Varnish 4 – Failover Director

vcl 4.0;
import directors;
backend static_server {
    .host = "aa.bb.cc.dd";
    .port = "80";
    .probe = { .url = "/"; .timeout = 200 ms; .interval = 1s; .window = 6; .threshold = 5; }
}
backend static_server_2 {
    .host = "aa.bb.cc.ee";
    .port = "80";
    .probe = { .url = "/"; .timeout = 200 ms; .interval = 1s; .window = 6; .threshold = 5; }
}


sub vcl_init {
    new cluster = directors.fallback();
    cluster.add_backend(static_server);
    cluster.add_backend(static_server_2);

}

sub vcl_recv {
    set req.backend_hint = cluster.backend();

    #unset req.http.Cookie;
    #unset req.http.Cache-Control;

}

Ref: http://smemoratesysop.blogspot.com/2015/05/failover-director-for-varnish-40.html