23.2 The PAM Configuration of sshd
To show how the theory behind PAM works, consider the PAM
configuration of sshd as a practical example:
Example 23-1
PAM Configuration for sshd
#%PAM-1.0
auth include common-auth
auth required pam_nologin.so
account include common-account
password include common-password
session include common-session
# Enable the following line to get resmgr support for
# ssh sessions (see /usr/share/doc/packages/resmgr/README.SuSE)
#session optional pam_resmgr.so fake_ttyname
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 calling
each module separately for each PAM application, 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 and
pam_unix2. See Example 23-2.
Example 23-2
Default Configuration for the auth Section
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.
After the modules specified in common-auth have been
successfully called, a third module called pam_nologin
checks whether the file /etc/nologin exists. If it
does, no user other than root may
log in. 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 23-3.
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 23-4.
Example 23-3
Default Configuration for the account
Section
account required pam_unix2.so
Example 23-4
Default Configuration for the password
Section
password required pam_pwcheck.so nullok
password required pam_unix2.so nullok use_first_pass use_authtok
#password required pam_make.so /var/yp
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 flag 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. This also makes it impossible to
circumvent the checks carried out by pam_pwcheck. The
modules of the password type should be used wherever the
preceding modules of the account or the
auth type are configured to complain about an expired
password.
Example 23-5
Default Configuration for the session
Section
session required pam_limits.so
session required pam_unix2.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.
Although pam_unix2 is processed again, it has no
practical consequences due to its none option specified in
the respective configuration file of this module,
pam_unix2.conf. The pam_limits
module loads the file /etc/security/limits.conf, which
may define limits on the use of certain system resources. The
session modules are called a second time when user logs
out.