By default, ip_unprivileged_port_start is set to 1024 in Linux. It defines the first unprivileged port in the network namespace. Privileged ports require root or CAP_NET_BIND_SERVICE in order to bind to them. This is why you cannot set web app server to use port 80, or any value below 1024 if you have default ip_unprivileged_port_start value.
To workaround this:
Use port forwarding with command "sudo iptables -t nat -A PREROUTING -p tcp --dport $external_port -j REDIRECT --to-ports $server_site_port". For example your web app server is set to port 9988 and you want to access it with port 80 in the url. The command is "sudo iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-ports 9988"
Alternatively, you can allow listening on all privileged ports using this commands "sudo sysctl net.ipv4.ip_unprivileged_port_start=0".
