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Samba HowTo Guide
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Name Service Switch

The NSS is a feature that is present in many UNIX operating systems. It allows system information such as hostnames, mail aliases, and user information to be resolved from different sources. For example, a standalone UNIX workstation may resolve system information from a series of flat files stored on the local file system. A networked workstation may first attempt to resolve system information from local files, and then consult an NIS database for user information or a DNS server for hostname information.

The NSS application programming interface allows Winbind to present itself as a source of system information when resolving UNIX usernames and groups. Winbind uses this interface and information obtained from a Windows NT server using MSRPC calls to provide a new source of account enumeration. Using standard UNIX library calls, you can enumerate the users and groups on a UNIX machine running Winbind and see all users and groups in an NT domain plus any trusted domain as though they were local users and groups.

The primary control file for NSS is /etc/nsswitch.conf. When a UNIX application makes a request to do a lookup, the C library looks in /etc/nsswitch.conf for a line that matches the service type being requested; for example, the “passwd” service type is used when user or group names are looked up. This config line specifies which implementations of that service should be tried and in what order. If the passwd config line is:

passwd: files example

then the C library will first load a module called /lib/libnss_files.so followed by the module /lib/libnss_example.so. The C library will dynamically load each of these modules in turn and call resolver functions within the modules to try to resolve the request. Once the request is resolved, the C library returns the result to the application.

This NSS interface provides an easy way for Winbind to hook into the operating system. All that needs to be done is to put libnss_winbind.so in /lib/ then add “winbind” into /etc/nsswitch.conf at the appropriate place. The C library will then call Winbind to resolve user and group names.

Samba HowTo Guide
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