Varnish probe

Your backend is responding with a 301 to the health check; only 200 is an expected "healthy" response by default.

.probe = {
    .url = "/";
    .timeout  = 1s;
    .interval = 10s;
    .window    = 10;
    .threshold = 8;
    .expected_response = 301;
}

https://serverfault.com/questions/451840/varnish-going-sick

ELBなどでhealth checkをする際に使える便利なempty_gif（Nginx） - Qiita

ヘルスチェック用のURLをnginxで用意する場合

    location = /healthcheck.html {
         empty_gif;
         access_log off;
         break;
    }

バックエンド確認 (varnish4)

varnishadm backend.list
(詳細)
varnishadm backend.list -p