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Yubico Pluggable Authentication Module (PAM)

Manual pages


Yubico PAM module

The Yubico PAM module provides an easy way to integrate the Yubikey into your existing user authentication infrastructure. PAM is used by GNU/Linux, Solaris and Mac OS X for user authentication, and by other specialized applications such as NCSA !MyProxy.

Status and Roadmap

The module is working for multi-user systems. The primary mode of operation is by doing online validation using a YubiKey validation service (such as the YubiCloud, or a private one configured using the urllist parameter).

In version 2.6, offline validation was also made possible through the use of HMAC-SHA1 Challenge-Response found in YubiKey 2.2 and later. This has introduced a dependency of libykpers-1 from the ykpersonalize package. Pass ‘--without-cr’ to ‘configure’ to avoid this dependency.

The development community is co-ordinated via GitHub :

The project is licensed under a BSD license. See the file COPYING for exact wording. For any copyright year range specified as YYYY-ZZZZ in this package note that the range specifies every single year in that closed interval.

Building from Git

Skip to the next section if you are using an official packaged version.

You may check out the sources using Git with the following command:

   $ git clone yubico-pam

This will create a directory yubico-pam.

Autoconf, automake and libtool must be installed to create a compilable source tree.

Generate the build system using:

   $ cd yubico-pam
   $ autoreconf --install


You will need to have libykclient (ykclient.h, and libpam-dev (security/pam_appl.h, installed. Get the ykclient library from:

It in turn requires Curl, which you need to have installed, and libyubikey. Get libyubikey from

The new Challenge-Response offline authentication requires libykpers-1 from the yubikey-personalization project :

The build system uses Autoconf, to set up the build system run:


Use --without-ldap to disable ldap support.

Then build the code, run the self-test and install the binaries:

  make check install


There is already a package in Fedora/EPEL of yubico-pam that can be installed by using yum : $ sudo yum install pam_yubico

Ubuntu PPA

There is an Ubuntu PPA (Personal Package Archive) for yubico-pam that can be installed using the following commands on reasonably modern Ubuntu platforms :

$ sudo add-apt-repository ppa:yubico/stable
$ sudo apt-get update
$ sudo apt-get install libpam-yubico

See the file /usr/share/doc/libpam-yubico/README.Debian after installing.

FreeBSD ports

yubico-pam and the supporting Yubico packages have corresponding FreeBSD ports. To install:

$ cd /usr/ports/security/pam_yubico
$ make install clean

Advanced configuration notes are available at


Install it in your PAM setup by adding a line to an appropriate file in /etc/pam.d/:

  auth sufficient id=16 debug

and move into /lib/security/ (or wherever PAM modules live in your system) :

  mv /usr/local/lib/security/ /lib/security/

For more information, see the project Wiki page.

Supported PAM module parameters are:

  "authfile":   to indicate the location of the file that holds the
                mappings of Yubikey token IDs to user names.

  "id":         to indicate your client identity.

  "key":        to indicate your client key in base64 format.
                The client key is also known as API key, and provides
                integrity in the communication between the client (you)
                and the validation server.
                If you want to get one for use with the default YubiCloud
                service, visit this URL :


  "debug":      to enable debug output to stdout.

  "alwaysok":   to enable all authentication attempts to succeed
                (aka presentation mode).

                Before prompting the user for their password, the module
                first tries the previous stacked module´s password in case
                that satisfies this module as well.

                The argument use_first_pass forces the module to use a previous
                stacked modules password and will never prompt the user - if no
                password is available or the password is not appropriate, the user
                will be denied access.

  "urllist":    List  of  URL  templates to be used. This is set by calling
                ykclient_set_url_bases. The list should be in the format :


  "url":        This option should not be used, please use the urllist
                option instead.
                Specify the URL template to use, this is set by calling
                yubikey_client_set_url_template, which defaults to:




                depending on your version of yubico-c-client.

  "capath":     specify the path where X509 certificates are stored. This is
                required if 'https' or 'ldaps' are used in 'url' and 'ldap_uri'

                This argument is used to show the OTP (One Time Password) when it
                is entered, i.e. to enable terminal echo of entered characters.
                You are advised to not use this, if you are using two factor
                authentication because that will display your password on the

                This requires the service using the PAM module to
                display custom fields.  For example, OpenSSH requires
                you to configure "ChallengeResponseAuthentication no".

  "ldap_uri":   specify the LDAP server URI (e.g. ldap://localhost).

  "ldapserver": specify the LDAP server host (default LDAP port is used).
                _Deprecated.  Use "ldap_uri" instead._

  "ldapdn":     specify the dn where the users are stored
                (eg: ou=users,dc=domain,dc=com).

  "user_attr":  specify the LDAP attribute used to store user names (eg:cn).

  "yubi_attr":  specify the LDAP attribute used to store the Yubikey id.

                specify the prefix of the LDAP attribute's value, in case
                of a generic attribute, used to store several types of ids.

                Length of ID prefixing the OTP (this is 12 if using the
                Mode of operation. Use "client" for online validation with
                a YubiKey validation service such as the YubiCloud, or use
                "challenge-response" for offline validation using YubiKeys
                with HMAC-SHA-1 Challenge-Response configurations. See the
                man-page ykpamcfg(1) for further details on how to configure
                offline Challenge-Response validation.

If you are using "debug" you may find it useful to create a world-writable log file:

  touch /var/run/pam-debug.log
  chmod go+w /var/run/pam-debug.log

Authorization Mapping Files

A mapping must be made between the Yubikey token ID and the user ID it is attached to. There are two ways to do this, either centrally in one file, or individually, where users can create the mapping in their home directories. If the central authorization mapping file is being used, user home directory mappings will not be used and the opposite applies if user home directory mappings are being used, the central authorization mappings file will not be used.

Central authorization mapping

Create a /etc/yubikey_mappings, the file must contain a user name and the Yubikey token ID separated by colons (same format as the passwd file) for each user you want to allow onto the system using a Yubikey.

The mappings should look like this, one per line:

   <first user name>:<Yubikey token ID1>:<Yubikey token ID2>:….
   <second user name>:<Yubikey token ID3>:<Yubikey token ID4>:….

Now add authfile=/etc/yubikey_mappings to your PAM configuration line, so it looks like:

   auth sufficient id=16 authfile=/etc/yubikey_mappings

Individual authorization mapping by user

Each user creates a ~/.yubico/authorized_yubikeys file inside of their home directory and places the mapping in that file, the file must have only one line:

   <user name>:<Yubikey token ID1>:<Yubikey token ID2>

This is much the same concept as the SSH authorized_keys file.

Obtaining the Yubikey token ID (a.k.a. public ID)

You can obtain the Yubikey token ID in several ways. One is by removing the last 32 characters of any OTP (One Time Password) generated with your Yubikey. Another is by using the modhex calculator located here:

Enter your Yubikey OTP and convert it, your Yubikey token ID is 12 characters and listed as:

Modhex encoded: XXXXXXX

Not sure what that last bit meant? Here is how to get a copy of your OTP.

Fast way

  • Open a terminal

  • Press yubikey button

It will output an OTP into the shell:

    $ cccccccgklgcvnkcvnnegrnhgrjkhlkfhdkclfncvlgj
   bash: cccccccgklgcvnkcvnnegrnhgrjkhlkfhdkclfncvlgj: command not found

This can be pasted into the Modhex_Calculator page.

Harder way

This requires you to have the pam module enabled with debug turned on. When prompted for the yubikey press the button. The pam module will print out debug information including the OTP and ID of your token to the shell - copy the ID into your config file and you should be up and going.

   Yubikey for `youruser':
   [pam_yubico.c:pam_sm_authenticate(867)] conv returned 44 bytes
   [pam_yubico.c:pam_sm_authenticate(885)] Skipping first 0 bytes. Length is 44, token_id set to 12 and token OTP always 32.
   [pam_yubico.c:pam_sm_authenticate(892)] OTP: ccccccclabcabkhbdncicglfltnukadfoifadfhhhhfe ID: cccccclabcab

Yubico PAM module and SELinux.

Users with SELinux in enforcing mode (the default on Fedora 17+) may experience login problems with services including those validated via polkit-agent-helper-1, sshd and login.

This is documented in the PAM Yubico issue tracker [1] and Red Hat bugzilla including a work around [2] for ssh (Equivalent files could be created for other services). Systems in permissive mode will generate AVC warnings but authentication will succeed.

To determine if you have SELinux enforcing or not run the sestatus command.


If you want to use the Yubikey to authenticate you on linux console logins, add the following to the top of /etc/pam.d/login:

   auth sufficient id=16 debug


If you want to discuss anything related to the Yubico PAM module, please e-mail the mailing list