19.2 The PAM Configuration of sshd

To show how the theory behind PAM works, consider the PAM configuration of sshd as a practical example:

#%PAM-1.0
auth     required       pam_nologin.so
auth     include        common-auth
account  include        common-account
password include        common-password
session  required       pam_loginuid.so
session  include        common-session
# Enable the following line to get resmgr support for
# ssh sessions (see /usr/share/doc/packages/resmgr/README)
#session  optional      pam_resmgr.so fake_ttyname

The first module that is called is pam_nologin. It checks whether the file /etc/nologin exists. If it does, no other user than root may log in.

The typical PAM configuration of an application (sshd, in this case) contains four include statements referring to the configuration files of four module types: common-auth, common-account, common-password, and common-session. These four files hold the default configuration for each module type. By including them instead of adding each module separately to the respective PAM configuration, you automatically get an updated PAM configuration if the administrator changes the defaults. In former times, you had to adjust all configuration files manually for all applications when changes to PAM occurred or a new application was installed. Now the PAM configuration is made with central configuration files and all changes are automatically inherited by the PAM configuration of each service.

The first include file (common-auth) calls two modules of the auth type: pam_env.so and pam_unix2.so. See Example 19-2.

auth    required        pam_env.so
auth    required        pam_unix2.so

The first one, pam_env, loads the file /etc/security/pam_env.conf to set the environment variables as specified in this file. This can be used to set the DISPLAY variable to the correct value, because the pam_env module knows about the location from which the login is taking place. The second one, pam_unix2, checks the user's login and password against /etc/passwd and /etc/shadow.

The whole stack of auth modules is processed before sshd gets any feedback about whether the login has succeeded. Given that all modules of the stack have the required control flag, they must all be processed successfully before sshd receives a message about the positive result. If one of the modules is not successful, the entire module stack is still processed and only then is sshd notified about the negative result.

As soon as all modules of the auth type have been successfully processed, another include statement is processed, in this case, that in Example 19-3. common-account contains just one module, pam_unix2. If pam_unix2 returns the result that the user exists, sshd receives a message announcing this success and the next stack of modules (password) is processed, shown in Example 19-4.

account required        pam_unix2.so

password requisite       pam_pwcheck.so  nullok cracklib
password required       pam_unix2.so    nullok use_authtok

Again, the PAM configuration of sshd involves just an include statement referring to the default configuration for password modules located in common-password. These modules must successfully be completed (control flags requisite and required) whenever the application requests the change of an authentication token.

Changing a password or another authentication token requires a security check. This is achieved with the pam_pwcheck module. The pam_unix2 module used afterwards carries over any old and new passwords from pam_pwcheck, so the user does not need to authenticate again after changing the password. This procedure makes it impossible to circumvent the checks carried out by pam_pwcheck. Whenever the account or the auth type are configured to complain about expired passwords, the password modules should also be used.

session required        pam_limits.so
session required        pam_unix2.so
session optional        pam_umask.so

As the final step, the modules of the session type, bundled in the common-session file are called to configure the session according to the settings for the user in question. The pam_limits module loads the file /etc/security/limits.conf, which may define limits on the use of certain system resources. The pam_unix2 module is processed again. The pam_umask module can be used to set the file mode creation mask. Since this module carries the optional flag, a failure of this module would not affect the successful completion of the entire session module stack. The session modules are called a second time when the user logs out.