ssh: add WithBannerError

Co-Authored-By: Maisem Ali <maisem@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/server.go

@@ -301,6 +301,19 @@ func (s *connection) serverHandshake(config *ServerConfig) (*Permissions, error)
 	return perms, err
 }
 
+// WithBannerError is an error wrapper type that can be returned from an authentication
+// function to additionally write out a banner error message.
+type WithBannerError struct {
+	Err     error
+	Message string
+}
+
+func (e WithBannerError) Unwrap() error {
+	return e.Err
+}
+
+func (e WithBannerError) Error() string { return e.Err.Error() }
+
 func checkSourceAddress(addr net.Addr, sourceAddrs string) error {
 	if addr == nil {
 		return errors.New("ssh: no address known for client, but source-address match required")
@@ -678,6 +691,13 @@ userAuthLoop:
 			break userAuthLoop
 		}
 
+		var w WithBannerError
+		if errors.As(authErr, &w) && w.Message != "" {
+			bannerMsg := &userAuthBannerMsg{Message: w.Message}
+			if err := s.transport.writePacket(Marshal(bannerMsg)); err != nil {
+				return nil, err
+			}
+		}
 		if errors.Is(authErr, ErrDenied) {
 			var failureMsg userAuthFailureMsg
 			if config.ImplictAuthMethod != "" {