I have taken a closer look and found that the error is the same as with an an address that does not exist on the system. So reason is probably that the network interface with the external address is not yet up.
The easy solution would be for the service to wait for the network- online target. However https://systemd.io/NETWORK_ONLINE/ tells me that this is a questionable solution. It is however acceptable since mosquitto would not delay boot. According to the above source IP_FREEBIND might be a better solution. This is however substantially harder to do. So I would suggest the attached patch without beeing sure about that. Unfortunately the bug is also no longer reproducible on my systems. So I can't verify the patch is working.
--- a/service/systemd/mosquitto.service.notify +++ b/service/systemd/mosquitto.service.notify @@ -1,8 +1,8 @@ [Unit] Description=Mosquitto MQTT Broker Documentation=man:mosquitto.conf(5) man:mosquitto(8) -After=network.target -Wants=network.target +After=network-online.target +Wants=network-online.target [Service] Type=notify --- a/service/systemd/mosquitto.service.simple +++ b/service/systemd/mosquitto.service.simple @@ -1,8 +1,8 @@ [Unit] Description=Mosquitto MQTT Broker Documentation=man:mosquitto.conf(5) man:mosquitto(8) -After=network.target -Wants=network.target +After=network-online.target +Wants=network-online.target [Service] ExecStart=/usr/sbin/mosquitto -c /etc/mosquitto/mosquitto.conf
