[mdns] allow service registration for local host (#11450)

This commit updates the mDNS service registration to allow services
for the local host. The `mHostName` field in an `otMdnsService`
structure can now be set to `NULL` to indicate that the service
if for the local host.

The `test_mdns` unit test is also updated to verify this new
functionality.

include/openthread/instance.h

@@ -52,7 +52,7 @@ extern "C" {
  *
  * @note This number versions both OpenThread platform and user APIs.
  */
-#define OPENTHREAD_API_VERSION (502)
+#define OPENTHREAD_API_VERSION (503)
 
 /**
  * @addtogroup api-instance

include/openthread/mdns.h

@@ -311,7 +311,8 @@ otError otMdnsUnregisterHost(otInstance *aInstance, const otMdnsHost *aHost);
  *   contain dot `.` character which is allowed in a service instance label.
  * - The `mServiceType` specifies the service type (e.g., "_tst._udp"). It is treated as multiple dot `.` separated
  *   labels. It MUST NOT contain the domain name.
- * - The `mHostName` field specifies the host name of the service. MUST NOT contain the domain name.
+ * - The `mHostName` field specifies the host name of the service if it is not NULL. Otherwise, if it is NULL, it
+ *   indicates that this service is for the local host (this device itself).
  * - The `mSubTypeLabels` is an array of strings representing sub-types associated with the service. Each array entry
  *   is a sub-type label. The `mSubTypeLabels can be NULL if there is no sub-type. Otherwise, the array length is
  *   specified by `mSubTypeLabelsLength`.

src/core/net/mdns.cpp

@@ -2332,7 +2332,8 @@ exit:
 
 void Core::ServiceEntry::Register(const Service &aService, const Callback &aCallback)
 {
-    uint32_t ttl = DetermineTtl(aService.mTtl, kDefaultTtl);
+    const char *hostName;
+    uint32_t    ttl = DetermineTtl(aService.mTtl, kDefaultTtl);
 
     if (GetState() == kRemoving)
     {
@@ -2377,8 +2378,10 @@ void Core::ServiceEntry::Register(const Service &aService, const Callback &aCall
 
     // Register SRV record info.
 
+    hostName = (aService.mHostName != nullptr) ? aService.mHostName : Get<Core>().mLocalHost.GetName();
+
     mSrvRecord.UpdateTtl(ttl);
-    mSrvRecord.UpdateProperty(mHostName, aService.mHostName);
+    mSrvRecord.UpdateProperty(mHostName, hostName);
     mSrvRecord.UpdateProperty(mPriority, aService.mPriority);
     mSrvRecord.UpdateProperty(mWeight, aService.mWeight);
     mSrvRecord.UpdateProperty(mPort, aService.mPort);

src/core/net/mdns.hpp

@@ -301,7 +301,8 @@ public:
      *   contain dot `.` character which is allowed in a service instance label.
      * - The `mServiceType` specifies the service type (e.g., "_tst._udp"). It is treated as multiple dot `.` separated
      *   labels. It MUST NOT contain the domain name.
-     * - The `mHostName` field specifies the host name of the service. MUST NOT contain the domain name.
+     * - The `mHostName` field specifies the host name of the service if it is not `nullptr`. Otherwise, if it is
+     *   `nullptr`, it indicates that this service is for the local host (this device itself).
      * - The `mSubTypeLabels` is an array of strings representing sub-types associated with the service. Each array
      *   entry is a sub-type label. The `mSubTypeLabels can be `nullptr` if there are no sub-types. Otherwise, the
      *   array length is specified by `mSubTypeLabelsLength`.

tests/unit/test_mdns.cpp

@@ -481,7 +481,8 @@ struct DnsRecords : public OwningList<DnsRecord>
         bool          contains = false;
         DnsNameString hostName;
 
-        hostName.Append("%s.local.", aService.mHostName);
+        hostName.Append("%s.local.",
+                        aService.mHostName != nullptr ? aService.mHostName : sInstance->Get<Core>().GetLocalHostName());
 
         for (const DnsRecord &record : *this)
         {
@@ -3144,6 +3145,28 @@ void TestServiceReg(void)
         sDnsMessages.Clear();
     }
 
+    Log("- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -");
+    Log("Update service host name to use local host and validate new announcements of SRV record");
+
+    service.mHostName = nullptr;
+
+    sRegCallbacks[1].Reset();
+    sDnsMessages.Clear();
+    SuccessOrQuit(mdns->RegisterService(service, 1, HandleSuccessCallback));
+
+    for (uint8_t anncCount = 0; anncCount < kNumAnnounces; anncCount++)
+    {
+        AdvanceTime((anncCount == 0) ? 0 : (1U << (anncCount - 1)) * 1000);
+        VerifyOrQuit(sRegCallbacks[1].mWasCalled);
+
+        VerifyOrQuit(!sDnsMessages.IsEmpty());
+        dnsMsg = sDnsMessages.GetHead();
+        dnsMsg->ValidateHeader(kMulticastResponse, /* Q */ 0, /* Ans */ 1, /* Auth */ 0, /* Addnl */ 1);
+        dnsMsg->Validate(service, kInAnswerSection, kCheckSrv);
+        VerifyOrQuit(dnsMsg->GetNext() == nullptr);
+        sDnsMessages.Clear();
+    }
+
     Log("- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -");
     Log("Update service host name and validate new announcements of SRV record");