问题
The Preamble
I start up my local SSH terminal at work behind a firewall, and connect to a remote server all the time without any problem.
The way Xdebug works, correct me if I'm wrong, is that it sends an "unsolicited" request to my network's port 9000. I actually initiated that action by sending the remote server an HTTP request through my browser with a POST/GET/COOKIE variable instructing xdebug to start up. But my network doesn't know that. All it knows is that it is getting a request on port 9000 from the internet. It doesn't know which computer in its private network to forward it to (without setting up port forwarding on the router), and can only ignore the request.
So if you can't do port forwarding, another option (and a much better one from what I can tell), is SSH tunneling. My computer sends the SSH request, the server responds. My router knows which computer in its network to route these responses to. Piggybacking on that SSH connection allows those "unsolicited" port 9000 requests from the remote server to get to me.
I think I understand that much.
I finally got tunneling to work, thanks to stackoverflow, but how it works is still fuzzy to me.
On the remote server, I tell Xdebug to connect to localhost (not to my ip via xdebug.remote_host=173.123.45.56, and not to xdebug.remote_connect_back=1 which also would end up at my IP) on port 9000. Connecting to localhost seems a bit weird, since I picture that as the server sending messages to its own IP address, as if it is sending messages into itself (but I think that connecting to localhost is probably fundamentally different than connecting to any other IP... I don't think the message gets routed out and back in to localhost).
On my computer at work, I open up an SSH connection on port 22, specifying a tunnel to/on port 9000, and remote port 9000. I've seen some explanations of the various settings here but still don't understand them. Some even seem to involve three machines. What seems to be happening though, is I'm connected as usual via port 22, but I've told the remote machine that I want to receive its port 9000 communications. I've specified "localhost" in my tunnel, and I suppose that might need to match the localhost in my xdebug.remote_host value. I wonder if I specified my IP address in both places (i.e. xdebug.remote_host=173.123.45.56 on the remote server, and same IP in my SSH terminal), would that work too?
So Xdebug on the remote server sends me a request to initiate a debug session. It comes through my port 22, but my SSH tunnel somehow makes it seem that it is coming in on port 9000. So my IDE that is listening on port 9000 receives the request and sends a response (also on 9000), which my SSH tunnel intercepts somehow and sends back to the remote server on port 22, where it is similarly spoofed into looking like port 9000 to xdebug.
The Crux
So what I'm really not clear on is, what exactly is the localhost in my SSH tunnel configuration referring to? Does it relate directly to the xdebug.remote_host=localhost value? Can I change them both to my IP address?
Are all of the remote server's outgoing communications on port 9000 being forwarded to me, or just some of them? E.g., if someone in Chattanooga initiates a debug session in their browser, will I receive Xdebug's response?
Are all of my outgoing communications on port 9000 being forwarded to that server? I.e. can I debug two applications on two different servers at the same time, with some of my port 9000 communications going one way and some the other, or would I need one port per local application? (I can use Google Chrome and Firefox browsers at the same time, both on port 80, for example.)
回答1:
The tunnel consists of an SSHD listening to port 9000 (as well as 22) at your end and an SSHD listening to port 22 at the other end. When you connect your XDebug to your local 9000, the SSHDs intercommunicate and the remote SSHD connects to port 9000 at the remote. Thereafter your local port 9000 behaves identically to the remote port 9000: all data written to either end appears at the other end.
来源：https://stackoverflow.com/questions/33424393/explain-ssh-tunneling-process-and-limitations-for-a-remote-xdebug-session