Google Authenticator PAM module

Example PAM module demonstrating two-factor authentication for logging into servers via SSH, OpenVPN, etc…

This project is not about logging in to Google, Facebook, or other TOTP/HOTP second factor systems, even if they recommend using the Google Authenticator apps.

HMAC-Based One-time Password (HOTP) is specified in RFC 4226 and Time-based One-time Password (TOTP) is specified in RFC 6238.

Build & install

./bootstrap.sh
./configure
make
sudo make install

If you don't have access to "sudo", you have to manually become "root" prior to calling "make install".

Setting up the PAM module for your system

For highest security, make sure that both password and OTP are being requested even if password and/or OTP are incorrect. This means that at least the first of pam_unix.so (or whatever other module is used to verify passwords) and pam_google_authenticator.so should be set as required, not requisite. It probably can't hurt to have both be required, but it could depend on the rest of your PAM config.

If you use HOTP (counter based as opposed to time based) then add the option no_increment_hotp to make sure the counter isn't incremented for failed attempts.

Add this line to your PAM configuration file:

auth required pam_google_authenticator.so no_increment_hotp

Setting up a user

Run the google-authenticator binary to create a new secret key in your home directory. These settings will be stored in ~/.google_authenticator.

If your system supports the "libqrencode" library, you will be shown a QRCode that you can scan using the Android "Google Authenticator" application.

If your system does not have this library, you can either follow the URL that google-authenticator outputs, or you have to manually enter the alphanumeric secret key into the Android "Google Authenticator" application.

In either case, after you have added the key, click-and-hold until the context menu shows. Then check that the key's verification value matches (this feature might not be available in all builds of the Android application).

Each time you log into your system, you will now be prompted for your TOTP code (time based one-time-password) or HOTP (counter-based), depending on options given to google-authenticator, after having entered your normal user id and your normal UNIX account password.

During the initial roll-out process, you might find that not all users have created a secret key yet. If you would still like them to be able to log in, you can pass the "nullok" option on the module's command line:

auth required pam_google_authenticator.so nullok

Encrypted home directories

If your system encrypts home directories until after your users entered their password, you either have to re-arrange the entries in the PAM configuration file to decrypt the home directory prior to asking for the OTP code, or you have to store the secret file in a non-standard location:

auth required pam_google_authenticator.so secret=/var/unencrypted-home/${USER}/.google_authenticator

would be a possible choice. Make sure to set appropriate permissions. You also have to tell your users to manually move their .google_authenticator file to this location.

In addition to "${USER}", the secret= option also recognizes both "~" and ${HOME} as short-hands for the user's home directory.

When using the secret= option, you might want to also set the user= option. The latter forces the PAM module to switch to a dedicated hard-coded user id prior to doing any file operations. When using the user= option, you must not include "~" or "${HOME}" in the filename.

The user= option can also be useful if you want to authenticate users who do not have traditional UNIX accounts on your system.

Module options

secret=/path/to/secret/file

See "encrypted home directories", above.

authtok_prompt=prompt

Overrides default token prompt. If you want to include spaces in the prompt, wrap the whole argument in square brackets:

auth required pam_google_authenticator.so [authtok_prompt=Your secret token: ]

user=some-user

Force the PAM module to switch to a hard-coded user id prior to doing any file operations. Commonly used with secret=.

no_strict_owner

DANGEROUS OPTION!

By default the PAM module requires that the secrets file must be owned the user logging in (or if user= is specified, owned by that user). This option disables that check.

This option can be used to allow daemons not running as root to still handle configuration files not owned by that user, for example owned by the users themselves.

allowed_perm=0nnn

DANGEROUS OPTION!

By default, the PAM module requires the secrets file to be readable only by the owner of the file (mode 0600 by default). In situations where the module is used in a non-default configuration, an administrator may need more lenient file permissions, or a specific setting for their use case.

debug

Enable more verbose log messages in syslog.

try_first_pass / use_first_pass / forward_pass

Some PAM clients cannot prompt the user for more than just the password. To work around this problem, this PAM module supports stacking. If you pass the forward_pass option, the pam_google_authenticator module queries the user for both the system password and the verification code in a single prompt. It then forwards the system password to the next PAM module, which will have to be configured with the use_first_pass option.

In turn, pam_google_authenticator module also supports both the standard use_first_pass and try_first_pass options. But most users would not need to set those on the pam_google_authenticator.

noskewadj

If you discover that your TOTP code never works, this is most commonly the result of the clock on your server being different from the one on your Android device. The PAM module makes an attempt to compensate for time skew. You can teach it about the amount of skew that you are experiencing, by trying to log it three times in a row. Make sure you always wait 30s (but not longer), so that you get three distinct TOTP codes.

Some administrators prefer that time skew isn't adjusted automatically, as doing so results in a slightly less secure system configuration. If you want to disable it, you can do so on the module command line:

auth required pam_google_authenticator.so noskewadj

no_increment_hotp

Don't increment the counter for failed HOTP attempts. Normally you should set this so failed password attempts by an attacker without a token don't lock out the authorized user.

nullok

Allow users to log in without OTP, if they haven't set up OTP yet.

PAM requires at least one SUCCESS answer from a module, and nullok causes this module to say IGNORE. This means that if this option is used at least one other module must have said SUCCESS. One way to do this is to add auth required pam_permit.so to the end of the PAM config.

echo_verification_code

By default, the PAM module does not echo the verification code when it is entered by the user. In some situations, the administrator might prefer a different behavior. Pass the echo_verification_code option to the module in order to enable echoing.

If you would like verification codes that are counter based instead of timebased, use the google-authenticator binary to generate a secret key in your home directory with the proper option. In this mode, clock skew is irrelevant and the window size option now applies to how many codes beyond the current one that would be accepted, to reduce synchronization problems.

grace_period=seconds

If present and non-zero, provide a grace period during which a second verification code will not be requested. Try setting seconds to 86400 to allow a full-day between requesting codes; or 3600 for an hour.

This works by adding an (IP address, timestamp) pair to the security file after a successful one-time-password login; only the last ten distinct IP addresses are tracked.

allow_readonly

DANGEROUS OPTION!

With this option an attacker with ability to fill up the filesystem (flood server with web requests, or if they have an account just fill the disk up) can force a situation where one-time-passwords can be reused, defeating the purpose of "one time".

By default, if the grace_period option is defined the PAM module requires some free space to store the IP address and timestamp of the last login. It could prevent access if a server has no free space or in case of an update config file error. With the allow_readonly option you can ignore any errors which could occur during config file update.

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