Merge pull request moby#47821 from robmry/internal_network_with_dns

Forward DNS requests into --internal networks

integration/networking/resolvconf_test.go

@@ -2,11 +2,14 @@ package networking
 
 import (
 	"context"
+	"os"
+	"path"
 	"strings"
 	"testing"
 	"time"
 
 	containertypes "github.com/docker/docker/api/types/container"
+	"github.com/docker/docker/api/types/mount"
 	"github.com/docker/docker/integration/internal/container"
 	"github.com/docker/docker/integration/internal/network"
 	"github.com/docker/docker/testutil/daemon"
@@ -134,6 +137,59 @@ func TestInternalNetworkDNS(t *testing.T) {
 	assert.Check(t, is.Contains(res.Stdout(), network.DNSRespAddr))
 }
 
+// Check that '--dns' can be used to name a server inside a '--internal' network.
+// Regression test for https://github.com/moby/moby/issues/47822
+func TestInternalNetworkLocalDNS(t *testing.T) {
+	skip.If(t, testEnv.DaemonInfo.OSType == "windows", "No internal networks on Windows")
+	skip.If(t, testEnv.IsRootless, "Can't write an accessible dnsd.conf in rootless mode")
+	ctx := setupTest(t)
+
+	d := daemon.New(t)
+	d.StartWithBusybox(ctx, t)
+	defer d.Stop(t)
+
+	c := d.NewClientT(t)
+	defer c.Close()
+
+	intNetName := "intnet"
+	network.CreateNoError(ctx, t, c, intNetName,
+		network.WithDriver("bridge"),
+		network.WithInternal(),
+	)
+	defer network.RemoveNoError(ctx, t, c, intNetName)
+
+	// Write a config file for busybox's dnsd.
+	td := t.TempDir()
+	fname := path.Join(td, "dnsd.conf")
+	err := os.WriteFile(fname, []byte("foo.example 192.0.2.42\n"), 0644)
+	assert.NilError(t, err)
+
+	// Start a DNS server on the internal network.
+	serverId := container.Run(ctx, t, c,
+		container.WithNetworkMode(intNetName),
+		container.WithMount(mount.Mount{
+			Type:   mount.TypeBind,
+			Source: fname,
+			Target: "/etc/dnsd.conf",
+		}),
+		container.WithCmd("dnsd"),
+	)
+	defer c.ContainerRemove(ctx, serverId, containertypes.RemoveOptions{Force: true})
+
+	// Get the DNS server's address.
+	inspect := container.Inspect(ctx, t, c, serverId)
+	serverIP := inspect.NetworkSettings.Networks[intNetName].IPAddress
+
+	// Query the internal network's DNS server (via the daemon's internal DNS server).
+	res := container.RunAttach(ctx, t, c,
+		container.WithNetworkMode(intNetName),
+		container.WithDNS([]string{serverIP}),
+		container.WithCmd("nslookup", "-type=A", "foo.example"),
+	)
+	defer c.ContainerRemove(ctx, res.ContainerID, containertypes.RemoveOptions{Force: true})
+	assert.Check(t, is.Contains(res.Stdout.String(), "192.0.2.42"))
+}
+
 // TestNslookupWindows checks that nslookup gets results from external DNS.
 // Regression test for https://github.com/moby/moby/issues/46792
 func TestNslookupWindows(t *testing.T) {

libnetwork/resolver.go

@@ -486,17 +486,15 @@ func (r *Resolver) serveDNS(w dns.ResponseWriter, query *dns.Msg) {
 		return
 	}
 
-	if r.proxyDNS.Load() {
-		// If the user sets ndots > 0 explicitly and the query is
-		// in the root domain don't forward it out. We will return
-		// failure and let the client retry with the search domain
-		// attached.
-		if (queryType == dns.TypeA || queryType == dns.TypeAAAA) && r.backend.NdotsSet() &&
-			!strings.Contains(strings.TrimSuffix(queryName, "."), ".") {
-			resp = createRespMsg(query)
-		} else {
-			resp = r.forwardExtDNS(ctx, w.LocalAddr().Network(), w.RemoteAddr(), query)
-		}
+	// If the user sets ndots > 0 explicitly and the query is
+	// in the root domain don't forward it out. We will return
+	// failure and let the client retry with the search domain
+	// attached.
+	if (queryType == dns.TypeA || queryType == dns.TypeAAAA) && r.backend.NdotsSet() &&
+		!strings.Contains(strings.TrimSuffix(queryName, "."), ".") {
+		resp = createRespMsg(query)
+	} else {
+		resp = r.forwardExtDNS(ctx, w.LocalAddr().Network(), w.RemoteAddr(), query)
 	}
 
 	if resp == nil {
@@ -541,10 +539,18 @@ func (r *Resolver) forwardExtDNS(ctx context.Context, proto string, remoteAddr n
 	ctx, span := otel.Tracer("").Start(ctx, "resolver.forwardExtDNS")
 	defer span.End()
 
+	proxyDNS := r.proxyDNS.Load()
 	for _, extDNS := range r.extDNS(netiputil.AddrPortFromNet(remoteAddr)) {
 		if extDNS.IPStr == "" {
 			break
 		}
+		// If proxyDNS is false, do not forward the request from the host's namespace
+		// (don't access an external DNS server from an internal network). But, it is
+		// safe to make the request from the container's network namespace - it'll fail
+		// if the DNS server is not accessible, but the server may be on-net.
+		if !proxyDNS && extDNS.HostLoopback {
+			continue
+		}
 
 		// limits the number of outstanding concurrent queries.
 		ctx, cancel := context.WithTimeout(ctx, extIOTimeout)