DirectAdmin can run on a LAN/NAT, but just requires some non-standard steps to get it running.
It requires the of loading the external IP into the internal/LAN device. Operating should still be fine via the local IPs.
echo 1 > /root/.lan
to have DA download the update.tar.gz and license.key more easily (skips the --bind-address option).lan_ip=192.168.1.2
where you'd replace 192.168.1.2 with the LAN IP value that should be used for outbound connections.If, for whatever reason, adding the public IP to the internal network simply cannot work, a new feature that enabled external checks can be tried. For this, you'd have your internal IP set on the main device (eth0), and do NOT need to setup the public IP on eth0:0. For the check to happen, you'd need to set lan_ip= to the same internal eth0 IP address.
Did you do step #2? This is the most commonly missed step. The public IP must be listed in the internal device (else, rely on the external checks instead)
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