From e039edde334541ecd4cfe50590f0e972180db114 Mon Sep 17 00:00:00 2001
From: Theo Arends <11044339+arendst@users.noreply.github.com>
Date: Mon, 26 Jul 2021 15:10:47 +0200
Subject: [PATCH] Enforce valid hostname

---
 tasmota/tasmota.ino | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tasmota/tasmota.ino b/tasmota/tasmota.ino
index aeab9e8ed..15a5cf519 100644
--- a/tasmota/tasmota.ino
+++ b/tasmota/tasmota.ino
@@ -382,7 +382,8 @@ void setup(void) {
   }
   char *s = TasmotaGlobal.hostname;
   while (*s) {
-    if (' ' == *s) { *s = '_'; }
+    if (!(isalnum(*s) || ('.' == *s))) { *s = '-'; }                 // Valid hostname chars are A..Z, a..z, 0..9, . and -
+    if ((s == TasmotaGlobal.hostname) && ('-' == *s)) { *s = 'x'; }  // First char cannot be a dash so replace by an x
     s++;
   }
   snprintf_P(TasmotaGlobal.mqtt_topic, sizeof(TasmotaGlobal.mqtt_topic), ResolveToken(TasmotaGlobal.mqtt_topic).c_str());