Chapter 4. Authentication

Table of Contents

4.1. Normal Unix authentication
4.2. Managing account and password information
4.3. Good password
4.4. Creating encrypted password
4.5. PAM and NSS
4.5.1. Configuration files accessed by the PAM and NSS
4.5.2. The modern centralized system management
4.5.3. "Why GNU su does not support the wheel group"
4.5.4. Stricter password rule
4.6. Other access controls
4.6.1. sudo
4.6.2. SELinux
4.6.3. Restricting access to some server services
4.7. Security of authentication
4.7.1. Secure password over the Internet
4.7.2. Secure Shell
4.7.3. Extra security measures for the Internet
4.7.4. Securing the root password

When a person (or a program) requests access to the system, authentication confirms the identity to be a trusted one.

[Warning] Warning

Configuration errors of PAM may lock you out of your own system. You must have a rescue CD handy or setup an alternative boot partition. To recover, boot the system with them and correct things from there.

4.1. Normal Unix authentication

Normal Unix authentication is provided by the pam_unix(8) module under the PAM (Pluggable Authentication Modules). Its 3 important configuration files, with ":" separated entries, are the following.

Table 4.1. 3 important configuration files for pam_unix(8)

file permission user group description
/etc/passwd -rw-r--r-- root root (sanitized) user account information
/etc/shadow -rw-r----- root shadow secure user account information
/etc/group -rw-r--r-- root root group information

"/etc/passwd" contains the following.

user1:x:1000:1000:User1 Name,,,:/home/user1:/bin/bash
user2:x:1001:1001:User2 Name,,,:/home/user2:/bin/bash

As explained in passwd(5), each ":" separated entry of this file means the following.

  • Login name
  • Password specification entry
  • Numerical user ID
  • Numerical group ID
  • User name or comment field
  • User home directory
  • Optional user command interpreter

The second entry of "/etc/passwd" was used for the encrypted password entry. After the introduction of "/etc/shadow", this entry is used for the password specification entry.

Table 4.2. The second entry content of "/etc/passwd"

content meaning
(empty) passwordless account
x the encrypted password is in "/etc/shadow"
* no login for this account
! no login for this account

"/etc/shadow" contains the following.


As explained in shadow(5), each ":" separated entry of this file means the following.

  • Login name
  • Encrypted password (The initial "$1$" indicates use of the MD5 encryption. The "*" indicates no login.)
  • Days since Jan 1, 1970 that password was last changed
  • Days before password may be changed
  • Days after which password must be changed
  • Days before password is to expire that user is warned

"/etc/group" contains the following.


As explained in shadow(5), each ":" separated entry of this file means the following.

  • Group name
  • Encrypted password (not really used)
  • Numerical group ID
  • "," separated list of user names
[Note] Note

"/etc/gshadow" provides the similar function as "/etc/shadow" for "/etc/group" but is not really used.

[Note] Note

The actual group membership of a user may be dynamically added if "auth optional" line is added to "/etc/pam.d/common-auth" and set it in "/etc/security/group.conf". See pam_group(8).

[Note] Note

The base-passwd package contains an authoritative list of the user and the group: "/usr/share/doc/base-passwd/users-and-groups.html".

4.2. Managing account and password information

Here are few notable commands to manage account information.

Table 4.3. List of commands to manage account information

command function
getent passwd <user_name> browse account information of "<user_name>"
getent shadow <user_name> browse shadowed account information of "<user_name>"
getent group <group_name> browse group information of "<group_name>"
passwd manage password for the account
passwd -e set one-time password for the account activation
chage manage password aging information

You may need to have the root privilege for some functions to work. See crypt(3) for the password and data encryption.

[Note] Note

On the system set up with PAM and NSS as the Debian alioth machine, the content of local "/etc/passwd", "/etc/group" and "/etc/shadow" may not be actively used by the system. Above commands are valid even under such environment.

4.3. Good password

When creating an account during your system installation or with the passwd(1) command, you should choose a good password which consists of 6 to 8 characters including one or more characters from each of the following sets according to passwd(1).

  • Lower case alphabetics
  • Digits 0 through 9
  • Punctuation marks
[Warning] Warning

Do not chose guessable words for the password.

4.4. Creating encrypted password

There are independent tools to generate encrypted password with salt.

Table 4.4. List of tools to generate password

package popcon size command function
whois * V:10, I:88 204 mkpasswd over-featured front end to the crypt(3) library
openssl * V:31, I:90 2352 openssl passwd compute password hashes (OpenSSL). passwd(1ssl)

4.5. PAM and NSS

Modern Unix-like systems such as the Debian system provide PAM (Pluggable Authentication Modules) and NSS (Name Service Switch) mechanism to the local system administrator to configure his system. The role of these can be summarizes as the following.

  • PAM offers a flexible authentication mechanism used by the application software thus involves password data exchange.
  • NSS offers a flexible name service mechanism which is frequently used by the C standard library to obtain the user and group name for programs such as ls(1) and id(1).

These PAM and NSS systems need to be configured consistently.

The notable packages of PAM and NSS systems are the following.

Table 4.5. List of notable PAM and NSS systems

package popcon size description
libpam-modules * V:81, I:99 972 Pluggable Authentication Modules (basic service)
libpam-ldap * V:1.9, I:4 404 Pluggable Authentication Module allowing LDAP interfaces
libpam-cracklib * V:0.8, I:1.8 132 Pluggable Authentication Module to enable cracklib support
libpam-doc * I:0.8 1160 Pluggable Authentication Modules (documentation in html and text)
libc6 * V:95, I:99 9916 GNU C Library: Shared libraries which also provides "Name Service Switch" service
glibc-doc * I:4 1948 GNU C Library: Manpages
glibc-doc-reference * I:1.5 12020 GNU C Library: Reference manual in info, pdf and html format (non-free)
libnss-mdns * I:53 116 NSS module for Multicast DNS name resolution
libnss-ldap * I:4 312 NSS module for using LDAP as a naming service
libnss-ldapd * V:0.17, I:0.2 364 NSS module for using LDAP as a naming service (new folk of libnss-ldap)

  • "The Linux-PAM System Administrators' Guide" in libpam-doc is essential for learning PAM configuration.
  • "System Databases and Name Service Switch" section in glibc-doc-reference is essential for learning NSS configuration.
[Note] Note

You can see more extensive and current list by "aptitude search 'libpam-|libnss-'" command. The acronym NSS may also mean "Network Security Service" which is different from "Name Service Switch".

[Note] Note

PAM is the most basic way to initialize environment variables for each program with the system wide default value.

4.5.1. Configuration files accessed by the PAM and NSS

Here are few notable configuration files accessed by the PAM.

Table 4.6. List of configuration files accessed by the PAM

configuration file function
/etc/pam.d/<program_name> set up PAM configuration for the "<program_name>" program; see pam(7) and pam.d(5)
/etc/nsswitch.conf set up NSS configuration with the entry for each service. See nsswitch.conf(5)
/etc/nologin limit the user login by the pam_nologin(8) module
/etc/securetty limit the tty for the root access by the pam_securetty(8) module
/etc/security/access.conf set access limit by the pam_access(8) module
/etc/security/group.conf set group based restraint by the pam_group(8) module
/etc/security/pam_env.conf set environment variables by the pam_env(8) module
/etc/environment set additional environment variables by the pam_env(8) module with the "readenv=1" argument
/etc/default/locale set locale by pam_env(8) module with the "readenv=1 envfile=/etc/default/locale" argument. (Debian)
/etc/security/limits.conf set resource restraint (ulimit, core, …) by the pam_linits(8) module
/etc/security/time.conf set time restraint by the pam_time(8) module

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] Tip

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

4.5.2. The modern centralized system management

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.).

  • You set up a centralized LDAP server by running program such as stand-alone LDAP daemon, slapd(8).
  • You change the PAM configuration files in the "/etc/pam.d/" directory to use "" instead of the default "".

    • Debian uses "/etc/pam_ldap.conf" as the configuration file for libpam-ldap and "/etc/pam_ldap.secret" as the file to store the password of the root.
  • You change the NSS configuration in the "/etc/nsswitch.conf" file to use "ldap" instead of the default ("compat" or "file").

    • Debian uses "/etc/libnss-ldap.conf" as the configuration file for libnss-ldap.
  • You must make libpam-ldap to use SSL (or TLS) connection for the security of password.
  • You may make libnss-ldap to use SSL (or TLS) connection to ensure integrity of data at the cost of the LDAP network overhead.
  • You should run nscd(8) locally to cache any LDAP search results in order to reduce the LDAP network traffic.

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.

4.5.3. "Why GNU su does not support the wheel group"

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 "" in "/etc/pam.d/su".

4.5.4. Stricter password rule

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 lenny:

password required retry=3 minlen=9 difok=3
password required use_authtok nullok md5

For squeeze:

password required retry=3 minlen=9 difok=3
password [success=1 default=ignore] use_authtok nullok md5
password requisite
password required

4.6. Other access controls

[Note] Note

See Section 9.5.15, “Alt-SysRq key” for restricting the kernel secure attention key (SAK) feature.

4.6.1. sudo

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".

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".

4.6.2. SELinux

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.

4.6.3. Restricting access to some server services

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 5.8, “Netfilter infrastructure”.

[Tip] Tip

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

[Tip] 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.)

4.7. Security of authentication

The information here may not be sufficient for your security needs but it should be a good start.

4.7.1. Secure password over the Internet

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.

Table 4.7. List of insecure and secure services and ports

insecure service name port secure service name port
www (http) 80 https 443
smtp (mail) 25 ssmtp (smtps) 465
ftp-data 20 ftps-data 989
ftp 21 ftps 990
telnet 23 telnets 992
imap2 143 imaps 993
pop3 110 pop3s 995
ldap 389 ldaps 636

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.)

4.7.2. Secure Shell

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)”.

4.7.3. Extra security measures for the Internet

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 5.8, “Netfilter infrastructure”) together with the following secure tools may improve the security situation.

Table 4.8. List of tools to provide extra security measures

package popcon size description
knockd * V:0.15, I:0.4 164 small port-knock daemon knocked(1) and client konck(1)
denyhosts * V:1.6, I:2 432 utility to help sysadmins thwart ssh hackers
fail2ban * V:3, I:4 554 ban IPs that cause multiple authentication errors
libpam-shield * V:0.01, I:0.05 172 lock out remote attackers trying password guessing

4.7.4. Securing the root password

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

  • Prevent physical access to the hard disk
  • Lock BIOS and prevent booting from the removable media
  • Set password for GRUB interactive session
  • Lock GRUB menu from editing

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

  1. Move the hard disk to a PC with CD bootable BIOS.
  2. Boot system with a rescue media (Debian boot disk, Knopix CD, GRUB CD, …).
  3. Mount root partition with read/write access.
  4. Edit "/etc/passwd" in the root partition and make the second entry for the root account empty.

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.

  1. Boot system with the kernel parameter changed to something like "root=/dev/hda6 rw init=/bin/sh".
  2. Edit "/etc/passwd" and make the second entry for the root account empty.
  3. Reboot system.

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

[Note] 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.