The purpose of this document is to guide readers through the configuration steps to use two factor authentication for SSH using YubiKey. This document assumes that the reader has advanced knowledge and experience in Linux system administration, particularly for how PAM authentication mechanism is configured on a Linux platform.
Successful configuration of the Yubico PAM module to support two factor authentication requires following prerequisites:
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Operating System
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Any Unix operating system which supports PAM (Pluggable Authentication Module) |
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Complier
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Yubico Client C library
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Version 1.5 or later |
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Yubico PAM Module
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Version 1.7 or later |
This document illustrates the configuration steps for Fedora Core 8 operating system. However, there steps should work on most other Linux distributions.
The Yubico PAM module for SSH can be downloaded from here.
The Yubico PAM module support two factor authentication for SSH. The two factor authentication module verifies the user name and password for the user and the One-Time Password (OTP) generated by YubiKey assigned to the user.
Build instructions for yubico-c-client and pam_yubico are found in their respective README.
There are two ways of user and YubiKey token ID mapping. It can be either done at administrative level or at individual user level.
In Administrative level, system administrators hold right to configure the user and YubiKey token ID mapping. Administrators can achieve this by creating a new file that contains information about the username and the corresponding IDs of YubiKey(s) assigned.
This file contains user name that is allowed to connect to the system over SSH and the token id of the YubiKey(s) assigned to that particular user. A user can be assigned multiple YubiKeys and this multi key mapping is supported by this file. However, presently there is no logic coded to detect or prevent use of same YubiKey ID for multiple users.
Each record in the file should begin on a new line. The parameters in each
record are separated by : character similar to /etc/passwd.
The contents of this file are as follows:
<user name>:<YubiKey token ID>:<YubiKey token ID>: ….
<user name>:<YubiKey token ID>:<YubiKey token ID>:…..
e.g.
paul:indvnvlcbdre:ldvglinuddek simon:uturrufnjder:hjturefjtehv kurt:ertbhunjimko
The mapping file must be created/updated manually before configuration of Yubico PAM module for SSH authentication.
Append the following line to the beginning of the /etc/pam.d/sshd file:
auth required pam_yubico.so id=16 debug authfile=/path/to/mapping/file
Make sure you set id=16 to the correct API-id for the yubico validation server.
After the above configuration changes, whenever a user connects to the server using any ssh client, the PAM authentication interface will pass the control to Yubico PAM module. The Yubico PAM module first checks the presence of authfile argument in PAM configuration. If authfile argument is present, it parses the corresponding mapping file and verifies the username with corresponding YubiKey token id as configured in the mapping file. If valid, the Yubico PAM module extracts the OTP string and sends it to the Yubico authentication server or else it reports failure. If authfile argument is present but the mapping file is not present at the provided path PAM module reports failure. After successful verification of OTP Yubico PAM module from the Yubico authentication server, a success code is returned.
In User level, individual users have the ability to configure YubiKey token
ID assigned to them. Users can achieve this by creating a new file
.yubico/authorized_yubikeys inside their home directories that contains
information about the username and the corresponding IDs of YubiKey(s) assigned
to them. A user can be assigned multiple YubiKeys and the multi key mapping is
supported by this file.
This file must contain only one record. The parameters in the record are
separated by : character similar to /etc/passwd. The contents of this file
are as shown below:
<user name>:<YubiKey token ID>:<YubiKey token ID>: ….
e.g.
paul:indvnvlcbdre:ldvglinuddek
The .yubico/authorized_yubikeys file must be created/updated manually and must
be placed inside user’s home directory before configuration of Yubico PAM
module for SSH authentication.
Append the following line to the beginning of the /etc/pam.d/sshd file:
auth required pam_yubico.so id=16 debug
After the above configuration changes, whenever a user connects to the server
using any SSH client, the PAM authentication interface will pass the control
to Yubico PAM module. The Yubico PAM module first verifies the username with
corresponding YubiKey token id as configured in the .yubico/authorized_yubikeys
file that present in the user’s home directory who is trying to assess server
through SSH. If valid, the Yubico PAM module extracts the OTP string and sends
it to the Yubico authentication server or else it reports failure. After
successful verification of OTP Yubico PAM module from the Yubico authentication
server, a success code is returned.
Append try_first_pass parameter to the pam_unix.so module to authenticate the user with password passed from the preceding auth module.
The pam_unix.so module used for authentication is generally located into
/etc/pam.d/system-auth for RedHat based Linux system and into
/etc/pam.d/common-auth for Debian based Linux systems.
Edit the sshd configuration file /etc/ssh/sshd_config`_ to disable challenge-
response passwords. Change `challenge-response passwords yes to
challenge-response passwords no.
Test setup for fedora 8 environment is as follows:
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OS Version
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Fedora release 8 (Werewolf) |
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Kernel Version
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Kernel version 2.6.23.1-42.fc8 |
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OpenSSH Version
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openssh-4.7p1-2.fc8 |
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Yubico PAM Version
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pam_yubico-1.7 |
Test setup for fedora 6 environment is as follows:
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OS Version
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Fedora Core release 6 (Zod) |
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Kernel Version
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Kernel version 2.6.18-1.2798.fc6 |
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OpenSSH Version
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openssh-4.3p2-10 |
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Yubico PAM Version
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pam_yubico-1.7 |
PAM configuration files in our testing environment are as follows:
auth required pam_yubico.so authfile=/etc/yubikeyid id=16 debug auth include system-auth account required pam_nologin.so account include system-auth password include system-auth session optional pam_keyinit.so force revoke session include system-auth session required pam_loginuid.so
root:indvnvlcbdre:ldvglinuddek test:ldvglinuddek
root:indvnvlcbdre:ldvglinuddek
Please change PAM configuration settings for SSH as shown above and test the configuration.
We assume that you have root and test user configured to access SSH on your test environment with password secret and pencil respectively.
Use any standard SSH client for testing (We used SSH command line utility).
Try to login to server with SSH client as configured user:
ssh -l test localhost Password: (enter 'pencil' and touch the ldvglinuddek YubiKey)
ssh -l root localhost Password: (enter 'secret' and touch the ldvglinuddek YubiKey)
ssh -l root localhost Password: (enter 'secret' and touch the indvnvlcbdre YubiKey)