IRULE to redirect the traffic to a under_maintenance.htm

https://devcentral.f5.com/wiki/irules.ltmmaintenancepage.ashx

https://devcentral.f5.com/wiki/iRules.LTMMaintenancePageLite.ashx

There are a few additional options to consider:

1. Fallback Host: Inside the HTTP profile is an option called Fallback Host. This option, when defined, will send a 302 redirect to the specified address if all pool members are down. No iRules needed.

2. active_members check: a simple active_members check of the pool and then whatever you need to happen if all members are down:

   when CLIENT_ACCEPTED {
       set default_pool [LB::server pool]
   }   
   when HTTP_REQUEST {
       if { [active_members $default_pool] < 1 } {
            send a redirect or output HTML content here
       }
   }
   

3. Time-based outage: based on the following post:

   https://devcentral.f5.com/questions/irule-to-disable-a-pool-to-new-connections

   when RULE_INIT {
       set static::START_OFF_TIME "Saturday 09:00 AM"
       set static::END_OFF_TIME "Saturday 11:00 AM"
   }
   when HTTP_REQUEST {
       set start_off_time [clock scan $static::START_OFF_TIME]
       set end_off_time [clock scan $static::END_OFF_TIME]
       set now [clock seconds]
   
       if { ( [expr $now > $start_off_time] ) and ( [expr $now < $end_off_time] ) } {
           HTTP::respond 200 content "Maintenance ModeMaintenance mode..."
       }
   }
   

Since the Fallback Host generates a 302 (possibly followed by a 200) this could do bad things to SEO. A 503 (Service Unavailable) would be preferable in many cases.