cmd/tailscale/cli: add routecheck command

Introduce a new `tailscale routecheck` command which prints a report
of high-availability routers that are reachable.

This command rhymes with the `tailscale netcheck` command and but
instead of reporting on local network conditions, `routecheck` reports
on remote connectivity.

Updates #17366
Updates tailscale/corp#33033

Signed-off-by: Simon Law <sfllaw@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -211,6 +211,7 @@ func noDupFlagify(c *ffcli.Command) {
 var (
 	fileCmd,
 	sysPolicyCmd,
+	maybeRoutecheckCmd,
 	maybeWebCmd,
 	maybeDriveCmd,
 	maybeNetlockCmd,
@@ -252,6 +253,7 @@ change in the future.
 			configureCmd(),
 			nilOrCall(sysPolicyCmd),
 			netcheckCmd,
+			nilOrCall(maybeRoutecheckCmd),
 			ipCmd,
 			dnsCmd,
 			statusCmd,

cmd/tailscale/cli/netcheck.go

@@ -143,7 +143,7 @@ func runNetcheck(ctx context.Context, args []string) error {
 		if err != nil {
 			return fmt.Errorf("netcheck: %w", err)
 		}
-		if err := printReport(dm, report); err != nil {
+		if err := printNetCheckReport(dm, report); err != nil {
 			return err
 		}
 		if netcheckArgs.every == 0 {
@@ -153,7 +153,7 @@ func runNetcheck(ctx context.Context, args []string) error {
 	}
 }
 
-func printReport(dm *tailcfg.DERPMap, report *netcheck.Report) error {
+func printNetCheckReport(dm *tailcfg.DERPMap, report *netcheck.Report) error {
 	var j []byte
 	var err error
 	switch netcheckArgs.format {

cmd/tailscale/cli/routecheck.go

@@ -0,0 +1,101 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_routecheck
+
+package cli
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"strings"
+	"text/tabwriter"
+	"time"
+
+	jsonv2 "github.com/go-json-experiment/json"
+	"github.com/go-json-experiment/json/jsontext"
+	"github.com/peterbourgon/ff/v3/ffcli"
+	"tailscale.com/ipn/routecheck"
+)
+
+func init() {
+	maybeRoutecheckCmd = routecheckCmd
+}
+
+var routecheckCmd = func() *ffcli.Command {
+	return &ffcli.Command{
+		Name:       "routecheck",
+		ShortUsage: "tailscale routecheck",
+		ShortHelp:  "Print a reachability report for routes with multiple paths",
+		Exec:       runRoutecheck,
+		FlagSet:    routecheckFlagSet,
+	}
+}
+
+var routecheckFlagSet = func() *flag.FlagSet {
+	fs := newFlagSet("routecheck")
+	fs.BoolVar(&routecheckArgs.force, "force", false, "force probe to generate a new reachability report")
+	fs.StringVar(&routecheckArgs.format, "format", "", `output format: empty (for human-readable), "json" or "json-line"`)
+	return fs
+}()
+
+var routecheckArgs struct {
+	force  bool
+	format string
+}
+
+func runRoutecheck(ctx context.Context, args []string) error {
+	rp, err := localClient.RouteCheck(ctx, routecheckArgs.force)
+	if err != nil {
+		return fmt.Errorf("routecheck: %w", err)
+	}
+	if err := printRouteCheckReport(rp); err != nil {
+		return err
+	}
+	return nil
+}
+
+func printRouteCheckReport(rp *routecheck.Report) error {
+	var enc *jsontext.Encoder
+	switch routecheckArgs.format {
+	case "":
+	case "json":
+		enc = jsontext.NewEncoder(Stdout, jsontext.WithIndent("\t"))
+	case "json-line":
+		enc = jsontext.NewEncoder(Stdout, jsontext.Multiline(false))
+	default:
+		return fmt.Errorf("unknown output format %q", routecheckArgs.format)
+	}
+
+	routes := rp.RoutablePrefixes()
+
+	if enc != nil {
+		out := struct {
+			Done   time.Time               `json:"done"`
+			Routes routecheck.RoutingTable `json:"routes"`
+		}{
+			Done:   rp.Done,
+			Routes: routes,
+		}
+		if err := jsonv2.MarshalEncode(enc, out); err != nil {
+			return err
+		}
+		if _, err := Stdout.Write([]byte("\n")); err != nil {
+			return err
+		}
+		return nil
+	}
+
+	w := tabwriter.NewWriter(Stdout, 10, 5, 5, ' ', 0)
+	defer w.Flush()
+	fmt.Fprintf(w, "\nReachable routers at %s:\n", rp.Done.UTC().Format(time.DateTime+"Z07:00"))
+	fmt.Fprintf(w, "\n %s\t%s\t%s\t", "PREFIX", "IP", "HOSTNAME")
+	for prefix, nodes := range routes {
+		for _, n := range nodes {
+			fmt.Fprintf(w, "\n %s\t%s\t%s\t", prefix, n.Addr, strings.Trim(n.Name, "."))
+		}
+	}
+	fmt.Fprintln(w)
+	return nil
+}

cmd/tailscale/depaware.txt

@@ -201,7 +201,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/ipn                                            from tailscale.com/client/local+
         tailscale.com/ipn/conffile                                   from tailscale.com/cmd/tailscale/cli
         tailscale.com/ipn/ipnstate                                   from tailscale.com/client/local+
-        tailscale.com/ipn/routecheck                                 from tailscale.com/client/local
+        tailscale.com/ipn/routecheck                                 from tailscale.com/client/local+
         tailscale.com/kube/kubetypes                                 from tailscale.com/envknob
         tailscale.com/licenses                                       from tailscale.com/client/web+
         tailscale.com/metrics                                        from tailscale.com/tsweb+