Currently, DDNS is tied to a particular port. If you have two gateway ports, you need to specify two different host names to update, which doesn't fit the failover model at all. I would want to have "my_company.com" updated to be whichever IP address was currently my primary gateway. If a failover to a secondary gateway occurs, the DDNS should update to reflect this.
It turns out that Google DDNS, at least, would allow Sophos to implement this -- for IPV4 at any rate -- with very little work.
If you contact Google DDNS to update the IP and you do not specify the IP address to set it, Google will use the IP from which the update is coming. (This works for IPv4 only, since the explicit IP address is mandatory for IPv6.) This would do what I'm asking: it would set the DDNS to the current primary gateway IP since the DDNS update would presumably be routed that way.
Thus, an additional radio button could be added to Network > Dynamic DNS > Host Configuration > IPv4 Address that would say "Include no address". Not sure if this works for non-Google DDNS, but at least in that case it would do what we want.
This isn't a final solution. DDNS updates should be explicitly aware of the status of gateways via Gateway link manager (or even SDWAN quality-based routing, maybe), and should update the far end DDNS with an IP address that is up. This might be done through a radio button that says "Use Primary Gateway Interface IP" or something like that.
Add another radio button idea.
[edited by: Wayne Folta at 7:25 PM (GMT -8) on 10 Jan 2022]