cmd/tailscale/cli: make cert fetch registration automatic, show valid domains

Updates #1235

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cert.go

@@ -23,11 +23,11 @@
 	"log"
 	"os"
 	"path/filepath"
-	"strconv"
 	"strings"
 
 	"golang.org/x/crypto/acme"
 	"tailscale.com/client/tailscale"
+	"tailscale.com/ipn/ipnstate"
 )
 
 func jout(v interface{}) {
@@ -38,7 +38,45 @@ func jout(v interface{}) {
 	fmt.Printf("%T: %s\n", v, j)
 }
 
+func checkCertDomain(st *ipnstate.Status, domain string) error {
+	if domain == "" {
+		return errors.New("missing domain name")
+	}
+	for _, d := range st.CertDomains {
+		if d == domain {
+			return nil
+		}
+	}
+	// Transitional way while server doesn't yet populate CertDomains: also permit the client
+	// attempting Self.DNSName.
+	okay := st.CertDomains[:len(st.CertDomains):len(st.CertDomains)]
+	if st.Self != nil {
+		if v := strings.Trim(st.Self.DNSName, "."); v != "" {
+			if v == domain {
+				return nil
+			}
+			okay = append(okay, v)
+		}
+	}
+	switch len(okay) {
+	case 0:
+		return errors.New("your Tailscale account does not support getting TLS certs")
+	case 1:
+		return fmt.Errorf("invalid domain %q; only %q is permitted", domain, okay[0])
+	default:
+		return fmt.Errorf("invalid domain %q; must be one of %q", domain, okay)
+	}
+}
+
 func debugGetCert(ctx context.Context, cert string) error {
+	st, err := tailscale.Status(ctx)
+	if err != nil {
+		return fmt.Errorf("getting tailscale status: %w", err)
+	}
+	if err := checkCertDomain(st, cert); err != nil {
+		return err
+	}
+
 	key, err := acmeKey()
 	if err != nil {
 		return err
@@ -46,18 +84,31 @@ func debugGetCert(ctx context.Context, cert string) error {
 	ac := &acme.Client{
 		Key: key,
 	}
-	d, err := ac.Discover(ctx)
-	if err != nil {
-		return err
-	}
-	jout(d)
 
-	if reg, _ := strconv.ParseBool(os.Getenv("TS_DEBUG_ACME_REGISTER")); reg {
-		acct, err := ac.Register(ctx, new(acme.Account), acme.AcceptTOS)
-		if err != nil {
-			return fmt.Errorf("Register: %v", err)
+	logf := log.Printf
+
+	a, err := ac.GetReg(ctx, "unused")
+	switch {
+	case err == nil:
+		// Great, already registered.
+		logf("Already had ACME account.")
+	case err == acme.ErrNoAccount:
+		a, err = ac.Register(ctx, new(acme.Account), acme.AcceptTOS)
+		if err == acme.ErrAccountAlreadyExists {
+			// Potential race. Double check.
+			a, err = ac.GetReg(ctx, "unused")
 		}
-		jout(acct)
+		if err != nil {
+			return fmt.Errorf("acme.Register: %w", err)
+		}
+		logf("Registered ACME account.")
+		jout(a)
+	default:
+		return fmt.Errorf("acme.GetReg: %w", err)
+
+	}
+	if a.Status != acme.StatusValid {
+		return fmt.Errorf("unexpected ACME account status %q", a.Status)
 	}
 
 	order, err := ac.AuthorizeOrder(ctx, []acme.AuthzID{{Type: "dns", Value: cert}})