VNC Remote Help Scripts

These are a set of scripts that permit customers who need remote help to share their desktops with trusted helpers in a semi-secure way.

Goals

• Minimize hassle for customers while protecting their security.
• Keep bandwidth use small, even at the expense of user experience.
• Require no port forwarding for either the customer or volunteer computers. Instead, use an SSH server on the Internet that both groups can log into.
• Lock down access to the SSH server by unlocking and relocking customer accounts as required.
• Use temporary passwords that are regenerated each session.
• Only allow access to customer machines if the customers explicitly allow it (ie there should be no backdoors).

Assumptions

• People with access to the technician accounts can be trusted, because they get passwordless sudo access to usermod and chpasswd

Overview

• The customer initiates a connection from his or her desktop computer. This prompts the customer for a (numerical) password.
• The technician SSHes into the SSH server. This unlocks a customer account and assigns a numerical password to this account.
• The technician reads the numerical password to the customer.
• The customer types in the numerical password, which creates an SSH tunnel to the SSH server. x11vnc tunnels through this SSH server, and sets a second numerical passcode to allow access to the session.
• The technician now makes a second connection to the SSH server to connect to the customer VNC session. The technician is first prompted for the SSH account password, and then is prompted for the VNC password.
• If the technician connects successfully, then the desktop is shared and usable by either user.
• The customer has the ability to end the session at any time by pressing a big "Close Connection" button.
• When the technician is done, he or she closes their first SSH session. This locks the customer account again, so the customer cannot reconnect (without going through the process again). If for some reason the technician does not lock the account, then the account should auto lock in two days.

It should be apparent that having a VNC session alone is not sufficient for this interaction to work. The technician has to interact with the customer in some other way (presumably by phone, although I guess some chat software would work as well).

Weaknesses:

• Setting login shells to shell scripts is a terrible idea and does not offer any more security than giving those users shell access.
• 256 colour output is ugly.
• Helpers need to enter too many passwords too many times, and in particular have to remember to enter the SSH password and VNC password in the correct sequence. This is a big pain. The solution is to use SSH keys for the helpers.
• The helper script does not validate input of the account number.

Future Work

• Set up corresponding Windows scripts. Allowing the helpers to use Windows should not be difficult. Allowing the customers to use Windows will require making new get-remote-help.sh scripts.
• Better error checking for missing dependencies.
• Better input validation.
• Allow more than 100 accounts (00-99?)

Initial server setup

Assume 10.10.10.x is the LAN subnet where technician machines live.

Install the unlock-customer-account.sh script on the server

• /usr/local/bin/unlock-customer-account.sh
• /etc/remote-help/remote-help.conf

Allow passwordless sudo access to unlock_accounts for following commands:

• usermod
• chpasswd

Create a group for the helpers:

addgroup unlock_accounts
adduser helper01 unlock_accounts
adduser helper02 unlock_accounts

In /etc/sudoers

%unlock_accounts ALL = NOPASSWD: /usr/sbin/usermod, /usr/sbin/chpasswd

In /etc/ssh/sshd_config:

PermitTunnel yes
AllowTcpForwarding yes
AllowUsers customer??  [email protected].*
PermitRootLogin no

Adding more user accounts

• Create helper account

• Set shell to /usr/local/bin/unlock-customer-account.sh usermod -s /usr/local/bin/unlock-customer-account.sh helper02

• Add corresponding customer account

• Set shell to /bin/false

On client machines

apt-get install zenity sshpass x11vnc bash

• /usr/share/pixmaps/get-remote-help.png
• /usr/local/bin/get-remote-help.sh
• /etc/remote-help/remote-help.conf
• /usr/share/applications/get-remote-help-local.desktop

On helper machine

apt-get install tightvnc

Make sure the following scripts are available in the same folder:

• setup-technician-remote-connection.sh
• remote-help.conf

Credits

• http:/www.karlrunge.com/x11vnc/faq.html#faq-firewall-out, which documents the procedure I implemented.
• https://openclipart.org/detail/3945/message-in-a-bottle for the bottle icon
• Members of http://kwlug.org community for suggestions and insight