Check for local dns server and output a warning

api.go

@@ -413,17 +413,23 @@ func postImagesPush(srv *Server, version float64, w http.ResponseWriter, r *http
 
 func postContainersCreate(srv *Server, version float64, w http.ResponseWriter, r *http.Request, vars map[string]string) error {
 	config := &Config{}
+	out := &APIRun{}
+
 	if err := json.NewDecoder(r.Body).Decode(config); err != nil {
 		return err
 	}
+
+	if len(config.Dns) == 0 && len(srv.runtime.Dns) == 0 && utils.CheckLocalDns() {
+		out.Warnings = append(out.Warnings, fmt.Sprintf("WARNING: Docker detected local DNS server on resolv.conf. Using default external servers: %v", defaultDns))
+		config.Dns = defaultDns
+	}
+
 	id, err := srv.ContainerCreate(config)
 	if err != nil {
 		return err
 	}
+	out.ID = id
 
-	out := &APIRun{
-		ID: id,
-	}
 	if config.Memory > 0 && !srv.runtime.capabilities.MemoryLimit {
 		log.Println("WARNING: Your kernel does not support memory limit capabilities. Limitation discarded.")
 		out.Warnings = append(out.Warnings, "Your kernel does not support memory limit capabilities. Limitation discarded.")
@@ -432,6 +438,7 @@ func postContainersCreate(srv *Server, version float64, w http.ResponseWriter, r
 		log.Println("WARNING: Your kernel does not support swap limit capabilities. Limitation discarded.")
 		out.Warnings = append(out.Warnings, "Your kernel does not support memory swap capabilities. Limitation discarded.")
 	}
+
 	b, err := json.Marshal(out)
 	if err != nil {
 		return err

builder.go

@@ -2,6 +2,7 @@ package docker
 
 import (
 	"fmt"
+	"github.com/dotcloud/docker/utils"
 	"os"
 	"path"
 	"time"
@@ -68,6 +69,11 @@ func (builder *Builder) Create(config *Config) (*Container, error) {
 		return nil, err
 	}
 
+	if len(config.Dns) == 0 && len(builder.runtime.Dns) == 0 && utils.CheckLocalDns() {
+		//"WARNING: Docker detected local DNS server on resolv.conf. Using default external servers: %v", defaultDns
+		builder.runtime.Dns = defaultDns
+	}
+
 	// If custom dns exists, then create a resolv.conf for the container
 	if len(config.Dns) > 0 || len(builder.runtime.Dns) > 0 {
 		var dns []string

utils/utils.go

@@ -620,3 +620,20 @@ func (sf *StreamFormatter) FormatProgress(action, str string) []byte {
 func (sf *StreamFormatter) Used() bool {
 	return sf.used
 }
+
+func CheckLocalDns() bool {
+	resolv, err := ioutil.ReadFile("/etc/resolv.conf")
+	if err != nil {
+		Debugf("Error openning resolv.conf: %s", err)
+		return false
+	}
+	for _, ip := range []string{
+		"127.0.0.1",
+		"127.0.1.1",
+	} {
+		if strings.Contains(string(resolv), ip) {
+			return true
+		}
+	}
+	return false
+}