Forward HTTP Proxy iRule - Multi DNS and adding URI Whitelist

1. I need to add a second DNS server (round robin is fine, though would prefer priority group failover).

the RESOLV::lookup command makes provisions for either a single DNS server IP or a virtual server name. So in this case, I'd create a port 53 internal VIP that load balances your DNS servers, and then specify that VIP name static::dns.

1. Use case is for access to 2 or 3 Domains only. Am I best setting up a 2nd iRule just for this (with simple HTTP Request URI verification, referencing a data class group)? Or can I add this in easily to existing v3.2 irule somewhere?

What do you want to do if the domain does not match? You could perhaps very simply add an if { [HTTP::host] equals... clause at the top of the HTTP_REQUEST.