Show maintenance page when everything is down

I was just working on this same problem. I came across a few sections in the DevCentral Codeshare:

• LTM Maintenance Page, and
• LTM Maintenance Page Lite.

Both had great tips. I ended up combining them, though, because I want to return a 503 instead of 200. The code I'm using now runs at the end of my iRules (priority 1000). Between my first HTTP_REQUEST block and that final one, I do bits of processing that may change the pool. So by the time I get to my error handling at the end, I want to make sure I'm only doing this for pages that would be sent to the default pool. I certainly don't want this happening for every image and style sheet request that comes through! I think I need to constrain this even more, though. One of the Codeshare examples filtered on GET requests; that's probably wise.

 do initial HTTP request processing
when HTTP_REQUEST priority 100 {
    set DEBUG 1

    set uri [HTTP::uri]

     Derive a default pool name from our virtual server name
     WARNING: This assumes our VS and pool use the same name.
    set virtual_name [virtual name]

    set default_pool [getfield $virtual_name "/" 3]
    ...
}

when HTTP_REQUEST priority 1000 {
    set log_label "(VS $virtual_name) final"
    set final_pool [getfield [LB::server pool] "/" 3]

     If our selected pool is down, show a maintenance page and trigger a retry in $retry_interval seconds
    if { $final_pool eq $default_pool && [active_members $final_pool] < 1 } {

         sets the timer to return client to host URL
        set retry_interval 30
        log local0. "$log_label: --> pool $final_pool down; retrying in $retry_interval seconds"

         Use the Host header value for the responses if it's set.  If not, use the VIP address.
        if {[string length [HTTP::host]]}{
            set retry_host [HTTP::host]
        } else {
            set retry_host [IP::local_addr]
        }

        set retry_uri [HTTP::uri]
        set iframe_src {https://MY_MAINTENANCE_LINK}
        set respond_content "Maintenance\n\nRetrying your request in $retry_interval seconds\n"

        HTTP::respond 503 content $respond_content {Content-Type} {text/html}
        return
    }
}

I have a bit of CSS in there, but the gist of the response is that it's sending back a 503 code, then following up with some short HTML to show a "retrying in $retry_interval seconds" message. Just below that, it includes an iframe that loads a maintenance page (see MY_MAINTENANCE_LINK above). I don't know if that's good or bad behavior for a 503 page, but it will let our web development team own the content of the maintenance page and let me handle how it's used.