The resolver setup file

The resolver setup file is an optional file (either an MVS™ data set or a z/OS® UNIX file) that contains resolver configuration statements that you can use to customize resolver functions.

If the resolver setup file is an MVS data set, the file must have the following characteristics:

• Use sequential (PS) or partitioned (PO) organization
• Use fixed (F) or fixed block (FB) format
• Contain a logical record length (LRECL) that is 80 - 256 bytes in length
• Contain any valid blocksize (BLKSIZE) for a fixed block
• If you think you will need to modify the setup file, use a member of an MVS partitioned data set

If the resolver setup file is a z/OS UNIX file, the file can be located in any directory. The maximum line length that is supported is 256 characters. If a line is longer than 256 characters, then the line is truncated to 256 characters before it is processed.

If you do not use a resolver setup file, the resolver uses the applicable native MVS or z/OS UNIX search order without any additional information and performs the following functions using the default settings:

• Enables system-wide caching (using the default maximum cache size and the default time-to-live [TTL] value)
• Monitors Domain Name System (DNS) name server responsiveness (using the default threshold setting)

The following statements are supported by the resolver setup file:

• Comments (; or #)
• CACHE

  Enables system-wide caching of DNS queries that have been resolved. System-wide caching is enabled by default, but you can explicitly enable it using this statement. For more information about caching, see Resolver caching.

• CACHEREORDER

  Enables system-wide reordering of a cached list of IP addresses in response to resolver API queries for the associated host name. If you do not specify this statement, system-wide reordering of a cached list is disabled by default.

  
• CACHESIZE

  Defines the amount of storage that can be allocated by the resolver to manage cached records.

  
• COMMONSEARCH

  Indicates that the same search order for local host files is used for both IPv4 and IPv6 name queries.

• DEFAULTIPNODES

  Identifies the default local host file.

• DEFAULTTCPIPDATA

  Identifies a default TCPIP.DATA file. The file that is specified by the DEFAULTTCPIPDATA statement becomes the last file that is searched by the resolver for resolver configuration information. If you do not specify the DEFAULTTCPIPDATA statement, the default file is TCPIP.TCPIP.DATA. For more information, see The resolver and the global TCPIP.DATA file.

• GLOBALIPNODES

  Identifies a local host file that contains hard-coded IP addresses and host names that can be used globally.

• GLOBALTCPIPDATA

  Identifies the file that is the first file that is searched by the resolver for resolver configuration information. Parameters that you specify in the file that is identified by the GLOBALTCPIPDATA statement become the global settings for the entire MVS image and for all TCP/IP stacks. For more information, see The resolver and the global TCPIP.DATA file.

  Restriction: You must code the GLOBALTCPIPDATA statement if the AUTOQUIESCE operand is coded on the UNRESPONSIVETHRESHOLD statement. If you cannot ensure that all DNS IP addresses are accessible from all your TCPIP stacks, you should not use a global TCPIP.DATA file, and you should not code AUTOQUIESCE on the UNRESPONSIVETHRESHOLD setup statement in your resolver setup file. For more information about the UNRESPONSIVETHRESHOLD statement, see z/OS Communications Server: IP Configuration Reference.
• MAXTTL

  Defines the amount of time that the resolver can use resource information that it receives from a name server.

• NOCACHE

  Disables system-wide caching of DNS response data. If you do not specify this statement, system-wide caching is enabled by default.

• NOCACHEREORDER

  Disables system-wide reordering of a cached list of IP addresses in response to resolver API queries for the associated host name. System-wide reordering of a cached list of IP addresses is disabled by default, but you can explicitly disable it by using this statement.

  
• NOCOMMONSEARCH

  Indicates that a different search order for local host files is used for IPv4 and IPv6 name queries.

• UNRESPONSIVETHRESHOLD

  Identifies the threshold value that determines when the resolver declares a DNS name server to be unresponsive. If the percentage of query failures to a name server during a fixed interval is greater than or equal to this threshold value, the resolver considers the name server to be unresponsive. The resolver might stop sending DNS queries that are generated by an application to unresponsive name servers, depending on the values that are coded on this statement. For more information, see Monitoring the responsiveness of Domain Name System name servers.

  Restriction: You must code the GLOBALTCPIPDATA statement if the AUTOQUIESCE operand is coded on the UNRESPONSIVETHRESHOLD statement. If you cannot ensure that all DNS IP addresses are accessible from all your TCPIP stacks, you should not use a global TCPIP.DATA file, and you should not code AUTOQUIESCE on the UNRESPONSIVETHRESHOLD setup statement in your resolver setup file. For more information about the UNRESPONSIVETHRESHOLD statement, see z/OS Communications Server: IP Configuration Reference.

For more information about resolver setup statements, see z/OS Communications Server: IP Configuration Reference.