perf: check internet with DNS lookup (#1034)

DNS lookups are much cheaper than HTTP requests since we only need to check if the Internet is available.

See: https://stackoverflow.com/a/50058255

dns/index.go

@@ -4,7 +4,7 @@ import (
 	"time"
 
 	"github.com/jeessy2/ddns-go/v6/config"
-	"github.com/jeessy2/ddns-go/v6/dns/internal"
+	"github.com/jeessy2/ddns-go/v6/dns/internet"
 	"github.com/jeessy2/ddns-go/v6/util"
 )
 
@@ -34,7 +34,7 @@ var (
 
 // RunTimer 定时运行
 func RunTimer(delay time.Duration) {
-	internal.WaitForNetworkConnected(addresses)
+	internet.Wait(addresses)
 
 	for {
 		RunOnce()

dns/internal/wait_net.go

@@ -1,48 +0,0 @@
-package internal
-
-import (
-	"strings"
-	"time"
-
-	"github.com/jeessy2/ddns-go/v6/util"
-)
-
-// waitForNetworkConnected 等待网络连接后继续
-//
-// addresses：用于测试网络是否连接的域名
-func WaitForNetworkConnected(addresses []string) {
-	// 延时 5 秒
-	timeout := time.Second * 5
-
-	loopbackServer := "[::1]:53"
-	find := false
-
-	for {
-		for _, addr := range addresses {
-			// https://github.com/jeessy2/ddns-go/issues/736
-			client := util.CreateHTTPClient()
-			resp, err := client.Get(addr)
-			if err != nil {
-
-				// 如果 err 包含回环地址（[::1]:53）则表示没有 DNS 服务器，设置 DNS 服务器
-				if strings.Contains(err.Error(), loopbackServer) && !find {
-					server := "1.1.1.1:53"
-					util.Log("本机DNS异常! 将默认使用 %s, 可参考文档通过 -dns 自定义 DNS 服务器", loopbackServer, server)
-					util.NewDialerResolver(server)
-					find = true
-					continue
-				}
-
-				util.Log("等待网络连接: %s", err)
-				util.Log("%s 后重试...", timeout)
-				// 等待 5 秒后重试
-				time.Sleep(timeout)
-				continue
-			}
-
-			// 网络已连接
-			resp.Body.Close()
-			return
-		}
-	}
-}

dns/internet/wait.go

@@ -0,0 +1,56 @@
+// Package internet implements utilities for checking the Internet connection.
+package internet
+
+import (
+	"strings"
+	"time"
+
+	"github.com/jeessy2/ddns-go/v6/util"
+)
+
+const (
+	// fallbackDNS used when a fallback occurs.
+	fallbackDNS = "1.1.1.1"
+
+	// delay is the delay time for each DNS lookup.
+	delay = time.Second * 5
+)
+
+// Wait blocks until the Internet is connected.
+//
+// See also:
+//
+//   - https://stackoverflow.com/a/50058255
+//   - https://github.com/ddev/ddev/blob/v1.22.7/pkg/globalconfig/global_config.go#L776
+func Wait(addresses []string) {
+	// fallbase in case loopback DNS is unavailable and only once.
+	fallback := false
+
+	for {
+		for _, addr := range addresses {
+			err := util.LookupHost(addr)
+			// Internet is connected.
+			if err == nil {
+				return
+			}
+
+			if !fallback && isLoopback(err) {
+				util.Log("本机DNS异常! 将默认使用 %s, 可参考文档通过 -dns 自定义 DNS 服务器", fallbackDNS)
+				util.SetDNS(fallbackDNS)
+
+				fallback = true
+				continue
+			}
+
+			util.Log("等待网络连接: %s", err)
+
+			util.Log("%s 后重试...", delay)
+			time.Sleep(delay)
+		}
+	}
+}
+
+// isLoopback checks if the error is a loopback error.
+func isLoopback(e error) bool {
+	return strings.Contains(e.Error(), "[::1]:53")
+}