Consider empty /etc/resolv.conf as local dns + add unit test

api.go

@@ -488,7 +488,12 @@ func postContainersCreate(srv *Server, version float64, w http.ResponseWriter, r
 		return err
 	}
 
-	if len(config.Dns) == 0 && len(srv.runtime.Dns) == 0 && utils.CheckLocalDns() {
+	resolvConf, err := utils.GetResolvConf()
+	if err != nil {
+		return err
+	}
+
+	if len(config.Dns) == 0 && len(srv.runtime.Dns) == 0 && utils.CheckLocalDns(resolvConf) {
 		out.Warnings = append(out.Warnings, fmt.Sprintf("Docker detected local DNS server on resolv.conf. Using default external servers: %v", defaultDns))
 		config.Dns = defaultDns
 	}

builder.go

@@ -80,7 +80,12 @@ func (builder *Builder) Create(config *Config) (*Container, error) {
 		return nil, err
 	}
 
-	if len(config.Dns) == 0 && len(builder.runtime.Dns) == 0 && utils.CheckLocalDns() {
+	resolvConf, err := utils.GetResolvConf()
+	if err != nil {
+		return nil, err
+	}
+
+	if len(config.Dns) == 0 && len(builder.runtime.Dns) == 0 && utils.CheckLocalDns(resolvConf) {
 		//"WARNING: Docker detected local DNS server on resolv.conf. Using default external servers: %v", defaultDns
 		builder.runtime.Dns = defaultDns
 	}

utils/utils.go

@@ -688,17 +688,29 @@ func IsGIT(str string) bool {
 	return strings.HasPrefix(str, "git://") || strings.HasPrefix(str, "github.com/")
 }
 
-func CheckLocalDns() bool {
+// GetResolvConf opens and read the content of /etc/resolv.conf.
+// It returns it as byte slice.
+func GetResolvConf() ([]byte, error) {
 	resolv, err := ioutil.ReadFile("/etc/resolv.conf")
 	if err != nil {
 		Debugf("Error openning resolv.conf: %s", err)
-		return false
+		return nil, err
 	}
-	for _, ip := range []string{
-		"127.0.0.1",
-		"127.0.1.1",
+	return resolv, nil
+}
+
+// CheckLocalDns looks into the /etc/resolv.conf,
+// it returns true if there is a local nameserver or if there is no nameserver.
+func CheckLocalDns(resolvConf []byte) bool {
+	if !bytes.Contains(resolvConf, []byte("nameserver")) {
+		return true
+	}
+
+	for _, ip := range [][]byte{
+		[]byte("127.0.0.1"),
+		[]byte("127.0.1.1"),
 	} {
-		if strings.Contains(string(resolv), ip) {
+		if bytes.Contains(resolvConf, ip) {
 			return true
 		}
 	}

utils/utils_test.go

@@ -303,3 +303,37 @@ func TestParseRepositoryTag(t *testing.T) {
 		t.Errorf("Expected repo: '%s' and tag: '%s', got '%s' and '%s'", "url:5000/repo", "tag", repo, tag)
 	}
 }
+
+func TestGetResolvConf(t *testing.T) {
+	resolvConfUtils, err := GetResolvConf()
+	if err != nil {
+		t.Fatal(err)
+	}
+	resolvConfSystem, err := ioutil.ReadFile("/etc/resolv.conf")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(resolvConfUtils) != string(resolvConfSystem) {
+		t.Fatalf("/etc/resolv.conf and GetResolvConf have different content.")
+	}
+}
+
+func TestCheclLocalDns(t *testing.T) {
+	for resolv, result := range map[string]bool{`# Dynamic
+nameserver 10.0.2.3
+search dotcloud.net`: false,
+		`# Dynamic
+nameserver 127.0.0.1
+search dotcloud.net`: true,
+		`# Dynamic
+nameserver 127.0.1.1
+search dotcloud.net`: true,
+		`# Dynamic
+`: true,
+		``: true,
+	} {
+		if CheckLocalDns([]byte(resolv)) != result {
+			t.Fatalf("Wrong local dns detection: {%s} should be %v", resolv, result)
+		}
+	}
+}