Here are few notable configuration files accessed by the PAM.

The limitation of the password selection is implemented by the PAM modules, pam_unix(8) and pam_cracklib(8). They can be configured by their arguments.

	Tip
	PAM modules use suffix ".so" for their filenames.

The modern centralized system management can be deployed using the centralized Lightweight Directory Access Protocol (LDAP) server to administer many Unix-like and non-Unix-like systems on the network. The open source implementation of the Lightweight Directory Access Protocol is OpenLDAP Software.

The LDAP server provides the account information through the use of PAM and NSS with libpam-ldap and libnss-ldap packages for the Debian system. Several actions are required to enable this (I have not used this setup and the following is purely secondary information. Please read this in this context.).

See documentations in pam_ldap.conf(5) and "/usr/share/doc/libpam-doc/html/" offered by the libpam-doc package and "info libc 'Name Service Switch'" offered by the glibc-doc package.

Similarly, you can set up alternative centralized systems with other methods.

This is the famous phrase at the bottom of the old "info su" page by Richard M. Stallman. Not to worry: the current su command in Debian uses PAM, so that one can restrict the ability to use su to the root group by enabling the line with "pam_wheel.so" in "/etc/pam.d/su".

Installing the libpam-cracklib package enables you to force stricter password rule, for example, by having active lines in "/etc/pam.d/common-password" as the following.

For squeeze:

password required pam_cracklib.so retry=3 minlen=9 difok=3
password [success=1 default=ignore] pam_unix.so use_authtok nullok md5
password requisite pam_deny.so
password required pam_permit.so

sudo(8) is a program designed to allow a sysadmin to give limited root privileges to users and log root activity. sudo requires only an ordinary user's password. Install sudo package and activate it by setting options in "/etc/sudoers". See configuration example at "/usr/share/doc/sudo/examples/sudoers" and Section 1.1.12, “sudo configuration”.

My usage of sudo for the single user system (see Section 1.1.12, “sudo configuration”) is aimed to protect myself from my own stupidity. Personally, I consider using sudo a better alternative to using the system from the root account all the time. For example, the following changes the owner of "<some_file>" to "<my_name>".

$ sudo chown <my_name> <some_file>

Of course if you know the root password (as self-installed Debian users do), any command can be run under root from any user's account using "su -c".

PolicyKit is an operating system component for controlling system-wide privileges in Unix-like operating systems.

Newer GUI applications are not designed to run as privileged processes. They talk to privileged processes via PolicyKit to perform administrative operations.

PolicyKit limits such operations to user accounts belonging to the sudo group on Debian system.

See polkit(8).

Security-Enhanced Linux (SELinux) is a framework to tighten privilege model tighter than the ordinary Unix-like security model with the mandatory access control (MAC) policies. The root power may be restricted under some conditions.

For system security, It is a good idea to disable as much server programs as possible. This becomes critical for network servers. Having unused servers, activated either directly as daemon or via super-server program, are considered security risks.

Many programs, such as sshd(8), use PAM based access control. There are many ways to restrict access to some server services.

See Section 3.5.3, “The runlevel management example”, Section 3.5.4, “The default parameter for each init script”, Section 4.5.1, “Configuration files accessed by the PAM and NSS”, Section 3.5.8, “Network service initialization”, and Section 5.9, “Netfilter infrastructure”.

	Tip
	Sun RPC services need to be active for NFS and other RPC based programs.

	Tip
	If you have problems with remote access in a recent Debian system, comment out offending configuration such as "ALL: PARANOID" in "/etc/hosts.deny" if it exists. (But you must be careful on security risks involved with this kind of action.)

Many popular transportation layer services communicate messages including password authentication in the plain text. It is very bad idea to transmit password in the plain text over the wild Internet where it can be intercepted. You can run these services over "Transport Layer Security" (TLS) or its predecessor, "Secure Sockets Layer" (SSL) to secure entire communication including password by the encryption.

The encryption costs CPU time. As a CPU friendly alternative, you can keep communication in plain text while securing just password with the secure authentication protocol such as "Authenticated Post Office Protocol" (APOP) for POP and "Challenge-Response Authentication Mechanism MD5" (CRAM-MD5) for SMTP and IMAP. (For sending mail messages over the Internet to your mail server from your mail client, it is recently popular to use new message submission port 587 instead of traditional SMTP port 25 to avoid port 25 blocking by the network provider while authenticating yourself with CRAM-MD5.)

The Secure Shell (SSH) program provides secure encrypted communications between two untrusted hosts over an insecure network with the secure authentication. It consists of the OpenSSH client, ssh(1), and the OpenSSH daemon, sshd(8). This SSH can be used to tunnel the insecure protocol communication such as POP and X securely over the Internet with the port forwarding feature.

The client tries to authenticate itself using host-based authentication, public key authentication, challenge-response authentication, or password authentication. The use of public key authentication enables the remote password-less login. See Section 6.9, “The remote access server and utility (SSH)”.

Even when you run secure services such as Secure Shell (SSH) and Point-to-point tunneling protocol (PPTP) servers, there are still chances for the break-ins using brute force password guessing attack etc. from the Internet. Use of the firewall policy (see Section 5.9, “Netfilter infrastructure”) together with the following secure tools may improve the security situation.

To prevent people to access your machine with root privilege, you need to make following actions.

With physical access to hard disk, resetting the password is relatively easy with following steps.

If you have the edit access to the GRUB menu entry (see Section 3.3, “Stage 2: the boot loader”) for grub-rescue-pc at the boot time, it is even easier with following steps.

The root shell of the system is now accessible without password.

	Note
	Once one has root shell access, he can access everything on the system and reset any passwords on the system. Further more, he may compromise password for all user accounts using brute force password cracking tools such as john and crack packages (see Section 9.6.11, “System security and integrity check”). This cracked password may lead to compromise other systems.

The only reasonable software solution to avoid all these concerns is to use software encrypted root partition (or "/etc" partition) using dm-crypt and initramfs (see Section 9.4, “Data encryption tips”). You always need password to boot the system, though.