Irule Error message

You're getting the TCL error because $host_pool isn't being set for every request--but is being referenced in every request.

This is your original rule, formatted for easier reading:

when HTTP_REQUEST {
   if {[string tolower [HTTP::header x-moz:] ] contains "prefetch" } {
      HTTP::respond 403 content "Forbidden"
   } else {
      set host_pool [findclass [HTTP::host] $::DSHostPoolMap " "]
   }
   if { $host_pool ne "" } {
      pool $host_pool
   }
}

Note that on every request, the $host_pool condition is being checked. It's only being set if the x-moz header check is false.

You might want to move the $host_pool check into the else. Or you could check to see that the $host_pool variable is set and not blank using [info exists host_pool].

You should also consider specifying what you want to happen if the [HTTP::host] value is not found in the datagroup.

Here's an example:

when HTTP_REQUEST {
   if {[string tolower [HTTP::header x-moz]] eq "prefetch" } {
      HTTP::respond 403 content "Forbidden"
   } else {
      set host_pool [findclass [string tolower [HTTP::host]] $::DSHostPoolMap " "]
      if { $host_pool ne "" } {
         pool $host_pool
      } else {
         pool default_pool
      } 
   }
}

Aaron