To retrieve IPv6 data from the correct name server, ensure that the resolver configuration data set points to name servers that can resolve the IPv6 queries. A resolver does not have to communicate with a name server over an IPv6 network in order to retrieve IPv6 Domain Name System (DNS) entries. The z/OS® resolver can use IPv4, IPv6, or both to communicate with a name server.

IPv6 resource records are larger than IPv4 resource records; therefore, DNS response messages are larger for IPv6 resources than for IPv4 resources. If the number of resource records in a DNS response message is large, the response message from the name server might exceed 512 bytes of data. If more than 512 bytes of data is needed to send the message, the message is truncated to fit in 512 bytes of UDP packet data. The resolver then resends the request using TCP protocols so that the name server can send the entire response message.

To eliminate the performance costs associated with switching from UDP to TCP protocols, the z/OS resolver can use Extension mechanisms for DNS (EDNS0). EDNS0 uses UDP protocols to accept messages that are greater than 512 bytes, when the name server that sends the response messages also supports EDNS0. The z/OS resolver can accept up to 3072 bytes of DNS response message data in a single UDP packet. (If the name server does not support EDNS0, responses that are larger than 512 bytes in length are truncated and resent using TCP protocols.)

The resolver dynamically determines which name servers support EDNS0 processing and modifies the DNS requests that it sends to the name server. If a name server is upgraded to support EDNS0, the resolver discovers this upgrade dynamically. The length of time that the discovery process takes depends on the frequency with which DNS responses are truncated to use UDP protocols. You can issue the MODIFY REFRESH command to cause the resolver to discover the upgrade more quickly. See z/OS Communications Server: IP System Administrator's Commands for the syntax and description of the MODIFY REFRESH command.