iRule to change the DNS response IP inside the header

you can try something like this:

when DNS_RESPONSE {
    set rrs [DNS::answer]
    DNS::answer clear    
    foreach rr $rrs {
        if {[lindex $rr 4] equals "4.3.2.1"} {
                DNS::answer insert "[lindex $rr 0] [lindex $rr 1] [lindex $rr 2] [lindex $rr 3] 1.2.3.4"
        } else {
            DNS::answer insert $rr
        }
    }

}