TMG --> Big IP migration

Create a datagroup with the below config

ltm data-group internal allowedsites{
    records {
        www.app1.com { }
        www.app2.com { }
    }
    type string
}

Then try the following iRule

when HTTP_REQUEST {
    if { not ([matchclass [HTTP::host] equals allowedsites ])} {
        drop   
    }
}

I don't have an environment that i can confirm it validates but it should work.

Alternatively if you wanted to respond a page instead of dropping the connection you could replace the drop line with:

HTTP::respond 404 -version 1.1 content {  }

HTH Andrew