Implement ssh port forwarding

[mr-null-byter]

Would it be possible to implement SSH port forwarding? The Blink app has this feature, so I think it should be achievable on iOS.

For instance, ssh -L 8888:localhost:8888 username@server_address.

Would be pretty useful for spinning up and accessing remote Jupyter servers.

[thekeith]

@pedromzadeh I’d go a step further and add ssh config to enable forwarding of multiple ports and agent forwarding as well. I raised this in #928 before I saw your issue

[bummoblizard]

This is doable but it's going to involve some work. We need to add the relevant features in our Objective-C libssh2 wrapper https://github.com/thebaselab/NMSSH to support this.

[bummoblizard]

https://libssh2.org/examples/tcpip-forward.html

[thekeith]

This is doable but it's going to involve some work. We need to add the relevant features in our Objective-C libssh2 wrapper https://github.com/thebaselab/NMSSH to support this.

I’ll go dig up my Objective-C skills…

[bummoblizard]

I’ll go dig up my Objective-C skills…

@thekeith I am working on it.

[bummoblizard]

Proposed NMSSH API:

class NMSSHChannel {
  // ...

  /* 
  Start listening on a local port and forward any TCP connection to the host, non-blocking
  queue: The queue where libssh2 calls are executed in (libssh2 and NMSSH are not thread-safe by design)
  */
  func bindLocalPortToRemoteHost(localListenIP: String, localPort: Int32, hostTargetIP: Int32, hostTargetPort: Int32, in queue: DispatchQueue) throws -> NMSSHSocket
}

class NMSSHSocket {
  // Underlying libssh2 socket
  private var sock: libssh2_socket_t 

  // To stop listening on a port
  func close()

  // TODO: Delegate / methods for status updates
}

bindLocalPortToRemoteHost():

• Initialise a POSIX socket
• Accept connections until the socket is closed from another thread. As accept() is a blocking call, it should be called in a new thread / DispatchGroup.
• Once a connection is accepted, an auxiliary function is called in the provided DispatchQueue
• The function sets up a new libssh2 channel using libssh2_channel_direct_tcpip_ex
• Receive the available data from the socket and forward it to the remote host.
• Forward the response back to the local socket.
• Close the channel (A channel only serves a single TCP connection)

[bummoblizard]

[thekeith]

Great to see progress here. How would it be configured?

One caveat is making sure that it doesn’t require a separate action to connect forwarded ports. In Termius for example you have to make a completely separate connection and initiate it for port forwarding which makes it a pain to set up, especially when you’re already in a session or want to forward multiple ports.

Ideally it would be great to allow users to do so via a UX which simply writes to a ~/.ssh/config file.

[bummoblizard]

Great to see progress here. How would it be configured?

One caveat is making sure that it doesn’t require a separate action to connect forwarded ports. In Termius for example you have to make a completely separate connection and initiate it for port forwarding which makes it a pain to set up, especially when you’re already in a session or want to forward multiple ports.

Ideally it would be great to allow users to do so via a UX which simply writes to a ~/.ssh/config file.

It would be configured through both GUI and config file. Let's support GUI first and deal with parsing .ssh/config in #928.