![]() The connection is then forwarded to a resource within the trusted internal network. Your local SSH client establishes a connection with the remote SSH server. Organizations usually solve this issue by setting up an intermediary SSH ‘jump’ server to accept remote SSH connections. This would be a severe limitation in a modern distributed environment. Valuable network resources do not generally allow remote SSH access. A reachable IP address or name of the remote/local server.An SSH client/server of your choice (OpenSSH or PuTTY).With that in place, I can point my web browser at localhost:9000, and my remote desktop client at localhost:9001. So, if I want to connect to both a web server (port 80) and a remote desktop server (port 5900), I can do this: I’ll close this post with the fact that ssh accepts multiple instances of the “-L” flag. It’s actually a remarkably stable and secure solution to this problem. While I’m connected, all of my traffic goes over port 22, and is encrypted in exactly the same way as the rest of my ssh traffic. As soon as I log out, my tunnel goes away. The connection will only stay open as long as the ssh connection is active. My ssh connection is catching those requests on port 9000, forwarding them along to the remote machine, and passing them to port 80. From the point of view of the web browser, I’m looking at port 9000 on my local machine. Once that tunnel is in place, I should be able to open a web browser and look at the URL below to see the web server on the remove machine: So, I’m setting up a connection from my local machine’s port 9000 to port 80 on. * Last is the port to connect to on that target machine. I think that this is the most frequent source of confusion with tunnels, figuring out the context in which hostnames are evaluated. In this simple example, I’m going to “localhost”, which really means “”. * The middle is the target hostname I want to connect to as it is seen on the remote machine. * First is the port on my local machine into which I peer ![]() ![]() The tunnel specification is in three parts: With that in hand, I am going to add the forward tunnel specification: Keep in mind that if that first command doesn’t work – there’s nothing I can do to make the rest work. I run this perfectly ordinary ssh command to log in, and it works: I will log into that remote machine with an account called “cdwan” Assume as well that we’ve already called up the network administrator, who said “no,” there are no exceptions to this security policy. ![]() I can ssh in, but I can’t really debug some problem with the web browser because port 80 is still blocked. However, because I’m not working onsite, her organization’s security scheme only allows me to get to port 22. That’s well and good, but in my work I often need to see web servers (port 80) and/or Apple’s Remote Desktop or VNC (port 5900).Īs a first puzzle, assume that a friendly administrator gave me an ssh account on her web server. A very reasonable policy might only allow access over ssh (port 22) from machines outside of a local network. I hope to shed some light on reverse tunnels in a future post.įirewalls and other network devices frequently limit the network ports that I am allowed to access. This post provides a very basic intro to forward tunnels. Even though I can be flippant about this, all of these techniques remain true to the spirit, if occasionally subverting the letter, of the network access requirements imposed by my customers. I have found that it is invariably easier to figure out how to construct a tunnel than it is to convince network security folks to open more ports in their firewalls. I use them daily to navigate the somewhat arbitrary networking requirements that I encounter from both software tools and network administrators. They allow me to connect to arbitrary ports on remote machines. SSH tunnels are some of the most powerful utilities in the entire command line arsenal.
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