Improved searching for provider-metadata.json

* Changes order to try the DNS path after security.txt.
* Add diagnostic output which URL is looked for.

cmd/csaf_checker/processor.go

@@ -729,6 +729,7 @@ func (p *processor) locateProviderMetadata(
 	client := p.httpClient()
 
 	tryURL := func(url string) (bool, error) {
+		log.Printf("Trying: %v\n", url)
 		res, err := client.Get(url)
 		if err != nil || res.StatusCode != http.StatusOK ||
 			res.Header.Get("Content-Type") != "application/json" {
@@ -762,33 +763,39 @@ func (p *processor) locateProviderMetadata(
 	// Read from security.txt
 
 	path := "https://" + domain + "/.well-known/security.txt"
+	log.Printf("Searching in: %v\n", path)
 	res, err := client.Get(path)
-	if err != nil {
-		return err
-	}
-
-	if res.StatusCode != http.StatusOK {
-		return nil
-	}
-
+	if err == nil && res.StatusCode == http.StatusOK {
 		loc, err := func() (string, error) {
 			defer res.Body.Close()
 			return p.extractProviderURL(res.Body)
 		}()
 
 		if err != nil {
-		log.Printf("error: %v\n", err)
-		return nil
+			log.Printf("did not find provider URL in /.well-known/security.txt, error: %v\n", err)
 		}
 
 		if loc != "" {
 			if _, err = tryURL(loc); err == errContinue {
 				err = nil
 			}
+			return err
+		}
 	}
 
+	// Read from DNS path
+
+	path = "https://csaf.data.security." + domain
+	ok, err := tryURL(path)
+	if err != nil {
 		return err
 	}
+	if ok {
+		return nil
+	}
+
+	return errStop
+}
 
 func (p *processor) extractProviderURL(r io.Reader) (string, error) {
 	urls, err := csaf.ExtractProviderURL(r, true)