PMDF System Manager's Guide

Some TCP/IP networks support the use of MX (mail forwarding) records and some do not. PMDF TCP/IP channel programs can be configured to not use MX records if they are not provided by the network to which the PMDF system is connected. Some PMDF TCP/IP channel programs can be configured to not do DNS (nameserver) lookups at all. randommx specifies that MX lookups should be done and MX record values of equal precedence should be processed in random order. nonrandommx specifies that MX lookups should be done and MX values of equal precedence should be processed in the same order in which they were received. The mx keyword is currently equivalent to nonrandommx; it may change to be equivalent to randommx in a future PMDF release. The nomx keyword disables MX lookups. The defaultmx keyword specifies that mx should be used if the network says that that MX records are supported.

defaultmx is the default on channels that support MX lookups in any form.

On UNIX, whether the underlying TCP/IP package's local host tables are used in addition to the DNS for lookups is up to the underlying TCP/IP package configuration. Generally, TCP/IP packages are configured so that local host tables will indeed be consulted. Consult your TCP/IP package documentation for details.

On OpenVMS, for PMDF's multithreaded TCP/IP channels, the underlying TCP/IP package's local host tables are not normally consulted during name lookups. In particular, note that specifying nomx on such a channel does not cause name lookups to refer to the underlying TCP/IP package's local host tables; specifying nomx merely causes such a channel to limit its DNS queries by not using MX records. But there is another keyword nodns, which goes further and disables DNS (nameserver) lookups entirely, causing all lookups to be done using the underlying TCP/IP package's local host tables; this keyword is only applicable on OpenVMS and only to multithreaded TCP/IP channels.

When nameserver lookups are being performed, that is, unless the nodns channel keyword is used on OpenVMS, or the nsswitch.conf file on UNIX or the NT TCP/IP configuration selects no use of nameservers, then the nameserver channel keyword may be used to specify a list of nameservers to consult rather than consulting the TCP/IP stack's own choice of nameservers. nameservers requires a space separated list of IP addresses for the nameservers, e.g.,

nameservers 1.2.3.1 1.2.3.2

defaultnameservers is the default, and means to use the TCP/IP stack's own choice of nameservers.