Irule Dns to handle nslookup dns from client

 , for something of this complexity, you may be well-served to engage F5 Professional Services. I will also say that the DNS Services module can handle this sort of case more natively.

Having said all of that, I assume the above rule works, but it is only returning a single A record. Is that the case? I also understand from your description that, if all three uplink checks (the LB::status node calls, that is) succeed, you want to return all three A records (essentially, all three class match -value $fqdn calls). What do you want to happen if two of the links are up? Return two A records? I will assume that is what you want.

I've not tested the following code, but assuming the code above works, this should get you close to the right answer:

when DNS_REQUEST {
    set return_a_records [list]
    if { [class match $fqdn equals whitelist] } {
        # does FQDN exist in our whitelist string:value datagroup for that site.
        if { [LB::status node 183.91.1.1] eq "up" } {
            # client made a DNS request for a Whitelist site.
            lappend return_a_records [class match -value $fqdn equals whitelist]
        }   
 
        if { [LB::status node 222.255.1.1] eq "up" } {
            lappend return_a_records [class match -value $fqdn equals vnpt_whitelist]
        }
 
        if {[LB::status node 118.69.1.1] eq "up" } {
            lappend return_a_records [class match -value $fqdn equals fpt_whitelist]
        }
    }
}   
 
when DNS_RESPONSE {
    if { $Whitelist_Match } {
        switch [DNS::question type] {
            "A" {
                foreach ip $return_a_records {
                    DNS::answer insert "$fqdn. $static::whitelist_ttl [DNS::question class] [DNS::question type] $ip"
                }
            }
        }
    }
}