cmd/tailscale/cli/web: add QNAP NAS_SID authentication

QTS 5.0 doesn't always pass a qtoken, in some circumstances
it sends a NAS_SID cookie for us to verify instead.

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

cmd/tailscale/cli/web.go

@@ -208,12 +208,20 @@ func qnapAuthn(r *http.Request) (string, *qnapAuthResponse, error) {
 		return "", nil, err
 	}
 	token, err := r.Cookie("qtoken")
-	if err != nil {
-		return "", nil, err
+	if err == nil {
+		return qnapAuthnQtoken(r, user.Value, token.Value)
+	}
+	sid, err := r.Cookie("NAS_SID")
+	if err == nil {
+		return qnapAuthnSid(r, user.Value, sid.Value)
 	}
+	return "", nil, fmt.Errorf("not authenticated by any mechanism")
+}
+
+func qnapAuthnQtoken(r *http.Request, user, token string) (string, *qnapAuthResponse, error) {
 	query := url.Values{
-		"qtoken": []string{token.Value},
-		"user":   []string{user.Value},
+		"qtoken": []string{token},
+		"user":   []string{user},
 	}
 	u := url.URL{
 		Scheme:   r.URL.Scheme,
@@ -221,7 +229,26 @@ func qnapAuthn(r *http.Request) (string, *qnapAuthResponse, error) {
 		Path:     "/cgi-bin/authLogin.cgi",
 		RawQuery: query.Encode(),
 	}
-	resp, err := http.Get(u.String())
+
+	return qnapAuthnFinish(user, u.String())
+}
+
+func qnapAuthnSid(r *http.Request, user, sid string) (string, *qnapAuthResponse, error) {
+	query := url.Values{
+		"sid": []string{sid},
+	}
+	u := url.URL{
+		Scheme:   r.URL.Scheme,
+		Host:     r.URL.Host,
+		Path:     "/cgi-bin/authLogin.cgi",
+		RawQuery: query.Encode(),
+	}
+
+	return qnapAuthnFinish(user, u.String())
+}
+
+func qnapAuthnFinish(user, url string) (string, *qnapAuthResponse, error) {
+	resp, err := http.Get(url)
 	if err != nil {
 		return "", nil, err
 	}
@@ -237,7 +264,7 @@ func qnapAuthn(r *http.Request) (string, *qnapAuthResponse, error) {
 	if authResp.AuthPassed == 0 {
 		return "", nil, fmt.Errorf("not authenticated")
 	}
-	return user.Value, authResp, nil
+	return user, authResp, nil
 }
 
 func synoAuthn() (string, error) {