Hostname creation cleanup, and a minimal variant or PR#5397 to fix bad DNS name

wled00/fcn_declare.h

@@ -409,7 +409,8 @@ size_t printSetFormValue(Print& settingsScript, const char* key, int val);
 size_t printSetFormValue(Print& settingsScript, const char* key, const char* val);
 size_t printSetFormIndex(Print& settingsScript, const char* key, int index);
 size_t printSetClassElementHTML(Print& settingsScript, const char* key, const int index, const char* val);
-void prepareHostname(char* hostname);
+void getWLEDhostname(char* hostname, size_t maxLen, bool preferMDNS=false); // maxLen = hostname buffer size including \0; if preferMDNSname=true, use mdns name (sanitized)
+void prepareHostname(char* hostname, size_t maxLen);                        // legacy function - not recommended for new code
 [[gnu::pure]] bool isAsterisksOnly(const char* str, byte maxLen);
 bool requestJSONBufferLock(uint8_t moduleID=JSON_LOCK_UNKNOWN);
 void releaseJSONBufferLock();

wled00/network.cpp

@@ -414,8 +414,8 @@ void WiFiEvent(WiFiEvent_t event)
         WiFi.disconnect(true); // disable WiFi entirely
       }
       // convert the "serverDescription" into a valid DNS hostname (alphanumeric)
-      char hostname[64];
-      prepareHostname(hostname);
+      char hostname[64] = {'\0'}; // any "hostname" within a Fully Qualified Domain Name (FQDN) must not exceed 63 characters
+      getWLEDhostname(hostname, sizeof(hostname));
       ETH.setHostname(hostname);
       showWelcomePage = false;
       break;

wled00/util.cpp

@@ -127,30 +127,61 @@ size_t printSetClassElementHTML(Print& settingsScript, const char* key, const in
 }
 
 
-
-void prepareHostname(char* hostname)
-{
-  sprintf_P(hostname, PSTR("wled-%*s"), 6, escapedMac.c_str() + 6);
-  const char *pC = serverDescription;
-  unsigned pos = 5;          // keep "wled-"
-  while (*pC && pos < 24) { // while !null and not over length
-    if (isalnum(*pC)) {     // if the current char is alpha-numeric append it to the hostname
-      hostname[pos] = *pC;
-      pos++;
-    } else if (*pC == ' ' || *pC == '_' || *pC == '-' || *pC == '+' || *pC == '!' || *pC == '?' || *pC == '*') {
-      hostname[pos] = '-';
-      pos++;
+// in-place hostname sanitizer, extracted from prepareHostname()
+static void sanitizeHostname(char* hostname, size_t maxLen) {
+  if (hostname == nullptr || maxLen < 1 || strlen(hostname) < 1) return;
+
+  char *pC = hostname;
+  unsigned pos = 0;
+  while (*pC && pos < maxLen) {
+    char c = *pC;
+    if (isalnum((unsigned char)c)) {
+      hostname[pos++] = c;
+    } else if (c == ' ' || c == '_' || c == '-' || c == '+' || c == '!' || c == '?' || c == '*') { // convert certain characters to hyphens
+      if (pos > 0 && hostname[pos -1] != '-') hostname[pos++] = '-'; // keep single non-leading hyphens only
     }
-    // else do nothing - no leading hyphens and do not include hyphens for all other characters.
+    // else: drop any character not valid in a DNS hostname label
     pC++;
   }
-  //last character must not be hyphen
-  if (pos > 5) {
-    while (pos > 4 && hostname[pos -1] == '-') pos--;
-    hostname[pos] = '\0'; // terminate string (leave at least "wled")
+  // Hostname must not end with a hyphen.
+  while (pos > 0 && hostname[pos -1] == '-') pos--;
+  hostname[min(pos, maxLen-1)] = '\0'; // terminate string
+}
+
+/*
+ * Stores sanitized hostname into buffer provided by caller
+ *   maxLen = hostname buffer size including \0
+ *   preferMDNSname -> use mDNS name if set, otherwise fall back to WLED "server description" name (legacy behaviour)
+ */
+void getWLEDhostname(char* hostname, size_t maxLen, bool preferMDNS) {
+  if (maxLen < 6) { strlcpy(hostname, "wled", maxLen); return; } // buffer too small (should not happen)
+  if (preferMDNS && strlen(cmDNS) > 0) {
+    strlcpy(hostname, cmDNS, maxLen);
+    sanitizeHostname(hostname, maxLen);  // sanitize cmDNS name
+    if (strlen(hostname) < 1) {          // if result is empty -> fall back to wled-MAC
+      snprintf_P(hostname, maxLen, PSTR("wled-%*s"), 6, escapedMac.c_str() + 6);
+      hostname[maxLen -1] = '\0';        // ensure string termination
+    }
+  } else {
+    prepareHostname(hostname, maxLen); // use legacy hostname based on "server description" - already sanitized
   }
 }
 
+/* Legacy hostname construction:
+ *   Start with "wled-" + serverDescription as suffix.
+ *   Sanitize only the suffix (always keep wled- prefix intact).
+ *   If the sanitized suffix ends up empty, fall back to wled-<mac>.
+ */
+void prepareHostname(char* hostname, size_t maxLen)
+{
+  if (maxLen < 6) { strlcpy(hostname, "wled", maxLen); return; } // buffer too small (should not happen)
+  snprintf_P(hostname, maxLen, PSTR("wled-%s"), serverDescription);
+  hostname[maxLen -1] = '\0';                         // ensure string termination
+  sanitizeHostname(hostname+5, maxLen-5);             // sanitize name, keep "wled-" intact
+  if (strlen(hostname) <= 5)
+    snprintf_P(hostname, maxLen, PSTR("wled-%*s"), 6, escapedMac.c_str() + 6); // fallback to wled-MAC
+}
+
 
 bool isAsterisksOnly(const char* str, byte maxLen)
 {

wled00/wled.cpp

@@ -719,8 +719,8 @@ void WLED::initConnection()
     DEBUG_PRINTF_P(PSTR("Connecting to %s...\n"), multiWiFi[selectedWiFi].clientSSID);
 
     // convert the "serverDescription" into a valid DNS hostname (alphanumeric)
-    char hostname[25];
-    prepareHostname(hostname);
+    char hostname[64] = {'\0'}; // any "hostname" within a Fully Qualified Domain Name (FQDN) must not exceed 63 characters
+    getWLEDhostname(hostname, sizeof(hostname));
 
 #ifdef WLED_ENABLE_WPA_ENTERPRISE
     if (multiWiFi[selectedWiFi].encryptionType == WIFI_ENCRYPTION_TYPE_PSK) {
@@ -865,6 +865,13 @@ void WLED::handleConnection()
   #endif
   const bool wifiConfigured = WLED_WIFI_CONFIGURED;
 
+  // The DNS name of the ESP must be set before the first connection to the DHCP server; otherwise, the default ESP name (such as "esp32s3-267D0C") will be used.
+  #ifdef ARDUINO_ARCH_ESP32
+    char hostname[64] = {'\0'}; // any "hostname" within a Fully Qualified Domain Name (FQDN) must not exceed 63 characters
+    getWLEDhostname(hostname, sizeof(hostname), true);   // create DNS name based on mDNS name if set, or fall back to standard WLED server name
+    WiFi.setHostname(hostname);
+  #endif
+
   // ignore connection handling if WiFi is configured and scan still running
   // or within first 2s if WiFi is not configured or AP is always active
   if ((wifiConfigured && multiWiFi.size() > 1 && WiFi.scanComplete() < 0) || (now < 2000 && (!wifiConfigured || apBehavior == AP_BEHAVIOR_ALWAYS)))