[mdns] allow hostnames beginning with a digit

A Matter device uses a hex as the hostname so it may begin with a digit
which wasn't allowed in NsdService. This CL looses this check to allow
Matter devices to register their services at Android Thread Border
Router.

Bug: 334828525
Test: CI
Change-Id: Ia779791e214158a08c2ec06adf8d5630ec8ea616

2 files changed, 41 insertions, 1 deletions

diff --git a/service-t/src/com/android/server/NsdService.java b/service-t/src/com/android/server/NsdService.java
index 8552eeca6b..7d077d5ae9 100644
--- a/service-t/src/com/android/server/NsdService.java
+++ b/service-t/src/com/android/server/NsdService.java
@@ -1825,12 +1825,20 @@ public class NsdService extends INsdManager.Stub {
      * <p>For now NsdService only allows single-label hostnames conforming to RFC 1035. In other
      * words, the hostname should be at most 63 characters long and it only contains letters, digits
      * and hyphens.
+     *
+     * <p>Additionally, this allows hostname starting with a digit to support Matter devices. Per
+     * Matter spec 4.3.1.1:
+     *
+     * <p>The target host name SHALL be constructed using one of the available link-layer addresses,
+     * such as a 48-bit device MAC address (for Ethernet and Wi‑Fi) or a 64-bit MAC Extended Address
+     * (for Thread) expressed as a fixed-length twelve-character (or sixteen-character) hexadecimal
+     * string, encoded as ASCII (UTF-8) text using capital letters, e.g., B75AFB458ECD.<domain>.
      */
     public static boolean checkHostname(@Nullable String hostname) {
         if (hostname == null) {
             return true;
         }
-        String HOSTNAME_REGEX = "^[a-zA-Z]([a-zA-Z0-9-_]{0,61}[a-zA-Z0-9])?$";
+        String HOSTNAME_REGEX = "^[a-zA-Z0-9]([a-zA-Z0-9-_]{0,61}[a-zA-Z0-9])?$";
         return Pattern.compile(HOSTNAME_REGEX).matcher(hostname).matches();
     }
 
diff --git a/tests/unit/java/com/android/server/NsdServiceTest.java b/tests/unit/java/com/android/server/NsdServiceTest.java
index d91e29c0de..78b9575c77 100644
--- a/tests/unit/java/com/android/server/NsdServiceTest.java
+++ b/tests/unit/java/com/android/server/NsdServiceTest.java
@@ -41,6 +41,7 @@ import static com.android.networkstack.apishim.api33.ConstantsShim.REGISTER_NSD_
 import static com.android.server.NsdService.DEFAULT_RUNNING_APP_ACTIVE_IMPORTANCE_CUTOFF;
 import static com.android.server.NsdService.MdnsListener;
 import static com.android.server.NsdService.NO_TRANSACTION;
+import static com.android.server.NsdService.checkHostname;
 import static com.android.server.NsdService.parseTypeAndSubtype;
 import static com.android.testutils.ContextUtils.mockService;
 
@@ -53,6 +54,7 @@ import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.assertFalse;
 import static org.mockito.ArgumentMatchers.anyInt;
 import static org.mockito.ArgumentMatchers.anyString;
 import static org.mockito.ArgumentMatchers.argThat;
@@ -1723,6 +1725,36 @@ public class NsdServiceTest {
     }
 
     @Test
+    public void TestCheckHostname() {
+        // Valid cases
+        assertTrue(checkHostname(null));
+        assertTrue(checkHostname("a"));
+        assertTrue(checkHostname("1"));
+        assertTrue(checkHostname("a-1234-bbbb-cccc000"));
+        assertTrue(checkHostname("A-1234-BBbb-CCCC000"));
+        assertTrue(checkHostname("1234-bbbb-cccc000"));
+        assertTrue(checkHostname("0123456789abcdef"
+                                + "0123456789abcdef"
+                                + "0123456789abcdef"
+                                + "0123456789abcde" // 63 characters
+                        ));
+
+        // Invalid cases
+        assertFalse(checkHostname("?"));
+        assertFalse(checkHostname("/"));
+        assertFalse(checkHostname("a-"));
+        assertFalse(checkHostname("B-"));
+        assertFalse(checkHostname("-A"));
+        assertFalse(checkHostname("-b"));
+        assertFalse(checkHostname("-1-"));
+        assertFalse(checkHostname("0123456789abcdef"
+                                + "0123456789abcdef"
+                                + "0123456789abcdef"
+                                + "0123456789abcdef" // 64 characters
+                        ));
+    }
+
+    @Test
     @EnableCompatChanges(ENABLE_PLATFORM_MDNS_BACKEND)
     public void testEnablePlatformMdnsBackend() {
         final NsdManager client = connectClient(mService);