Dynamic SSH proxies forward application connections using the SOCKS4 or SOCKS5 protocol. The third kind of tunnel we’ll cover is a dynamic proxy. A more realistic use for remote tunnels will be given below in the examples. The tunnel’s ingress is on bastion-host’s localhost:8080 and the egress points to bastion-host: $ curl This is a somewhat contrived remote tunnel example, since we might just as easily have made a local tunnel routed from 8080 to port 80 over localhost on the tunnel host itself. If we wanted to make available to a remote server via the remote server’s localhost:8080, we might do something like this: The basic syntax of a remote tunnel is identical to local tunnel syntax except for the -R option:Ĭlient-host: $ ssh -R port:host:hostport tunnel-host -N Remote tunnels are created with the -R option. Remote tunnelsĪ remote tunnel is a tunnel whose ingress is located on the tunnel host. You: $ curl The HTTP request goes through the tunnel host bastion-host the tunnel egress is at Keep in mind that the SSH portion of the tunnel is between the client host and the tunnel host traffic between the tunnel host and is not protected by SSH. If our client host were named you, we might make a connection to a web server through a bastion host:Īnd to use the tunnel we connect our web client to the tunnel’s ingress at localhost:8080: The basic syntax of a local tunnel looks like this:Ĭlient-host: $ ssh -L port:host:hostport tunnel-host -N Local tunnels are created with the -L option. See An Illustrated Guide to SSH Agent Fowarding and Using SSH Agent Forwarding for more information 1.Ī local tunnel is a tunnel whose ingress is located on the client host.

You should never enable forwarding on hosts you do not trust or otherwise do not wish to use agent forwarding on.

N indicates that we will not be sending any commands over SSH and that ssh should not open stdin or execute any commands on the tunnel host. We’ll also cover some general ssh options that are often used in SSH tunnels: We’ll cover the basics of tunnel syntax here, copied more or less from the ssh(1) man page. Some complex tunneling involves multiple client hosts, tunnel hosts and ingress/egress pairs. We may find that our way is sometimes blocked by firewall rules or network topologies 1. To create an SSH tunnel, the tunnel host must be able to reach the egress host:hostport via TCP. Using TunnelsĪny service using the tunnel must connect to the ingress port and the connection will come out at the egress host:hostport. TCP traffic past the tunnel host is not secured by SSH 1 and any security would be determined by the protocol being tunneled (e.g., HTTPS, IMAPS, etc.), if any. The SSH portion of the tunnel ends at the host where the final SSH connection is made, though the tunnel host will forward the TCP connection to the specified host:hostport if host is not localhost. The tunnel egress can be a little confusing. The ingress bind-address is nearly always localhost 1 and since this is the default in ssh, it is often omitted and we have only the ingress port. In ssh syntax, the bind-address:port pair is always the ingress and the host:hostport pair is always the egress. We will call the initiating side, entrance, or “listening” end of the tunnel ingress and the terminating or exit end of the tunnel egress.

With SSH tunnels, the ends of the tunnel are not interchangeable as connections may be initiated at only one end. In the physical world of plumbing we have pipes, and one end of a pipe is usually as good as the other. To build an SSH tunnel, the client host must be able to reach the tunnel host via SSH. The second host is the tunnel host where the tunnel connections are created. The first host is the client host, or the host from which we run ssh to define the tunnel. SSH tunnels always have at least two hosts involved 1. Client-host: $ ssh -L 8080:www:80 tunnel-host -N

Tell specific all to use ssh tunnel