diff --git a/irc/client.go b/irc/client.go
index 8d4f7827..4f4f4cb0 100644
--- a/irc/client.go
+++ b/irc/client.go
@@ -111,6 +111,8 @@ type Session struct {
 	capState     caps.State
 	capVersion   caps.Version
 
+	registrationMessages int
+
 	resumeID         string
 	resumeDetails    *ResumeDetails
 	zncPlaybackTimes *zncPlaybackTimes
@@ -396,6 +398,15 @@ func (client *Client) run(session *Session) {
 			}
 		}
 
+		// DoS hardening, #505
+		if !client.registered {
+			session.registrationMessages++
+			if client.server.Config().Limits.RegistrationMessages < session.registrationMessages {
+				client.Send(nil, client.server.name, ERR_UNKNOWNERROR, "*", client.t("You have sent too many registration messages"))
+				break
+			}
+		}
+
 		msg, err := ircmsg.ParseLineStrict(line, true, maxlenRest)
 		if err == ircmsg.ErrorLineIsEmpty {
 			continue
diff --git a/irc/config.go b/irc/config.go
index 965aa9d5..5750276f 100644
--- a/irc/config.go
+++ b/irc/config.go
@@ -214,16 +214,17 @@ type LineLenLimits struct {
 
 // Various server-enforced limits on data size.
 type Limits struct {
-	AwayLen        int           `yaml:"awaylen"`
-	ChanListModes  int           `yaml:"chan-list-modes"`
-	ChannelLen     int           `yaml:"channellen"`
-	IdentLen       int           `yaml:"identlen"`
-	KickLen        int           `yaml:"kicklen"`
-	LineLen        LineLenLimits `yaml:"linelen"`
-	MonitorEntries int           `yaml:"monitor-entries"`
-	NickLen        int           `yaml:"nicklen"`
-	TopicLen       int           `yaml:"topiclen"`
-	WhowasEntries  int           `yaml:"whowas-entries"`
+	AwayLen              int           `yaml:"awaylen"`
+	ChanListModes        int           `yaml:"chan-list-modes"`
+	ChannelLen           int           `yaml:"channellen"`
+	IdentLen             int           `yaml:"identlen"`
+	KickLen              int           `yaml:"kicklen"`
+	LineLen              LineLenLimits `yaml:"linelen"`
+	MonitorEntries       int           `yaml:"monitor-entries"`
+	NickLen              int           `yaml:"nicklen"`
+	TopicLen             int           `yaml:"topiclen"`
+	WhowasEntries        int           `yaml:"whowas-entries"`
+	RegistrationMessages int           `yaml:"registration-messages"`
 }
 
 // STSConfig controls the STS configuration/
@@ -532,6 +533,9 @@ func LoadConfig(filename string) (config *Config, err error) {
 	if config.Limits.NickLen < 1 || config.Limits.ChannelLen < 2 || config.Limits.AwayLen < 1 || config.Limits.KickLen < 1 || config.Limits.TopicLen < 1 {
 		return nil, ErrLimitsAreInsane
 	}
+	if config.Limits.RegistrationMessages == 0 {
+		config.Limits.RegistrationMessages = 1024
+	}
 	if config.Server.STS.Enabled {
 		config.Server.STS.Duration, err = custime.ParseDuration(config.Server.STS.DurationString)
 		if err != nil {
diff --git a/oragono.yaml b/oragono.yaml
index 9c965b23..35754dea 100644
--- a/oragono.yaml
+++ b/oragono.yaml
@@ -557,6 +557,10 @@ limits:
         # configurable length for the rest of the message:
         rest: 2048
 
+    # maximum number of messages to accept during registration (prevents
+    # DoS / resource exhaustion attacks):
+    registration-messages: 1024
+
 # fakelag: prevents clients from spamming commands too rapidly
 fakelag:
     # whether to enforce fakelag