i rule to failover between external and intrenal dns

hi all,

 

i am new to gtm and trying to set up the below desing , please let me know if its fesable

 

we have file share server which can be access locally and externally. so as per client requirement we need to set up an architecture, the file server always access internally but can be access exteranlly too if internally goes down.

 

so we need to set up a GTM with wide ip or Dns which can point to both dns below and can switch aotumatically to external if internal goes down.

 

File server is nated too external IP for oustside access

 

There are two dns :

 

Dns a points user to https://10.132.80.10/abc/xyz------------------- for internal access

 

Dns b points user to same server but with differents dns https://test.viainternet.com/abc/xyz------for external ip range of 202.203.X.X

 

Any solution will be highly appreciated

 

Also , i am triyng to devlop logic that an i rtule can be written which will take dns request and point it to local dns , if conncetion drop on local than traffic can point to external.