fix(network): rebuild the DHCP hostname when the node is renamed

The hostname was built once in begin() from the node name, so `set name` left
the node advertising its old name to DHCP until the next reflash. It is now
rebuilt on rename through a new CommonCLICallbacks::onNodeNameChanged() hook,
and begin() shares the same helper.

What this cannot do is rename a live lease. IDF 4.4 states that a hostname
changed after the interface is up "would only be reflected once the interface
restarts/reconnects", and Arduino 2.x's WiFi.setHostname() does not touch a
running netif at all — it writes a static string that the netif reads at
bring-up. Forcing the issue would mean bouncing the link, which costs every
MQTT slot a reconnect; that is not a reasonable price for a rename, so the new
name lands at the next reconnect or boot instead.

Only automatic (Ethernet-preferred) links carry a hostname today, exactly as
before. Giving plain Wi-Fi observers one would change the DHCP identity of the
existing fleet and is a separate decision.

examples/simple_repeater/MyMesh.cpp

@@ -1123,6 +1123,27 @@ MyMesh::MyMesh(mesh::MainBoard &board, mesh::Radio &radio, mesh::MillisecondCloc
   memset(default_scope.key, 0, sizeof(default_scope.key));
 }
 
+#if defined(WITH_MQTT_BRIDGE) && defined(ESP_PLATFORM)
+// Only an automatic link carries a hostname today (see the header note). The
+// value reaches DHCP when the link next starts or reconnects: IDF 4.4 states
+// plainly that a hostname changed after the interface is up "would only be
+// reflected once the interface restarts/reconnects", and Arduino 2.x's
+// WiFi.setHostname() does not even touch a running netif — it writes a static
+// string the netif reads at bring-up. So this cannot rename a live lease, and
+// deliberately does not bounce the link to force one: dropping an observer's
+// route mid-operation costs every MQTT slot a reconnect, which is not a
+// reasonable price for a cosmetic rename.
+void MyMesh::applyNetworkHostname(const char* when) {
+  NetworkLink& link = activeNetworkLink();
+  if (!link.isAutomatic()) return;
+  char network_hostname[NetworkHostname::kBufferSize];
+  NetworkHostname::build(network_hostname, sizeof(network_hostname),
+                         _prefs.node_name, self_id.pub_key, PUB_KEY_SIZE);
+  link.setHostname(network_hostname);
+  Serial.printf("Network: hostname %s%s\n", network_hostname, when);
+}
+#endif
+
 void MyMesh::begin(FILESYSTEM *fs) {
   mesh::Mesh::begin();
   _fs = fs;
@@ -1151,11 +1172,7 @@ void MyMesh::begin(FILESYSTEM *fs) {
 #if defined(WITH_MQTT_BRIDGE) && defined(ESP_PLATFORM)
   NetworkLink& boot_network = activeNetworkLink();
   if (boot_network.isAutomatic()) {
-    char network_hostname[NetworkHostname::kBufferSize];
-    NetworkHostname::build(network_hostname, sizeof(network_hostname),
-                           _prefs.node_name, self_id.pub_key, PUB_KEY_SIZE);
-    boot_network.setHostname(network_hostname);
-    Serial.printf("Network: hostname %s\n", network_hostname);
+    applyNetworkHostname("");
     MQTTPrefs* obs = _cli.getObserverPrefs();
     Serial.printf("Network: probing Ethernet for up to %lums\n",
                   (unsigned long)NETWORK_ETHERNET_BOOT_WAIT_MS);

examples/simple_repeater/MyMesh.h

@@ -274,10 +274,23 @@ protected:
 
   void sendFloodReply(mesh::Packet* packet, unsigned long delay_millis, uint8_t path_hash_size);
 
+  // Build the DHCP hostname from the current node name and hand it to the
+  // link. Only automatic links carry one: a plain Wi-Fi observer keeps the
+  // framework default, and changing that for the existing fleet is a separate
+  // decision.
+  void applyNetworkHostname(const char* when);
+
 public:
   MyMesh(mesh::MainBoard& board, mesh::Radio& radio, mesh::MillisecondClock& ms, mesh::RNG& rng, mesh::RTCClock& rtc, mesh::MeshTables& tables);
 
   void begin(FILESYSTEM* fs);
+
+  // CommonCLICallbacks: `set name` has just rewritten node_name.
+  void onNodeNameChanged() override {
+#if defined(WITH_MQTT_BRIDGE) && defined(ESP_PLATFORM)
+    applyNetworkHostname(" (renamed)");
+#endif
+  }
   void sendNodeDiscoverReq();
   const char* getFirmwareVer() override { return FIRMWARE_VERSION; }
   const char* getBuildDate() override { return FIRMWARE_BUILD_DATE; }

examples/simple_room_server/MyMesh.cpp

@@ -943,6 +943,27 @@ MyMesh::MyMesh(mesh::MainBoard &board, mesh::Radio &radio, mesh::MillisecondCloc
   memset(default_scope.key, 0, sizeof(default_scope.key));
 }
 
+#if defined(WITH_MQTT_BRIDGE) && defined(ESP_PLATFORM)
+// Only an automatic link carries a hostname today (see the header note). The
+// value reaches DHCP when the link next starts or reconnects: IDF 4.4 states
+// plainly that a hostname changed after the interface is up "would only be
+// reflected once the interface restarts/reconnects", and Arduino 2.x's
+// WiFi.setHostname() does not even touch a running netif — it writes a static
+// string the netif reads at bring-up. So this cannot rename a live lease, and
+// deliberately does not bounce the link to force one: dropping an observer's
+// route mid-operation costs every MQTT slot a reconnect, which is not a
+// reasonable price for a cosmetic rename.
+void MyMesh::applyNetworkHostname(const char* when) {
+  NetworkLink& link = activeNetworkLink();
+  if (!link.isAutomatic()) return;
+  char network_hostname[NetworkHostname::kBufferSize];
+  NetworkHostname::build(network_hostname, sizeof(network_hostname),
+                         _prefs.node_name, self_id.pub_key, PUB_KEY_SIZE);
+  link.setHostname(network_hostname);
+  Serial.printf("Network: hostname %s%s\n", network_hostname, when);
+}
+#endif
+
 void MyMesh::begin(FILESYSTEM *fs) {
   mesh::Mesh::begin();
   _fs = fs;
@@ -952,11 +973,7 @@ void MyMesh::begin(FILESYSTEM *fs) {
 #if defined(WITH_MQTT_BRIDGE) && defined(ESP_PLATFORM)
   NetworkLink& boot_network = activeNetworkLink();
   if (boot_network.isAutomatic()) {
-    char network_hostname[NetworkHostname::kBufferSize];
-    NetworkHostname::build(network_hostname, sizeof(network_hostname),
-                           _prefs.node_name, self_id.pub_key, PUB_KEY_SIZE);
-    boot_network.setHostname(network_hostname);
-    Serial.printf("Network: hostname %s\n", network_hostname);
+    applyNetworkHostname("");
     MQTTPrefs* obs = _cli.getObserverPrefs();
     Serial.printf("Network: probing Ethernet for up to %lums\n",
                   (unsigned long)NETWORK_ETHERNET_BOOT_WAIT_MS);

examples/simple_room_server/MyMesh.h

@@ -281,10 +281,23 @@ protected:
 
   void sendFloodReply(mesh::Packet* packet, unsigned long delay_millis, uint8_t path_hash_size);
 
+  // Build the DHCP hostname from the current node name and hand it to the
+  // link. Only automatic links carry one: a plain Wi-Fi observer keeps the
+  // framework default, and changing that for the existing fleet is a separate
+  // decision.
+  void applyNetworkHostname(const char* when);
+
 public:
   MyMesh(mesh::MainBoard& board, mesh::Radio& radio, mesh::MillisecondClock& ms, mesh::RNG& rng, mesh::RTCClock& rtc, mesh::MeshTables& tables);
 
   void begin(FILESYSTEM* fs);
+
+  // CommonCLICallbacks: `set name` has just rewritten node_name.
+  void onNodeNameChanged() override {
+#if defined(WITH_MQTT_BRIDGE) && defined(ESP_PLATFORM)
+    applyNetworkHostname(" (renamed)");
+#endif
+  }
   void addSystemPost(const char* postData);
 
   const char* getFirmwareVer() override { return FIRMWARE_VERSION; }

src/helpers/CommonCLI.cpp

@@ -1583,6 +1583,7 @@ void CommonCLI::handleSetCmd(uint32_t sender_timestamp, char* command, char* rep
     if (isValidName(&config[5])) {
       StrHelper::strncpy(_prefs->node_name, &config[5], sizeof(_prefs->node_name));
       savePrefs();
+      _callbacks->onNodeNameChanged();
       strcpy(reply, "OK");
     } else {
       strcpy(reply, "Error, bad chars");

src/helpers/CommonCLI.h

@@ -274,6 +274,10 @@ struct LegacyObserverTail {
 class CommonCLICallbacks {
 public:
   virtual void savePrefs() = 0;
+  // The node name feeds the DHCP hostname, which is built once when the link
+  // starts. Renaming a running node has to rebuild it or the node keeps
+  // advertising the old name to DHCP until the next reflash.
+  virtual void onNodeNameChanged() {}
 #ifdef WITH_MQTT_BRIDGE
   virtual bool saveObserverPrefs() = 0;
 #else