Remove nameserver 127.0.0.1 line rather then dumping resolv.conf

We have a bug report complaining about docker dumping the contents of the
hosts resolv.conf if it container 127.0.0.1.  They asked that instead
of dropping the file altogether, that we just remove the line.

This patch removes the 127.0.0.1 lines, if they exist and then
checks if any nameserver lines exist.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)

daemon/daemon.go

@@ -1044,6 +1044,7 @@ func (daemon *Daemon) checkLocaldns() error {
 	if err != nil {
 		return err
 	}
+	resolvConf = utils.RemoveLocalDns(resolvConf)
 	if len(daemon.config.Dns) == 0 && utils.CheckLocalDns(resolvConf) {
 		log.Infof("Local (127.0.0.1) DNS resolver found in resolv.conf and containers can't use it. Using default external servers : %v", DefaultDns)
 		daemon.config.Dns = DefaultDns

daemon/utils_test.go

@@ -27,3 +27,34 @@ func TestMergeLxcConfig(t *testing.T) {
 		t.Fatalf("expected %s got %s", expected, cpuset)
 	}
 }
+
+func TestRemoveLocalDns(t *testing.T) {
+	ns0 := "nameserver 10.16.60.14\nnameserver 10.16.60.21\n"
+
+	if result := utils.RemoveLocalDns([]byte(ns0)); result != nil {
+		if ns0 != string(result) {
+			t.Fatalf("Failed No Localhost: expected \n<%s> got \n<%s>", ns0, string(result))
+		}
+	}
+
+	ns1 := "nameserver 10.16.60.14\nnameserver 10.16.60.21\nnameserver 127.0.0.1\n"
+	if result := utils.RemoveLocalDns([]byte(ns1)); result != nil {
+		if ns0 != string(result) {
+			t.Fatalf("Failed Localhost: expected \n<%s> got \n<%s>", ns0, string(result))
+		}
+	}
+
+	ns1 = "nameserver 10.16.60.14\nnameserver 127.0.0.1\nnameserver 10.16.60.21\n"
+	if result := utils.RemoveLocalDns([]byte(ns1)); result != nil {
+		if ns0 != string(result) {
+			t.Fatalf("Failed Localhost: expected \n<%s> got \n<%s>", ns0, string(result))
+		}
+	}
+
+	ns1 = "nameserver 127.0.1.1\nnameserver 10.16.60.14\nnameserver 10.16.60.21\n"
+	if result := utils.RemoveLocalDns([]byte(ns1)); result != nil {
+		if ns0 != string(result) {
+			t.Fatalf("Failed Localhost: expected \n<%s> got \n<%s>", ns0, string(result))
+		}
+	}
+}

utils/utils.go

@@ -13,6 +13,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"runtime"
 	"strconv"
 	"strings"
@@ -332,6 +333,16 @@ func CheckLocalDns(resolvConf []byte) bool {
 	return true
 }
 
+var (
+	localHostRx = regexp.MustCompile(`(?m)^nameserver 127[^\n]+\n*`)
+)
+
+// RemoveLocalDns looks into the /etc/resolv.conf,
+// and removes any local nameserver entries.
+func RemoveLocalDns(resolvConf []byte) []byte {
+	return localHostRx.ReplaceAll(resolvConf, []byte{})
+}
+
 // GetLines parses input into lines and strips away comments.
 func GetLines(input []byte, commentMarker []byte) [][]byte {
 	lines := bytes.Split(input, []byte("\n"))