HAProxy default backends

[robert199311]

HI all,

I'm trying to setup HAproxy but in OPNsense but I can't figure out a way to setup a default backend.
F.E. I have 3 servers running. server 1 & 2 have specific domains but server 3 is a shared hosting platform with hundreds of domains which change on daily basis.

I want HAproxy to serve server 1 or 2, but if none of these conditions are met it has to redirect to server 3. How can I set this up?

Kind regards,

Robert