Update container resolv.conf when host network changes /etc/resolv.conf

Only modifies non-running containers resolv.conf bind mount, and only if
the container has an unmodified resolv.conf compared to its contents at
container start time (so we don't overwrite manual/automated changes
within the container runtime). For containers which are running when
the host resolv.conf changes, the update will only be applied to the
container version of resolv.conf when the container is "bounced" down
and back up (e.g. stop/start or restart)

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)

networkfs/resolvconf/resolvconf.go

@@ -5,13 +5,25 @@ import (
 	"io/ioutil"
 	"regexp"
 	"strings"
+	"sync"
+
+	log "github.com/Sirupsen/logrus"
+	"github.com/docker/docker/utils"
 )
 
 var (
+	defaultDns      = []string{"8.8.8.8", "8.8.4.4"}
+	localHostRegexp = regexp.MustCompile(`(?m)^nameserver 127[^\n]+\n*`)
 	nsRegexp        = regexp.MustCompile(`^\s*nameserver\s*(([0-9]+\.){3}([0-9]+))\s*$`)
 	searchRegexp    = regexp.MustCompile(`^\s*search\s*(([^\s]+\s*)*)$`)
 )
 
+var lastModified struct {
+	sync.Mutex
+	sha256   string
+	contents []byte
+}
+
 func Get() ([]byte, error) {
 	resolv, err := ioutil.ReadFile("/etc/resolv.conf")
 	if err != nil {
@@ -20,6 +32,57 @@ func Get() ([]byte, error) {
 	return resolv, nil
 }
 
+// Retrieves the host /etc/resolv.conf file, checks against the last hash
+// and, if modified since last check, returns the bytes and new hash.
+// This feature is used by the resolv.conf updater for containers
+func GetIfChanged() ([]byte, string, error) {
+	lastModified.Lock()
+	defer lastModified.Unlock()
+
+	resolv, err := ioutil.ReadFile("/etc/resolv.conf")
+	if err != nil {
+		return nil, "", err
+	}
+	newHash, err := utils.HashData(bytes.NewReader(resolv))
+	if err != nil {
+		return nil, "", err
+	}
+	if lastModified.sha256 != newHash {
+		lastModified.sha256 = newHash
+		lastModified.contents = resolv
+		return resolv, newHash, nil
+	}
+	// nothing changed, so return no data
+	return nil, "", nil
+}
+
+// retrieve the last used contents and hash of the host resolv.conf
+// Used by containers updating on restart
+func GetLastModified() ([]byte, string) {
+	lastModified.Lock()
+	defer lastModified.Unlock()
+
+	return lastModified.contents, lastModified.sha256
+}
+
+// RemoveReplaceLocalDns looks for localhost (127.*) entries in the provided
+// resolv.conf, removing local nameserver entries, and, if the resulting
+// cleaned config has no defined nameservers left, adds default DNS entries
+// It also returns a boolean to notify the caller if changes were made at all
+func RemoveReplaceLocalDns(resolvConf []byte) ([]byte, bool) {
+	changed := false
+	cleanedResolvConf := localHostRegexp.ReplaceAll(resolvConf, []byte{})
+	// if the resulting resolvConf is empty, use defaultDns
+	if !bytes.Contains(cleanedResolvConf, []byte("nameserver")) {
+		log.Infof("No non-localhost DNS nameservers are left in resolv.conf. Using default external servers : %v", defaultDns)
+		cleanedResolvConf = append(cleanedResolvConf, []byte("\nnameserver "+strings.Join(defaultDns, "\nnameserver "))...)
+	}
+	if !bytes.Equal(resolvConf, cleanedResolvConf) {
+		changed = true
+	}
+	return cleanedResolvConf, changed
+}
+
 // getLines parses input into lines and strips away comments.
 func getLines(input []byte, commentMarker []byte) [][]byte {
 	lines := bytes.Split(input, []byte("\n"))

networkfs/resolvconf/resolvconf_test.go

@@ -156,3 +156,34 @@ func TestBuildWithZeroLengthDomainSearch(t *testing.T) {
 		t.Fatalf("Expected to not find '%s' got '%s'", notExpected, content)
 	}
 }
+
+func TestRemoveReplaceLocalDns(t *testing.T) {
+	ns0 := "nameserver 10.16.60.14\nnameserver 10.16.60.21\n"
+
+	if result, _ := RemoveReplaceLocalDns([]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, _ := RemoveReplaceLocalDns([]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, _ := RemoveReplaceLocalDns([]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, _ := RemoveReplaceLocalDns([]byte(ns1)); result != nil {
+		if ns0 != string(result) {
+			t.Fatalf("Failed Localhost: expected \n<%s> got \n<%s>", ns0, string(result))
+		}
+	}
+}