pfSense

miniupnp allow to use external STUN server to learn WAN IP address in case of NAT 1:1,
this may be useful for double nat cases (many mobile ISP)

https://github.com/miniupnp/miniupnp/blob/master/miniupnpd/miniupnpd.conf:

#ext_ip=
# WAN interface must have public IP address. Otherwise it is behind NAT
# and port forwarding is impossible. In some cases WAN interface can be
# behind unrestricted NAT 1:1 when all incoming traffic is NAT-ed and
# routed to WAN interfaces without any filtering. In this cases miniupnpd
# needs to know public IP address and it can be learnt by asking external
# server via STUN protocol. Following option enable retrieving external
# public IP address from STUN server and detection of NAT type. You need
# to specify also external STUN server in stun_host option below.
# This option is disabled by default.
#ext_perform_stun=yes
# Specify STUN server, either hostname or IP address
# Some public STUN servers:
#  stun.stunprotocol.org
#  stun.sipgate.net
#  stun.xten.com
#  stun.l.google.com (on non standard port 19302)
#ext_stun_host=stun.stunprotocol.org
# Specify STUN UDP port, by default it is standard port 3478.
#ext_stun_port=3478

see also https://forum.openwrt.org/t/miniupnpd-in-trunk-stun-setup/18688