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up vote 4 down vote favorite

When I request X forwarding from SSH server, then SSH server sets a $DISPLAY variable with value localhost:10.0. In addition, it starts to listen on 127.0.0.1 port 6010(and also ::1 port 6010 for IPv6):

Netid  State      Recv-Q Send-Q                                     Local Address:Port                                                      Peer Address:Port                
LISTEN     0      128                                               127.0.0.1:6010                                                                    *:*                     users:(("sshd",pid=11405,fd=10))

How do X clients know that they will need to connect to TCP port 6010? Does this work in a way that by default they connect to TCP port 6000+<display number> and as display number is in this example 10, then they will connect to TCP port 6010?

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A term that might be helpful is well-known port. – chrylis 2 hours ago
up vote 12 down vote

It’s part of the X11 protocol (search for "6000") and is documented e.g. in Xorg(1):

Xorg listens on port 6000+n, where n is the display number. This connection type can be disabled with the -nolisten option (see the Xserver(1) man page for details).

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up vote 2 down vote

Yes.

Just as browsers know to connect to a webserver on port 80 (unless otherwise specified), X clients know to talk to X servers on port 6000 + display number (unless otherwise specified).

These expectations are what standards (and, more broadly, documentation) give us — else, interaction between computers would be extremely cumbersome.

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I can make an educated guess whom your pointed comment is aimed at, but it could look like you are having a go at the questioner, which (if my guess is correct) is not the case at all. It is best to make that clear. – JdeBP 3 hours ago
    
@JdeBP: Eh? What "pointed comment"? Where am I "having a go" at anyone?? I'm very simply answering the question... – Lightness Races in Orbit 2 hours ago
    
Your pointed comment about standards, of course. – JdeBP 52 mins ago
    
@JdeBP: I think you must be misreading it, because I have absolutely no idea what you're talking about. That web browsers should talk to webservers on port 80 is given by as close to a standard as there is: the RFC(s) on HTTP. Similar "legislation" is passed for all other protocols. That's how networks are defined. Similarly, that's how X clients "know" which ports to use, which was the question posted above. What's "pointed" about that? – Lightness Races in Orbit 47 mins ago

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