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Summary

There should be a simple, secure, robust way for a non-technical user to allow a more technical user to connect to their computer and get root access, using only instructions that can be described simply, in layman's terms, over a poor quality phone line. The facility to allow the technical user access should be available, and easily visible, in the default install.

In this document, the technical user is referred to as the "helper", and the non-technical user as the "friend".

Rationale

For experienced Linux users, over-the-phone tech support for a non-technical friend is a common use case. It's normally an unpleasant experience, for the following reasons:

• Telephones often have poor audio quality. For example, it's hard for the friend to tell whether you're saying "less" or "ls"
• Describing what command lines to type can be socially awkward. For example, do you tell them where to put spaces and when to press enter? Making the wrong decisions will either insult your friend or make them feel even more helpless than they do already
• It's very difficult for a friend to accurately describe what they're seeing on their screen. For example, most people don't know how to pronounce "~" or what a backtick is

Other Projects

Several projects aim to provide complete application suites that handle tasks such as remote support. Included among them are:

• [https://launchpad.net/locoremotesupport/ LoCo Remote]

• [https://launchpad.net/telepathy Telepathy]

This project would not provide a complete suite, just a tool to enable remote connections to be made. Complete solutions aren't appropriate to a support request made over the phone.

Overview of the problem

Both helper and friend might be behind a NAT router or a firewall beyond their control. It's more likely that the helper would be able to configure their network so as to allow incoming connections on specified ports, but this can't be assumed in the general case.

If the friend has an X session running, the helper should be given access to it over a VNC connection. Otherwise, the friend should be given a login shell on the user's machine. [https://launchpad.net/screen screen] is the preferred tool for this, because it can provide a multi-user session encapsulated within a Unix socket that can be forwarded over an SSH connection.

The friend's computer can't be assumed to have a particularly large set of packages installed and functioning, as they might need help because they've broken important packages, or might have decided that they didn't like the look of some packages and decided to uninstall them. At best, we can assume that specified packages in main have been installed, and may or may not have functioning configuration files. Any system more broken than this would be better served by (semi-)automated recovery scripts that can solve specific problems.

The helper's computer can be assumed to have a much more complete system, because people with sufficient expertise and patience to help out can reasonably be asked to install packages outside of main, and not to find imaginative ways of breaking those packages.

Man-in-the-middle attacks are a serious security issue here. If the helper and friend are talking to one another on the phone while setting up the connection, they have a largely tamper-proof connection - albeit one where bandwidth is severely limited. If they're communicating over some other system (such as instant messaging), there's no tamper-proof connection between them. If helper and friend haven't already exchanged security information (such as SSH keys), there is no full-proof defence against MITM attacks without a tamper-proof connection available. Therefore, the following security precautions should be taken:

• The helper should never be able to do anything behind the friend's back. Anything the helper does should be visible to the friend
• passwords and other important security information should never be transferred over the connection
• The friend should have an easy way of terminating the session, and should be aware of that method

The first of these conditions have two important side-effects, one positive, one negative. The positive side-effect is that the friend has the opportunity to learn a little by watching the helper. The negative side-effect is that there's no easy way to transfer files between the computers. It's possible to transer files by pasting base64-encoded text, which accomplishes the same goal and leaves the friend with the ability to check the process.

The second condition can be met by ensuring that, as well as reading anything the helper does, the friend can write anywhere that the helper can write. Therefore, if the helper needs a password, the friend can type it in without telling the helper.

Over a VNC connection, these conditions are trivially met - both users share a session, and the friend can terminate the session by pressing ctrl-alt-backspace. Over a [https://launchpad.net/screen screen], these conditions can be met using a multi-user screen, and by ensuring that the screen session is terminated when the friend disconnects from it.

Design

On the friend's computer, a Perl script will be used that depends on openssh-client, x11vnc, and screen. The x11vnc package is currently in universe, the others are in main. The script will ask the friend for certain details (such as the helper's IP address), and will describe what will happen as it tries to set up a screen or VNC session. A Perl script is suggested because (according to [http://popcon.ubuntu.com/ the popularity contest]) Perl is one of the most widely installed packages in Ubuntu, and the friend can download and run it in an emergency without needing to compile it for their particular architecture.

On the helper's computer, a program will attempt to let the friend connect to a special-purpose SSH daemon. While doing so, it will tell the helper what the friend is seeing on their screen, and what information the helper should give to the friend.

In order to make it easier to access, the Perl script on the friend's computer should be available through the recovery menu in single user mode, and through System Tools->Share my Desktop from the GUI.

The computers will attempt to connect three ways:

1. the friend's computer will try to connect directly to the helper's computer
2. both computers will ask if there is a shared server they can exchange messages through
3. the friend's computer will ask permission to install an SSH daemon that the helper can connect to

Note that in the final case, the connection from the helper to the friend's computer would only be used to forward a port that lets the friend SSH in to the helper's computer. Although a secure way could be found to only use a single connection, the extra complexity isn't worth the minor speed advantage it would gain.

Once the connection is made, the friend's computer will (optionally) send their public key for future use, request either a screen or VNC session, then start that session.

Implementation

This system will take the form of two interacting programs on two computers.

Helper's computer

1. When the program on the helper's computer is run, it will ask whether the friend has connected to the system before, and offer a selection of known users.
2. It will then start a special-purpose SSH daemon that allows a heavily locked-down user to connect
   1. if the friend has connected before, it will allow access with the friend's (pre-shared) public key
   2. otherwise, it will require a random one-time password consisting of numbers and lower-case letters, which is [#Communicate communicated] to the friend
3. It will ask what IP address the client should connect to. This is normally one of the computer's IP addresses, but might be the IP address of a router if the computer is behind a NAT
4. Then it will tell the helper to [#Communicate communicate] that IP address to the friend
5. It will then wait indefinitely for the friend to connect
6. When the friend has connected, the user account will run a special program, rather than a login shell. Because this account isn't fully trusted, the following security precautions will apply:
   • The SSH server will not allow forwarding of local or remote ports or X connections
   • The program that's run will only be allowed to listen on TCP port 5901 on localhost,

   • The program will only be allowed to read and write to the /var/run/screen/S-$helpers_username/$PID.remote-help.$helpers_computer_name Unix socket

   • The above restrictions will be enforced by AppArmor

   • The program will not be put in a chroot jail, as it was felt not to provide sufficient extra protection, given the above measures and the complexities of maintaining the jail.

7. When a connection is made, the program immediately stops the server, so as to reject any further connections.
8. If the friend logged in with a password, the friend's computer must immediately send a public key for future use.
9. The helper's computer then sends a space-separated line of session types that it supports. Currently, "vnc" and "screen" are defined, although helpers without X, without a VNC client, or who have uninstalled screen, might not send one or other session type
10. The friend's computer replies with a line containing one of those choices.

11. The input from, and output to, the SSH connection are piped through to either a server listening on port 5901, or a Unix socket at /var/run/screen/S-$helpers_username/$PID.remote-help.$helpers_computer_name

12. The helper will be asked either to point their favourite VNC client at localhost:5901, or to attach to the specified screen

Friend's computer

1. When the script is run, it will print the following warning:

   • This program gives complete control of your computer to someone else
     on the Internet, so that they can help solve any problems you are
     having.  In order to keep your computer safe, remember:
     
     *** Never run this program because somebody online asked you to.
       + Only run it as a request made over the telephone,
         from somebody you trust not to break your computer.
     
     *** Never tell the other person your password.
       + If you need to, you can type it in yourself.
     
     There's no security risk if you quit now.

2. The friend is asked if they want to continue.
3. The program [#Communicate asks] for the IP address of the friend's computer
4. The program [#Communicate asks] for an optional password
5. The program attempts to SSH to the specified IP address using its public key, and the specified password if one was given.
6. The program sends its public key if a password was given
7. The program receives the space-separated list of session types, and returns its preferred type

8. The program starts either x11vncserver or screen, and connects the standard input and output from the shell to a forwarder that communicates with either TCP port 5901, or a Unix socket at /var/run/screen/S-$friends_username/$PID.remote-help.$friends_computer_name

Communicating technical data

Anchor(Communicate)

At several points during the connection process, users will be asked to send out-of-band information to one another - for example, the friend will need to be told the IP address of the helper. Because this might involve correct spelling over a phone line, users will need to be given the text normally and in the [wiki:WikiPedia/NATO_phonetic_alphabet NATO phonetic alphabet]. As an additional safety measure, messages are also given a checksum, consisting of the number of characters sent, a dash, then the sum of Unicode code points, in hexadecimal. This checksum is cryptographically worthless, but strong enough to guard against typos.

When a message is sent, the sending user should be shown a message like the following:

Please pass the following message to your friend.

If speaking to them, tell them to type:
        Alpha Bravo Charlie One Two Three ENTER
        Six Dash One Bravo Charlie ENTER
Otherwise, tell them to type:
        abc123 <enter>
        6-1bc <enter>

It doesn't matter whether you type this in upper or lower case.

When a message is received to be verified, the receiving user should be shown a message like the following:

Please verify the message your friend reads out to you.

If they're speaking to you, they should say:
        Alpha Bravo Charlie One Two Three ENTER
        Six Dash One Bravo Charlie ENTER
Otherwise, they should type:
        abc123 <enter>
        6-1bc <enter>

When a message is received to be typed, the receiving user should be shown a message like the following:

Your friend will now tell you to type two lines of text.  If speaking
to you, your friend will spell the message out phonetically.  Here's
the alphabet that will be used:

A = Alpha       N = November    0 = Zero
B = Bravo       O = Oscar       1 = One
C = Charlie     P = Papa        2 = Two
D = Delta       Q = Quebec      3 = Three
E = Echo        R = Romeo       4 = Four
F = Foxtrot     S = Sierra      5 = Five
G = Golf        T = Tango       6 = Six
H = Hotel       U = Uniform     7 = Seven
I = India       V = Victor      8 = Eight
J = Juliet      W = Whiskey     9 = Nine
K = Kilo        X = X-ray
L = Lima        Y = Yankee
M = Mike        Z = Zulu

It doesn't matter whether you type this in upper or lower case.

Message:

See also

• RecoveryMode

• [https://lists.ubuntu.com/archives/ubuntu-devel-discuss/2008-May/004078.html Discussion on the ubuntu-devel-discuss mailing list]

CategoryNetworking CategoryRecovery