookup DNS names), that resolver will cache the record for this period of time (in seconds). This has many advantages, load is distributed and concurrent DNS lookups of the same domain is done much faster improving the user experience. The downside is that when changing a DNS record, it will take this long before you can be sure that the change is visible worldwide. We recommend an hour (so 3600) as a well rounded TTL.

Please see below for information on how to setup a PTR record on a floating IP using the various management tools.

Adding PTR records using the cloud management portal¶

To assign a PTR record to a floating IP using the Cloud management portal, follow these steps:

• Press “Network” and then “Floating IPs” in the sidebar menu.

• On the floating IP that you want to assign the PTR to, press the small “cog” icon “set PTR”.

• In the “domain” field, enter the full PTR (for instance “smtp.binero.com.”. Note the final dot which needs to be there (this is the root zone that is most often left out).

• In the description field, enter an optional description of the IP (for instance “external mail relay”).

• In the TTL field, enter the TTL (in seconds) you prefer.

• Press “save”.

Adding PTR records using OpenStack Horizon¶

To assign a PTR record to a floating IP using the OpenStack Horizon, follow these steps:

• Under “project”, click “DNS” and then “Reverse DNS” in the sidebar menu.

• On the floating IP that you want to assign the PTR to, press the “Set” button.

• In the “domain” field, enter the full PTR (for instance “smtp.binero.com.”. Note the final dot which needs to be there (this is the root zone that is most often left out).

• In the description field, enter an optional description of the IP (for instance “external mail relay”).

• In the TTL field, enter the TTL (in seconds) you prefer.

• Press “save”.

• You are also able to unset (as in completely remove) the record using the small arrow next to the “Set” button as per above.

Adding PTR records using OpenStack terminal client¶

To assign a PTR record to a floating IP using the OpenStack Terminal Client, follow these steps:

• Run this command: openstack floating ip list. Save the UUID (id column) of the IP you want to set PTR on.

• Run this command: openstack ptr record list. Find the row that matches the UUID from previous command. Save the complete id from this command (could look something like this: europe-se-1:2e6a89c0-d8ae-471e-ae28-e858c63c4c6e).

• Run this command: openstack ptr record set --description [DESCRIPTION] --ttl 3600 [FLOATING IP ID] [PTR DOMAIN].. Take care to end the PTR name (the domain), with a dot (“.”) and replace the values within angle brackets with their corresponding values. Description is optional.

• You are also able to unset (as in completely remove) the record using the following command: openstack ptr record unset [FLOATING IP ID]