How the resolver polls unresponsive name servers

The resolver sends DNS polling queries to unresponsive name servers every 6 seconds. The resolver uses the value specified on the RESOLVERTIMEOUT statement in the global TCPIP.DATA file to determine how long to wait for a response to a given DNS polling query. If the RESOLVERTIMEOUT value is greater than 6, multiple DNS polling queries can be outstanding to a given name server.

Rule: If you change the value of the RESOLVERTIMEOUT statement, only the timeout value of subsequent DNS polling queries is affected. For more information about the RESOLVERTIMEOUT statement, see z/OS Communications Server: IP Configuration Reference.

The resolver continues to poll unresponsive name servers until one of the following events occur:

The name server responds to all DNS queries generated by an application and to all DNS polling queries.
If a name server is unresponsive, the only queries that the resolver sends to the name server are DNS polling queries. The resolver sends DNS queries generated by an application to the name server as soon as the name server is responsive to DNS polling queries, but the resolver continues polling a name server as long as some DNS queries result in failures. The resolver considers a name server to be responsive to DNS polling queries if one of the following situations occur:
- If the name server had responded to DNS queries that are generated by an application prior to becoming 100% unresponsive to these queries, a single positive response to a DNS polling query is sufficient for the name server to be considered responsive.
- In all other cases, the name server must respond to a sufficient percentage of DNS polling queries to ensure that the following conditions are true:
  - At least 10 DNS polling queries received positive responses or timed out within the last 60 seconds
  - The failure rate for this name server in terms of responding to DNS polling queries is less than the UNRESPONSIVETHRESHOLD percentage.
Any response from the name server to a DNS polling query is considered to be a positive response, including responses that specify NXDOMAIN, SERVFAIL, or similar responses.
The autonomic quiescing of unresponsive name servers is stopped with a MODIFY REFRESH,SETUP command. For more information about the MODIFY command for the resolver address space, see z/OS Communications Server: IP System Administrator's Commands.
The name server is removed from the list of name servers that is specified on the NSINTERADDR statements in the global TCPIP.DATA file. The resolver stops polling this name server whether the server was explicitly removed from the list by the network operator using a MODIFY REFRESH command or implicitly removed as a result of a DNS query generated by an application. For more information about the MODIFY command for the resolver address space, see z/OS Communications Server: IP System Administrator's Commands.
The resolver is stopped.