URL rewrite (or HTTP forwarding) using iRule?

First, I think you have to configure the virtual Server on Port 80 - no magic here. Second, you can create a datagroup like this:

class header_mapping {
headervalue1 /test
headervalue2 /dev/test2
...
}

Third, You can create an iRule like this:

WHEN_HTTP_REQUEST {
 set location [findclass [HTTP::header "header_1"] header_mapping " "]
 if { ($location ne "") } {
  HTTP::respond 301 Location "http://www.domain.de$location"
  return
 }
}

See also https://devcentral.f5.com/wiki/iRules.findclass.ashx