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    <p>Assuming you ran this command: <code>ssh -f user@mysql-server.com -L 3306:mysql-server.com:3306 -N</code> as described in the post you linked. </p> <p>A breakdown of the command:</p> <ol> <li><code>ssh</code>: that's pretty self-explanatory. Invokes <code>ssh</code>. </li> <li><p><code>-f</code>: (From the <code>man ssh</code> page)</p> <blockquote> <p>Requests ssh to go to background just before command execution. This is useful if ssh is going to ask for passwords or passphrases, but the user wants it in the background.</p> </blockquote> <p>Essentially, send <code>ssh</code> to background once you've entered any passwords to establish the connection; it gives the shell prompt back to you at <code>localhost</code> rather than logging you in to <code>remote-host</code>. </p></li> <li><code>user@mysql-server.com</code>: the remote server you'd like to log into. </li> <li><p><code>-L 3306:mysql-server.com:3306</code>: This is the interesting bit. <code>-L</code> (from the <code>man ssh</code> page):</p> <blockquote> <p>[bind_address:]port:host:hostport Specifies that the given port on the local (client) host is to be forwarded to the given host and port on the remote side.</p> </blockquote> <p>So <code>-L 3306:mysql-server.com:3306</code> binds the <em>local</em> port <code>3306</code> to the <em>remote port</em> <code>3306</code> on host <code>mysql-server.com</code>. </p> <p>When you connect to <em>local</em> port <code>3306</code>, the connection is forwarded over the secure channel to <code>mysql-server.com</code>. The <em>remote host</em>, <code>mysql-server.com</code> then connects to <code>mysql-server.com</code> on port <code>3306</code>. </p></li> <li><p><code>-N</code>: don't execute a command. This is useful for "just forwarding ports" (quoting the man page). </p></li> </ol> <blockquote> <p>Does this command affect anything on the server? </p> </blockquote> <p>Yes, it establishes a connection between <em>localhost</em> and <em>mysql-server.com</em> on port <em>3306</em>. </p> <blockquote> <p>And how do I close this tunnel...</p> </blockquote> <p>If you've used <code>-f</code>, you'll notice that the <code>ssh</code> process you've opened heads into the background. The nicer method of closing it is to run <code>ps aux | grep 3306</code>, find the <code>pid</code> of the <code>ssh -f ... -L 3306:mysql-server.com:3306 -N</code>, and <code>kill &lt;pid&gt;</code>. (Or maybe <code>kill -9 &lt;pid&gt;</code>; I forget if just <code>kill</code> works). That has the beautiful benefit of <strong>not</strong> killing all your other <code>ssh</code> connections; if you've got more than one, re-establishing them can be a slight ... pain. </p> <blockquote> <p>... because now I can't use my local mysql properly.</p> </blockquote> <p>This is because you've effectively "captured" the <em>local</em> <code>mysql</code> process and forwarded any traffic that attempts to connect to it, off to the <em>remote</em> <code>mysql</code> process. A <em>much nicer</em> solution would be to <strong>not use local port 3306</strong> in the port-forward. Use something that's not used, like 33060. (Higher numbers are generally less used; it's pretty common to port-forward a combination like this: "2525->25", "8080->80", "33060->3306" or similar. Makes remembering slightly easier). </p> <p>So, if you used <code>ssh -f user@mysql-server.com -L 33060:mysql-server.com:3306 -N</code>, you'd then point your Zend connect-to-mysql function to <code>localhost</code> on port <code>33060</code>, which would connect to <code>mysql-server.com</code> on port <code>3306</code>. You can obviously still connect to <code>localhost</code> on port <code>3306</code>, so you can still use the local <code>mysql</code> server. </p>
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    1. POHow to close this ssh tunnel?
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    1. COBest explanation I read in a while. This is very useful when accessing a remote database from locally installed environments like R for example. Works well with public / private key authentication. Not with passwords because I did not find a way to pass the passwords.
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    2. COAccepting this answer as best because of the thorough explanation.
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