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 email relay. This is because email servers normally use their domain name when communicating with each other via the so called HELO (or EHLO) message.

For anti spam purposes, its common on the internet to require that the HELO message (which might be for example 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 (asking what IP-address smpt.binero.com points to), its less likely that the email is spam.

Aside from this, when doing a traceroute (or other diagnostics involving IP addresses and routing) its good practice to add a PTR record to understand the routing path by using more human naming.

Using PTR in Binero cloud

PTR is (via the platform) only available on Floating IP addresses (not on Directly attached IP addresses).

While you would be able to setup a local DNS server on your project to service PTR records for your internal IP addresses, 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).

The TTL determines how long DNS servers should cache the DNS record. What this means is that when a lookup of the PTR record 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 amount of time, in seconds.

This has many advantages, load becomes distributed and concurrent DNS resolutions of the same domain is 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 (3600 seconds) as a well rounded TTL.

Note

Setting less than 3600 seconds might appear to work but our backend changes this to 3600 as this is the smallest number we allow.

Please see below for information on how to setup a PTR record on a floating IP.

Adding PTR records using the cloud management portal

To assign a PTR record to a floating IP by using the Cloud management portal

  • 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 example smtp.binero.com.). Note the final dot in the example 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 example external mail relay).

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

  • Press Save

Note

You are not able to unset a DNS record by using the cloud management portal (but you can change it). This can be done using either horizon or the terminal client, see below.

Adding PTR records using OpenStack Horizon

To assign a PTR record to a floating IP by using the OpenStack Horizon

  • 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 example smtp.binero.com.). Note the final dot in the example 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 example external mail relay).

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

  • Press Save

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

Adding PTR records using OpenStack terminal client

To assign a PTR record to a floating IP by using the OpenStack Terminal Client

  • 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 (such as 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 remove) the record using the following command: openstack ptr record unset [FLOATING IP ID]

Note

To be able to run above terminal client commands, you might need to install the Designate client to add the commands to the OpenStack terminal client.

You can do this by running pip install python-openstackclient python-designateclient.

See also