dns: support multiple upstream servers

dns/client.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"net"
 	"strings"
+	"time"
 
 	"github.com/nadoo/glider/common/log"
 	"github.com/nadoo/glider/proxy"
@@ -105,7 +106,6 @@ func (c *Client) Exchange(reqBytes []byte, clientAddr string, preferTCP bool) ([
 func (c *Client) exchange(qname string, reqBytes []byte, preferTCP bool) (server, network string, respBytes []byte, err error) {
 	// use tcp to connect upstream server default
 	network = "tcp"
-
 	dialer := c.dialer.NextDialer(qname + ":53")
 
 	// If client uses udp and no forwarders specified, use udp
@@ -113,21 +113,30 @@ func (c *Client) exchange(qname string, reqBytes []byte, preferTCP bool) (server
 		network = "udp"
 	}
 
-	server = c.GetServer(qname)
+	servers := c.GetServers(qname)
-	rc, err := dialer.Dial(network, server)
+	for _, server = range servers {
-	if err != nil {
+		rc, err := dialer.Dial(network, server)
-		log.F("[dns] failed to connect to server %v: %v", server, err)
+		// TODO: check the timeout setting here, some dns client has 5 seconds timeout
-		return
+		rc.SetDeadline(time.Now().Add(time.Duration(3) * time.Second))
+		if err != nil {
+			log.F("[dns] failed to connect to server %v: %v", server, err)
+			continue
+		}
+
+		switch network {
+		case "tcp":
+			respBytes, err = c.exchangeTCP(rc, reqBytes)
+		case "udp":
+			respBytes, err = c.exchangeUDP(rc, reqBytes)
+		}
+
+		if err == nil {
+			break
+		}
+		log.F("[dns] failed to exchange with server %v: %v", server, err)
 	}
 
-	switch network {
+	return server, network, respBytes, err
-	case "tcp":
-		respBytes, err = c.exchangeTCP(rc, reqBytes)
-	case "udp":
-		respBytes, err = c.exchangeUDP(rc, reqBytes)
-	}
-
-	return
 }
 
 // exchangeTCP exchange with server over tcp
@@ -181,20 +190,20 @@ func (c *Client) SetServer(domain string, servers ...string) {
 	c.upServerMap[domain] = append(c.upServerMap[domain], servers...)
 }
 
-// GetServer .
+// GetServers .
-func (c *Client) GetServer(domain string) string {
+func (c *Client) GetServers(domain string) []string {
 	domainParts := strings.Split(domain, ".")
 	length := len(domainParts)
 	for i := length - 2; i >= 0; i-- {
 		domain := strings.Join(domainParts[i:length], ".")
 
 		if servers, ok := c.upServerMap[domain]; ok {
-			return servers[0]
+			return servers
 		}
 	}
 
 	// TODO:
-	return c.upServers[0]
+	return c.upServers
 }
 
 // AddHandler .