sshportsocks

SSH Socks Server

I'm trying to run a ssh SOCKS server on Windows 7 (listening on port 12345). Here's the output I get on Cygwin:

$ ssh -v -D 12345 localhost
OpenSSH_6.8p1, OpenSSL 1.0.2c 12 Jun 2015
debug1: Reading configuration data /etc/ssh_config
debug1: Connecting to localhost [::1] port 22.
debug1: connect to address ::1 port 22: Connection refused
debug1: Connecting to localhost [127.0.0.1] port 22.
debug1: connect to address 127.0.0.1 port 22: Connection refused
ssh: connect to host localhost port 22: Connection refused

Why is it trying to connect to localhost:22? Looks like it's trying to reach sshd running on localhost. I thought the ssh client was enough to set up a local SOCKS server. If it isn't, why do I need sshd running?

From https://help.ubuntu.com/community/SSH/OpenSSH/PortForwarding:

"Dynamic port forwarding turns your SSH client into a SOCKS proxy server"

Solution

  • To explain why you need a (remote) ssh server, ssh can do three (or four) kinds of forwarding; quoting the man page:

    -L Specifies that the given port on the local (client) host is to be forwarded to the given host and port on the remote side. 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 a connection is made to host port hostport from the remote machine. [...]

    -R Specifies that the given port on the remote (server) host is to be forwarded to the given host and port on the local side. This works by allocating a socket to listen to port on the remote side, and whenever a connection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host port hostport from the local machine. [...]

    -D 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. [...]

    -X and -Y enable forwarding for X11. This is a small but convenient variation of -R.

    Note that in all cases the data is forwarded over the ssh tunnel, from the local machine to the ssh server or the reverse, and (therefore) the ssh tunnel must exist for the data to be forwarded over. The only difference between -L and -D is that -D uses SOCKS4/5 on the local end to specify where the remote end connects to.

    If you want a SOCKS proxy that connects directly from the proxy to the destination, not over an ssh tunnel, you need a plain SOCKS proxy, not ssh+sshd.