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PAM_GOOGLE_AUTHENTICATOR(8) PAM_GOOGLE_AUTHENTICATOR(8)

NAME
pam_google_authenticator - PAM module for Google two-factor authentication

SYNOPSIS
pam_google_authenticator.so [secret=file] [authtok_prompt=prompt] [user=username]
[no_strict_owner] [allowed_perm=0nnn] [debug] [try_first_pass|use_first_pass|forward_pass]
[noskewadj] [no_increment_hotp] [nullok] [echo_verification_code]

DESCRIPTION
The pam_google_authenticator module is designed to protect user authentication with a second
factor, either time-based (TOTP) or counter-based (HOTP). Prior logging in, the user will be
asked for both its password and a one-time code. Such one-time codes can be generated with
the Google Authenticator application, installed on the user's Android device. To respective‐
ly generate and verify those one-time codes, a secret key (randomly generated) must be shared
between the device on which one-time codes are generated and the system on which this PAM
module is enabled.

Depending on its configuration (see options section), this module requires that a secret file
is manually set up for each account on the system. This secret file holds the secret key and
user-specific options (see google-authenticator(1)). Unless the nullok option is used, au‐
thentication tries will be rejected if such secret file doesn't exist. Alternatively, a sys‐
tem administrator may create those secret files on behalf of the users and then communicates
to them the secret keys.

OPTIONS
secret=file
Specify a non-standard file location for the secret file.

By default, the PAM module looks for the secret file in the .google_authenticator file
within the home of the user logging in. This option overrides this location.

The provided location may include the following short-hands:

• ${USER} that will be interpreted as the username.

• ${HOME} and ~ that will be interpreted as the user's home directory.

authtok_prompt=prompt
Override default token prompt.

Note that if spaces are present in the provided prompt, the whole argument must be
wrapped in square brackets.

user=username
Switch to a hard-coded user prior to doing any file operation.

no_strict_owner
Disable the check against the secret file's owner.

By default, the secret file must be owned by the user logging in. This option dis‐
ables this check.

allowed_perm=0nnn
Override checked permissions of the secret file.

By default, the secret file must be readable only by its owner (ie. mode 0600). This
option allows a different mode to be specified for this file.

debug Enable more verbose log messages in syslog.

try_first_pass|use_first_pass|forward_pass
Stacking options for this PAM module.

Because some PAM clients cannot prompt the user for more than just the password, the
following stacking options may be used:

• try_first_pass: before prompting the user for the one-time code, this module first
tries the previous stacked module's password in case that satisfies this module as
well.

• use_first_pass: force this module to use a previous stacked modules password. With
this option, this module will never prompt the user for the one-time code. Thus, if
no valid one-time code is available, the user will be denied access.

• forward_pass: query the user for both the system password and the verification code
in a single prompt. The system password is then forwarded to the next PAM module,
which will have to be configured with either the use_first_pass option, or the
try_first_pass option.

noskewadj
Don't adjust time skew automatically.

By default, the PAM module makes an attempt to compensate for time skew between the
server and the device on which one-time passcodes are generated. This option disable
this behavior.

Note that this option is only relevant for time-based (TOTP) mode.

no_increment_hotp
Don't increment the counter for failed attempts.

In some circonstance, failed passwords still get an OTP prompt. This option disables
counter incrementation is such situations.

Note that this option is only relevant for counter-based (HOTP) mode.

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

During the initial roll-out process, all users may not have created a secret key yet.
This option allows them to log in, even if the secret file doesn't exist.

echo_verification_code
Echo the verification code when it is entered by the user.

MODULE TYPE PROVIDED
Only the auth module type is provided.

RETURN VALUES
PAM_SUCCESS
Either the provided one-time code is correct or is a valid emergency code.

PAM_IGNORE
This module is ignored.

PAM_AUTH_ERR
The provided one-time code isn't correct and isn't a valid emergency code, or an error
was encountered.

EXAMPLES
The following lines may be used to enable this PAM module:

• auth required pam_google_authenticator.so no_increment_hotp # Make sure the counter (for
HOTP mode) isn't incremented for failed attempts.

• auth required pam_google_authenticator.so nullok # Allow users to log in if their secret
files don't exist

• auth required pam_google_authenticator.so secret=/var/unencrypted-home/${USER}/.google_au‐
thenticator # Store secret files in a specific location

• auth required pam_google_authenticator.so [authtok_prompt=Your secret token: ] # Use a spe‐
cific prompt

• auth required pam_google_authenticator.so noskewadj # Don't compensate time skew automati‐
cally

SECURITY NOTES
For highest security, make sure that both password and one-time code are being requested even
if password and/or one-time code 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_authentica‐
tor.so should be set as required, not requisite.