cmd/containerboot: load auth key from file if TS_AUTHKEY_FILE is set

This changes adds a new environment variable `TS_AUTHKEY_FILE`, that
can be used instead of `TS_AUTHKEY` to specify the auth key to use for
login. TS_AUTHKEY takes precedence over TS_AUTHKEY_FILE, if both are
set.

This is useful for cases where the auth key is stored in a docker secret
or similar, and makes it possible to avoid storing the auth key in a
docker compose file.

Fixes <https://github.com/tailscale/tailscale/issues/14210>

Signed-off-by: Josh McKinney <joshka@users.noreply.github.com>

cmd/containerboot/main.go

@@ -12,6 +12,7 @@
 // variables. All configuration is optional.
 //
 //   - TS_AUTHKEY: the authkey to use for login.
+//   - TS_AUTHKEY_FILE: the path to a file containing the authkey to use for login.
 //   - TS_HOSTNAME: the hostname to request for the node.
 //   - TS_ROUTES: subnet routes to advertise. Explicitly setting it to an empty
 //     value will cause containerboot to stop acting as a subnet router for any

cmd/containerboot/main_test.go

@@ -262,6 +262,29 @@ func TestContainerBoot(t *testing.T) {
 				},
 			}
 		},
+		"authkey_file": func(env *testEnv) testCase {
+			authFile := filepath.Join(env.d, "authkey.txt")
+			if err := os.WriteFile(authFile, []byte("tskey-key"), 0600); err != nil {
+				t.Fatalf("Failed to write authkey file: %v", err)
+			}
+			return testCase{
+				// Userspace mode, ephemeral storage, authkey provided via file.
+				Env: map[string]string{
+					"TS_AUTHKEY_FILE": authFile,
+				},
+				Phases: []phase{
+					{
+						WantCmds: []string{
+							"/usr/bin/tailscaled --socket=/tmp/tailscaled.sock --state=mem: --statedir=/tmp --tun=userspace-networking",
+							"/usr/bin/tailscale --socket=/tmp/tailscaled.sock up --accept-dns=false --authkey=tskey-key",
+						},
+					},
+					{
+						Notify: runningNotify,
+					},
+				},
+			}
+		},
 		"authkey_disk_state": func(env *testEnv) testCase {
 			return testCase{
 				Env: map[string]string{

cmd/containerboot/settings.go

@@ -85,7 +85,7 @@ type settings struct {
 
 func configFromEnv() (*settings, error) {
 	cfg := &settings{
-		AuthKey:                               defaultEnvs([]string{"TS_AUTHKEY", "TS_AUTH_KEY"}, ""),
+		AuthKey:                               defaultEnvs([]string{"TS_AUTHKEY", "TS_AUTH_KEY", "TS_AUTHKEY_FILE", "TS_AUTH_KEY_FILE"}, ""),
 		Hostname:                              defaultEnv("TS_HOSTNAME", ""),
 		Routes:                                defaultEnvStringPointer("TS_ROUTES"),
 		ServeConfigPath:                       defaultEnv("TS_SERVE_CONFIG", ""),
@@ -365,9 +365,21 @@ func defaultEnvBoolPointer(name string) *bool {
 	return &ret
 }
 
+// defaultEnvs returns the value of the first envvar in names that is set,
+// or defVal if none are set. If the envvar ends with "_FILE", it reads the
+// value from the file specified by the envvar.
 func defaultEnvs(names []string, defVal string) string {
 	for _, name := range names {
-		if v, ok := os.LookupEnv(name); ok {
+		if strings.HasSuffix(name, "_FILE") {
+			if filepath, ok := os.LookupEnv(name); ok {
+				data, err := os.ReadFile(filepath)
+				if err != nil {
+					log.Printf("error reading env var %s from file %s: %v", name, filepath, err)
+					continue
+				}
+				return strings.TrimSpace(string(data))
+			}
+		} else if v, ok := os.LookupEnv(name); ok {
 			return v
 		}
 	}