Report ssh command failures to the surface (#1959)

Report ssh command failures to the surface (#1959)

The ssh debug commands told the user when something went wrong but
never told the surface that ran them, so `ssh host cmd` always exited 0
and a few failures were not reported at all. A failed command now
returns its error, a failure the user has already heard about is a
commands.Failure, and the ssh exec path exits 1 for either. The command
bodies also lose their ssh prefixes and the duplication they carried
over from ssh.go; both goldens pin that nothing else changed.

- Dispatch returns the callback's error, or shlex's; the ssh session
  logs it at debug, and not at all for a Failure the user already saw
- commands.Fail writes a message and returns it as a Failure in one step;
  an unknown command, `help` for a command that is not there, a failed
  reload and an unbalanced quote are failures
- ssh exec exit-status is 1 for a command, flag or failure error, 0
  otherwise; the e2e golden records the exit status for every case
- Clone rebinds help to the copy, so an ssh session's help lists logout
  and `help logout` and `logout -h` answer; sshd.LogoutCommand is the
  session's logout, exported for tests
- "did not understand" names the first word only, the one looked up;
  DispatchArgs takes just the tokens
- log-format writes its refusal to the user, as log-level does
- print-relays emits the relay error as a string and omits the key when
  there is none, where it printed {} or "Error":null
- list-hostmap and list-lighthouse-addrmap -json return the encode
  error; print-cert reports a certificate that will not marshal
- a flags type that is not the one registered is an internal error, not
  a silent success
- command functions and flag types are <name>Command and <name>Flags;
  parseVpnAddr and listHostMapFlagSet replace the repeated parse block
  and flag set; close-tunnel sends through sendCloseTunnel

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

commands.go

@@ -3,7 +3,6 @@ package nebula
 import (
 	"bytes"
 	"encoding/json"
-	"errors"
 	"flag"
 	"fmt"
 	"log/slog"
@@ -14,42 +13,51 @@ import (
 
 	"github.com/slackhq/nebula/commands"
 	"github.com/slackhq/nebula/config"
-	"github.com/slackhq/nebula/header"
 	"github.com/slackhq/nebula/logging"
 )
 
 // The commands nebula serves. attachCommands builds the one registry every surface
 // reads. See the commands package for what a Command is.
 
-type sshListHostMapFlags struct {
+type listHostMapFlags struct {
 	Json    bool
 	Pretty  bool
 	ByIndex bool
 }
 
-type sshPrintCertFlags struct {
+// listHostMapFlagSet is the flag set list-hostmap and list-pending-hostmap share.
+func listHostMapFlagSet() (*flag.FlagSet, any) {
+	fl := flag.NewFlagSet("", flag.ContinueOnError)
+	s := listHostMapFlags{}
+	fl.BoolVar(&s.Json, "json", false, "outputs as json with more information")
+	fl.BoolVar(&s.Pretty, "pretty", false, "pretty prints json, assumes -json")
+	fl.BoolVar(&s.ByIndex, "by-index", false, "gets all hosts in the hostmap from the index table")
+	return fl, &s
+}
+
+type printCertFlags struct {
 	Json   bool
 	Pretty bool
 	Raw    bool
 }
 
-type sshPrintTunnelFlags struct {
+type printTunnelFlags struct {
 	Pretty bool
 }
 
-type sshChangeRemoteFlags struct {
+type changeRemoteFlags struct {
 	Address string
 }
 
-type sshCloseTunnelFlags struct {
+type closeTunnelFlags struct {
 	LocalOnly bool
 }
 
-type sshCreateTunnelFlags struct {
+type createTunnelFlags struct {
 	Address string
 }
 
-type sshDeviceInfoFlags struct {
+type deviceInfoFlags struct {
 	Json   bool
 	Pretty bool
 }
@@ -58,32 +66,18 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 	reg.Register(&commands.Command{
 		Name:             commands.ListHostmap,
 		ShortDescription: "List all known previously connected hosts",
-		Flags: func() (*flag.FlagSet, any) {
+		Flags:            listHostMapFlagSet,
-			fl := flag.NewFlagSet("", flag.ContinueOnError)
-			s := sshListHostMapFlags{}
-			fl.BoolVar(&s.Json, "json", false, "outputs as json with more information")
-			fl.BoolVar(&s.Pretty, "pretty", false, "pretty prints json, assumes -json")
-			fl.BoolVar(&s.ByIndex, "by-index", false, "gets all hosts in the hostmap from the index table")
-			return fl, &s
-		},
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshListHostMap(f.hostMap, fs, w)
+			return listHostMapCommand(f.hostMap, fs, w)
 		},
 	})
 
 	reg.Register(&commands.Command{
 		Name:             commands.ListPendingHostmap,
 		ShortDescription: "List all handshaking hosts",
-		Flags: func() (*flag.FlagSet, any) {
+		Flags:            listHostMapFlagSet,
-			fl := flag.NewFlagSet("", flag.ContinueOnError)
-			s := sshListHostMapFlags{}
-			fl.BoolVar(&s.Json, "json", false, "outputs as json with more information")
-			fl.BoolVar(&s.Pretty, "pretty", false, "pretty prints json, assumes -json")
-			fl.BoolVar(&s.ByIndex, "by-index", false, "gets all hosts in the hostmap from the index table")
-			return fl, &s
-		},
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshListHostMap(f.handshakeManager, fs, w)
+			return listHostMapCommand(f.handshakeManager, fs, w)
 		},
 	})
 
@@ -92,13 +86,13 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		ShortDescription: "List all lighthouse map entries",
 		Flags: func() (*flag.FlagSet, any) {
 			fl := flag.NewFlagSet("", flag.ContinueOnError)
-			s := sshListHostMapFlags{}
+			s := listHostMapFlags{}
 			fl.BoolVar(&s.Json, "json", false, "outputs as json with more information")
 			fl.BoolVar(&s.Pretty, "pretty", false, "pretty prints json, assumes -json")
 			return fl, &s
 		},
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshListLighthouseMap(f.lightHouse, fs, w)
+			return listLighthouseAddrmapCommand(f.lightHouse, fs, w)
 		},
 	})
 
@@ -106,7 +100,7 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		Name:             commands.Reload,
 		ShortDescription: "Reloads configuration from disk, same as sending HUP to the process",
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshReload(l, c, w)
+			return reloadCommand(l, c, w)
 		},
 	})
 
@@ -114,7 +108,7 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		Name:             commands.LogLevel,
 		ShortDescription: "Gets or sets the current log level",
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshLogLevel(l, fs, a, w)
+			return logLevelCommand(l, fs, a, w)
 		},
 	})
 
@@ -122,7 +116,7 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		Name:             commands.LogFormat,
 		ShortDescription: "Gets or sets the current log format",
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshLogFormat(l, fs, a, w)
+			return logFormatCommand(l, fs, a, w)
 		},
 	})
 
@@ -130,7 +124,7 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		Name:             commands.Version,
 		ShortDescription: "Prints the currently running version of nebula",
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshVersion(f, fs, a, w)
+			return versionCommand(f, fs, a, w)
 		},
 	})
 
@@ -139,13 +133,13 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		ShortDescription: "Prints information about the network device.",
 		Flags: func() (*flag.FlagSet, any) {
 			fl := flag.NewFlagSet("", flag.ContinueOnError)
-			s := sshDeviceInfoFlags{}
+			s := deviceInfoFlags{}
 			fl.BoolVar(&s.Json, "json", false, "outputs as json with more information")
 			fl.BoolVar(&s.Pretty, "pretty", false, "pretty prints json, assumes -json")
 			return fl, &s
 		},
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshDeviceInfo(f, fs, w)
+			return deviceInfoCommand(f, fs, w)
 		},
 	})
 
@@ -154,14 +148,14 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		ShortDescription: "Prints the current certificate being used or the certificate for the provided vpn addr",
 		Flags: func() (*flag.FlagSet, any) {
 			fl := flag.NewFlagSet("", flag.ContinueOnError)
-			s := sshPrintCertFlags{}
+			s := printCertFlags{}
 			fl.BoolVar(&s.Json, "json", false, "outputs as json")
 			fl.BoolVar(&s.Pretty, "pretty", false, "pretty prints json, assumes -json")
 			fl.BoolVar(&s.Raw, "raw", false, "raw prints the PEM encoded certificate, not compatible with -json or -pretty")
 			return fl, &s
 		},
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshPrintCert(f, fs, a, w)
+			return printCertCommand(f, fs, a, w)
 		},
 	})
 
@@ -170,12 +164,12 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		ShortDescription: "Prints json details about a tunnel for the provided vpn addr",
 		Flags: func() (*flag.FlagSet, any) {
 			fl := flag.NewFlagSet("", flag.ContinueOnError)
-			s := sshPrintTunnelFlags{}
+			s := printTunnelFlags{}
 			fl.BoolVar(&s.Pretty, "pretty", false, "pretty prints json")
 			return fl, &s
 		},
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshPrintTunnel(f, fs, a, w)
+			return printTunnelCommand(f, fs, a, w)
 		},
 	})
 
@@ -184,12 +178,12 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		ShortDescription: "Prints json details about all relay info",
 		Flags: func() (*flag.FlagSet, any) {
 			fl := flag.NewFlagSet("", flag.ContinueOnError)
-			s := sshPrintTunnelFlags{}
+			s := printTunnelFlags{}
 			fl.BoolVar(&s.Pretty, "pretty", false, "pretty prints json")
 			return fl, &s
 		},
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshPrintRelays(f, fs, a, w)
+			return printRelaysCommand(f, fs, a, w)
 		},
 	})
 
@@ -198,12 +192,12 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		ShortDescription: "Changes the remote address used in the tunnel for the provided vpn addr",
 		Flags: func() (*flag.FlagSet, any) {
 			fl := flag.NewFlagSet("", flag.ContinueOnError)
-			s := sshChangeRemoteFlags{}
+			s := changeRemoteFlags{}
 			fl.StringVar(&s.Address, "address", "", "The new remote address, ip:port")
 			return fl, &s
 		},
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshChangeRemote(f, fs, a, w)
+			return changeRemoteCommand(f, fs, a, w)
 		},
 	})
 
@@ -212,12 +206,12 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		ShortDescription: "Closes a tunnel for the provided vpn addr",
 		Flags: func() (*flag.FlagSet, any) {
 			fl := flag.NewFlagSet("", flag.ContinueOnError)
-			s := sshCloseTunnelFlags{}
+			s := closeTunnelFlags{}
 			fl.BoolVar(&s.LocalOnly, "local-only", false, "Disables notifying the remote that the tunnel is shutting down")
 			return fl, &s
 		},
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshCloseTunnel(f, fs, a, w)
+			return closeTunnelCommand(f, fs, a, w)
 		},
 	})
 
@@ -227,12 +221,12 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		Help:             "The lighthouses will be queried for real addresses but you can provide one as well.",
 		Flags: func() (*flag.FlagSet, any) {
 			fl := flag.NewFlagSet("", flag.ContinueOnError)
-			s := sshCreateTunnelFlags{}
+			s := createTunnelFlags{}
 			fl.StringVar(&s.Address, "address", "", "Optionally provide a real remote address, ip:port ")
 			return fl, &s
 		},
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshCreateTunnel(f, fs, a, w)
+			return createTunnelCommand(f, fs, a, w)
 		},
 	})
 
@@ -241,15 +235,15 @@ func attachCommands(l *slog.Logger, c *config.C, reg *commands.Registry, f *Inte
 		ShortDescription: "Query the lighthouses for the provided vpn address",
 		Help:             "This command is asynchronous. Only currently known udp addresses will be printed.",
 		Callback: func(fs any, a []string, w commands.StringWriter) error {
-			return sshQueryLighthouse(f, fs, a, w)
+			return queryLighthouseCommand(f, fs, a, w)
 		},
 	})
 }
 
-func sshListHostMap(hl controlHostLister, a any, w commands.StringWriter) error {
+func listHostMapCommand(hl controlHostLister, a any, w commands.StringWriter) error {
-	fs, ok := a.(*sshListHostMapFlags)
+	fs, ok := a.(*listHostMapFlags)
 	if !ok {
-		return nil
+		return fmt.Errorf("internal error: expected *listHostMapFlags but was %T", a)
 	}
 
 	var hm []ControlHostInfo
@@ -269,9 +263,8 @@ func sshListHostMap(hl controlHostLister, a any, w commands.StringWriter) error
 			js.SetIndent("", "    ")
 		}
 
-		err := js.Encode(hm)
+		if err := js.Encode(hm); err != nil {
-		if err != nil {
+			return err
-			return nil
 		}
 
 	} else {
@@ -286,10 +279,10 @@ func sshListHostMap(hl controlHostLister, a any, w commands.StringWriter) error
 	return nil
 }
 
-func sshListLighthouseMap(lightHouse *LightHouse, a any, w commands.StringWriter) error {
+func listLighthouseAddrmapCommand(lightHouse *LightHouse, a any, w commands.StringWriter) error {
-	fs, ok := a.(*sshListHostMapFlags)
+	fs, ok := a.(*listHostMapFlags)
 	if !ok {
-		return nil
+		return fmt.Errorf("internal error: expected *listHostMapFlags but was %T", a)
 	}
 
 	type lighthouseInfo struct {
@@ -319,9 +312,8 @@ func sshListLighthouseMap(lightHouse *LightHouse, a any, w commands.StringWriter
 			js.SetIndent("", "    ")
 		}
 
-		err := js.Encode(addrMap)
+		if err := js.Encode(addrMap); err != nil {
-		if err != nil {
+			return err
-			return nil
 		}
 
 	} else {
@@ -340,22 +332,30 @@ func sshListLighthouseMap(lightHouse *LightHouse, a any, w commands.StringWriter
 	return nil
 }
 
-func sshVersion(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
+// parseVpnAddr parses the vpn address a command takes as its first operand, failing to w
-	return w.WriteLine(fmt.Sprintf("%s", ifce.version))
+// when there is none or it does not parse. noun is how the command's messages name it,
-}
+// "vpn address" or "vpn addr", which the output golden pins per command.
-
+func parseVpnAddr(a []string, noun string, w commands.StringWriter) (netip.Addr, error) {
-func sshQueryLighthouse(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
 	if len(a) == 0 {
-		return w.WriteLine("No vpn address was provided")
+		return netip.Addr{}, commands.Fail(w, "No vpn address was provided")
 	}
 
 	vpnAddr, err := netip.ParseAddr(a[0])
 	if err != nil {
-		return w.WriteLine(fmt.Sprintf("The provided vpn address could not be parsed: %s", a[0]))
+		return netip.Addr{}, commands.Fail(w, fmt.Sprintf("The provided %s could not be parsed: %s", noun, a[0]))
 	}
 
-	if !vpnAddr.IsValid() {
+	return vpnAddr, nil
-		return w.WriteLine(fmt.Sprintf("The provided vpn address could not be parsed: %s", a[0]))
+}
+
+func versionCommand(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
+	return w.WriteLine(ifce.version)
+}
+
+func queryLighthouseCommand(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
+	vpnAddr, err := parseVpnAddr(a, "vpn address", w)
+	if err != nil {
+		return err
 	}
 
 	var cm *CacheMap
@@ -366,80 +366,56 @@ func sshQueryLighthouse(ifce *Interface, fs any, a []string, w commands.StringWr
 	return json.NewEncoder(w.GetWriter()).Encode(cm)
 }
 
-func sshCloseTunnel(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
+func closeTunnelCommand(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
-	flags, ok := fs.(*sshCloseTunnelFlags)
+	flags, ok := fs.(*closeTunnelFlags)
 	if !ok {
-		return nil
+		return fmt.Errorf("internal error: expected *closeTunnelFlags but was %T", fs)
 	}
 
-	if len(a) == 0 {
+	vpnAddr, err := parseVpnAddr(a, "vpn address", w)
-		return w.WriteLine("No vpn address was provided")
-	}
-
-	vpnAddr, err := netip.ParseAddr(a[0])
 	if err != nil {
-		return w.WriteLine(fmt.Sprintf("The provided vpn address could not be parsed: %s", a[0]))
+		return err
-	}
-
-	if !vpnAddr.IsValid() {
-		return w.WriteLine(fmt.Sprintf("The provided vpn address could not be parsed: %s", a[0]))
 	}
 
 	hostInfo := ifce.hostMap.QueryVpnAddr(vpnAddr)
 	if hostInfo == nil {
-		return w.WriteLine(fmt.Sprintf("Could not find tunnel for vpn address: %v", a[0]))
+		return commands.Fail(w, fmt.Sprintf("Could not find tunnel for vpn address: %v", a[0]))
 	}
 
 	if !flags.LocalOnly {
-		ifce.send(
+		ifce.sendCloseTunnel(hostInfo)
-			header.CloseTunnel,
-			0,
-			hostInfo.ConnectionState,
-			hostInfo,
-			[]byte{},
-			make([]byte, 12, 12),
-			make([]byte, mtu),
-		)
 	}
 
 	ifce.closeTunnel(hostInfo)
 	return w.WriteLine("Closed")
 }
 
-func sshCreateTunnel(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
+func createTunnelCommand(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
-	flags, ok := fs.(*sshCreateTunnelFlags)
+	flags, ok := fs.(*createTunnelFlags)
 	if !ok {
-		return nil
+		return fmt.Errorf("internal error: expected *createTunnelFlags but was %T", fs)
 	}
 
-	if len(a) == 0 {
+	vpnAddr, err := parseVpnAddr(a, "vpn address", w)
-		return w.WriteLine("No vpn address was provided")
-	}
-
-	vpnAddr, err := netip.ParseAddr(a[0])
 	if err != nil {
-		return w.WriteLine(fmt.Sprintf("The provided vpn address could not be parsed: %s", a[0]))
+		return err
-	}
-
-	if !vpnAddr.IsValid() {
-		return w.WriteLine(fmt.Sprintf("The provided vpn address could not be parsed: %s", a[0]))
 	}
 
 	hostInfo := ifce.hostMap.QueryVpnAddr(vpnAddr)
 	if hostInfo != nil {
-		return w.WriteLine(fmt.Sprintf("Tunnel already exists"))
+		return commands.Fail(w, "Tunnel already exists")
 	}
 
 	hostInfo = ifce.handshakeManager.QueryVpnAddr(vpnAddr)
 	if hostInfo != nil {
-		return w.WriteLine(fmt.Sprintf("Tunnel already handshaking"))
+		return commands.Fail(w, "Tunnel already handshaking")
 	}
 
 	var addr netip.AddrPort
 	if flags.Address != "" {
 		addr, err = netip.ParseAddrPort(flags.Address)
 		if err != nil {
-			return w.WriteLine("Address could not be parsed")
+			return commands.Fail(w, "Address could not be parsed")
 		}
 	}
 
@@ -455,50 +431,42 @@ func sshCreateTunnel(ifce *Interface, fs any, a []string, w commands.StringWrite
 	return w.WriteLine("Created")
 }
 
-func sshChangeRemote(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
+func changeRemoteCommand(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
-	flags, ok := fs.(*sshChangeRemoteFlags)
+	flags, ok := fs.(*changeRemoteFlags)
 	if !ok {
-		return nil
+		return fmt.Errorf("internal error: expected *changeRemoteFlags but was %T", fs)
 	}
 
-	if len(a) == 0 {
+	vpnAddr, err := parseVpnAddr(a, "vpn address", w)
-		return w.WriteLine("No vpn address was provided")
+	if err != nil {
+		return err
 	}
 
 	if flags.Address == "" {
-		return w.WriteLine("No address was provided")
+		return commands.Fail(w, "No address was provided")
 	}
 
 	addr, err := netip.ParseAddrPort(flags.Address)
 	if err != nil {
-		return w.WriteLine("Address could not be parsed")
+		return commands.Fail(w, "Address could not be parsed")
-	}
-
-	vpnAddr, err := netip.ParseAddr(a[0])
-	if err != nil {
-		return w.WriteLine(fmt.Sprintf("The provided vpn address could not be parsed: %s", a[0]))
-	}
-
-	if !vpnAddr.IsValid() {
-		return w.WriteLine(fmt.Sprintf("The provided vpn address could not be parsed: %s", a[0]))
 	}
 
 	hostInfo := ifce.hostMap.QueryVpnAddr(vpnAddr)
 	if hostInfo == nil {
-		return w.WriteLine(fmt.Sprintf("Could not find tunnel for vpn address: %v", a[0]))
+		return commands.Fail(w, fmt.Sprintf("Could not find tunnel for vpn address: %v", a[0]))
 	}
 
 	hostInfo.SetRemote(addr)
 	return w.WriteLine("Changed")
 }
 
-func sshLogLevel(l *slog.Logger, fs any, a []string, w commands.StringWriter) error {
+func logLevelCommand(l *slog.Logger, fs any, a []string, w commands.StringWriter) error {
 	ctrl, ok := l.Handler().(interface {
 		GetLevel() slog.Level
 		SetLevel(slog.Level)
 	})
 	if !ok {
-		return w.WriteLine("Log level is not reconfigurable on this logger")
+		return commands.Fail(w, "Log level is not reconfigurable on this logger")
 	}
 
 	if len(a) == 0 {
@@ -507,20 +475,20 @@ func sshLogLevel(l *slog.Logger, fs any, a []string, w commands.StringWriter) er
 
 	level, err := logging.ParseLevel(strings.ToLower(a[0]))
 	if err != nil {
-		return w.WriteLine(fmt.Sprintf("Unknown log level %s. Possible log levels: trace, debug, info, warn, error", a))
+		return commands.Fail(w, fmt.Sprintf("Unknown log level %s. Possible log levels: trace, debug, info, warn, error", a))
 	}
 
 	ctrl.SetLevel(level)
 	return w.WriteLine(fmt.Sprintf("Log level is: %s", logging.LevelName(ctrl.GetLevel())))
 }
 
-func sshLogFormat(l *slog.Logger, fs any, a []string, w commands.StringWriter) error {
+func logFormatCommand(l *slog.Logger, fs any, a []string, w commands.StringWriter) error {
 	ctrl, ok := l.Handler().(interface {
 		GetFormat() string
 		SetFormat(string) error
 	})
 	if !ok {
-		return w.WriteLine("Log format is not reconfigurable on this logger")
+		return commands.Fail(w, "Log format is not reconfigurable on this logger")
 	}
 
 	if len(a) == 0 {
@@ -528,31 +496,27 @@ func sshLogFormat(l *slog.Logger, fs any, a []string, w commands.StringWriter) e
 	}
 
 	if err := ctrl.SetFormat(strings.ToLower(a[0])); err != nil {
-		return err
+		return commands.Fail(w, fmt.Sprintf("Unable to set log format: %s", err))
 	}
 	return w.WriteLine(fmt.Sprintf("Log format is: %s", ctrl.GetFormat()))
 }
 
-func sshPrintCert(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
+func printCertCommand(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
-	args, ok := fs.(*sshPrintCertFlags)
+	args, ok := fs.(*printCertFlags)
 	if !ok {
-		return nil
+		return fmt.Errorf("internal error: expected *printCertFlags but was %T", fs)
 	}
 
 	cert := ifce.pki.getCertState().GetDefaultCertificate()
 	if len(a) > 0 {
-		vpnAddr, err := netip.ParseAddr(a[0])
+		vpnAddr, err := parseVpnAddr(a, "vpn addr", w)
 		if err != nil {
-			return w.WriteLine(fmt.Sprintf("The provided vpn addr could not be parsed: %s", a[0]))
+			return err
-		}
-
-		if !vpnAddr.IsValid() {
-			return w.WriteLine(fmt.Sprintf("The provided vpn addr could not be parsed: %s", a[0]))
 		}
 
 		hostInfo := ifce.hostMap.QueryVpnAddr(vpnAddr)
 		if hostInfo == nil {
-			return w.WriteLine(fmt.Sprintf("Could not find tunnel for vpn addr: %v", a[0]))
+			return commands.Fail(w, fmt.Sprintf("Could not find tunnel for vpn addr: %v", a[0]))
 		}
 
 		cert = hostInfo.GetCert().Certificate
@@ -561,16 +525,15 @@ func sshPrintCert(ifce *Interface, fs any, a []string, w commands.StringWriter)
 	if args.Json || args.Pretty {
 		b, err := cert.MarshalJSON()
 		if err != nil {
-			return nil
+			return commands.Fail(w, fmt.Sprintf("Unable to marshal the certificate: %s", err))
 		}
 
 		if args.Pretty {
 			buf := new(bytes.Buffer)
-			err := json.Indent(buf, b, "", "    ")
+			if err := json.Indent(buf, b, "", "    "); err != nil {
-			b = buf.Bytes()
+				return commands.Fail(w, fmt.Sprintf("Unable to marshal the certificate: %s", err))
-			if err != nil {
-				return nil
 			}
+			b = buf.Bytes()
 		}
 
 		return w.WriteBytes(b)
@@ -579,7 +542,7 @@ func sshPrintCert(ifce *Interface, fs any, a []string, w commands.StringWriter)
 	if args.Raw {
 		b, err := cert.MarshalPEM()
 		if err != nil {
-			return nil
+			return commands.Fail(w, fmt.Sprintf("Unable to marshal the certificate: %s", err))
 		}
 
 		return w.WriteBytes(b)
@@ -588,20 +551,18 @@ func sshPrintCert(ifce *Interface, fs any, a []string, w commands.StringWriter)
 	return w.WriteLine(cert.String())
 }
 
-func sshPrintRelays(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
+func printRelaysCommand(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
-	args, ok := fs.(*sshPrintTunnelFlags)
+	args, ok := fs.(*printTunnelFlags)
 	if !ok {
-		w.WriteLine(fmt.Sprintf("sshPrintRelays failed to convert args type"))
+		return fmt.Errorf("internal error: expected *printTunnelFlags but was %T", fs)
-		return nil
 	}
 
-	relays := map[uint32]*HostInfo{}
+	ifce.hostMap.RLock()
-	ifce.hostMap.Lock()
+	relays := maps.Clone(ifce.hostMap.Relays)
-	maps.Copy(relays, ifce.hostMap.Relays)
+	ifce.hostMap.RUnlock()
-	ifce.hostMap.Unlock()
 
 	type RelayFor struct {
-		Error          error
+		Error          string `json:",omitempty"`
 		Type           string
 		State          string
 		PeerAddr       netip.Addr
@@ -632,11 +593,11 @@ func sshPrintRelays(ifce *Interface, fs any, a []string, w commands.StringWriter
 		co.Relays = append(co.Relays, &ro)
 		relayHI := ifce.hostMap.QueryVpnAddr(v.vpnAddrs[0])
 		if relayHI == nil {
-			ro.RelayForAddrs = append(ro.RelayForAddrs, RelayFor{Error: errors.New("could not find hostinfo")})
+			ro.RelayForAddrs = append(ro.RelayForAddrs, RelayFor{Error: "could not find hostinfo"})
 			continue
 		}
 		for _, vpnAddr := range relayHI.relayState.CopyRelayForIps() {
-			rf := RelayFor{Error: nil}
+			rf := RelayFor{}
 			r, ok := relayHI.relayState.GetRelayForByAddr(vpnAddr)
 			if ok {
 				t := ""
@@ -665,7 +626,7 @@ func sshPrintRelays(ifce *Interface, fs any, a []string, w commands.StringWriter
 				rf.Type = t
 				rf.State = s
 				if rf.LocalIndex != k {
-					rf.Error = fmt.Errorf("hostmap LocalIndex '%v' does not match RelayState LocalIndex", k)
+					rf.Error = fmt.Sprintf("hostmap LocalIndex '%v' does not match RelayState LocalIndex", k)
 				}
 			}
 			relayedHI := ifce.hostMap.QueryVpnAddr(vpnAddr)
@@ -676,35 +637,23 @@ func sshPrintRelays(ifce *Interface, fs any, a []string, w commands.StringWriter
 			ro.RelayForAddrs = append(ro.RelayForAddrs, rf)
 		}
 	}
-	err := enc.Encode(co)
+	return enc.Encode(co)
+}
+
+func printTunnelCommand(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
+	args, ok := fs.(*printTunnelFlags)
+	if !ok {
+		return fmt.Errorf("internal error: expected *printTunnelFlags but was %T", fs)
+	}
+
+	vpnAddr, err := parseVpnAddr(a, "vpn addr", w)
 	if err != nil {
 		return err
 	}
-	return nil
-}
-
-func sshPrintTunnel(ifce *Interface, fs any, a []string, w commands.StringWriter) error {
-	args, ok := fs.(*sshPrintTunnelFlags)
-	if !ok {
-		return nil
-	}
-
-	if len(a) == 0 {
-		return w.WriteLine("No vpn address was provided")
-	}
-
-	vpnAddr, err := netip.ParseAddr(a[0])
-	if err != nil {
-		return w.WriteLine(fmt.Sprintf("The provided vpn addr could not be parsed: %s", a[0]))
-	}
-
-	if !vpnAddr.IsValid() {
-		return w.WriteLine(fmt.Sprintf("The provided vpn addr could not be parsed: %s", a[0]))
-	}
 
 	hostInfo := ifce.hostMap.QueryVpnAddr(vpnAddr)
 	if hostInfo == nil {
-		return w.WriteLine(fmt.Sprintf("Could not find tunnel for vpn addr: %v", a[0]))
+		return commands.Fail(w, fmt.Sprintf("Could not find tunnel for vpn addr: %v", a[0]))
 	}
 
 	enc := json.NewEncoder(w.GetWriter())
@@ -715,7 +664,11 @@ func sshPrintTunnel(ifce *Interface, fs any, a []string, w commands.StringWriter
 	return enc.Encode(copyHostInfo(hostInfo, ifce.hostMap.GetPreferredRanges()))
 }
 
-func sshDeviceInfo(ifce *Interface, fs any, w commands.StringWriter) error {
+func deviceInfoCommand(ifce *Interface, fs any, w commands.StringWriter) error {
+	flags, ok := fs.(*deviceInfoFlags)
+	if !ok {
+		return fmt.Errorf("internal error: expected flags to be deviceInfoFlags but was %+v", fs)
+	}
 
 	data := struct {
 		Name string         `json:"name"`
@@ -727,11 +680,6 @@ func sshDeviceInfo(ifce *Interface, fs any, w commands.StringWriter) error {
 
 	copy(data.Cidr, ifce.inside.Networks())
 
-	flags, ok := fs.(*sshDeviceInfoFlags)
-	if !ok {
-		return fmt.Errorf("internal error: expected flags to be sshDeviceInfoFlags but was %+v", fs)
-	}
-
 	if flags.Json || flags.Pretty {
 		js := json.NewEncoder(w.GetWriter())
 		if flags.Pretty {
@@ -744,13 +692,19 @@ func sshDeviceInfo(ifce *Interface, fs any, w commands.StringWriter) error {
 	}
 }
 
-// sshReload reloads the config and writes the result to the client. It reloads even if the client has gone away
+// reloadCommand reloads the config and writes the result to the client. It reloads even if the client has gone away
-func sshReload(l *slog.Logger, c *config.C, w commands.StringWriter) error {
+func reloadCommand(l *slog.Logger, c *config.C, w commands.StringWriter) error {
 	writeErr := w.WriteLine("Reloading config")
 	res := c.ReloadConfig()
 	res.Log(l)
 	if err := w.WriteLine(res.String()); writeErr == nil {
 		writeErr = err
 	}
+	if writeErr != nil {
 		return writeErr
 	}
+	if res.Err != nil || res.Failed {
+		return &commands.Failure{Msg: res.String()}
+	}
+	return nil
+}

commands/command.go

@@ -9,12 +9,14 @@ import (
 type CommandFlags func() (*flag.FlagSet, any)
 
 // CommandCallback is called when your command should execute.
-// fs will be a a pointer to the struct provided by Command.Flags callback, if there was one. -h and -help are reserved
+// fs will be a pointer to the struct provided by Command.Flags callback, if there was one. -h and -help are reserved
 // and handled automatically for you.
 // a will be any unconsumed arguments, if no Command.Flags was available this will be all the flags passed in.
 // w is the writer to use when sending messages back to the client.
-// If an error is returned by the callback it is logged locally, the callback should handle messaging errors to the user
+// An error returned by the callback is handed back to whoever dispatched the command, which reports a failure: exit
-// where appropriate
+// status 1 over ssh. The error is not shown to the user. For a failure the user caused or can act on, write the
+// message and return it as a *Failure, which Fail does in one step; the surface then reports the failure without
+// logging what the user was already told. Any other error is logged by the surface as well.
 type CommandCallback func(fs any, a []string, w StringWriter) error
 
 type Command struct {
@@ -26,6 +28,27 @@ type Command struct {
 	Callback CommandCallback
 }
 
+// Failure is the error a command returns for a failure it has already explained to the user:
+// the message it wrote is the whole story. A surface reports it as a failure and does not
+// log it.
+type Failure struct {
+	Msg string
+}
+
+func (f *Failure) Error() string {
+	return f.Msg
+}
+
+// Fail writes msg to w and returns it as a *Failure, so a command reports a failure to the
+// user and to its surface in one line. If the write itself fails, that error is returned
+// instead: the user was not told anything.
+func Fail(w StringWriter, msg string) error {
+	if err := w.WriteLine(msg); err != nil {
+		return err
+	}
+	return &Failure{Msg: msg}
+}
+
 func execCommand(c *Command, args []string, w StringWriter) error {
 	var (
 		fl *flag.FlagSet

commands/registry.go

@@ -38,7 +38,8 @@ func (r *Registry) registerHelp() {
 	})
 }
 
-// Register adds a command that can be run by a user.
+// Register adds a command that can be run by a user. A command registered under a name
+// already held replaces it; see Clone for what that means for Help.
 func (r *Registry) Register(c *Command) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
@@ -77,29 +78,32 @@ func (r *Registry) All() []*Command {
 	return cmds
 }
 
-// Clone returns a copy holding the same commands.
+// Clone returns a copy holding the same commands, with help bound to the copy: a
+// command registered on the clone alone is listed and described there, and one the
+// clone withholds is not. That rebinding replaces whatever was registered under Help on
+// the source, built-in or not.
 func (r *Registry) Clone() *Registry {
 	r.mu.RLock()
-	defer r.mu.RUnlock()
+	c := &Registry{commands: maps.Clone(r.commands)}
-	return &Registry{commands: maps.Clone(r.commands)}
+	r.mu.RUnlock()
+	c.registerHelp()
+	return c
 }
 
-// Dispatch parses a command line and runs it, reporting problems to w.
+// Dispatch parses a command line and runs it, reporting problems to w. The error is for
-func (r *Registry) Dispatch(line string, w StringWriter) {
+// the caller to log, not to show; see CommandCallback.
-	// A line shlex cannot split, an unbalanced quote, gets no answer. sshd was silent
+func (r *Registry) Dispatch(line string, w StringWriter) error {
-	// here too; this is a relocation, so it stays silent.
+	// A line shlex cannot split, an unbalanced quote.
 	args, err := shlex.Split(line, true)
 	if err != nil {
-		return
+		return Fail(w, "could not parse the command line: "+err.Error())
 	}
 
-	_ = r.DispatchArgs(args, line, w)
+	return r.DispatchArgs(args, w)
 }
 
-// DispatchArgs runs an already tokenized command line. orig is what the user typed,
+// DispatchArgs runs an already tokenized command line. The error is as for Dispatch.
-// used only for the "did not understand" message; callers that have no raw line may
+func (r *Registry) DispatchArgs(args []string, w StringWriter) error {
-// pass "" and the joined args are used instead.
-func (r *Registry) DispatchArgs(args []string, orig string, w StringWriter) error {
 	if len(args) == 0 {
 		r.dumpCommands(w)
 		return nil
@@ -107,16 +111,14 @@ func (r *Registry) DispatchArgs(args []string, orig string, w StringWriter) erro
 
 	c := r.Get(Name(args[0]))
 	if c == nil {
-		if orig == "" {
+		// Only the first word was looked up, so only it is named.
-			orig = strings.Join(args, " ")
+		msg := fmt.Sprintf("did not understand: %s", args[0])
-		}
+		if err := w.WriteLine(msg); err != nil {
-
-		if err := w.WriteLine(fmt.Sprintf("did not understand: %s", orig)); err != nil {
 			return err
 		}
 
 		r.dumpCommands(w)
-		return nil
+		return &Failure{Msg: msg}
 	}
 
 	if checkHelpArgs(args) {
@@ -154,7 +156,7 @@ func (r *Registry) help(a []string, w StringWriter) error {
 	// We are printing a specific commands help text
 	cmd := r.Get(Name(a[0]))
 	if cmd == nil {
-		return w.WriteLine("Command not available " + a[0])
+		return Fail(w, "Command not available "+a[0])
 	}
 
 	if err := w.WriteLine(fmt.Sprintf("%s - %s", cmd.Name, cmd.ShortDescription)); err != nil {

commands/registry_test.go

@@ -2,6 +2,7 @@ package commands
 
 import (
 	"bytes"
+	"errors"
 	"flag"
 	"strings"
 	"testing"
@@ -45,18 +46,41 @@ func TestFlagUsageGoesToTheWriter(t *testing.T) {
 	})
 
 	merged := &bytes.Buffer{}
-	require.Error(t, r.DispatchArgs([]string{"emit", "-nope"}, "", NewStringWriter(merged)))
+	require.Error(t, r.DispatchArgs([]string{"emit", "-nope"}, NewStringWriter(merged)))
 	assert.Contains(t, merged.String(), "flag provided but not defined")
 }
 
-// TestDispatchDropsUnparseableLine pins sshd's behaviour for a line shlex rejects: no
+// TestDispatchReportsUnparseableLine: a line shlex rejects, an unbalanced quote, is a
-// answer at all. An operator with an unbalanced quote gets silence, as they always have.
+// failure the user is told about.
-func TestDispatchDropsUnparseableLine(t *testing.T) {
+func TestDispatchReportsUnparseableLine(t *testing.T) {
 	out := &bytes.Buffer{}
-	testRegistry().Dispatch(`served "unterminated`, NewStringWriter(out))
+	err := testRegistry().Dispatch(`served "unterminated`, NewStringWriter(out))
-	assert.Empty(t, out.String())
+
+	var f *Failure
+	require.ErrorAs(t, err, &f)
+	assert.Equal(t, f.Msg+"\n", out.String())
+	assert.Contains(t, out.String(), "could not parse the command line: ")
 }
 
+// TestFailTellsTheUserAndTheSurface: Fail writes the message and returns it as a Failure;
+// when the write fails, the user heard nothing, so the write error is what comes back.
+func TestFailTellsTheUserAndTheSurface(t *testing.T) {
+	out := &bytes.Buffer{}
+	err := Fail(NewStringWriter(out), "no such tunnel")
+	var f *Failure
+	require.ErrorAs(t, err, &f)
+	assert.Equal(t, "no such tunnel", f.Msg)
+	assert.Equal(t, "no such tunnel\n", out.String())
+
+	err = Fail(NewStringWriter(failingWriter{}), "no such tunnel")
+	require.EqualError(t, err, "gone")
+	assert.NotErrorAs(t, err, &f)
+}
+
+type failingWriter struct{}
+
+func (failingWriter) Write([]byte) (int, error) { return 0, errors.New("gone") }
+
 // TestAllSortsByName: registration order does not matter.
 func TestAllSortsByName(t *testing.T) {
 	r := New()
@@ -72,8 +96,8 @@ func TestAllSortsByName(t *testing.T) {
 }
 
 // TestHelpFlagAgreesWithHelpCommand pins that -h answers through the registered help
-// command. On a clone, help is bound to the source, so a command registered only on the
+// command, and that help on a clone describes the clone: a command registered only there
-// clone is known to neither -h nor `help <name>`, and the two never disagree.
+// is known to both -h and `help <name>`, and one withheld from it is known to neither.
 func TestHelpFlagAgreesWithHelpCommand(t *testing.T) {
 	r := testRegistry()
 	c := r.Clone()
@@ -88,10 +112,65 @@ func TestHelpFlagAgreesWithHelpCommand(t *testing.T) {
 	cmd := &bytes.Buffer{}
 	c.Dispatch("help logout", NewStringWriter(cmd))
 	assert.Equal(t, cmd.String(), flag.String())
-	assert.Equal(t, "Command not available logout\n", flag.String())
+	assert.Equal(t, "logout - ends the session\n", flag.String())
+
+	// The source never learns of it.
+	src := &bytes.Buffer{}
+	r.Dispatch("help logout", NewStringWriter(src))
+	assert.Equal(t, "Command not available logout\n", src.String())
 
 	// And a command both hold is described either way.
 	flag.Reset()
 	c.Dispatch("served -h", NewStringWriter(flag))
 	assert.Equal(t, "served - a command this surface serves\n", flag.String())
 }
+
+// TestCloneHelpListsTheClone: the command list help prints is the clone's, so a clone
+// that withholds a command does not name it, and the source still does.
+func TestCloneHelpListsTheClone(t *testing.T) {
+	r := testRegistry()
+	c := New()
+	c.Register(r.Get("served"))
+	c = c.Clone()
+
+	out := &bytes.Buffer{}
+	c.Dispatch("help", NewStringWriter(out))
+	assert.Equal(t, "Available commands:\n"+
+		"help - prints available commands or help <command> for specific usage info\n"+
+		"served - a command this surface serves\n\n", out.String())
+
+	out.Reset()
+	c.Dispatch("help withheld", NewStringWriter(out))
+	assert.Equal(t, "Command not available withheld\n", out.String())
+
+	out.Reset()
+	r.Dispatch("help withheld", NewStringWriter(out))
+	assert.Equal(t, "withheld - a command this surface withholds\n", out.String())
+}
+
+// TestDispatchReturnsTheCommandsError: what the callback returns reaches the caller, so
+// a surface can log it.
+func TestDispatchReturnsTheCommandsError(t *testing.T) {
+	r := New()
+	r.Register(&Command{
+		Name: "fails",
+		Callback: func(fs any, a []string, w StringWriter) error {
+			return errors.New("boom")
+		},
+	})
+
+	out := &bytes.Buffer{}
+	require.EqualError(t, r.Dispatch("fails", NewStringWriter(out)), "boom")
+	assert.Empty(t, out.String(), "a command's own error is not shown; the command speaks for itself")
+}
+
+// TestUnknownCommandNamesTheWord: only the first word is looked up, so only it is
+// reported, followed by the command list, and the line is a failure to the surface.
+func TestUnknownCommandNamesTheWord(t *testing.T) {
+	out := &bytes.Buffer{}
+	err := testRegistry().Dispatch(`nope "with args"`, NewStringWriter(out))
+	var f *Failure
+	require.ErrorAs(t, err, &f)
+	assert.Equal(t, "did not understand: nope", f.Msg)
+	assert.True(t, strings.HasPrefix(out.String(), "did not understand: nope\nAvailable commands:\n"), out.String())
+}

commands_test.go

@@ -0,0 +1,67 @@
+package nebula
+
+import (
+	"bytes"
+	"io"
+	"net/netip"
+	"testing"
+
+	"github.com/slackhq/nebula/commands"
+	"github.com/slackhq/nebula/logging"
+	"github.com/slackhq/nebula/test"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestLogFormatRefusalIsShown: a format the logger does not know is reported to the user
+// as a failure, rather than returned bare, which over ssh was silence.
+func TestLogFormatRefusalIsShown(t *testing.T) {
+	l := logging.NewLogger(io.Discard)
+	out := &bytes.Buffer{}
+	w := commands.NewStringWriter(out)
+
+	var f *commands.Failure
+	require.ErrorAs(t, logFormatCommand(l, nil, []string{"bogus"}, w), &f)
+	assert.Equal(t, "Unable to set log format: unknown log format `bogus`. possible formats: [text json]\n", out.String())
+
+	out.Reset()
+	require.NoError(t, logFormatCommand(l, nil, []string{"JSON"}, w))
+	assert.Equal(t, "Log format is: json\n", out.String())
+}
+
+// TestPrintRelaysReportsErrorsAsText: the Error field was an error value, which encodes
+// as {} and lost the message. It is the message now, and absent when there is none.
+func TestPrintRelaysReportsErrorsAsText(t *testing.T) {
+	l := test.NewLogger()
+	hm := newHostMap(l)
+	relay := netip.MustParseAddr("10.0.0.1")
+	peer := netip.MustParseAddr("10.0.0.2")
+
+	// A relay host that is in Relays but not in Hosts, so its hostinfo cannot be found.
+	hm.Relays[1] = &HostInfo{vpnAddrs: []netip.Addr{relay}}
+	ifce := &Interface{hostMap: hm}
+
+	out := &bytes.Buffer{}
+	require.NoError(t, printRelaysCommand(ifce, &printTunnelFlags{}, nil, commands.NewStringWriter(out)))
+	assert.JSONEq(t, `{"Relays":[{"NebulaAddr":"10.0.0.1","RelayForAddrs":[{"Error":"could not find hostinfo","Type":"","State":"","PeerAddr":"","LocalIndex":0,"RemoteIndex":0,"RelayedThrough":null}]}]}`, out.String())
+
+	// The same relay host, found, relaying for a peer whose index matches: no Error key.
+	hi := hm.Relays[1]
+	r := &Relay{Type: ForwardingType, State: Established, PeerAddr: peer, LocalIndex: 1, RemoteIndex: 2}
+	hi.relayState = RelayState{
+		relayForByAddr: map[netip.Addr]*Relay{peer: r},
+		relayForByIdx:  map[uint32]*Relay{r.LocalIndex: r},
+	}
+	hm.Hosts[relay] = hi
+
+	out.Reset()
+	require.NoError(t, printRelaysCommand(ifce, &printTunnelFlags{}, nil, commands.NewStringWriter(out)))
+	assert.JSONEq(t, `{"Relays":[{"NebulaAddr":"10.0.0.1","RelayForAddrs":[{"Type":"forwarding","State":"established","PeerAddr":"10.0.0.2","LocalIndex":1,"RemoteIndex":2,"RelayedThrough":null}]}]}`, out.String())
+
+	// And an index mismatch names it.
+	hm.Relays[7] = hi
+	delete(hm.Relays, 1)
+	out.Reset()
+	require.NoError(t, printRelaysCommand(ifce, &printTunnelFlags{}, nil, commands.NewStringWriter(out)))
+	assert.Contains(t, out.String(), `"Error":"hostmap LocalIndex '7' does not match RelayState LocalIndex"`)
+}

control.go

@@ -374,6 +374,9 @@ func (c *Control) CloseAllTunnels(excludeLighthouses bool) (closed int) {
 	return
 }
 
+// Commands returns the registry the daemon's surfaces serve from. It is the live
+// registry, not a copy: a command registered on it is served to every ssh session that
+// starts afterwards. Clone it to run commands without that reach.
 func (c *Control) Commands() *commands.Registry {
 	return c.commands
 }

e2e/commands_output_test.go

@@ -14,6 +14,7 @@ import (
 	"github.com/slackhq/nebula/cert"
 	"github.com/slackhq/nebula/cert_test"
 	"github.com/slackhq/nebula/commands"
+	"github.com/slackhq/nebula/sshd"
 	"github.com/slackhq/nebula/test"
 	"github.com/stretchr/testify/require"
 )
@@ -64,16 +65,22 @@ func TestCommandOutput(t *testing.T) {
 	control.Start()
 	t.Cleanup(func() { control.Stop() })
 
+	// As an ssh session does: a clone of the daemon's registry with logout added, which
+	// help on the clone lists and describes. There is no session to end here.
 	reg := control.Commands().Clone()
+	reg.Register(sshd.LogoutCommand(func() {}))
 
-	run := func(args ...string) string {
+	// run dispatches as the ssh exec path does and returns the output and the error, which
+	// over ssh is exit status 1.
+	run := func(args ...string) (string, error) {
 		buf := &bytes.Buffer{}
-		_ = reg.DispatchArgs(args, strings.Join(args, " "), commands.NewStringWriter(buf))
+		err := reg.DispatchArgs(args, commands.NewStringWriter(buf))
-		return buf.String()
+		return buf.String(), err
 	}
 
 	// Seed a pending tunnel so the listing and per-address commands have something to show.
-	seed := run("create-tunnel", "10.222.0.9")
+	seed, err := run("create-tunnel", "10.222.0.9")
+	require.NoError(t, err)
 	require.Equal(t, "Created\n", seed, "seeding a tunnel should succeed before anything else is recorded")
 
 	cases := []outputCase{
@@ -120,6 +127,9 @@ func TestCommandOutput(t *testing.T) {
 		{"close-tunnel", []string{"not-an-address"}},
 		{"help", nil},
 		{"help", []string{"print-cert"}},
+		{"help", []string{"logout"}},
+		{"logout", []string{"-h"}},
+		{"not-a-command", []string{"10.222.0.9"}},
 		// reload is last so that whatever a reload does to the daemon cannot leak into an
 		// earlier case. In this harness it fails: the config was loaded from a string and
 		// has no path to re-read, which is what the recorded result says.
@@ -133,13 +143,19 @@ func TestCommandOutput(t *testing.T) {
 			args = append(args, c.args...)
 
 			fmt.Fprintf(got, "$ %s\n", strings.Join(args, " "))
-			out := normalise(run(args...))
+			out, err := run(args...)
+			out = normalise(out)
 			got.WriteString(out)
 			// Output that does not end in a newline is recorded as such, the way diff marks
 			// it, so the golden sees that byte too and the next prompt still starts a line.
 			if out != "" && !strings.HasSuffix(out, "\n") {
 				got.WriteString("\\ No newline at end of output\n")
 			}
+			// A failed command is recorded as the exit status an ssh caller sees, so the
+			// golden pins which outcomes count as failures.
+			if err != nil {
+				got.WriteString("exit status 1\n")
+			}
 			got.WriteString("\n")
 		}
 	}

e2e/testdata/commands_output.golden

@@ -158,66 +158,84 @@ $ print-cert -pretty -raw
 
 $ print-cert 10.222.0.9
 Could not find tunnel for vpn addr: 10.222.0.9
+exit status 1
 
 $ print-cert -json 10.222.0.9
 Could not find tunnel for vpn addr: 10.222.0.9
+exit status 1
 
 $ print-cert -pretty 10.222.0.9
 Could not find tunnel for vpn addr: 10.222.0.9
+exit status 1
 
 $ print-cert 10.222.0.250
 Could not find tunnel for vpn addr: 10.222.0.250
+exit status 1
 
 $ print-cert -json 10.222.0.250
 Could not find tunnel for vpn addr: 10.222.0.250
+exit status 1
 
 $ print-cert -pretty 10.222.0.250
 Could not find tunnel for vpn addr: 10.222.0.250
+exit status 1
 
 $ print-cert not-an-address
 The provided vpn addr could not be parsed: not-an-address
+exit status 1
 
 $ print-cert -json not-an-address
 The provided vpn addr could not be parsed: not-an-address
+exit status 1
 
 $ print-cert -pretty not-an-address
 The provided vpn addr could not be parsed: not-an-address
+exit status 1
 
 $ print-tunnel
 No vpn address was provided
+exit status 1
 
 $ print-tunnel -json
 flag provided but not defined: -json
 Usage:
   -pretty
     	pretty prints json
+exit status 1
 
 $ print-tunnel -pretty
 No vpn address was provided
+exit status 1
 
 $ print-tunnel 10.222.0.9
 Could not find tunnel for vpn addr: 10.222.0.9
+exit status 1
 
 $ print-tunnel -json 10.222.0.9
 flag provided but not defined: -json
 Usage:
   -pretty
     	pretty prints json
+exit status 1
 
 $ print-tunnel -pretty 10.222.0.9
 Could not find tunnel for vpn addr: 10.222.0.9
+exit status 1
 
 $ print-tunnel 10.222.0.250
 Could not find tunnel for vpn addr: 10.222.0.250
+exit status 1
 
 $ print-tunnel -json 10.222.0.250
 flag provided but not defined: -json
 Usage:
   -pretty
     	pretty prints json
+exit status 1
 
 $ print-tunnel -pretty 10.222.0.250
 Could not find tunnel for vpn addr: 10.222.0.250
+exit status 1
 
 $ print-relays
 {"Relays":null}
@@ -227,6 +245,7 @@ flag provided but not defined: -json
 Usage:
   -pretty
     	pretty prints json
+exit status 1
 
 $ print-relays -pretty
 {
@@ -235,183 +254,230 @@ $ print-relays -pretty
 
 $ query-lighthouse
 No vpn address was provided
+exit status 1
 
 $ query-lighthouse -json
 The provided vpn address could not be parsed: -json
+exit status 1
 
 $ query-lighthouse -pretty
 The provided vpn address could not be parsed: -pretty
+exit status 1
 
 $ query-lighthouse 10.222.0.9
 {}
 
 $ query-lighthouse -json 10.222.0.9
 The provided vpn address could not be parsed: -json
+exit status 1
 
 $ query-lighthouse -pretty 10.222.0.9
 The provided vpn address could not be parsed: -pretty
+exit status 1
 
 $ query-lighthouse not-an-address
 The provided vpn address could not be parsed: not-an-address
+exit status 1
 
 $ query-lighthouse -json not-an-address
 The provided vpn address could not be parsed: -json
+exit status 1
 
 $ query-lighthouse -pretty not-an-address
 The provided vpn address could not be parsed: -pretty
+exit status 1
 
 $ log-level
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-level -json
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-level -pretty
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-level debug
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-level -json debug
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-level -pretty debug
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-level bogus
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-level -json bogus
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-level -pretty bogus
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-level info
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-level -json info
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-level -pretty info
 Log level is not reconfigurable on this logger
+exit status 1
 
 $ log-format
 Log format is not reconfigurable on this logger
+exit status 1
 
 $ log-format -json
 Log format is not reconfigurable on this logger
+exit status 1
 
 $ log-format -pretty
 Log format is not reconfigurable on this logger
+exit status 1
 
 $ log-format json
 Log format is not reconfigurable on this logger
+exit status 1
 
 $ log-format -json json
 Log format is not reconfigurable on this logger
+exit status 1
 
 $ log-format -pretty json
 Log format is not reconfigurable on this logger
+exit status 1
 
 $ log-format text
 Log format is not reconfigurable on this logger
+exit status 1
 
 $ log-format -json text
 Log format is not reconfigurable on this logger
+exit status 1
 
 $ log-format -pretty text
 Log format is not reconfigurable on this logger
+exit status 1
 
 $ change-remote
 No vpn address was provided
+exit status 1
 
 $ change-remote -json
 flag provided but not defined: -json
 Usage:
   -address string
     	The new remote address, ip:port
+exit status 1
 
 $ change-remote -pretty
 flag provided but not defined: -pretty
 Usage:
   -address string
     	The new remote address, ip:port
+exit status 1
 
 $ change-remote 10.222.0.9
 No address was provided
+exit status 1
 
 $ change-remote -json 10.222.0.9
 flag provided but not defined: -json
 Usage:
   -address string
     	The new remote address, ip:port
+exit status 1
 
 $ change-remote -pretty 10.222.0.9
 flag provided but not defined: -pretty
 Usage:
   -address string
     	The new remote address, ip:port
+exit status 1
 
 $ change-remote -address 10.94.0.10:4242 10.222.0.9
 Could not find tunnel for vpn address: 10.222.0.9
+exit status 1
 
 $ change-remote -json -address 10.94.0.10:4242 10.222.0.9
 flag provided but not defined: -json
 Usage:
   -address string
     	The new remote address, ip:port
+exit status 1
 
 $ change-remote -pretty -address 10.94.0.10:4242 10.222.0.9
 flag provided but not defined: -pretty
 Usage:
   -address string
     	The new remote address, ip:port
+exit status 1
 
 $ change-remote -address 10.94.0.10:4242 10.222.0.250
 Could not find tunnel for vpn address: 10.222.0.250
+exit status 1
 
 $ change-remote -json -address 10.94.0.10:4242 10.222.0.250
 flag provided but not defined: -json
 Usage:
   -address string
     	The new remote address, ip:port
+exit status 1
 
 $ change-remote -pretty -address 10.94.0.10:4242 10.222.0.250
 flag provided but not defined: -pretty
 Usage:
   -address string
     	The new remote address, ip:port
+exit status 1
 
 $ create-tunnel
 No vpn address was provided
+exit status 1
 
 $ create-tunnel -json
 flag provided but not defined: -json
 Usage:
   -address string
     	Optionally provide a real remote address, ip:port 
+exit status 1
 
 $ create-tunnel -pretty
 flag provided but not defined: -pretty
 Usage:
   -address string
     	Optionally provide a real remote address, ip:port 
+exit status 1
 
 $ create-tunnel 10.222.0.9
 Tunnel already handshaking
+exit status 1
 
 $ create-tunnel -json 10.222.0.9
 flag provided but not defined: -json
 Usage:
   -address string
     	Optionally provide a real remote address, ip:port 
+exit status 1
 
 $ create-tunnel -pretty 10.222.0.9
 flag provided but not defined: -pretty
 Usage:
   -address string
     	Optionally provide a real remote address, ip:port 
+exit status 1
 
 $ create-tunnel -address 10.94.0.10:4242 10.222.0.10
 Created
@@ -421,72 +487,86 @@ flag provided but not defined: -json
 Usage:
   -address string
     	Optionally provide a real remote address, ip:port 
+exit status 1
 
 $ create-tunnel -pretty -address 10.94.0.10:4242 10.222.0.10
 flag provided but not defined: -pretty
 Usage:
   -address string
     	Optionally provide a real remote address, ip:port 
+exit status 1
 
 $ create-tunnel not-an-address
 The provided vpn address could not be parsed: not-an-address
+exit status 1
 
 $ create-tunnel -json not-an-address
 flag provided but not defined: -json
 Usage:
   -address string
     	Optionally provide a real remote address, ip:port 
+exit status 1
 
 $ create-tunnel -pretty not-an-address
 flag provided but not defined: -pretty
 Usage:
   -address string
     	Optionally provide a real remote address, ip:port 
+exit status 1
 
 $ close-tunnel
 No vpn address was provided
+exit status 1
 
 $ close-tunnel -json
 flag provided but not defined: -json
 Usage:
   -local-only
     	Disables notifying the remote that the tunnel is shutting down
+exit status 1
 
 $ close-tunnel -pretty
 flag provided but not defined: -pretty
 Usage:
   -local-only
     	Disables notifying the remote that the tunnel is shutting down
+exit status 1
 
 $ close-tunnel 10.222.0.250
 Could not find tunnel for vpn address: 10.222.0.250
+exit status 1
 
 $ close-tunnel -json 10.222.0.250
 flag provided but not defined: -json
 Usage:
   -local-only
     	Disables notifying the remote that the tunnel is shutting down
+exit status 1
 
 $ close-tunnel -pretty 10.222.0.250
 flag provided but not defined: -pretty
 Usage:
   -local-only
     	Disables notifying the remote that the tunnel is shutting down
+exit status 1
 
 $ close-tunnel not-an-address
 The provided vpn address could not be parsed: not-an-address
+exit status 1
 
 $ close-tunnel -json not-an-address
 flag provided but not defined: -json
 Usage:
   -local-only
     	Disables notifying the remote that the tunnel is shutting down
+exit status 1
 
 $ close-tunnel -pretty not-an-address
 flag provided but not defined: -pretty
 Usage:
   -local-only
     	Disables notifying the remote that the tunnel is shutting down
+exit status 1
 
 $ help
 Available commands:
@@ -500,6 +580,7 @@ list-lighthouse-addrmap - List all lighthouse map entries
 list-pending-hostmap - List all handshaking hosts
 log-format - Gets or sets the current log format
 log-level - Gets or sets the current log level
+logout - Ends the current session
 print-cert - Prints the current certificate being used or the certificate for the provided vpn addr
 print-relays - Prints json details about all relay info
 print-tunnel - Prints json details about a tunnel for the provided vpn addr
@@ -510,9 +591,11 @@ version - Prints the currently running version of nebula
 
 $ help -json
 Command not available -json
+exit status 1
 
 $ help -pretty
 Command not available -pretty
+exit status 1
 
 $ help print-cert
 print-cert - Prints the current certificate being used or the certificate for the provided vpn addr
@@ -525,19 +608,113 @@ print-cert - Prints the current certificate being used or the certificate for th
 
 $ help -json print-cert
 Command not available -json
+exit status 1
 
 $ help -pretty print-cert
 Command not available -pretty
+exit status 1
+
+$ help logout
+logout - Ends the current session
+
+$ help -json logout
+Command not available -json
+exit status 1
+
+$ help -pretty logout
+Command not available -pretty
+exit status 1
+
+$ logout -h
+logout - Ends the current session
+
+$ logout -json -h
+logout - Ends the current session
+
+$ logout -pretty -h
+logout - Ends the current session
+
+$ not-a-command 10.222.0.9
+did not understand: not-a-command
+Available commands:
+change-remote - Changes the remote address used in the tunnel for the provided vpn addr
+close-tunnel - Closes a tunnel for the provided vpn addr
+create-tunnel - Creates a tunnel for the provided vpn address
+device-info - Prints information about the network device.
+help - prints available commands or help <command> for specific usage info
+list-hostmap - List all known previously connected hosts
+list-lighthouse-addrmap - List all lighthouse map entries
+list-pending-hostmap - List all handshaking hosts
+log-format - Gets or sets the current log format
+log-level - Gets or sets the current log level
+logout - Ends the current session
+print-cert - Prints the current certificate being used or the certificate for the provided vpn addr
+print-relays - Prints json details about all relay info
+print-tunnel - Prints json details about a tunnel for the provided vpn addr
+query-lighthouse - Query the lighthouses for the provided vpn address
+reload - Reloads configuration from disk, same as sending HUP to the process
+version - Prints the currently running version of nebula
+
+exit status 1
+
+$ not-a-command -json 10.222.0.9
+did not understand: not-a-command
+Available commands:
+change-remote - Changes the remote address used in the tunnel for the provided vpn addr
+close-tunnel - Closes a tunnel for the provided vpn addr
+create-tunnel - Creates a tunnel for the provided vpn address
+device-info - Prints information about the network device.
+help - prints available commands or help <command> for specific usage info
+list-hostmap - List all known previously connected hosts
+list-lighthouse-addrmap - List all lighthouse map entries
+list-pending-hostmap - List all handshaking hosts
+log-format - Gets or sets the current log format
+log-level - Gets or sets the current log level
+logout - Ends the current session
+print-cert - Prints the current certificate being used or the certificate for the provided vpn addr
+print-relays - Prints json details about all relay info
+print-tunnel - Prints json details about a tunnel for the provided vpn addr
+query-lighthouse - Query the lighthouses for the provided vpn address
+reload - Reloads configuration from disk, same as sending HUP to the process
+version - Prints the currently running version of nebula
+
+exit status 1
+
+$ not-a-command -pretty 10.222.0.9
+did not understand: not-a-command
+Available commands:
+change-remote - Changes the remote address used in the tunnel for the provided vpn addr
+close-tunnel - Closes a tunnel for the provided vpn addr
+create-tunnel - Creates a tunnel for the provided vpn address
+device-info - Prints information about the network device.
+help - prints available commands or help <command> for specific usage info
+list-hostmap - List all known previously connected hosts
+list-lighthouse-addrmap - List all lighthouse map entries
+list-pending-hostmap - List all handshaking hosts
+log-format - Gets or sets the current log format
+log-level - Gets or sets the current log level
+logout - Ends the current session
+print-cert - Prints the current certificate being used or the certificate for the provided vpn addr
+print-relays - Prints json details about all relay info
+print-tunnel - Prints json details about a tunnel for the provided vpn addr
+query-lighthouse - Query the lighthouses for the provided vpn address
+reload - Reloads configuration from disk, same as sending HUP to the process
+version - Prints the currently running version of nebula
+
+exit status 1
 
 $ reload
 Reloading config
 Config reload failed, the new config did not load: no config files found at 
+exit status 1
 
 $ reload -json
 Reloading config
 Config reload failed, the new config did not load: no config files found at 
+exit status 1
 
 $ reload -pretty
 Reloading config
 Config reload failed, the new config did not load: no config files found at 
+exit status 1
 

ssh_reload_test.go

@@ -7,6 +7,7 @@ import (
 	"path/filepath"
 	"testing"
 
+	"github.com/slackhq/nebula/commands"
 	"github.com/slackhq/nebula/config"
 	"github.com/slackhq/nebula/test"
 	"github.com/stretchr/testify/assert"
@@ -26,7 +27,7 @@ func (w *lineWriter) Write(s string) error      { return w.WriteLine(s) }
 func (w *lineWriter) WriteBytes(b []byte) error { return w.WriteLine(string(b)) }
 func (w *lineWriter) GetWriter() io.Writer      { return io.Discard }
 
-func TestSSHReloadReportsTheResult(t *testing.T) {
+func TestReloadCommandReportsTheResult(t *testing.T) {
 	dir := t.TempDir()
 	write := func(body string) {
 		require.NoError(t, os.WriteFile(filepath.Join(dir, "config.yml"), []byte(body), 0o600))
@@ -43,32 +44,42 @@ func TestSSHReloadReportsTheResult(t *testing.T) {
 		}
 	})
 
-	reload := func() []string {
+	// reload returns what the client was told. A reload that failed is a commands.Failure
+	// to the surface, exit 1 over ssh, with the result line as its message; one that
+	// merely needs a restart is not.
+	reload := func(failed bool) []string {
 		w := &lineWriter{}
-		require.NoError(t, sshReload(l, c, w))
+		err := reloadCommand(l, c, w)
+		if failed {
+			var f *commands.Failure
+			require.ErrorAs(t, err, &f)
+			assert.Equal(t, w.lines[len(w.lines)-1], f.Msg)
+		} else {
+			require.NoError(t, err)
+		}
 		return w.lines
 	}
 
 	write("a: 2\nb: 1")
-	assert.Equal(t, []string{"Reloading config", "Config reload complete"}, reload())
+	assert.Equal(t, []string{"Reloading config", "Config reload complete"}, reload(false))
 
 	fail = true
 	write("a: 3\nb: 1")
-	assert.Equal(t, []string{"Reloading config", "Config reload completed with errors, review the logs"}, reload())
+	assert.Equal(t, []string{"Reloading config", "Config reload completed with errors, review the logs"}, reload(true))
 
 	fail = false
 	write("a: 3\nb: 2")
-	assert.Equal(t, []string{"Reloading config", "Config reload was completed but requires a restart to fully apply, keys [b]"}, reload())
+	assert.Equal(t, []string{"Reloading config", "Config reload was completed but requires a restart to fully apply, keys [b]"}, reload(false))
 
 	write("a: [")
-	lines := reload()
+	lines := reload(true)
 	require.Len(t, lines, 2)
 	assert.Contains(t, lines[1], "Config reload failed, the new config did not load")
 	assert.Contains(t, lines[1], dir, "names the config that did not load")
 }
 
 // A client that went away still gets the reload it asked for
-func TestSSHReloadReloadsWhenTheClientIsGone(t *testing.T) {
+func TestReloadCommandReloadsWhenTheClientIsGone(t *testing.T) {
 	dir := t.TempDir()
 	require.NoError(t, os.WriteFile(filepath.Join(dir, "config.yml"), []byte("a: 1"), 0o600))
 	l := test.NewLogger()
@@ -78,6 +89,6 @@ func TestSSHReloadReloadsWhenTheClientIsGone(t *testing.T) {
 	c.RegisterReloadCallback(func(*config.C, *config.Reload) { reloaded = true })
 
 	gone := errors.New("client is gone")
-	require.ErrorIs(t, sshReload(l, c, &lineWriter{err: gone}), gone)
+	require.ErrorIs(t, reloadCommand(l, c, &lineWriter{err: gone}), gone)
 	assert.True(t, reloaded)
 }

sshd/session.go

@@ -1,6 +1,7 @@
 package sshd
 
 import (
+	"errors"
 	"log/slog"
 	"strings"
 
@@ -25,19 +26,25 @@ func NewSession(reg *commands.Registry, conn *ssh.ServerConn, chans <-chan ssh.N
 		cancel:   cancel,
 	}
 
-	s.commands.Register(&commands.Command{
+	s.commands.Register(LogoutCommand(s.Close))
-		Name:             commands.Logout,
-		ShortDescription: "Ends the current session",
-		Callback: func(a any, args []string, w commands.StringWriter) error {
-			s.Close()
-			return nil
-		},
-	})
 
 	go s.handleChannels(chans)
 	return s
 }
 
+// LogoutCommand is the logout command a session registers on its clone of the registry,
+// with end as what it does. Exported so a test can serve what a session serves.
+func LogoutCommand(end func()) *commands.Command {
+	return &commands.Command{
+		Name:             commands.Logout,
+		ShortDescription: "Ends the current session",
+		Callback: func(fs any, a []string, w commands.StringWriter) error {
+			end()
+			return nil
+		},
+	}
+}
+
 func (s *session) handleChannels(chans <-chan ssh.NewChannel) {
 	defer s.Close()
 	for newChannel := range chans {
@@ -84,9 +91,13 @@ func (s *session) handleRequests(in <-chan *ssh.Request, channel ssh.Channel) {
 			}
 
 			req.Reply(true, nil)
-			s.dispatchCommand(payload.Value, commands.NewStringWriter(channel))
+			err := s.dispatchCommand(payload.Value, commands.NewStringWriter(channel))
 
+			// A command that failed exits 1, so `ssh host cmd` callers can tell.
 			status := struct{ Status uint32 }{uint32(0)}
+			if err != nil {
+				status.Status = 1
+			}
 			channel.SendRequest("exit-status", false, ssh.Marshal(status))
 			channel.Close()
 			return
@@ -135,8 +146,20 @@ func (s *session) handleInput() {
 	}
 }
 
-func (s *session) dispatchCommand(line string, w commands.StringWriter) {
+// dispatchCommand runs line and returns the command's error, which it has logged unless
-	s.commands.Dispatch(line, w)
+// the command already told the user everything: the log line is for the operator reading
+// the daemon's log, who learns nothing from a failure the user caused.
+func (s *session) dispatchCommand(line string, w commands.StringWriter) error {
+	err := s.commands.Dispatch(line, w)
+	if err == nil {
+		return nil
+	}
+
+	var f *commands.Failure
+	if !errors.As(err, &f) {
+		s.l.Debug("Command returned an error", "line", line, "error", err)
+	}
+	return err
 }
 
 func (s *session) Close() {

sshd_reload_test.go

@@ -6,10 +6,12 @@ import (
 	"crypto/rand"
 	"crypto/rsa"
 	"encoding/pem"
+	"errors"
 	"fmt"
 	"net"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -24,6 +26,7 @@ import (
 
 type sshdReloadHarness struct {
 	c       *config.C
+	reg     *commands.Registry
 	server  *sshd.SSHServer
 	keyFile string
 	signer  ssh.Signer
@@ -45,7 +48,8 @@ func newSSHDReloadHarness(t *testing.T) *sshdReloadHarness {
 
 	h.c = config.NewC(l)
 	require.NoError(t, h.c.LoadString(h.config()))
-	h.server, err = sshd.NewSSHServer(t.Context(), l, commands.New())
+	h.reg = commands.New()
+	h.server, err = sshd.NewSSHServer(t.Context(), l, h.reg)
 	require.NoError(t, err)
 	wireSSHReload(l, h.server, h.c)
 	run, err := configSSH(l, h.server, h.c, nil)
@@ -239,3 +243,39 @@ func TestSSHDReload_SkippedEntriesFail(t *testing.T) {
 		})
 	}
 }
+
+// TestSSHExecExitStatusReflectsTheCommand: an exec request exits 1 when the command
+// returned an error and 0 otherwise, with the command's output either way.
+func TestSSHExecExitStatusReflectsTheCommand(t *testing.T) {
+	h := newSSHDReloadHarness(t)
+	h.reg.Register(&commands.Command{
+		Name: "fails",
+		Callback: func(fs any, a []string, w commands.StringWriter) error {
+			_ = w.WriteLine("nope")
+			return errors.New("failed")
+		},
+	})
+	client, _ := h.connect(t)
+
+	sess, err := client.NewSession()
+	require.NoError(t, err)
+	out, err := sess.CombinedOutput("fails")
+	var exit *ssh.ExitError
+	require.ErrorAs(t, err, &exit)
+	assert.Equal(t, 1, exit.ExitStatus())
+	assert.Equal(t, "nope\n", string(out))
+
+	sess, err = client.NewSession()
+	require.NoError(t, err)
+	out, err = sess.CombinedOutput("help fails")
+	require.NoError(t, err)
+	assert.Equal(t, "fails - \n", string(out))
+
+	// A word that is not a command is a failure too, with the command list as before.
+	sess, err = client.NewSession()
+	require.NoError(t, err)
+	out, err = sess.CombinedOutput("nope")
+	require.ErrorAs(t, err, &exit)
+	assert.Equal(t, 1, exit.ExitStatus())
+	assert.True(t, strings.HasPrefix(string(out), "did not understand: nope\nAvailable commands:\n"), string(out))
+}