libnss_mapuser v1.0.1
Dave Olson <[email protected]>
June 15, 2017

This NSS module has one and only one purpose.  It allows getpwnam() and getpwuid()
lookups for arbitrary usernames, with explict matching to a named account.
The mapped lookup is only done if the requested name is not already present
in /etc/passwd (no other lookup, such as LDAP, is done).

It exists as two separate plugins, because the name lookup should be last
in the passwd database lookup in nsswitch.conf (so any other valid lookup
matches first), while the UID lookup should be first, so that a lookup on
the UID of the mapped user returns the mapped name.

It's intended for use with authentication mechanisms such as RADIUS, where
it is not possible to determine if a username is valid without authenticating
at the same time.

The mapping is done to a single account specified in the configuration
file /etc/nss_mapuser.conf.

The returned passwd field is always filled in as 'x', so that authentication
of the base account is not possible through PAM.  Only the mapped accounts
are able to login, typically through PAM, such as pam_radius.so.

The GECOS field is filled in as 'USERNAME mapped user' and the home directory
uses the same path as the user from /etc/passwd, with the last component replaced
by the passed in username.   The uid, gid, and shell fields are copied directly
from the map_user account passwd dataa.

For example, if the passed in username is 'olsonr', the result of running
   getent -s mapuser passwd olsonr
will be something like this:
   olsonr:x:1017:1017:olsonr mapped user:/home/olsonr:/bin/bash
if the map_user field is set to radius_user, and the radius_user entry in
/etc/passwd is:
   radius_user:x:1017:1017:radius_user,,,:/home/radius_user:/bin/bash

This package will create the radius_user account with adduser if it does not
already exist, and that is the default mapping in the configuration, and will
add the group radius_users with the addgroup command.

The mapping can be changed in the configuration file /etc/nss_mapuser.conf.
In that case, the account must already exist, or should be created with
a command similar to:
   adduser --quiet --firstuid 1000 --disabled-login --ingroup GROUP \
     --gecos "radius user" USERNAME

On install, this package will edit /etc/nsswitch.conf to add the two plugins,
so that it looks similar to:
   passwd:         mapuid compat  mapname
if these plugins are not already present.