hotspot_ynh/conf/openvpn_90-hotspot

#!/bin/bash

is_nat_set() {
  local gateway_interface=${1}
  iptables -w -nvt nat -L POSTROUTING | grep MASQUERADE | grep -q "${gateway_interface}"
}

unset_nat() {
  local gateway_interface=${1}
  iptables -w -t nat -D POSTROUTING -o "${gateway_interface}" -j MASQUERADE
}

set_nat() {
  local gateway_interface=${1}
  iptables -w -t nat -A POSTROUTING -o "${gateway_interface}" -j MASQUERADE
}

ynh_hotspot_state=$(systemctl is-active __SERVICE_NAME__)
if [[ "${ynh_hotspot_state}" == "active" || "${ynh_hotspot_state}" == "activating" ]]; then
  old_gateway_interface=$(yunohost app setting __APP__ gateway_interface)
  new_gateway_interface=$(ip route get 1.2.3.4 | awk '{ print $5; }')

  if [[ -n "$old_gateway_interface" ]] && [[ "$old_gateway_interface" != "$new_gateway_interface" ]] && is_nat_set "$old_gateway_interface"; then
    unset_nat "${old_gateway_interface}"
  fi

  if [[ -n "$new_gateway_interface" ]] && ! is_nat_set $new_gateway_interface; then
    set_nat "${new_gateway_interface}"
  fi

  yunohost app setting __APP__ gateway_interface --value "${new_gateway_interface}"
fi