cmd/tailscale/cli: add "configure kubeconfig" subcommand

It takes in a node hostname and configures the local kubeconfig file to
point to that.

Updates #7220

Signed-off-by: Maisem Ali <maisem@tailscale.com>

5 files changed, 361 insertions, 4 deletions

diff --git a/cmd/tailscale/cli/configure-kube.go b/cmd/tailscale/cli/configure-kube.go
new file mode 100644
index 000000000..65ff85e14
--- /dev/null
+++ b/cmd/tailscale/cli/configure-kube.go
@@ -0,0 +1,152 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+//go:build !ts_omit_kube
+
+package cli
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/peterbourgon/ff/v3/ffcli"
+	"golang.org/x/exp/slices"
+	"k8s.io/client-go/util/homedir"
+	"sigs.k8s.io/yaml"
+)
+
+func init() {
+	configureCmd.Subcommands = append(configureCmd.Subcommands, configureKubeconfigCmd)
+}
+
+var configureKubeconfigCmd = &ffcli.Command{
+	Name:       "kubeconfig",
+	ShortHelp:  "Configure kubeconfig to use Tailscale",
+	ShortUsage: "kubeconfig <hostname-or-fqdn>",
+	LongHelp: strings.TrimSpace(`
+Run this command to configure your kubeconfig to use Tailscale for authentication to a Kubernetes cluster.
+
+The hostname argument should be set to the Tailscale hostname of the peer running as an auth proxy in the cluster.
+`),
+	FlagSet: (func() *flag.FlagSet {
+		fs := newFlagSet("kubeconfig")
+		return fs
+	})(),
+	Exec: runConfigureKubeconfig,
+}
+
+func runConfigureKubeconfig(ctx context.Context, args []string) error {
+	if len(args) != 1 {
+		return errors.New("unknown arguments")
+	}
+	hostOrFQDN := args[0]
+
+	st, err := localClient.Status(ctx)
+	if err != nil {
+		return err
+	}
+	if st.BackendState != "Running" {
+		return errors.New("Tailscale is not running")
+	}
+	targetFQDN, ok := nodeDNSNameFromArg(st, hostOrFQDN)
+	if !ok {
+		return fmt.Errorf("no peer found with hostname %q", hostOrFQDN)
+	}
+	targetFQDN = strings.TrimSuffix(targetFQDN, ".")
+	confPath := filepath.Join(homedir.HomeDir(), ".kube", "config")
+	if err := setKubeconfigForPeer(targetFQDN, confPath); err != nil {
+		return err
+	}
+	printf("kubeconfig configured for %q\n", hostOrFQDN)
+	return nil
+}
+
+// appendOrSetNamed finds a map with a "name" key matching name in dst, and
+// replaces it with val. If no such map is found, val is appended to dst.
+func appendOrSetNamed(dst []any, name string, val map[string]any) []any {
+	if got := slices.IndexFunc(dst, func(m any) bool {
+		if m, ok := m.(map[string]any); ok {
+			return m["name"] == name
+		}
+		return false
+	}); got != -1 {
+		dst[got] = val
+	} else {
+		dst = append(dst, val)
+	}
+	return dst
+}
+
+var errInvalidKubeconfig = errors.New("invalid kubeconfig")
+
+func updateKubeconfig(cfgYaml []byte, fqdn string) ([]byte, error) {
+	var cfg map[string]any
+	if len(cfgYaml) > 0 {
+		if err := yaml.Unmarshal(cfgYaml, &cfg); err != nil {
+			return nil, errInvalidKubeconfig
+		}
+	}
+	if cfg == nil {
+		cfg = map[string]any{
+			"apiVersion": "v1",
+			"kind":       "Config",
+		}
+	} else if cfg["apiVersion"] != "v1" || cfg["kind"] != "Config" {
+		return nil, errInvalidKubeconfig
+	}
+
+	var clusters []any
+	if cm, ok := cfg["clusters"]; ok {
+		clusters = cm.([]any)
+	}
+	cfg["clusters"] = appendOrSetNamed(clusters, fqdn, map[string]any{
+		"name": fqdn,
+		"cluster": map[string]string{
+			"server": "https://" + fqdn,
+		},
+	})
+
+	var users []any
+	if um, ok := cfg["users"]; ok {
+		users = um.([]any)
+	}
+	cfg["users"] = appendOrSetNamed(users, "tailscale-auth", map[string]any{
+		// We just need one of these, and can reuse it for all clusters.
+		"name": "tailscale-auth",
+		"user": map[string]string{
+			// We do not use the token, but if we do not set anything here
+			// kubectl will prompt for a username and password.
+			"token": "unused",
+		},
+	})
+
+	var contexts []any
+	if cm, ok := cfg["contexts"]; ok {
+		contexts = cm.([]any)
+	}
+	cfg["contexts"] = appendOrSetNamed(contexts, fqdn, map[string]any{
+		"name": fqdn,
+		"context": map[string]string{
+			"cluster": fqdn,
+			"user":    "tailscale-auth",
+		},
+	})
+	cfg["current-context"] = fqdn
+	return yaml.Marshal(cfg)
+}
+
+func setKubeconfigForPeer(fqdn, filePath string) error {
+	b, err := os.ReadFile(filePath)
+	if err != nil && !os.IsNotExist(err) {
+		return err
+	}
+	b, err = updateKubeconfig(b, fqdn)
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(filePath, b, 0600)
+}
diff --git a/cmd/tailscale/cli/configure-kube_test.go b/cmd/tailscale/cli/configure-kube_test.go
new file mode 100644
index 000000000..0f326cd64
--- /dev/null
+++ b/cmd/tailscale/cli/configure-kube_test.go
@@ -0,0 +1,196 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+//go:build !ts_omit_kube
+
+package cli
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestKubeconfig(t *testing.T) {
+	const fqdn = "foo.tail-scale.ts.net"
+	tests := []struct {
+		name    string
+		in      string
+		want    string
+		wantErr error
+	}{
+		{
+			name: "invalid-yaml",
+			in: `apiVersion: v1
+kind: ,asdf`,
+			wantErr: errInvalidKubeconfig,
+		},
+		{
+			name: "invalid-cfg",
+			in: `apiVersion: v1
+kind: Pod`,
+			wantErr: errInvalidKubeconfig,
+		},
+		{
+			name: "empty",
+			in:   "",
+			want: `apiVersion: v1
+clusters:
+- cluster:
+    server: https://foo.tail-scale.ts.net
+  name: foo.tail-scale.ts.net
+contexts:
+- context:
+    cluster: foo.tail-scale.ts.net
+    user: tailscale-auth
+  name: foo.tail-scale.ts.net
+current-context: foo.tail-scale.ts.net
+kind: Config
+users:
+- name: tailscale-auth
+  user:
+    token: unused`,
+		},
+		{
+			name: "already-configured",
+			in: `apiVersion: v1
+clusters:
+- cluster:
+    server: https://foo.tail-scale.ts.net
+  name: foo.tail-scale.ts.net
+contexts:
+- context:
+    cluster: foo.tail-scale.ts.net
+    user: tailscale-auth
+  name: foo.tail-scale.ts.net
+kind: Config
+current-context: foo.tail-scale.ts.net
+users:
+- name: tailscale-auth
+  user:
+    token: unused`,
+			want: `apiVersion: v1
+clusters:
+- cluster:
+    server: https://foo.tail-scale.ts.net
+  name: foo.tail-scale.ts.net
+contexts:
+- context:
+    cluster: foo.tail-scale.ts.net
+    user: tailscale-auth
+  name: foo.tail-scale.ts.net
+current-context: foo.tail-scale.ts.net
+kind: Config
+users:
+- name: tailscale-auth
+  user:
+    token: unused`,
+		},
+		{
+			name: "other-cluster",
+			in: `apiVersion: v1
+clusters:
+- cluster:
+    server: https://192.168.1.1:8443
+  name: some-cluster
+contexts:
+- context:
+    cluster: some-cluster
+    user: some-auth
+  name: some-cluster
+kind: Config
+current-context: some-cluster
+users:
+- name: some-auth
+  user:
+    token: asdfasdf`,
+			want: `apiVersion: v1
+clusters:
+- cluster:
+    server: https://192.168.1.1:8443
+  name: some-cluster
+- cluster:
+    server: https://foo.tail-scale.ts.net
+  name: foo.tail-scale.ts.net
+contexts:
+- context:
+    cluster: some-cluster
+    user: some-auth
+  name: some-cluster
+- context:
+    cluster: foo.tail-scale.ts.net
+    user: tailscale-auth
+  name: foo.tail-scale.ts.net
+current-context: foo.tail-scale.ts.net
+kind: Config
+users:
+- name: some-auth
+  user:
+    token: asdfasdf
+- name: tailscale-auth
+  user:
+    token: unused`,
+		},
+		{
+			name: "already-using-tailscale",
+			in: `apiVersion: v1
+clusters:
+- cluster:
+    server: https://bar.tail-scale.ts.net
+  name: bar.tail-scale.ts.net
+contexts:
+- context:
+    cluster: bar.tail-scale.ts.net
+    user: tailscale-auth
+  name: bar.tail-scale.ts.net
+kind: Config
+current-context: bar.tail-scale.ts.net
+users:
+- name: tailscale-auth
+  user:
+    token: unused`,
+			want: `apiVersion: v1
+clusters:
+- cluster:
+    server: https://bar.tail-scale.ts.net
+  name: bar.tail-scale.ts.net
+- cluster:
+    server: https://foo.tail-scale.ts.net
+  name: foo.tail-scale.ts.net
+contexts:
+- context:
+    cluster: bar.tail-scale.ts.net
+    user: tailscale-auth
+  name: bar.tail-scale.ts.net
+- context:
+    cluster: foo.tail-scale.ts.net
+    user: tailscale-auth
+  name: foo.tail-scale.ts.net
+current-context: foo.tail-scale.ts.net
+kind: Config
+users:
+- name: tailscale-auth
+  user:
+    token: unused`,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := updateKubeconfig([]byte(tt.in), fqdn)
+			if err != nil {
+				if err != tt.wantErr {
+					t.Fatalf("updateKubeconfig() error = %v, wantErr %v", err, tt.wantErr)
+				}
+				return
+			} else if tt.wantErr != nil {
+				t.Fatalf("updateKubeconfig() error = %v, wantErr %v", err, tt.wantErr)
+			}
+			got = bytes.TrimSpace(got)
+			want := []byte(strings.TrimSpace(tt.want))
+			if d := cmp.Diff(want, got); d != "" {
+				t.Errorf("Kubeconfig() mismatch (-want +got):\n%s", d)
+			}
+		})
+	}
+}
diff --git a/cmd/tailscale/cli/configure-synology.go b/cmd/tailscale/cli/configure-synology.go
index 3c3ed8e14..86c739f2e 100644
--- a/cmd/tailscale/cli/configure-synology.go
+++ b/cmd/tailscale/cli/configure-synology.go
@@ -18,9 +18,9 @@ import (
 	"tailscale.com/version/distro"
 )
 
-// configureHostCmd is the "tailscale configure-host" command which
-// was once used to configure Synology devices, but is now a redirect
-// to "tailscale configure synology".
+// configureHostCmd is the "tailscale configure-host" command which was once
+// used to configure Synology devices, but is now a compatibility alias to
+// "tailscale configure synology".
 var configureHostCmd = &ffcli.Command{
 	Name:      "configure-host",
 	Exec:      runConfigureSynology,
diff --git a/cmd/tailscale/cli/configure.go b/cmd/tailscale/cli/configure.go
index f8aaad449..a40039081 100644
--- a/cmd/tailscale/cli/configure.go
+++ b/cmd/tailscale/cli/configure.go
@@ -7,13 +7,19 @@ import (
 	"context"
 	"flag"
 	"runtime"
+	"strings"
 
 	"github.com/peterbourgon/ff/v3/ffcli"
 	"tailscale.com/version/distro"
 )
 
 var configureCmd = &ffcli.Command{
-	Name: "configure",
+	Name:      "configure",
+	ShortHelp: "Configure the host to enable more Tailscale features",
+	LongHelp: strings.TrimSpace(`
+The 'configure' command is intended to provide a way to configure different
+services on the host to enable more Tailscale features.
+`),
 	FlagSet: (func() *flag.FlagSet {
 		fs := newFlagSet("configure")
 		return fs
diff --git a/cmd/tailscale/depaware.txt b/cmd/tailscale/depaware.txt
index 98c5c4b00..a926d9eae 100644
--- a/cmd/tailscale/depaware.txt
+++ b/cmd/tailscale/depaware.txt
@@ -40,9 +40,12 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
      💣 go4.org/mem                                                  from tailscale.com/derp+
         go4.org/netipx                                               from tailscale.com/wgengine/filter
    W 💣 golang.zx2c4.com/wireguard/windows/tunnel/winipcfg           from tailscale.com/net/interfaces+
+        gopkg.in/yaml.v2                                             from sigs.k8s.io/yaml
+        k8s.io/client-go/util/homedir                                from tailscale.com/cmd/tailscale/cli
         nhooyr.io/websocket                                          from tailscale.com/derp/derphttp+
         nhooyr.io/websocket/internal/errd                            from nhooyr.io/websocket
         nhooyr.io/websocket/internal/xsync                           from nhooyr.io/websocket
+        sigs.k8s.io/yaml                                             from tailscale.com/cmd/tailscale/cli
         software.sslmate.com/src/go-pkcs12                           from tailscale.com/cmd/tailscale/cli
         software.sslmate.com/src/go-pkcs12/internal/rc2              from software.sslmate.com/src/go-pkcs12
         tailscale.com                                                from tailscale.com/version