Add tsh command to resolve a single host

`tsh resolve` allows identifying a single host either directly by
hostname, or via custom search/predicate expression from a matched
proxy template. The main use case is to provide a simple command
for users that wish to use `Match exec` in their SSH config. Today,
all matching in SSH config must be done via DNS and requires users
to add some Teleport specific suffix/prefix, or use a wildcard entry
to invoke a tsh proxy command.

For example, the SSH config generated today via `tsh config` is the
following:

```
# Common flags for all local.dev hosts
Host *.cluster-name proxy.example.com
    UserKnownHostsFile "/Users/tim/.tsh/known_hosts"
    IdentityFile "/Users/tim/.tsh/keys/proxy.example.com/tim"
    CertificateFile "/Users/tim/.tsh/keys/proxy.example.com/tim-ssh/cluster-name-cert.pub"

# Flags for all local.dev hosts except the proxy
Host *.cluster-name
    Port 3022 !proxy.example.com
    ProxyCommand "tsh" proxy ssh --cluster=cluster-name --proxy=proxy.example.com:443 %r@%h:%p
```

This allows connections to the Teleport SSH service without using tsh directly
via `ssh foo.cluster-name`. However, when migrating to Teleport, that requires
the user to alter their existing workflow to include the cluster-name suffix.
To remedy this, users can now augment their SSH config to utilize `Match exec`
instead of globbing on the cluster name suffix with the following:

```
Match exec "tsh resolve -q %h"
    ProxyCommand "tsh" proxy ssh --cluster=cluster-name --proxy=proxy.example.com:443 %r@%h:%p
```

By default tsh resolve will output the matching host, if one was
found, but if the `-q` flag provided like in the example above
the output will be silenced. If no matches are found, or multiple
matches are found, `tsh resolve` will exit with a non-zero exit
code as per the `Match exec` requirements. If and only if a single
host is resolved will `tsh resolve` exit with a zero exit code.

There are performance concerns that need to be taken into account
before users adopt this in their SSH config. First, this may cause
`tsh resolve` to be invoked on any SSH request. For example, even
doing git pull/git push may now first require `tsh resolve` to
exit with a non-zero exit code before interact with the git remote.
Additionally, when `tsh resolve` finds a match, any connections to
the node will require _two_ connections to the cluster and _two_
ListUnifiedResourcesRequests to resolve the host since the
invocation of `tsh resolve` and `tsh proxy ssh` or `tbot proxy ssh`
do not share any resources. For the reasons mentioned above, the
SSH configuration generated by `tsh config` was not updated to
include this new command. If users want to opt into this behavior
they must acknowledge the latency concerns by manually editting
the config.

tool/tsh/common/tsh.go

@@ -802,6 +802,11 @@ func Run(ctx context.Context, args []string, opts ...CliOption) error {
 	ssh.Flag("no-resume", "Disable SSH connection resumption").Envar(noResumeEnvVar).BoolVar(&cf.DisableSSHResumption)
 	ssh.Flag("relogin", "Permit performing an authentication attempt on a failed command").Default("true").BoolVar(&cf.Relogin)
 
+	resolve := app.Command("resolve", "Resolves an SSH host.")
+	resolve.Arg("host", "Remote hostname to resolve").Required().StringVar(&cf.UserHost)
+	resolve.Flag("quiet", "Quiet mode").Short('q').BoolVar(&cf.Quiet)
+	resolve.Flag("format", defaults.FormatFlagDescription(defaults.DefaultFormats...)).Short('f').Default(teleport.Text).EnumVar(&cf.Format, defaults.DefaultFormats...)
+
 	// Daemon service for teleterm client
 	daemon := app.Command("daemon", "Daemon is the tsh daemon service.").Hidden()
 	daemonStart := daemon.Command("start", "Starts tsh daemon service.").Hidden()
@@ -1361,6 +1366,18 @@ func Run(ctx context.Context, args []string, opts ...CliOption) error {
 		err = onVersion(&cf)
 	case ssh.FullCommand():
 		err = onSSH(&cf)
+	case resolve.FullCommand():
+		err = onResolve(&cf)
+		// If quiet was specified for this command and
+		// an error occurred, exit with a non-zero exit
+		// code without emitting any other messaging.
+		// In this case, the command was likely invoked
+		// via a Match exec block from an SSH config and
+		// if no matches were found, we should not add
+		// additional spam to stderr.
+		if err != nil && cf.Quiet {
+			err = trace.Wrap(&common.ExitCodeError{Code: 1})
+		}
 	case latencySSH.FullCommand():
 		err = onSSHLatency(&cf)
 	case benchSSH.FullCommand():
@@ -3595,6 +3612,95 @@ func runLocalCommand(hostLogin string, command []string) error {
 	return cmd.Run()
 }
 
+// onResolve executes `tsh resolve`, a command that
+// attempts to resolve a single host from a provided
+// hostname. The host information provided may be
+// interpolated by proxy templates and converted
+// from a hostname into a fuzzy search, or predicate query.
+// Errors are returned if unable to connect to the cluster,
+// no matching hosts were found, or multiple matching hosts
+// were found. This is primarily meant to be used as a command
+// for a match exec block in an SSH config.
+func onResolve(cf *CLIConf) error {
+	tc, err := makeClient(cf)
+	if err != nil {
+		return trace.Wrap(err)
+	}
+
+	req := proto.ListUnifiedResourcesRequest{
+		Kinds:               []string{types.KindNode},
+		Labels:              tc.Labels,
+		SearchKeywords:      tc.SearchKeywords,
+		PredicateExpression: tc.PredicateExpression,
+		UseSearchAsRoles:    tc.UseSearchAsRoles,
+		SortBy:              types.SortBy{Field: types.ResourceKind},
+		// Limit to 2 so we can check for an ambiguous result
+		Limit: 2,
+	}
+
+	// If no search criteria were explicitly provided, then match exclusively
+	// on the hostname of the server. Otherwise, this would end up listing
+	// the first two servers that the user has access to and yield unexpected results.
+	if len(tc.Labels) == 0 && len(tc.SearchKeywords) == 0 && tc.PredicateExpression == "" {
+		req.PredicateExpression = fmt.Sprintf(`name == "%s"`, tc.Host)
+	}
+
+	// Only enable the re-authentication behavior if not invoked with `-q`. When
+	// in quiet mode, this command is likely being invoked via ssh and
+	// the login prompt will not be able to be presented to users anyway.
+	executor := client.RetryWithRelogin
+	if cf.Quiet {
+		executor = func(ctx context.Context, teleportClient *client.TeleportClient, f func() error, option ...client.RetryWithReloginOption) error {
+			return f()
+		}
+	}
+
+	var page []*types.EnrichedResource
+	if err := executor(cf.Context, tc, func() error {
+		clt, err := tc.ConnectToCluster(cf.Context)
+		if err != nil {
+			return trace.Wrap(err)
+		}
+
+		defer clt.Close()
+
+		page, _, err = apiclient.GetUnifiedResourcePage(cf.Context, clt.AuthClient, &req)
+		if err != nil {
+			return trace.Wrap(err)
+		}
+
+		return nil
+	}); err != nil {
+		return trace.Wrap(err)
+	}
+
+	switch len(page) {
+	case 1:
+	case 0:
+		return trace.NotFound("no matching hosts found")
+	default:
+		return trace.BadParameter("multiple matching hosts found")
+	}
+
+	if cf.Quiet {
+		return nil
+	}
+
+	format := strings.ToLower(cf.Format)
+	switch format {
+	case teleport.Text, "":
+		printNodesAsText(cf.Stdout(), []types.Server{page[0].ResourceWithLabels.(types.Server)}, true)
+	case teleport.JSON:
+		utils.WriteJSON(cf.Stdout(), page[0].ResourceWithLabels)
+	case teleport.YAML:
+		utils.WriteYAML(cf.Stdout(), page[0].ResourceWithLabels)
+	default:
+		return trace.BadParameter("unsupported format %q", cf.Format)
+	}
+
+	return nil
+}
+
 // onSSH executes 'tsh ssh' command
 func onSSH(cf *CLIConf) error {
 	tc, err := makeClient(cf)

tool/tsh/common/tsh_test.go

@@ -4012,6 +4012,25 @@ func setCopyStdout(stdout io.Writer) CliOption {
 func setHomePath(path string) CliOption {
 	return func(cf *CLIConf) error {
 		cf.HomePath = path
+		// The TSHConfig is populated prior to applying any options, and
+		// if the home directory is being overridden, then any proxy templates
+		// and aliases that may have been loaded from the default home directory
+		// should be returned to default to avoid altering tests run from machines
+		// that may have a tsh config file preset in the home directory.
+		cf.TSHConfig = client.TSHConfig{}
+		return nil
+	}
+}
+
+func setTSHConfig(cfg client.TSHConfig) CliOption {
+	return func(cf *CLIConf) error {
+		for _, template := range cfg.ProxyTemplates {
+			if err := template.Check(); err != nil {
+				return err
+			}
+		}
+
+		cf.TSHConfig = cfg
 		return nil
 	}
 }
@@ -6527,3 +6546,175 @@ func TestRolesToString(t *testing.T) {
 		})
 	}
 }
+
+// TestResolve tests that host resolution works for various inputs and
+// that proxy templates are respected.
+func TestResolve(t *testing.T) {
+	modules.SetTestModules(t, &modules.TestModules{TestBuildType: modules.BuildEnterprise})
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	accessRoleName := "access"
+	sshHostname := "test-ssh-server"
+
+	accessUser, err := types.NewUser(accessRoleName)
+	require.NoError(t, err)
+	accessUser.SetRoles([]string{accessRoleName})
+
+	user, err := user.Current()
+	require.NoError(t, err)
+	accessUser.SetLogins([]string{user.Username})
+
+	traits := map[string][]string{
+		constants.TraitLogins: {user.Username},
+	}
+	accessUser.SetTraits(traits)
+
+	connector := mockConnector(t)
+	rootServerOpts := []testserver.TestServerOptFunc{
+		testserver.WithBootstrap(connector, accessUser),
+		testserver.WithHostname(sshHostname),
+		testserver.WithClusterName(t, "root"),
+		testserver.WithSSHPublicAddrs("127.0.0.1:0"),
+		testserver.WithConfig(func(cfg *servicecfg.Config) {
+			cfg.SSH.Enabled = true
+			cfg.SSH.PublicAddrs = []utils.NetAddr{cfg.SSH.Addr}
+			cfg.SSH.DisableCreateHostUser = true
+			cfg.SSH.Labels = map[string]string{
+				"animal": "llama",
+				"env":    "dev",
+			}
+		}),
+	}
+	rootServer := testserver.MakeTestServer(t, rootServerOpts...)
+
+	node := testserver.MakeTestServer(t,
+		testserver.WithConfig(func(cfg *servicecfg.Config) {
+			cfg.SetAuthServerAddresses(rootServer.Config.AuthServerAddresses())
+			cfg.Hostname = "second-node"
+			cfg.Auth.Enabled = false
+			cfg.Proxy.Enabled = false
+			cfg.SSH.Enabled = true
+			cfg.SSH.DisableCreateHostUser = true
+			cfg.SSH.Labels = map[string]string{
+				"animal": "shark",
+				"env":    "dev",
+			}
+		}))
+
+	rootProxyAddr, err := rootServer.ProxyWebAddr()
+	require.NoError(t, err)
+
+	require.EventuallyWithT(t, func(t *assert.CollectT) {
+		found, err := rootServer.GetAuthServer().GetNodes(ctx, apidefaults.Namespace)
+		if !assert.NoError(t, err) || !assert.Len(t, found, 2) {
+			return
+		}
+	}, 10*time.Second, 100*time.Millisecond)
+
+	tmpHomePath := t.TempDir()
+	rootAuth := rootServer.GetAuthServer()
+
+	err = Run(ctx, []string{
+		"login",
+		"--insecure",
+		"--proxy", rootProxyAddr.String(),
+		"--user", user.Username,
+	}, setHomePath(tmpHomePath), setMockSSOLogin(rootAuth, accessUser, connector.GetName()))
+	require.NoError(t, err)
+
+	tests := []struct {
+		name      string
+		hostname  string
+		quiet     bool
+		assertion require.ErrorAssertionFunc
+	}{
+		{
+			name:      "resolved without using templates",
+			hostname:  sshHostname,
+			assertion: require.NoError,
+		},
+		{
+			name:      "resolved via predicate from template",
+			hostname:  "2.3.4.5",
+			assertion: require.NoError,
+		},
+		{
+			name:      "resolved via search from template",
+			hostname:  "llama.example.com:3023",
+			assertion: require.NoError,
+		},
+		{
+			name:     "no matching host",
+			hostname: "asdf",
+			assertion: func(tt require.TestingT, err error, i ...interface{}) {
+				require.Error(tt, err, i...)
+				require.ErrorContains(tt, err, "no matching hosts", i...)
+			},
+		},
+		{
+			name:      "quiet prevents output",
+			hostname:  node.Config.Hostname,
+			quiet:     true,
+			assertion: require.NoError,
+		},
+		{
+			name:     "multiple matching hosts",
+			hostname: "dev.example.com",
+			assertion: func(tt require.TestingT, err error, i ...interface{}) {
+				require.Error(tt, err, i...)
+				require.ErrorContains(tt, err, "multiple matching hosts", i...)
+			},
+		},
+	}
+
+	for _, test := range tests {
+		test := test
+		ctx := context.Background()
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			stdout := &output{buf: bytes.Buffer{}}
+			stderr := &output{buf: bytes.Buffer{}}
+
+			args := []string{"resolve"}
+			if test.quiet {
+				args = append(args, "-q")
+			}
+			args = append(args, test.hostname)
+
+			err := Run(ctx, args,
+				setHomePath(tmpHomePath),
+				setTSHConfig(client.TSHConfig{
+					ProxyTemplates: client.ProxyTemplates{
+						{
+							Template: `^([0-9\.]+):\d+$`,
+							Query:    `labels["animal"] == "llama"`,
+						},
+						{
+							Template: `^(.*).example.com:\d+$`,
+							Search:   "$1",
+						},
+					},
+				}),
+				func(conf *CLIConf) error {
+					conf.overrideStdin = &bytes.Buffer{}
+					conf.OverrideStdout = stdout
+					conf.overrideStderr = stderr
+					return nil
+				},
+			)
+
+			test.assertion(t, err)
+			if err != nil {
+				return
+			}
+
+			if test.quiet {
+				require.Empty(t, stdout.String())
+			} else {
+				require.Contains(t, stdout.String(), sshHostname)
+			}
+		})
+	}
+}