Ssh – Directx application using wine over ssh -x

Originally posted on Ask Ubuntu

If you have ruled out any "external" factors, the following set of steps usually helps to narrow it down. So while this doesn't directly answer your question, it may help tracking down the error cause.

Troubleshooting sshd

What I find generally very useful in any such cases is to start sshd without letting it daemonize. The problem in my case was that neither syslog nor auth.log showed anything meaningful.

When I started it from the terminal I got:

# $(which sshd) -Ddp 10222
/etc/ssh/sshd_config line 8: address family must be specified before ListenAddress.

Much better! This error message allowed me to see what's wrong and fix it. Neither of the log files contained this output.

NB: at least on Ubuntu the $(which sshd) is the best method to satisfy sshd requirement of an absolute path. Otherwise you'll get the following error: sshd re-exec requires execution with an absolute path. The -p 10222 makes sshd listen on that alternative port, overriding the configuration file - this is so that it doesn't clash with potentially running sshd instances. Make sure to choose a free port here.

Finally: connect to the alternative port (ssh -p 10222 user@server).

This method has helped me many many times in finding issues, be it authentication issues or other types. To get really verbose output to stdout, use $(which sshd) -Ddddp 10222 (note the added dd to increase verbosity). For more debugging goodness check man sshd.

The main advantage of this method is that it allows you to check the sshd configuration without having to restart the sshd on the default port. Normally this should not interfere with existing SSH-connections, but I've seen it. So this allows one to validate the configuration file prior to - potentially - cutting off ones access to a remote server (for example I have that for some VPS and even for physical servers where I need to pay extra to get out-of-band access to the machine).

Performing SSH tunneling can get a bit confusing with all the terminology, but there is a complementary feature to -L, which provides you the ability to "dynamically" assign ports by allocating a socket locally, instead of a single port.

From the man page:

-D [bind_address:]port
    Specifies a local ``dynamic'' application-level port forwarding.  This works by 
    allocating a socket to listen to port on the local side, optionally bound to the 
    specified bind_address.  Whenever a connection is made to this port, the connection 
    is forwarded over the secure channel, and the application protocol is then used to 
    determine where to connect to from the remote machine.  Currently the SOCKS4 and 
    SOCKS5 protocols are supported, and ssh will act as a SOCKS server.  Only root 
    can forward privileged ports. Dynamic port forwardings can also be specified in 
    the configuration file.

By allocating a socket, all the traffic can be funneled through to the remote site, including DNS queries.

How to use it

For starters you'll need to open up a connection to your LAN (through its public IP address on the internet) like so:

$ ssh -D 1234 myserver

NOTE: This assumes that you have the ability to SSH into a server that's accessible through your public internet IP address.

Once that's setup, in another terminal, you'll want to configure your web browser to make use of this tunnel. NOTE: This type of tunnel is providing you a socket, so to connect to it, you need to tell your web browser to proxy all of its traffic via this socket. This is typically shown as a SOCKS or SOCKS v5 type of connection for your proxy.

An example

In this example I'll show how you can do it using Chromium, via the CLI:

$ ./Chromium --proxy-server="socks5://localhost:1234"

Here I'm launching Chromium and pointing it to the SSH tunnel which we earlier configured on our localhost's port 1234. And with this, if I then attempt to visit a URL for a server that's configured on my LAN, I'm directed to it:

Proxying with other browsers

All the major browsers provide this feature and it's covered pretty extensively on other SE sites such as SuperUser:

• How can I change the proxy settings for Google Chrome?
• How to setup proxy settings in firefox?

You can even make use of extensions to the various browsers which allow you to selectively proxy only certain traffic, while allowing you to route everything else out over your normal connection to the internet.

For example, you can use ProxySwitchy! with Chrome to do exactly that: