On Unix systems, non-root processes cannot bind to sockets on port numbers under 1024. Since running an HTTP server as root is a potential security risk, a typical setup runs an HTTP server under an ordinary user account, set up to listen on a higher port number such as 8080. Then, the HTTP port is redirected to 8080. On Linux, this might be done using commands such as the following:
echo 1 > /proc/sys/net/ipv4/ip_forward iptables -t nat -F iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 443 -j DNAT --to :8443 iptables -A PREROUTING -t nat -i eth0 -p tcp --dport 80 -j DNAT --to :8080

However, the HTTP server is unaware of the forwarding, and still believes that it is listening on port 8080 and 8443, respectively. This can be a problem if a responder wishes to redirect the user to a secure page; they will be sent to port 8443 and not 443 as one would expect.

The http.server.remapping vocabulary defines a variable which may store an assoc of port mappings:

For example, with the above setup, we would set it as follows:
{ { 8080 80 } { 8443 443 } } port-remapping set-global