I would like to know if and how I might be able to use a sub domain with my current domain name to have the ability to access my home server but at the same time if the home server is down to simply fall back to the sub domain directory on my hosted domain.
I'm asuming you're home is connected via DSL/dial-in connection in that case your public IP would change every time you reconnect (i.e. most providers in germany force-reconnect every 24h). Asuming that is the case, the answer for your problem is dyndns or ddns.
A fallback can not be done without a service monitoring your home network connectivity and reconfiguring your dns settings once that downtime happens.
I am using dsl and was thinking of setting it up using a sub domain to my current domain name and using a service like no.ip.com linking my home server. The isp here does renew ip's but that should not be a problem with the software setup on linux and the router.
What I need to know is what I have to do to get it to work so that if the home linux system is offline, the redirection will fallback to my sub domain name attached to the my regular host.
In other words, I can currently connect my linux server using my router through the no.ip.com service but how do I change the dns to make it see that instead of my hosting service account?