Get the software:

<code>
apt-get install gcc mercurial libpam0g-dev -y
</code>

Compile the PAM module:

<code>
cd google-authenticator/libpam/
make install
</code>

Now apply the PAM module to one of your authentication mechanisms (sshd, ppp, samba or what ever you have in /etc/pam.d):

<code>
vi /etc/pam.d/sshd
</code>

and append this to the file sshd:

<file>
auth required pam_google_authenticator.so
</file>


You can disable the 2-factor login by appending these two lines:

<file>
auth [success=1 default=ignore] pam_access.so accessfile=/etc/security/access-local.conf
auth       required     pam_google_authenticator.so
</file>

In the file /etc/security/access-local.conf you can add the following to skip 2-factor for the network 10.0.0.0/24:

<file>
+ : ALL : 10.0.0.0/24
+ : ALL : LOCAL
- : ALL : ALL
</file>