HTTP:: redirection is not working in iRule

It matches all the conditions in the below irule except http::redirect condition.

what host (fqdn) did you test?

 configuration

[root@ve11a:Active:In Sync] config  tmsh list ltm rule qux
ltm rule qux {
    when HTTP_REQUEST {
  if { [string tolower [HTTP::host]] equals "x.y.com" } {
    pool x-pool
  } elseif { [string tolower [HTTP::host]] equals "xdev.y.com" } {
    pool xdev-pool
  } elseif { [string tolower [HTTP::host]] equals "xuat.y.com" } {
    pool xuat-pool
  } else {
    HTTP::redirect http://1.1.1.1:65535/
  }
}
}

 test

[root@ve11a:Active:In Sync] config  curl -I http://172.28.24.10 -H "Host: something"
HTTP/1.0 302 Found
Location: http://1.1.1.1:65535/
Server: BigIP
Connection: Keep-Alive
Content-Length: 0

Requirement : The condition is if all pools are down, it should redirect to server Maintenance page

the irule does not match the requirement (i.e. it does redirect when fqdn does not match predefined names). you can use active_members to check number of active pool members.

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