D.3 K Command's -d switch

Old Sun configuration files could transfer all locally deliverable mail to a central mail hub. The assumption was that the mail hub possessed a superset of all aliases and so would be authoritative about final delivery. Unfortunately, this assumption tended to cause unnecessary traffic to be sent to the mail hub and could cause congestion at the mail hub.

In an NIS or NIS+ environment the assumption that the mail hub is authoritative may not be valid, because (in theory) every client machine can have equal access to authoritative aliasing information. With "domainwide" alias support, local addresses can be resolved at the sending client. The mail hub is then free to just handle Internet traffic.

The -d switch to the K configuration command (see Section 33.3) declares a map to be domainwide. Such a map is presumed to be available through out an NIS or NIS+ domain. The -d switch is useful only with alias maps and affects sendmail's behavior in two ways:

Address qualification

An unqualified recipient address is one that lacks an @host part. Such addresses should not be propagated in an unqualified form, so if the -d switch is specified for a map and if a username is found in that map, the name is fully qualified by appending an @ and the NIS or NIS+ domain.[2] Otherwise, if a username is found in a local alias map, the name is fully qualified by appending an @ and the hostname of the local host. Otherwise, the username is fully qualified by appending an @ and the hostname of the mail hub.

[2] This gives addresses a special property. If an address is qualified with an NIS or NIS+ domain, Sun's sendmail presumes that the name can be resolved correctly anywhere in the specified domain.

Selective alias expansion

If an address is qualified with an NIS or NIS+ domain, the user part should be alias expanded only by looking it up in a domainwide alias map. Otherwise, if an address is unqualified or an address is qualified with the local hostname, the user part should be alias expanded only by looking it up in the manner specified in the /etc/nsswitch.conf file. Otherwise, if an address is qualified with a nonlocal hostname, the mail message is forwarded to that specified host for resolution.