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
nonrandommx; it may change to be equivalent
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
such a channel does not cause name lookups to refer to the underlying
TCP/IP package's local host tables; specifying
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
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
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 184.108.40.206 220.127.116.11
defaultnameserversis the default, and means to use the TCP/IP stack's own choice of nameservers.