IRule Redirect URL to other URL for maintenance

If you are running 11.4 or higher, then I recommend using a Local Traffic Policy:

• Sol 15085: Overview of the Local Traffic Policies feature

I assume that you only want requests for '/' to redirect. If so, the Local Traffic Policy definition is:

ltm policy maintenance_redirect {
    controls { forwarding }
    requires { http }
    rules {
        rule01 {
            actions {
                0 {
                    http-reply
                    redirect
                    location /app_offline_abd.htm
                }
            }
            conditions {
                0 {
                    http-host
                    host
                    values { branch.com }
                }
                1 {
                    http-uri
                    path
                    values { / }
                }
            }
            ordinal 1
        }
    }
    strategy first-match
}

If you want any URI path to redirect there:

ltm policy foo {
    controls { forwarding }
    requires { http }
    rules {
        rule01 {
            actions {
                0 {
                    http-reply
                    redirect
                    location /app_offline_abd.htm
                }
            }
            conditions {
                0 {
                    http-host
                    host
                    values { branch.com }
                }
                1 {
                    http-uri
                    path
                    not
                    values { /app_offline_abd.htm }
                }
            }
            ordinal 1
        }
    }
    strategy first-match
}

This redirects a request for anything but the target of the redirection.

Either of these can be loaded by saving them to a file on the BIG-IP (say, /var/tmp/mr.txt) and merging them into the configuration:

tmsh load sys config file /var/tmp/mr.txt merge

When you want to utilize the policy, apply it to the appropriate Virtual Servers (in the Web UI, select the Virtual Server, then "Resources" at the top).

If you are using a version before 11.4, you can employ an HTTP Class:

• (SOL7280: Overview of the HTTP Class profile)[https://support.f5.com/kb/en-us/solutions/public/7000/200/sol7280.html]

There's no particular need or advantage to using an iRule for this case, but if you decide that is what you want, then:

when HTTP_REQUEST {
    if { [HTTP::host] eq "branch.com" and [HTTP::path] eq "/" } {
        HTTP::redirect "/app_offline_abd.htm"
    }
}

or, to redirect everything:

when HTTP_REQUEST {
    if { [HTTP::host] eq "branch.com" and [HTTP::path] ne "/app_offline_abd.htm" } {
        HTTP::redirect "/app_offline_abd.htm"
    }
}