How to create a custom ssh shell in elixir?

Questions / Help
1

My Google-Fu has failed me and i am stuck if any one can help

I am trying to create a custom ssh shell in elixir using the erlang :ssh library so that i can ssh into my application with a restricted set of options/commands available. I can log in successfully (using passwords is fine for this use) but at that point i can not send anything to the client or receive anything from the client (at least i can not work out how).

I have read the erlang ssh library document and looked at an example library (esshd) but the information i need is not clear and the library does not work for me (and no dev help). Please see attached example code.

def init(_opts) do
      Logger.info("--> Starting Communications.SSHD process")

      # start the erlang library
      :ssh.start()
      # config the server
      {ok, sshd2} = :ssh.daemon(10022, 
                              shell: &on_shell/2,
                              system_dir: './priv/daemon', 
                              user_passwords: [{'test', 'testpass'}], 
                              user_dir: './') 

      # link the ssh daemon pid to ourselves
      Process.link sshd2

      {:ok, []}
    end

    def on_shell(username, {ip, port} = peer_address) do
      Logger.info("Onshell Called")
      # spawn a thread to handle the shell
      spawn(__MODULE__, handle_shell, [])
    end

    # handle the shell commands
    def handle_shell() do
      Logger.info("Shell staretd #{inspect Process.group_leader}")
      _ = :io.setopts(Process.group_leader, binary: true, encoding: :unicode)  

      # this never appears on client or server console

      IO.puts "Interactive example SSH shell - type exit ENTER to quit"
    end

Any ideas?

Thanks

Tom

2

Go to hex.pm and search for “ssh” and have a look at all the various packages. Maybe one of them already does what you want or something very similar.

3

Yes i have already tried that by looking at esshd and ssh_echo but can’t work out why its not working. I can connect, auth and see the shell started but have no idea how to read from or write to the ssh socket connection for the connected client. Following what the above libraries have done does not seem to work!

4

Hi, Did you ever make any progress with this.
Regards,
Brian

5

HI Brian,

sorry - no progress - i gave up in the end

Shifters

6

Hello,

I’ve implemented (in erlang) a custom ssh shell:


Based on:

https://www.erlang-solutions.com/blog/secure-shell-for-your-erlang-node.html

I also wrote a post (in pt_BR) related to it https://beam-mignon.netlify.app/posts/mignon_21/

Is that what you were looking for ?

3 Likes
7

Thanks for sharing that - i will look into your code in more detail when i revisit my project

cheers

shifters