diff --git a/derp/derp_server.go b/derp/derp_server.go
index 369cd69d3..7ff6a9fa0 100644
--- a/derp/derp_server.go
+++ b/derp/derp_server.go
@@ -86,7 +86,7 @@ func (s *Server) Accept(netConn net.Conn, conn *bufio.ReadWriter) {
 
 func (s *Server) accept(netConn net.Conn, conn *bufio.ReadWriter) error {
 	netConn.SetDeadline(time.Now().Add(10 * time.Second))
-	if err := s.sendServerKey(conn); err != nil {
+	if err := s.sendServerKey(conn.Writer); err != nil {
 		return fmt.Errorf("send server key: %v", err)
 	}
 	netConn.SetDeadline(time.Now().Add(10 * time.Second))
@@ -150,7 +150,7 @@ func (s *Server) accept(netConn net.Conn, conn *bufio.ReadWriter) error {
 	}
 
 	for {
-		dstKey, contents, err := s.recvPacket(c.conn)
+		dstKey, contents, err := s.recvPacket(c.conn.Reader)
 		if err != nil {
 			return fmt.Errorf("client %x: recv: %v", c.key, err)
 		}
@@ -187,17 +187,17 @@ func (s *Server) verifyClient(clientKey key.Public, info *clientInfo) error {
 	return nil
 }
 
-func (s *Server) sendServerKey(conn *bufio.ReadWriter) error {
-	if err := putUint32(conn, magic); err != nil {
+func (s *Server) sendServerKey(bw *bufio.Writer) error {
+	if err := putUint32(bw, magic); err != nil {
 		return err
 	}
-	if err := typeServerKey.Write(conn); err != nil {
+	if err := typeServerKey.Write(bw); err != nil {
 		return err
 	}
-	if _, err := conn.Write(s.publicKey[:]); err != nil {
+	if _, err := bw.Write(s.publicKey[:]); err != nil {
 		return err
 	}
-	return conn.Flush()
+	return bw.Flush()
 }
 
 func (s *Server) sendServerInfo(conn *bufio.ReadWriter, clientKey key.Public) error {
@@ -223,20 +223,24 @@ func (s *Server) sendServerInfo(conn *bufio.ReadWriter, clientKey key.Public) er
 	return conn.Flush()
 }
 
-func (s *Server) recvClientKey(conn *bufio.ReadWriter) (clientKey key.Public, info *clientInfo, err error) {
-	if _, err := io.ReadFull(conn, clientKey[:]); err != nil {
+// recvClientKey reads the client's hello (its proof of identity) upon its initial connection.
+// It should be considered especially untrusted at this point.
+func (s *Server) recvClientKey(br *bufio.ReadWriter) (clientKey key.Public, info *clientInfo, err error) {
+	if _, err := io.ReadFull(br, clientKey[:]); err != nil {
 		return key.Public{}, nil, err
 	}
 	var nonce [24]byte
-	if _, err := io.ReadFull(conn, nonce[:]); err != nil {
+	if _, err := io.ReadFull(br, nonce[:]); err != nil {
 		return key.Public{}, nil, fmt.Errorf("nonce: %v", err)
 	}
-	msgLen, err := readUint32(conn, oneMB)
+	// We don't trust the client at all yet, so limit its input size to limit
+	// things like JSON resource exhausting (http://github.com/golang/go/issues/31789).
+	msgLen, err := readUint32(br, 256<<10)
 	if err != nil {
 		return key.Public{}, nil, fmt.Errorf("msglen: %v", err)
 	}
 	msgbox := make([]byte, msgLen)
-	if _, err := io.ReadFull(conn, msgbox); err != nil {
+	if _, err := io.ReadFull(br, msgbox); err != nil {
 		return key.Public{}, nil, fmt.Errorf("msgbox: %v", err)
 	}
 	msg, ok := box.Open(nil, msgbox, &nonce, (*[32]byte)(&clientKey), s.privateKey.B32())
@@ -263,19 +267,19 @@ func (s *Server) sendPacket(bw *bufio.Writer, srcKey key.Public, contents []byte
 	return bw.Flush()
 }
 
-func (s *Server) recvPacket(conn *bufio.ReadWriter) (dstKey key.Public, contents []byte, err error) {
-	if err := readType(conn.Reader, typeSendPacket); err != nil {
+func (s *Server) recvPacket(br *bufio.Reader) (dstKey key.Public, contents []byte, err error) {
+	if err := readType(br, typeSendPacket); err != nil {
 		return key.Public{}, nil, err
 	}
-	if _, err := io.ReadFull(conn, dstKey[:]); err != nil {
+	if _, err := io.ReadFull(br, dstKey[:]); err != nil {
 		return key.Public{}, nil, err
 	}
-	packetLen, err := readUint32(conn.Reader, oneMB)
+	packetLen, err := readUint32(br, oneMB)
 	if err != nil {
 		return key.Public{}, nil, err
 	}
 	contents = make([]byte, packetLen)
-	if _, err := io.ReadFull(conn, contents); err != nil {
+	if _, err := io.ReadFull(br, contents); err != nil {
 		return key.Public{}, nil, err
 	}
 	return dstKey, contents, nil
diff --git a/derp/derphttp/derphttp_test.go b/derp/derphttp/derphttp_test.go
index 4354ce2d8..73885681d 100644
--- a/derp/derphttp/derphttp_test.go
+++ b/derp/derphttp/derphttp_test.go
@@ -5,6 +5,7 @@
 package derphttp
 
 import (
+	"context"
 	crand "crypto/rand"
 	"crypto/tls"
 	"net"
@@ -77,6 +78,9 @@ func TestSendRecv(t *testing.T) {
 		if err != nil {
 			t.Fatalf("client %d: %v", i, err)
 		}
+		if err := c.Connect(context.Background()); err != nil {
+			t.Fatalf("client %d Connect: %v", i, err)
+		}
 		clients = append(clients, c)
 		recvChs = append(recvChs, make(chan []byte))