Chapter 7: Forwarding connections (tunnel)

Forwarding connections

Port forwarding comes in SSH protocol in two different flavours: direct or reverse port forwarding. Direct port forwarding is also named local port forwardind, and reverse port forwarding is also called remote port forwarding. SSH also allows X11 tunnels.

Direct port forwarding

Direct port forwarding is from client to server. The client opens a tunnel, and forwards whatever data to the server. Then, the server connects to an end point. The end point can reside on another machine or on the SSH server itself.

Example of use of direct port forwarding:

Mail client application   Google Mail
         |                    ^
     5555 (arbitrary)         |
         |                143 (IMAP2)
         V                    |
    SSH client   =====>   SSH server

Legend:
--P-->: port connexion through port P
=====>: SSH tunnel

A mail client connects to port 5555 of a client. An encrypted tunnel is established to the server. The server connects to port 143 of Google Mail (the end point). Now the local mail client can retreive mail.

Reverse port forwarding

The reverse forwarding is slightly different. It goes from server to client, even though the client has the initiative of establishing the tunnel. Once the tunnel is established, the server will listen on a port. Whenever a connection to this port is made, the server forwards the data to the client.

Example of use of reverse port forwarding:

 Local mail server    Mail client application
         ^                     |
         |               5555 (arbitrary)
     143 (IMAP2)               |
         |                     V
    SSH client   <=====   SSH server

Legend:
--P-->: port connexion through port P
=====>: SSH tunnel

In this example, the SSH client establishes the tunnel, but it is used to forward the connections established at the server to the client.

X11 tunnels

X11 tunnels allow a remote application to display locally.

Example of use of X11 tunnels:

   Local display     Graphical application
   (X11 server)          (X11 client)
         ^                     |
         |                     V
    SSH client   <=====   SSH server

Legend:
----->: X11 connection through X11 display number
=====>: SSH tunnel

The SSH tunnel is established by the client.

How to establish X11 tunnels with libssh has already been described in this tutorial.

Doing direct port forwarding with libssh

To do direct port forwarding, call function ssh_channel_open_forward() :

you need a separate channel for the tunnel as first parameter;
second and third parameters are the remote endpoint;
fourth and fifth parameters are sent to the remote server so that they can be logged on that server.

If you don’t plan to forward the data you will receive to any local port, just put fake values like “localhost” and 5555 as your local host and port.

The example below shows how to open a direct channel that would be used to retrieve google’s home page from the remote SSH server.

int direct_forwarding(ssh_session session)
{
  ssh_channel forwarding_channel;
  int rc;
  char *http_get = "GET / HTTP/1.1\nHost: www.google.com\n\n";
  int nbytes, nwritten;

  forwarding_channel = ssh_channel_new(session);
  if (forwarding_channel == NULL) {
      return rc;
  }

  rc = ssh_channel_open_forward(forwarding_channel,
                                "www.google.com", 80,
                                "localhost", 5555);
  if (rc != SSH_OK)
  {
    ssh_channel_free(forwarding_channel);
    return rc;
  }

  nbytes = strlen(http_get);
  nwritten = ssh_channel_write(forwarding_channel,
                           http_get,
                           nbytes);
  if (nbytes != nwritten)
  {
    ssh_channel_free(forwarding_channel);
    return SSH_ERROR;
  }

  ...

  ssh_channel_free(forwarding_channel);
  return SSH_OK;
}

The data sent by Google can be retrieved for example with ssh_select() and ssh_channel_read(). Goggle’s home page can then be displayed on the local SSH client, saved into a local file, made available on a local port, or whatever use you have for it.

Doing reverse port forwarding with libssh

To do reverse port forwarding, call ssh_channel_listen_forward(), then ssh_channel_accept_forward().

When you call ssh_channel_listen_forward(), you can let the remote server chose the non-priviledged port it should listen to. Otherwise, you can chose your own priviledged or non-priviledged port. Beware that you should have administrative priviledges on the remote server to open a priviledged port (port number < 1024).

Below is an example of a very rough web server waiting for connections on port 8080 of remote SSH server. The incoming connections are passed to the local libssh application, which handles them:

int web_server(ssh_session session)
{
  int rc;
  ssh_channel channel;
  char buffer[256];
  int nbytes, nwritten;
  int port = 0;
  char *helloworld = ""
"HTTP/1.1 200 OK\n"
"Content-Type: text/html\n"
"Content-Length: 113\n"
"\n"
"<html>\n"
"  <head>\n"
"    <title>Hello, World!</title>\n"
"  </head>\n"
"  <body>\n"
"    <h1>Hello, World!</h1>\n"
"  </body>\n"
"</html>\n";

  rc = ssh_channel_listen_forward(session, NULL, 8080, NULL);
  if (rc != SSH_OK)
  {
    fprintf(stderr, "Error opening remote port: %s\n",
            ssh_get_error(session));
    return rc;
  }

  channel = ssh_channel_accept_forward(session, 60000, &port);
  if (channel == NULL)
  {
    fprintf(stderr, "Error waiting for incoming connection: %s\n",
            ssh_get_error(session));
    return SSH_ERROR;
  }

  while (1)
  {
    nbytes = ssh_channel_read(channel, buffer, sizeof(buffer), 0);
    if (nbytes < 0)
    {
      fprintf(stderr, "Error reading incoming data: %s\n",
              ssh_get_error(session));
      ssh_channel_send_eof(channel);
      ssh_channel_free(channel);
      return SSH_ERROR;
    }
    if (strncmp(buffer, "GET /", 5)) continue;

    nbytes = strlen(helloworld);
    nwritten = ssh_channel_write(channel, helloworld, nbytes);
    if (nwritten != nbytes)
    {
      fprintf(stderr, "Error sending answer: %s\n",
              ssh_get_error(session));
      ssh_channel_send_eof(channel);
      ssh_channel_free(channel);
      return SSH_ERROR;
    }
    printf("Sent answer\n");
  }

  ssh_channel_send_eof(channel);
  ssh_channel_free(channel);
  return SSH_OK;
}