Steps for resolving the hostname

Determine why a hostname failed to resolve.

1. Diagnose why the hostname failed to resolve by pinging the hostname. Base your next course of action on the following conditions:

   If ping for the hostname...	Then...	Solution
   Succeeds, but another application fails when resolving the same hostname	One or more of the following may be the problem: The resolver configuration for the application in the users environment. The resolver cache has information about the hostname, and one application is able to access the resolver cache and the other is not. The resolver cache has saved different information about the hostname as provided by different name servers, and the applications are using different cached information.	Use the Trace Resolver to solve the problem.
   Fails, but the hostname is converted to an IP address	The resolution is successful but the host is not reachable or active.	See Diagnosing network connectivity problems to continue researching the problem.
   Fails to convert the name to an IP address	The problem might be with the resolver configuration, querying the resolver cache, searching local host files, or using DNS.	Use Trace Resolver to solve the problem. Note: You can use the LOOKUP option in TCPIP.DATA to specify local searching before or instead of asking DNS.

2. Determine whether the name or address being queried is known to DNS if you expect to resolve the hostname using DNS. The following example looks for the name www.johndoe.com from the DNS at IP address 1.2.3.4:

   dig @1.2.3.4 www.johndoe.com -t any

   The command should return all resource records of any type from the DNS at 1.2.3.4 for www.johndoe.com. For more information about dig, see z/OS Communications Server: IP System Administrator's Commands.

3. If dig does not return all resource records, base your next course of action on the following conditions:

   If dig. . .	Then . . .	Solution
   Fails because it cannot contact DNS	You need to check your link to the DNS IP address.	See Diagnosing network connectivity problems to continue researching the problem.
   Fails because DNS reports that the resource was not found	www.johndoe.com is not a resource record known to DNS.	See the DNS administrator to add the name. As a temporary work-around, you might want to add the name to a local host file that the resolver searches. See z/OS Communications Server: IP Configuration Guide for information about local host files.
   Succeeds	The problem in resolving the hostname using ping or another application might be in configuring the resolver, or might involve the contents of the resolver cache, or might result from the resolver treating the name server as unresponsive.	The dig command bypasses the resolver cache, search orders, local host files, and domain names that are appended by the resolver. The best way to check the configuration is to start the Trace Resolver. It is important to use the Trace Resolver in the environment where the application is failing because the application might be using a different TCPIP.DATA file, environment variables, or search order than the environment where the dig command was issued. The application might also be accessing the resolver cache, which might have inaccurate or outdated information. See Steps for resolving caching problems for more cache information. The resolver might not be sending DNS queries generated by applications because the name server is considered to be unresponsive. See Steps for responding to message EZZ9311E for more information.

4. If the Trace Resolver output indicates errors in the resolver configuration, issue the MODIFY RESOLVER,DISPLAY command and base your course of action on the following conditions:

   If MODIFY RESOLVER,DISPLAY output...	Then...	Solution
   Includes message EZD2039I WARNINGS ISSUED DURING RESOLVER INITIALIZATION	The resolver detected syntax errors or unrecognized setup statements when it was processing the resolver setup file statements during address space initialization. These errors might cause the resolver to use different settings than you intended.	Correct the errors in the resolver setup file. Issue the MODIFY RESOLVER,REFRESH,SETUP command to correct the resolver configuration.
   Does not include message EZD2039I.	No syntax errors were in the resolver setup file. The configuration error might still be a value that is incorrectly specified on a resolver setup statement, or it might be an error in the application TCPIP.DATA file.	Correct the errors in the resolver setup file or application TCPIP.DATA file. Issue the MODIFY RESOLVER,REFRESH command to correct the configuration errors.