Log config warnings in the logs, not on stderr

server/opts.go

@@ -511,6 +511,9 @@ type Options struct {
 
 	// configDigest represents the state of configuration.
 	configDigest string
+
+	// configWarnings holds warnings from config parsing to be logged once the server starts.
+	configWarnings []error
 }
 
 // WebsocketOpts are options for websocket
@@ -1028,6 +1031,9 @@ func (o *Options) processConfigFile(configFile string, m map[string]any) error {
 		}
 	}
 
+	// Store warnings in Options so they can be logged once the server starts.
+	o.configWarnings = warnings
+
 	if len(errors) > 0 || len(warnings) > 0 {
 		return &processConfigErr{
 			errors:   errors,
@@ -6186,8 +6192,8 @@ func ConfigureOptions(fs *flag.FlagSet, args []string, printVersion, printHelp,
 			if cerr, ok := err.(*processConfigErr); !ok || len(cerr.Errors()) != 0 {
 				return nil, err
 			}
-			// If we get here we only have warnings and can still continue
-			fmt.Fprint(os.Stderr, err)
+			// If we get here we only have warnings; they are stored in opts.configWarnings
+			// and will be logged when the server starts.
 		} else if opts.CheckConfig {
 			// Report configuration file syntax test was successful and exit.
 			return opts, nil

server/reload.go

@@ -1174,6 +1174,12 @@ func (s *Server) ReloadOptions(newOpts *Options) error {
 		return err
 	}
 
+	// Log any warnings from config parsing.
+	for _, warn := range newOpts.configWarnings {
+		s.Warnf("Configuration warning: %v", warn)
+	}
+	newOpts.configWarnings = nil
+
 	s.recheckPinnedCerts(curOpts, newOpts)
 
 	s.mu.Lock()

server/server.go

@@ -2332,6 +2332,12 @@ func (s *Server) Start() {
 		s.Noticef("Using configuration file: %s %s", opts.ConfigFile, cd)
 	}
 
+	// Log any warnings from config parsing.
+	for _, warn := range opts.configWarnings {
+		s.Warnf("Configuration warning: %v", warn)
+	}
+	opts.configWarnings = nil
+
 	hasOperators := len(opts.TrustedOperators) > 0
 	if hasOperators {
 		s.Noticef("Trusted Operators")