revnetgroup processes the contents of a file in
netgroup(5) format into what is called
reverse netgroup form. That is, where the original file shows netgroup memberships in terms of which members reside in a particular group, the reverse netgroup format specifies what groups are associated with a particular member. This information is used to generate the
netgroup.byuser and
netgroup.byhosts NIS maps. These reverse netgroup maps are used to help speed up netgroup lookups, particularly for the
innetgr() library function.
For example, the standard
/etc/netgroup file may list a netgroup and a list of its members. Here, the netgroup is considered the
key and the member names are the
data. By contrast, the reverse
netgroup.byusers database lists each unique member as the key and the netgroups to which the members belong become the data. Separate databases are created to hold information pertaining to users and hosts; this allows netgroup username lookups and netgroup hostname lookups to be performed using independent keyspaces.
By constructing these reverse netgroup databases (and the corresponding NIS maps) in advance, the
getnetgrent(3) library functions are spared from having to work out the dependencies themselves on the fly. This is important on networks with large numbers of users and hosts, since it can take a considerable amount of time to process very large netgroup databases.
The
revnetgroup command prints its results on the standard output. It is usually called only by
/var/yp/<domain>/Makefile when rebuilding the NIS netgroup maps.