Clarify the function of Port to avoid problems like PR#459 and PR#485.

<strong>Context:</strong> server config<br>

<strong>Status:</strong> core<p>

The Port directive sets the network port on which the server listens.

<em>Num</em> is a number from 0 to 65535; some port numbers (especially below

<em>Number</em> is a number from 0 to 65535; some port numbers (especially below

1024) are reserved for particular protocols. See <code>/etc/services</code>

for a list of some defined ports; the standard port for the http protocol

is 80.<p>

The Port directive has two behaviours, the first of which is necessary for

NCSA backwards compatibility (and which is confusing in the context of

Apache).<p>

<ul>

<li>

In the absence of any <a href="#listen">Listen</a> or

<a href="#bindaddress">BindAddress</a> directives specifying a port number,

the Port directive sets the network port on which the server listens.

If there are any Listen or BindAddress directives specifying

<code>:number</code> then Port has no effect on what address the server

listens at.

<li>The Port directive

sets the <code>SERVER_PORT</code> environment variable (for

<a href="mod_cgi.html">CGI</a> and <a href="mod_include.html">SSI</a>),

and is used when the server must generate a URL that refers to itself

(for example when creating an external redirect to itself).  

</ul>

In no event does a Port setting affect

what ports a <a href="#virtualhost">VirtualHost</a> responds on, the

VirtualHost directive itself is used for that.<p>

The primary behaviour of Port should be considered to be similar to that of

the <a href="#servername">ServerName</a> directive.  The ServerName

and Port together specify what you consider to be the <i>canonical</i>

address of the server.<p>

Port 80 is one of Unix's special ports. All ports numbered

below 1024 are reserved for system use, i.e. regular (non-root) users cannot

make use of them; instead they can only use higher port numbers.<p>

make use of them; instead they can only use higher port numbers.

To use port 80, you must start the server from the root account.

After binding to the port and before accepting requests, Apache will change

to a low privileged user as set by the <A HREF="#user">User directive</A>.<p>