cmd/get-authkey: add an OAuth API client to produce an authkey

Updates https://github.com/tailscale/tailscale/issues/3243

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

cmd/get-authkey/.gitignore

@@ -0,0 +1 @@
+get-authkey

cmd/get-authkey/main.go

@@ -0,0 +1,72 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// get-authkey allocates an authkey using an OAuth API client
+// https://tailscale.com/kb/1215/oauth-clients/ and prints it
+// to stdout for scripts to capture and use.
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log"
+	"os"
+	"strings"
+
+	"golang.org/x/oauth2/clientcredentials"
+	"tailscale.com/client/tailscale"
+)
+
+func main() {
+	// Required to use our client API. We're fine with the instability since the
+	// client lives in the same repo as this code.
+	tailscale.I_Acknowledge_This_API_Is_Unstable = true
+
+	reusable := flag.Bool("reusable", false, "allocate a reusable authkey")
+	ephemeral := flag.Bool("ephemeral", false, "allocate an ephemeral authkey")
+	preauth := flag.Bool("preauth", true, "set the authkey as pre-authorized")
+	tags := flag.String("tags", "", "comma-separated list of tags to apply to the authkey")
+	flag.Parse()
+
+	clientId := os.Getenv("TS_API_CLIENT_ID")
+	clientSecret := os.Getenv("TS_API_CLIENT_SECRET")
+	if clientId == "" || clientSecret == "" {
+		log.Fatal("TS_API_CLIENT_ID and TS_API_CLIENT_SECRET must be set")
+	}
+
+	baseUrl := os.Getenv("TS_BASE_URL")
+	if baseUrl == "" {
+		baseUrl = "https://api.tailscale.com"
+	}
+
+	credentials := clientcredentials.Config{
+		ClientID:     clientId,
+		ClientSecret: clientSecret,
+		TokenURL:     baseUrl + "/api/v2/oauth/token",
+		Scopes:       []string{"device"},
+	}
+
+	ctx := context.Background()
+	tsClient := tailscale.NewClient("-", nil)
+	tsClient.HTTPClient = credentials.Client(ctx)
+	tsClient.BaseURL = baseUrl
+
+	caps := tailscale.KeyCapabilities{
+		Devices: tailscale.KeyDeviceCapabilities{
+			Create: tailscale.KeyDeviceCreateCapabilities{
+				Reusable:      *reusable,
+				Ephemeral:     *ephemeral,
+				Preauthorized: *preauth,
+				Tags:          strings.Split(*tags, ","),
+			},
+		},
+	}
+
+	authkey, _, err := tsClient.CreateKey(ctx, caps)
+	if err != nil {
+		log.Fatal(err.Error())
+	}
+
+	fmt.Println(authkey)
+}