- The DNS server is incorrectly specified in the Operating System's configuration.
- The DNS server is correctly specified in the Operating System's configuration, but the DNS server itself is misconfigured
- The hostname that JIRA is attempting to resolve is incorrect
- There is some other network error causing the JIRA instance to be unable to reach the DNS server
You can enable the java networking logging and take a closer look at the traffic.
Alternatively you can switch over to a different DNS server.