/usr/lib/swipl/library/http/http_host.pl
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 http_public_host(?Request, -Hostname, -Port, +Options) is det
Current global host and port of the HTTP server. This is the basis to form absolute address, which we need for redirection based interaction such as the OpenID protocol. Options are:
global(+Bool)
If true (default false), try to replace a local hostname by a world-wide accessible name.

This predicate performs the following steps to find the host and port:

  1. Use the settings http:public_host and http:public_port
  2. Use X-Forwarded-Host header, which applies if this server runs behind a proxy.
  3. Use the Host header, which applies for HTTP 1.1 if we are contacted directly.
  4. Use gethostname/1 to find the host and http_current_server/2 to find the port.
Arguments:
Request- is the current request. If it is left unbound, and the request is needed, it is obtained with http_current_request/1.