Improved two factor authentication with DuoSecurity

duo_auth.sh

###########################################################
# Two factor authentication with DuoSecurity              #
#                                                         #
# (c) 2012 P. Mathis <pmathis@snapserv.net>               #
###########################################################
# This script will improve the normal UNIX integration    #
# of DuoSecurity. You can specify for each SSH subsystem  #
# if the two factor authentication is required.           #
#                                                         #
# I am not responsible for lost or breached servers,      #
# broken hearts, thermonuclear war, massive traffic costs #
# or unfulfilled sexual partners. Please do some research #
# if you have any concerns about features included in     #
# this script before using it!                            #
#                                                         #
# But don't worry, the worst thing that could happen      #
# is that your two factor authentication would always be  #
# bypassed, as long as you don't do other modifications.  #
###########################################################

###########################################################
# Configuration                                           #
###########################################################

# Specifies if the the two factor authentication should be
# used for normal SSH sessions or not. Please use yes / no
# Recommended: yes
SSH_ENFORCE_DS=yes

# You can allow specific SSH keyfile users to bypass the
# two factor authentication. To use that feature, you must
# do a few things:
# 1) You must set "PermitUserEnvironment" to "yes" in your
#    SSH configuration. Otherwise the keyfile won't work
#    anymore. (And you're locked out, if password
#    authentication is disabled.
#
# 2) Add the following text in front of every SSH key which
#    can bypass the two factor authentication. For example,
#    you can add the following text in front of your
#    ~/.ssh/authorized_keys file
#
#    environment="DUOSEC=BYPASS"
#
#    After this option the normal "ssh-rsa ..." text should
#    follow. Don't forget a space after the environment
#    option, otherwise your key won't work again.
#
# 3) Set the following option to "yes"
SSH_ALLOW_KEY_BYPASS=yes

# Specifies if the two factor authentication should be
# used for RSYNC connections.
# Recommended: no
RSYNC_ENFORCE_DS=no

# Specifies if the two factor authentication should be
# used for SSHFS / SFTP connections.
# Recommended: no
SFTP_ENFORCE_DS=no

###########################################################
# End of configuration - Do not touch anything below      #
###########################################################

# Disconnect clients when they quit the script with Ctrl+C
trap jail INT
jail() {
        echo "You must not quit this script."
        kill -9 $PPID
        exit 0
}

# SSH connections (normal SSH shell)
if [ -z "$SSH_ORIGINAL_COMMAND" ]; then
        if [ "$SSH_ENFORCE_DS" = "yes" ]; then
                if [ "$DUOSEC" = "BYPASS" ] && [ "$SSH_ALLOW_KEY_BYPASS" = "yes" ]; then
                        # Give the user their shell
                        $SHELL -l
                        exit 0
                else
                        # Enforce DuoSecurity authentication
                        /usr/sbin/login_duo
                        exit 0
                fi
        else
                # Give the user their shell
                $SHELL -l
                exit 0
        fi
fi

# RSYNC connections
if [ `echo $SSH_ORIGINAL_COMMAND | awk '{print $1}'` = rsync ]; then
        if [ "$RSYNC_ENFORCE_DS" = "yes" ]; then
                # Enforce DuoSecurity authentication
                /usr/sbin/login_duo
                exit 0
        else
                # Allow the RSYNC connection directly
                $SHELL -c "$SSH_ORIGINAL_COMMAND"
                exit 0
        fi
fi

# SFTP / SSHFS connections
if [ `echo $SSH_ORIGINAL_COMMAND | awk '{print $1}'` = "/usr/lib/openssh/sftp-server" ]; then
        if [ "$SFTP_ENFORCE_DS" = "yes" ]; then
                # Enforce DuoSecurity authentication
                /usr/sbin/login_duo
                exit 0
        else
                # Allow the SSH / SFTP connection directly
                $SHELL -c "$SSH_ORIGINAL_COMMAND"
                exit 0
        fi
fi

# Invalid subsystem - deny it by default
kill -9 $PPID
exit 0

Installation

1. Install DuoSecurity according to their instructions, but do not do the part where the "ForceCommand" entry should be added.
2. Copy & paste the above script into /etc/ssh/duo_auth.sh
3. Add the following lines to your SSH configuration (/etc/ssh/sshd_config)
   ForceCommand /etc/ssh/duo_auth.sh PermitUserEnvironment yes
4. You may configure and test it now. Please do not close your current SSH session as long as you're not sure if everything works!

How about adding a #!/bin/bash as the first line for non-bash shell accounts?

Thanks for sharing, this has been a great help.