How to increase max connections for PDNS

Symptoms

Connections to your DNS server are failing or timing out, and your server's system log shows errors similar to the following:

Jan 1 00:00:00 hostname pdns_server: Limit of simultaneous TCP connections reached - raise max-tcp-connections

Description 

A large number of simultaneous connections can exhaust the PDNS server's connection limit, and it will not be able to respond until a connection is freed.

Workaround

We can change the setting within the config file for PDNS:

1. Connect to the server over SSH as root.
2. Open /etc/pdns/pdns.conf in your text editor of choice.
3. Find the following entry. By default, it is commented out:

   # max-tcp-connections=20

4. Remove the comment, and update the number to be higher. Start out by raising it in small increments of 5 or 10:

   max-tcp-connections=25

5. Save your file and close the editor.
6. Restart the PDNS service to load the changes:

   /scripts/restartsrv_pdns

If you continue to see connection issues after raising the limit a small amount, you may try raising it again until the connections can be made reliably.