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