Setting up a pam_userdb.so based authentication

The pam_userdb.so authentication module uses a Berkeley format DB to hold passwords associated with user names.

To create this DB, you need to install the libdb3-util package, which includes the necessary tools.

$/!\$ Please bear in mind that the passwords in this file are *not* encrypted. Anybody with read access to the DB will be able to dump the passwords.

Here is how you can setup the pam_userdb.so based authentication:

Create a text file with the username and password pairs: This file will list all username and password pairs, each username being on a first line, with the associated password on the following line. For example:
```
user_1
password_of_user_1
user_2
password_of_user_2
...
```
Convert this text file to a Berkeley DB:
```
db3_load -T -f users_and_passwords_text_file.txt -t hash userpass.db
```
This will create a Berkeley DB called "userpass.db" which we can use for PAM authentication.
Make sure that only root can read and change this file:
```
chown root.root userpass.db
chmod 0600 userpass.db
```
Add the call to the PAM module in the appropriate file in /etc/pam.d/:
```
auth   required    pam_userdb.so    db=/full/path/to/userpass
```
Please note that the ".db" extension is omitted in the parameter given to the PAM module.