Multiple Entries and Priority

For each DNS server that you enter, you can specify a numeric priority. AsyncOS attempts to use the DNS server with the priority closest to 0. If that DNS server is not responding, AsyncOS attempts to use the server at the next priority. If you specify multiple entries for DNS servers with the same priority, the system randomizes the list of DNS servers at that priority every time it performs a query. The system then waits a short amount of time for the first query to expire or “time out” and then a slightly longer amount of time for the second, and so on. The amount of time depends on the exact total number of DNS servers and priorities that have been configured. The timeout length is the same for all IP addresses at any particular priority. The first priority gets the shortest timeout; each subsequent priority gets a longer timeout. Further, the timeout period is roughly 60 seconds. If you have one priority, the timeout for each server at that priority is 60 seconds. If you have two priorities, the timeout for each server at the first priority is 15 seconds, and each server at the second priority is 45 seconds. For three priorities, the timeouts are 5, 10, 45.

For example, suppose you configure four DNS servers, with two of them at priority 0, one at priority 1, and one at priority 2:

AsyncOS randomly chooses between the two servers at priority 0. If one of the priority 0 servers is down, the other is used. If both of the priority 0 servers are down, the priority 1 server (1.2.3.6) is used, and then, finally, the priority 2 (1.2.3.7) server.

The timeout period is the same for both priority 0 servers, longer for the priority 1 server, and longer still for the priority 2 server.