From ba7c06785a03f1f7c570b572e5aa677d0ed9d073 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Thu, 30 Apr 2026 19:14:55 -0500
Subject: [PATCH] [mdns] Broadcast config_hash TXT record on _esphomelib._tcp
 (#16145)

---
 esphome/components/mdns/mdns_component.cpp | 40 ++++++++++++++++++++--
 esphome/components/mdns/mdns_component.h   | 33 +++++-------------
 esphome/components/mdns/mdns_host.cpp      |  7 +++-
 3 files changed, 52 insertions(+), 28 deletions(-)

diff --git a/esphome/components/mdns/mdns_component.cpp b/esphome/components/mdns/mdns_component.cpp
index e05373ac5d3..9bf27e71e4c 100644
--- a/esphome/components/mdns/mdns_component.cpp
+++ b/esphome/components/mdns/mdns_component.cpp
@@ -39,7 +39,39 @@ MDNS_STATIC_CONST_CHAR(SERVICE_TCP, "_tcp");
 // Wrap build-time defines into flash storage
 MDNS_STATIC_CONST_CHAR(VALUE_VERSION, ESPHOME_VERSION);
 
-void MDNSComponent::compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services, char *mac_address_buf) {
+void MDNSComponent::setup_buffers_and_register_(PlatformRegisterFn platform_register) {
+#ifdef USE_MDNS_STORE_SERVICES
+  auto &services = this->services_;
+#else
+  StaticVector<MDNSService, MDNS_SERVICE_COUNT> services_storage;
+  auto &services = services_storage;
+#endif
+
+#ifdef USE_API
+#ifdef USE_MDNS_STORE_SERVICES
+  get_mac_address_into_buffer(this->mac_address_);
+  char *mac_ptr = this->mac_address_;
+  format_hex_to(this->config_hash_str_, App.get_config_hash());
+  char *cfg_ptr = this->config_hash_str_;
+#else
+  char mac_address[MAC_ADDRESS_BUFFER_SIZE];
+  char config_hash_str[CONFIG_HASH_STR_SIZE];
+  get_mac_address_into_buffer(mac_address);
+  format_hex_to(config_hash_str, App.get_config_hash());
+  char *mac_ptr = mac_address;
+  char *cfg_ptr = config_hash_str;
+#endif
+#else
+  char *mac_ptr = nullptr;
+  char *cfg_ptr = nullptr;
+#endif
+
+  this->compile_records_(services, mac_ptr, cfg_ptr);
+  platform_register(this, services);
+}
+
+void MDNSComponent::compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services, char *mac_address_buf,
+                                     char *config_hash_buf) {
   // IMPORTANT: The #ifdef blocks below must match COMPONENTS_WITH_MDNS_SERVICES
   // in mdns/__init__.py. If you add a new service here, update both locations.
 
@@ -47,6 +79,7 @@ void MDNSComponent::compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUN
   MDNS_STATIC_CONST_CHAR(SERVICE_ESPHOMELIB, "_esphomelib");
   MDNS_STATIC_CONST_CHAR(TXT_FRIENDLY_NAME, "friendly_name");
   MDNS_STATIC_CONST_CHAR(TXT_VERSION, "version");
+  MDNS_STATIC_CONST_CHAR(TXT_CONFIG_HASH, "config_hash");
   MDNS_STATIC_CONST_CHAR(TXT_MAC, "mac");
   MDNS_STATIC_CONST_CHAR(TXT_PLATFORM, "platform");
   MDNS_STATIC_CONST_CHAR(TXT_BOARD, "board");
@@ -63,7 +96,7 @@ void MDNSComponent::compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUN
     bool friendly_name_empty = friendly_name.empty();
 
     // Calculate exact capacity for txt_records
-    size_t txt_count = 3;  // version, mac, board (always present)
+    size_t txt_count = 4;  // version, config_hash, mac, board (always present)
     if (!friendly_name_empty) {
       txt_count++;  // friendly_name
     }
@@ -91,6 +124,9 @@ void MDNSComponent::compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUN
     }
     txt_records.push_back({MDNS_STR(TXT_VERSION), MDNS_STR(VALUE_VERSION)});
 
+    // Config hash: passed from caller (either member buffer or stack buffer depending on USE_MDNS_STORE_SERVICES)
+    txt_records.push_back({MDNS_STR(TXT_CONFIG_HASH), MDNS_STR(config_hash_buf)});
+
     // MAC address: passed from caller (either member buffer or stack buffer depending on USE_MDNS_STORE_SERVICES)
     txt_records.push_back({MDNS_STR(TXT_MAC), MDNS_STR(mac_address_buf)});
 
diff --git a/esphome/components/mdns/mdns_component.h b/esphome/components/mdns/mdns_component.h
index 798af0e0bf5..e981a659419 100644
--- a/esphome/components/mdns/mdns_component.h
+++ b/esphome/components/mdns/mdns_component.h
@@ -70,6 +70,9 @@ class MDNSComponent final : public Component
   void setup() override;
   void dump_config() override;
 
+  /// Size of buffer required for config hash hex string (8 hex chars + null terminator)
+  static constexpr size_t CONFIG_HASH_STR_SIZE = format_hex_size(sizeof(uint32_t));
+
 #ifdef USE_MDNS_EVENT_DRIVEN_POLLING
   // LEAmDNS has meaningful work only during the probe+announce phase (3×250ms probes +
   // 8×1000ms announces, ~9s). Afterwards every internal timer is resetToNeverExpires()
@@ -124,30 +127,7 @@ class MDNSComponent final : public Component
   /// Helper to set up services and MAC buffers, then call platform-specific registration
   using PlatformRegisterFn = void (*)(MDNSComponent *, StaticVector<MDNSService, MDNS_SERVICE_COUNT> &);
 
-  void setup_buffers_and_register_(PlatformRegisterFn platform_register) {
-#ifdef USE_MDNS_STORE_SERVICES
-    auto &services = this->services_;
-#else
-    StaticVector<MDNSService, MDNS_SERVICE_COUNT> services_storage;
-    auto &services = services_storage;
-#endif
-
-#ifdef USE_API
-#ifdef USE_MDNS_STORE_SERVICES
-    get_mac_address_into_buffer(this->mac_address_);
-    char *mac_ptr = this->mac_address_;
-#else
-    char mac_address[MAC_ADDRESS_BUFFER_SIZE];
-    get_mac_address_into_buffer(mac_address);
-    char *mac_ptr = mac_address;
-#endif
-#else
-    char *mac_ptr = nullptr;
-#endif
-
-    this->compile_records_(services, mac_ptr);
-    platform_register(this, services);
-  }
+  void setup_buffers_and_register_(PlatformRegisterFn platform_register);
 
 #ifdef USE_MDNS_DYNAMIC_TXT
   /// Storage for runtime-generated TXT values from user lambdas
@@ -159,6 +139,8 @@ class MDNSComponent final : public Component
 #if defined(USE_API) && defined(USE_MDNS_STORE_SERVICES)
   /// Fixed buffer for MAC address (only needed when services are stored)
   char mac_address_[MAC_ADDRESS_BUFFER_SIZE];
+  /// Fixed buffer for config hash hex string (only needed when services are stored)
+  char config_hash_str_[CONFIG_HASH_STR_SIZE];
 #endif
 #ifdef USE_MDNS_STORE_SERVICES
   StaticVector<MDNSService, MDNS_SERVICE_COUNT> services_{};
@@ -167,7 +149,8 @@ class MDNSComponent final : public Component
   // RP2040 defers MDNS.begin() until the first IP-up event; this tracks that.
   bool initialized_{false};
 #endif
-  void compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services, char *mac_address_buf);
+  void compile_records_(StaticVector<MDNSService, MDNS_SERVICE_COUNT> &services, char *mac_address_buf,
+                        char *config_hash_buf);
 };
 
 }  // namespace esphome::mdns
diff --git a/esphome/components/mdns/mdns_host.cpp b/esphome/components/mdns/mdns_host.cpp
index 4d902319b88..1e66a10df09 100644
--- a/esphome/components/mdns/mdns_host.cpp
+++ b/esphome/components/mdns/mdns_host.cpp
@@ -3,6 +3,8 @@
 
 #include "esphome/components/network/ip_address.h"
 #include "esphome/components/network/util.h"
+#include "esphome/core/application.h"
+#include "esphome/core/helpers.h"
 #include "esphome/core/log.h"
 #include "mdns_component.h"
 
@@ -13,10 +15,13 @@ void MDNSComponent::setup() {
 #ifdef USE_API
   get_mac_address_into_buffer(this->mac_address_);
   char *mac_ptr = this->mac_address_;
+  format_hex_to(this->config_hash_str_, App.get_config_hash());
+  char *cfg_ptr = this->config_hash_str_;
 #else
   char *mac_ptr = nullptr;
+  char *cfg_ptr = nullptr;
 #endif
-  this->compile_records_(this->services_, mac_ptr);
+  this->compile_records_(this->services_, mac_ptr, cfg_ptr);
 #endif
   // Host platform doesn't have actual mDNS implementation
 }