Sanitize SSH server hostnames

Prevents any invalid and malicious hostnames, but replacing them with
known valid data already associated with the host. This was chosen
instead of rejecting to persist the server resource in an attempt to
continue providing access to the host in order to remedy the invalid
hostname.

Any servers that represent a Teleport ssh_service with an invalid
hostname will be replaced by the host UUID. Any static OpenSSH servers
will have invalid hostnames replaced with the address. This will continue
to allow the hosts to be dialable. In order to make these hosts
discoverable, the invalid hostname will be set in the
"teleport.internal/invalid-hostname" label.

Updates gravitational/teleport-private#1676.

lib/auth/auth.go

@@ -39,7 +39,9 @@ import (
 	"log/slog"
 	"math/big"
 	insecurerand "math/rand"
+	"net"
 	"os"
+	"regexp"
 	"slices"
 	"sort"
 	"strconv"
@@ -1470,6 +1472,25 @@ func (a *Server) runPeriodicOperations() {
 								if services.NodeHasMissedKeepAlives(srv) {
 									missedKeepAliveCount++
 								}
+
+								// TODO(tross) DELETE in v20.0.0 - all invalid hostnames should have been sanitized by then.
+								if !validServerHostname(srv.GetHostname()) {
+									if srv.GetSubKind() != types.SubKindOpenSSHNode {
+										return false, nil
+									}
+
+									// Any existing static hosts will not have their
+									// hostname sanitized since they don't heartbeat.
+									if err := sanitizeHostname(srv); err != nil {
+										a.logger.WarnContext(a.closeCtx, "failed to sanitize static SSH server hostname", "error", err, "server", srv.GetName())
+										return false, nil
+									}
+
+									if _, err := a.Services.UpsertNode(a.closeCtx, srv); err != nil {
+										a.logger.WarnContext(a.closeCtx, "failed to update SSH server hostname", "error", err, "server", srv.GetName())
+									}
+								}
+
 								return false, nil
 							},
 							req,
@@ -5618,9 +5639,64 @@ func (a *Server) KeepAliveServer(ctx context.Context, h types.KeepAlive) error {
 	return nil
 }
 
+const (
+	serverHostnameMaxLen       = 256
+	serverHostnameRegexPattern = `^[a-zA-Z0-9]([\.-]?[a-zA-Z0-9]+)*$`
+	replacedHostnameLabel      = types.TeleportInternalLabelPrefix + "invalid-hostname"
+)
+
+var serverHostnameRegex = regexp.MustCompile(serverHostnameRegexPattern)
+
+// validServerHostname returns false if the hostname is longer than 256 characters or
+// does not entirely consist of alphanumeric characters as well as '-' and '.'. A valid hostname also
+// cannot begin with a symbol, and a symbol cannot be followed immediately by another symbol.
+func validServerHostname(hostname string) bool {
+	return len(hostname) <= serverHostnameMaxLen && serverHostnameRegex.MatchString(hostname)
+}
+
+func sanitizeHostname(server types.Server) error {
+	invalidHostname := server.GetHostname()
+
+	replacedHostname := server.GetName()
+	if server.GetSubKind() == types.SubKindOpenSSHNode {
+		host, _, err := net.SplitHostPort(server.GetAddr())
+		if err != nil || !validServerHostname(host) {
+			id, err := uuid.NewRandom()
+			if err != nil {
+				return trace.Wrap(err)
+			}
+
+			host = id.String()
+		}
+
+		replacedHostname = host
+	}
+
+	switch s := server.(type) {
+	case *types.ServerV2:
+		s.Spec.Hostname = replacedHostname
+
+		if s.Metadata.Labels == nil {
+			s.Metadata.Labels = map[string]string{}
+		}
+
+		s.Metadata.Labels[replacedHostnameLabel] = invalidHostname
+	default:
+		return trace.BadParameter("invalid server provided")
+	}
+
+	return nil
+}
+
 // UpsertNode implements [services.Presence] by delegating to [Server.Services]
 // and potentially emitting a [usagereporter] event.
 func (a *Server) UpsertNode(ctx context.Context, server types.Server) (*types.KeepAlive, error) {
+	if !validServerHostname(server.GetHostname()) {
+		if err := sanitizeHostname(server); err != nil {
+			return nil, trace.Wrap(err)
+		}
+	}
+
 	lease, err := a.Services.UpsertNode(ctx, server)
 	if err != nil {
 		return nil, trace.Wrap(err)

lib/auth/auth_test.go

@@ -19,6 +19,7 @@
 package auth
 
 import (
+	"cmp"
 	"context"
 	"crypto/rand"
 	"crypto/x509"
@@ -34,7 +35,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/google/go-cmp/cmp"
+	gocmp "github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
 	"github.com/google/uuid"
 	"github.com/gravitational/license"
@@ -307,7 +308,7 @@ func TestSessions(t *testing.T) {
 			require.NoError(t, err)
 			assert.Empty(t, out.GetSSHPriv())
 			assert.Empty(t, out.GetTLSPriv())
-			assert.Empty(t, cmp.Diff(ws, out,
+			assert.Empty(t, gocmp.Diff(ws, out,
 				cmpopts.IgnoreFields(types.Metadata{}, "Revision"),
 				cmpopts.IgnoreFields(types.WebSessionSpecV2{}, "Priv", "TLSPriv")))
 
@@ -1655,7 +1656,7 @@ func TestServer_AugmentContextUserCertificates(t *testing.T) {
 				AssetTag:     test.opts.DeviceExtensions.AssetTag,
 				CredentialId: test.opts.DeviceExtensions.CredentialID,
 			}
-			if diff := cmp.Diff(want, got); diff != "" {
+			if diff := gocmp.Diff(want, got); diff != "" {
 				t.Errorf("certEvent.Identity.DeviceExtensions mismatch (-want +got)\n%s", diff)
 			}
 		}
@@ -2301,12 +2302,12 @@ func TestServer_ExtendWebSession_deviceExtensions(t *testing.T) {
 		// Assert TLS extensions.
 		_, newIdentity := parseX509PEMAndIdentity(t, newSession.GetTLSCert())
 		wantExts := tlsca.DeviceExtensions(*deviceExts)
-		if diff := cmp.Diff(wantExts, newIdentity.DeviceExtensions); diff != "" {
+		if diff := gocmp.Diff(wantExts, newIdentity.DeviceExtensions); diff != "" {
 			t.Errorf("newSession.TLSCert DeviceExtensions mismatch (-want +got)\n%s", diff)
 		}
 
 		// Assert SSH extensions.
-		if diff := cmp.Diff(wantExts, parseSSHDeviceExtensions(t, newSession.GetPub())); diff != "" {
+		if diff := gocmp.Diff(wantExts, parseSSHDeviceExtensions(t, newSession.GetPub())); diff != "" {
 			t.Errorf("newSession.Pub DeviceExtensions mismatch (-want +got)\n%s", diff)
 		}
 	})
@@ -2545,7 +2546,7 @@ func TestGenerateUserCertWithCertExtension(t *testing.T) {
 	// Validate audit event.
 	lastEvent := p.mockEmitter.LastEvent()
 	require.IsType(t, &apievents.CertificateCreate{}, lastEvent)
-	require.Empty(t, cmp.Diff(
+	require.Empty(t, gocmp.Diff(
 		&apievents.CertificateCreate{
 			Metadata: apievents.Metadata{
 				Type: events.CertificateCreateEvent,
@@ -3801,15 +3802,15 @@ func compareDevices(t *testing.T, ignoreUpdateAndCounter bool, got []*types.MFAD
 	}
 
 	// Ignore LastUsed and SignatureCounter?
-	var opts []cmp.Option
+	var opts []gocmp.Option
 	if ignoreUpdateAndCounter {
-		opts = append(opts, cmp.FilterPath(func(path cmp.Path) bool {
+		opts = append(opts, gocmp.FilterPath(func(path gocmp.Path) bool {
 			p := path.String()
 			return p == "LastUsed" || p == "Device.Webauthn.SignatureCounter"
-		}, cmp.Ignore()))
+		}, gocmp.Ignore()))
 	}
 
-	if diff := cmp.Diff(want, got, opts...); diff != "" {
+	if diff := gocmp.Diff(want, got, opts...); diff != "" {
 		t.Errorf("compareDevices mismatch (-want +got):\n%s", diff)
 	}
 }
@@ -4444,3 +4445,120 @@ func newGlobalNotificationWithExpiry(t *testing.T, title string, expires *timest
 
 	return &notification
 }
+
+// TestServerHostnameSanitization tests that persisting servers with
+// "invalid" hostnames results in the hostname being sanitized and the
+// illegal name being placed in a label.
+func TestServerHostnameSanitization(t *testing.T) {
+	t.Parallel()
+	ctx := context.Background()
+	srv, err := NewTestAuthServer(TestAuthServerConfig{Dir: t.TempDir()})
+	require.NoError(t, err)
+
+	cases := []struct {
+		name            string
+		hostname        string
+		addr            string
+		invalidHostname bool
+		invalidAddr     bool
+	}{
+		{
+			name:     "valid dns hostname",
+			hostname: "llama.example.com",
+		},
+		{
+			name:     "valid friendly hostname",
+			hostname: "llama",
+		},
+		{
+			name:     "uuid hostname",
+			hostname: uuid.NewString(),
+		},
+		{
+			name:     "uuid dns hostname",
+			hostname: uuid.NewString() + ".example.com",
+		},
+		{
+			name:            "empty hostname",
+			hostname:        "",
+			invalidHostname: true,
+		},
+		{
+			name:            "exceptionally long hostname",
+			hostname:        strings.Repeat("a", serverHostnameMaxLen*2),
+			invalidHostname: true,
+		},
+		{
+			name:            "invalid dns hostname",
+			hostname:        "llama..example.com",
+			invalidHostname: true,
+		},
+		{
+			name:            "spaces in hostname",
+			hostname:        "the quick brown fox jumps over the lazy dog",
+			invalidHostname: true,
+		},
+		{
+			name:            "invalid addr",
+			hostname:        "..",
+			addr:            "..:2345",
+			invalidHostname: true,
+			invalidAddr:     true,
+		},
+	}
+
+	for _, test := range cases {
+		t.Run(test.name, func(t *testing.T) {
+			for _, subKind := range []string{types.KindNode, types.SubKindOpenSSHNode} {
+				t.Run(subKind, func(t *testing.T) {
+					server := &types.ServerV2{
+						Kind:    types.KindNode,
+						SubKind: subKind,
+						Metadata: types.Metadata{
+							Name: uuid.NewString(),
+						},
+						Spec: types.ServerSpecV2{
+							Hostname: test.hostname,
+							Addr:     cmp.Or(test.addr, "abcd:1234"),
+						},
+					}
+					if subKind == types.KindNode {
+						server.SubKind = ""
+					}
+
+					_, err = srv.AuthServer.UpsertNode(ctx, server)
+					require.NoError(t, err)
+
+					replacedValue, _ := server.GetLabel("teleport.internal/invalid-hostname")
+					if !test.invalidHostname {
+						assert.Equal(t, test.hostname, server.GetHostname())
+						assert.Empty(t, replacedValue)
+						return
+					}
+
+					assert.Equal(t, test.hostname, replacedValue)
+					switch subKind {
+					case types.SubKindOpenSSHNode:
+						host, _, err := net.SplitHostPort(server.GetAddr())
+						assert.NoError(t, err)
+						if !test.invalidAddr {
+							// If the address is valid, then the hostname should be set
+							// to the host of the addr field.
+							assert.Equal(t, host, server.GetHostname())
+						} else {
+							// If the address is not valid, then the hostname should be
+							// set to a UUID.
+							assert.NotEqual(t, host, server.GetHostname())
+							assert.NotEqual(t, server.GetName(), server.GetHostname())
+
+							_, err := uuid.Parse(server.GetHostname())
+							require.NoError(t, err)
+						}
+					default:
+						assert.Equal(t, server.GetName(), server.GetHostname())
+					}
+				})
+			}
+		})
+	}
+}

lib/auth/grpcserver_test.go

@@ -2955,9 +2955,9 @@ func TestNodesCRUD(t *testing.T) {
 	require.NoError(t, err)
 
 	// node1 and node2 will be added to default namespace
-	node1, err := types.NewServerWithLabels("node1", types.KindNode, types.ServerSpecV2{}, nil)
+	node1, err := types.NewServerWithLabels("node1", types.KindNode, types.ServerSpecV2{Hostname: "node1"}, nil)
 	require.NoError(t, err)
-	node2, err := types.NewServerWithLabels("node2", types.KindNode, types.ServerSpecV2{}, nil)
+	node2, err := types.NewServerWithLabels("node2", types.KindNode, types.ServerSpecV2{Hostname: "node2"}, nil)
 	require.NoError(t, err)
 
 	t.Run("CreateNode", func(t *testing.T) {

lib/services/suite/suite.go

@@ -488,6 +488,7 @@ func (s *ServicesTestSuite) ServerCRUD(t *testing.T) {
 	require.Empty(t, out)
 
 	srv := NewServer(types.KindNode, "srv1", "127.0.0.1:2022", apidefaults.Namespace)
+	srv.Spec.Hostname = "llama"
 	_, err = s.PresenceS.UpsertNode(ctx, srv)
 	require.NoError(t, err)
 
@@ -513,6 +514,7 @@ func (s *ServicesTestSuite) ServerCRUD(t *testing.T) {
 	require.Empty(t, out)
 
 	proxy := NewServer(types.KindProxy, "proxy1", "127.0.0.1:2023", apidefaults.Namespace)
+	proxy.Spec.Hostname = "proxy.llama"
 	require.NoError(t, s.PresenceS.UpsertProxy(ctx, proxy))
 
 	out, err = s.PresenceS.GetProxies()
@@ -533,6 +535,7 @@ func (s *ServicesTestSuite) ServerCRUD(t *testing.T) {
 	require.Empty(t, out)
 
 	auth := NewServer(types.KindAuthServer, "auth1", "127.0.0.1:2025", apidefaults.Namespace)
+	auth.Spec.Hostname = "auth.llama"
 	require.NoError(t, s.PresenceS.UpsertAuthServer(ctx, auth))
 
 	out, err = s.PresenceS.GetAuthServers()