11.4 node/pool select according to domains list

As an iRule, this sounds like an excellent candidate for a data group.

Step 1: create a string-based data group consisting of domain names and corresponding nodes. Example (my_domain_dg):

"www.domain1.com" := "10.10.10.11 80"
"www.domain2.com" := "10.10.10.12 80"
"www.domain3.com" := "10.10.10.13 80"
"www.domain4.com" := "10.10.10.14 80"
...
"www.domain500.com" := "10.10.10.510 80"

And a reasonably simple iRule:

when HTTP_REQUEST {
    if { [class match [string tolower [HTTP::host]] equals my_domain_dg] } {
        set my_node [split [class match -value [string tolower [HTTP::host]] equals my_domain_dg]]
        node [lindex $my_node 0] [lindex $my_node 1]
    }
}

You can also use this method with pools, which may be a better option in the future if you're going to need to actually load balance and scale out services. The beauty of this approach is that you manage a single data group and don't really have to touch the iRule.

...And you can also do this with policies in 11.4