Setting the domain of a cookie

You could use the domain command to do this:

http://devcentral.f5.com/wiki/default.aspx/iRules/domain

[domain www.sub.my.domain.com 2] -> domain.com

You might want to check that the host header isn't an empty or an IP address first. Here is an untested example:

when HTTP_REQUEST {

   if { [string match {*[a-zA-Z]*} [HTTP::host]]}{
      set domain [domain [HTTP::host] 2]
   } else {
      set domain ""
   }
}
when HTTP_RESPONSE {

    Check if the persistence cookie exists in the response
   if {[HTTP::cookie exists "BIGipServer[LB::server pool]"] and $domain ne ""} {
       set the domain attribute on the persistence cookie
      HTTP::cookie domain "BIGipServer[LB::server pool]" $domain
   }
}

Aaron