user131151: Bitcoin Core listens on three ports, as can be seen by running the following command after it has started:

sudo netstat -tulpn | grep LISTEN

By default, the ports it listens on are:

8332 for the RPC client (i.e. bitcoin-cli)
8333 for bitcoin clients (other nodes who want to contact this node)
8334 for ????

Searching for information on port 8334 does not seem to turn up anything useful.
By observation, if the port is already bound to, it does not cause a fatal error. Bitcoin Core continues to start, although the debug.log shows the following:

Unable to bind to 127.0.0.1:8334 on this computer. Bitcoin Core is probably already running.

What does Bitcoin Core use port 8334 for?
Furthermore, other ports can be overridden using command-line arguments such as -rpcport=18332 and -port=18333 or similar configuration options in bitcoin.conf. How can Bitcoin Core be made to listen on a different port number than 8334 for whatever it uses that port for?

user51948: It is used for incoming Tor connections.
See here and here for more info.
You can use -bind=127.0.0.1:XXXX=onion to use port XXXX instead.

131151: <p>Bitcoin Core listens on three ports, as can be seen by running the following command after it has started:</p>
<blockquote>
<p><code>sudo netstat -tulpn | grep LISTEN</code></p>
</blockquote>
<p>By default, the ports it listens on are:</p>
<ul>
<li>8332 for the RPC client (i.e. <code>bitcoin-cli</code>)</li>
<li>8333 for bitcoin clients (other nodes who want to contact this node)</li>
<li>8334 for ????</li>
</ul>
<p>Searching for information on port 8334 does not seem to turn up anything useful.</p>
<p>By observation, if the port is already bound to, it does not cause a fatal error. Bitcoin Core continues to start, although the <code>debug.log</code> shows the following:</p>
<blockquote>
<p><code>Unable to bind to 127.0.0.1:8334 on this computer. Bitcoin Core is probably already running.</code></p>
</blockquote>
<p>What does Bitcoin Core use port 8334 for?</p>
<p>Furthermore, other ports can be overridden using command-line arguments such as <code>-rpcport=18332</code> and <code>-port=18333</code> or similar configuration options in <code>bitcoin.conf</code>. How can Bitcoin Core be made to listen on a different port number than 8334 for whatever it uses that port for?</p>
 51948: <p>It is used for incoming Tor connections.</p>
<p>See <a href="https://github.com/bitcoin/bitcoin/blob/6ef84e0503994c53f77093e7ea5951a7b0edea7a/doc/tor.md" rel="noreferrer">here</a> and <a href="https://github.com/bitcoin/bitcoin/blob/623745ca74cf3f54b474dac106f5802b7929503f/src/chainparamsbase.cpp#L40" rel="noreferrer">here</a> for more info.</p>
<p>You can use <code>-bind=127.0.0.1:XXXX=onion</code> to use port <code>XXXX</code> instead.</p>
