Q. How to setup a connection sharing (“hotspot”) for a wired interface ?
A) First make sure to install dnsmasq-base (but not dnsmasq unless you disable the system daemon in /etc/default/dnsmasq). Then use nm-connection-editor and setup a new profile using “shared to other computer” as the method for IPv4 (not possible from Gnome3’s gnome-control-center). This hides all the complexity (dnsmasq, iptables, sysctl). Finally, use that profile for the network interface connected to the shared network. For Wi-Fi interfaces, a hotspot functionality is provided which uses the same functionality besides setting up an ad-hoc WiFi network.
デフォルトのサブネット：10.42.0.x/24 を 192.168.5.1/24 変更する場合
$ sudo nmcli con modify WIFI_AP ipv4.addr 192.168.5.1/24
Note that the package “dnsmasq” interferes with Network Manager which can use “dnsmasq-base” to provide DHCP services when sharing an internet connection. Therefore, if you use network manager (fine in simple set-ups only), then install dnsmasq-base, but not dnsmasq. If you have a more complicated set-up, uninstall network manager, use dnsmasq, or similar software (bind9, dhcpd, etc), and configure things by hand.