[v15] Display more user friendly error for invalid os logins in Web UI

lib/web/apiserver_test.go

@@ -7535,6 +7535,10 @@ type authProviderMock struct {
 	server types.ServerV2
 }
 
+func (mock authProviderMock) ListUnifiedResources(ctx context.Context, req *authproto.ListUnifiedResourcesRequest) (*authproto.ListUnifiedResourcesResponse, error) {
+	return nil, nil
+}
+
 func (mock authProviderMock) GetNode(ctx context.Context, namespace, name string) (types.Server, error) {
 	return &mock.server, nil
 }

lib/web/terminal.go

@@ -22,6 +22,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io"
 	"net"
 	"net/http"
@@ -104,6 +105,7 @@ type UserAuthClient interface {
 	CreateAuthenticateChallenge(ctx context.Context, req *authproto.CreateAuthenticateChallengeRequest) (*authproto.MFAAuthenticateChallenge, error)
 	GenerateUserCerts(ctx context.Context, req authproto.UserCertsRequest) (*authproto.Certs, error)
 	MaintainSessionPresence(ctx context.Context) (authproto.AuthService_MaintainSessionPresenceClient, error)
+	ListUnifiedResources(ctx context.Context, req *authproto.ListUnifiedResourcesRequest) (*authproto.ListUnifiedResourcesResponse, error)
 }
 
 // NewTerminal creates a web-based terminal based on WebSockets and returns a
@@ -911,6 +913,21 @@ func (t *sshBaseHandler) connectToNode(ctx context.Context, ws terminal.WSConn,
 		// The close error is ignored instead of using [trace.NewAggregate] because
 		// aggregate errors do not allow error inspection with things like [trace.IsAccessDenied].
 		_ = conn.Close()
+
+		// Since connection attempts are made via UUID and not hostname, any access denied errors
+		// will not contain the resolved host address. To provide an easier troubleshooting experience
+		// for users, attempt to resolve the hostname of the server and augment the error message with it.
+		if trace.IsAccessDenied(err) {
+			if resp, err := t.userAuthClient.ListUnifiedResources(ctx, &authproto.ListUnifiedResourcesRequest{
+				SortBy:              types.SortBy{Field: types.ResourceKind},
+				Kinds:               []string{types.KindNode},
+				Limit:               1,
+				PredicateExpression: fmt.Sprintf(`resource.metadata.name == "%s"`, t.sessionData.ServerID),
+			}); err == nil && len(resp.Resources) > 0 {
+				return nil, trace.AccessDenied("access denied to %q connecting to %v", sshConfig.User, resp.Resources[0].GetNode().GetHostname())
+			}
+		}
+
 		return nil, trace.Wrap(err)
 	}