Use server returned TTL for cache, instead of 600

transport/internet/tls/ech.go

@@ -26,7 +26,7 @@ func ApplyECH(c *Config, config *tls.Config) error {
 		if err != nil {
 			return errors.New("invalid ECH config")
 		}
-	} else {
+	} else { // ECH config > DOH lookup
 		if c.ServerName == "" {
 			return errors.New("Using DOH for ECH needs serverName")
 		}
@@ -58,54 +58,58 @@ func QueryRecord(domain string, server string) (string, error) {
 	}
 	mutex.Lock()
 	defer mutex.Unlock()
-	record, err := dohQuery(server, domain)
+	record, ttl, err := dohQuery(server, domain)
 	if err != nil {
 		return "", err
 	}
+	// Use TTL for good, but many HTTPS records have TTL 60, too short
+	if ttl < 600 {
+		ttl = 600
+	}
 	rec.record = record
-	rec.expire = time.Now().Add(time.Second * 600)
+	rec.expire = time.Now().Add(time.Second * time.Duration(ttl))
 	dnsCache[domain] = rec
 	return record, nil
 }
 
-func dohQuery(server string, domain string) (string, error) {
+func dohQuery(server string, domain string) (string, uint32, error) {
 	m := new(dns.Msg)
 	m.SetQuestion(dns.Fqdn(domain), dns.TypeHTTPS)
 	msg, err := m.Pack()
 	if err != nil {
-		return "", err
+		return "", 0, err
 	}
 	client := &http.Client{
 		Timeout: 5 * time.Second,
 	}
 	req, err := http.NewRequest("POST", server, bytes.NewReader(msg))
 	if err != nil {
-		return "", err
+		return "", 0, err
 	}
 	req.Header.Set("Content-Type", "application/dns-message")
 	resp, err := client.Do(req)
 	if err != nil {
-		return "", err
+		return "", 0, err
 	}
 	defer resp.Body.Close()
 	respBody, err := io.ReadAll(resp.Body)
 	if err != nil {
-		return "", err
+		return "", 0, err
 	}
 	if resp.StatusCode != http.StatusOK {
-		return "", errors.New("query failed with response code:", resp.StatusCode)
+		return "", 0, errors.New("query failed with response code:", resp.StatusCode)
 	}
 	respMsg := new(dns.Msg)
 	err = respMsg.Unpack(respBody)
 	if err != nil {
-		return "", err
+		return "", 0, err
 	}
 	if len(respMsg.Answer) > 0 {
 		re := regexp.MustCompile(`ech="([^"]+)"`)
 		match := re.FindStringSubmatch(respMsg.Answer[0].String())
 		if match[1] != "" {
-			return match[1], nil
+			return match[1], respMsg.Answer[0].Header().Ttl, nil
 		}
 	}
-	return "", errors.New("no ech record found")
+	return "", 0, errors.New("no ech record found")
 }