Check if host works first before assigning random host

pkg/http/client.go

@@ -198,11 +198,12 @@ func (r *HTTPClient) replaceHostIfNeeded(req *http.Request) {
 			break
 		}
 	}
-	if !found {
-		r.log.Debug("Not found, assigning random IPv6 host")
-		// random IPv6 host
+	if !found && !r.CanFetchFirstByte(req.URL.String()) {
 		req.Host = r.ipv6Hosts[rand.Intn(len(r.ipv6Hosts))]
 		req.URL.Host = req.Host
+		r.log.Debugf("Host is not a valid IPv6 host, assigning a random IPv6 host: %s", req.Host)
+	} else {
+		r.ipv6Hosts = append(r.ipv6Hosts, req.Host)
 	}
 }
 
@@ -267,3 +268,28 @@ func backoffFunc(attempt int) time.Duration {
 	}
 	return time.Duration(backoff) * time.Second
 }
+
+func (r *HTTPClient) CanFetchFirstByte(url string) bool {
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		return false
+	}
+
+	req.Header.Set("Range", "bytes=0-0")
+
+	resp, err := r.client.Do(req)
+	if err != nil {
+		return false
+	}
+	defer resp.Body.Close()
+
+	// If server supports partial content
+	if resp.StatusCode/100 == 2 {
+		buffer := make([]byte, 1)
+		_, err = resp.Body.Read(buffer)
+		if err == nil {
+			return true
+		}
+	}
+	return false
+}