SlackBuildsOrg/system/nss-mysql

The NSS-MySQL name service switch library

libnss_mysql retrieves user account information from a MySQL database server.
Per default stored in /etc/{group,passwd,shadow} on Unix-like systems. Which
(partly) is similar in functionality to NIS, LDAP, RADIUS, Hesoid, winbindd

When used over a remote-network uttermost care should be taken in the security
of this (such as TLS/SSL encrypting the connection) and even then storing any
valid password for such 'virtual' users is probably a bad idea in itself.

Kerberos provides for a far superior single-sign-on authentication system
(the 'shadow' part in these setups) in both performance and security.

Besides, this may break stuff so bad you'd need console access to fix it...
For some basic installation instructions though see README.SLACKWARE