update

main.go

@@ -87,8 +87,17 @@ func init() {
 		if len(agentConfig.DNS) > 0 {
 			dnsServers = agentConfig.DNS
 		}
-		dnsServer := dnsServers[time.Now().Unix()%int64(len(dnsServers))]
+		index := int(time.Now().Unix()) % int(len(dnsServers))
-		return d.DialContext(ctx, "udp", dnsServer)
+		queue := generateQueue(index, len(dnsServers))
+		var conn net.Conn
+		var err error
+		for i := 0; i < len(queue); i++ {
+			conn, err = d.DialContext(ctx, "udp", dnsServers[queue[i]])
+			if err == nil {
+				return conn, nil
+			}
+		}
+		return nil, err
 	}
 	flag.CommandLine.ParseErrorsWhitelist.UnknownFlags = true
 
@@ -636,3 +645,15 @@ func println(v ...interface{}) {
 		fmt.Println(v...)
 	}
 }
+
+func generateQueue(start int, size int) []int {
+	var result []int
+	for i := start; i < start+size; i++ {
+		if i < size {
+			result = append(result, i)
+		} else {
+			result = append(result, i-size)
+		}
+	}
+	return result
+}

main_test.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"reflect"
+	"testing"
+)
+
+func Test(t *testing.T) {
+	cases := []struct {
+		start, size int
+		want        []int
+	}{
+		{0, 2, []int{0, 1}},
+		{1, 2, []int{1, 0}},
+		{0, 3, []int{0, 1, 2}},
+		{1, 3, []int{1, 2, 0}},
+		{2, 3, []int{2, 0, 1}},
+	}
+
+	for _, c := range cases {
+		if !reflect.DeepEqual(c.want, generateQueue(c.start, c.size)) {
+			t.Errorf("generateQueue(%d, %d) == %d, want %d", c.start, c.size, generateQueue(c.start, c.size), c.want)
+		}
+	}
+}