player: restrict name length

player.go

@@ -11,8 +11,13 @@ import (
 	//"errors"
 )
 
+const (
+	nameMaxLen = 16
+	nameWhitelistChars = "a-zA-Z0-9_"
+)
+
 var (
-	nameWhitelist = regexp.MustCompile(".*[^a-zA-Z0-9_].*") // replace all characters if any 'evil' one is present
+	nameWhitelist = regexp.MustCompile("[^"+nameWhitelistChars+"]")
 )
 
 type Context interface {
@@ -52,11 +57,19 @@ func NewPlayer(conn net.Conn) (p *Player) {
 	if err != nil {
 		log.Println(err)
 	}
-	if len(nameWhitelist.ReplaceAllString(p.name, "")) == 0 {
+	reason := ""
+	if len(p.name) == 0 {
+		reason = "Must set username. `telnet -l <name>`\n"
+	} else if len(p.name) > nameMaxLen {
+		reason = fmt.Sprintf("Username must contain at most %d characters.\n", nameMaxLen)
+	} else if len(nameWhitelist.ReplaceAllString(p.name, "")) != len(p.name) {
+		reason = fmt.Sprintf("Username must consist only of %s\n", nameWhitelistChars)
+	}
+	if reason != "" {
 		p.inputKill <- struct{}{}
 		close(p.inputKill)
 		close(p.inputEvent)
-		conn.Write([]byte(fmt.Sprintf("Invalid username >%s<\n", p.name)))
+		conn.Write([]byte(reason))
 		conn.Close()
 		return nil
 	}