[trel] require link-local for peer discovered by mDNS (#2709)

Author: jwhui

.github/workflows/border_router.yml

@@ -73,14 +73,6 @@ jobs:
             otbr_mdns: "mDNSResponder"
             cert_scripts: ./tests/scripts/thread-cert/border_router/*.py
             packet_verification: 2
-          - name: "Border Router TREL (Avahi)"
-            otbr_options: "-DOT_DUA=ON -DOT_ECDSA=ON -DOT_MLR=ON -DOT_SERVICE=ON -DOT_SRP_SERVER=ON -DOTBR_COVERAGE=ON -DOTBR_DUA_ROUTING=ON -DOTBR_TREL=ON -DOTBR_DNS_UPSTREAM_QUERY=ON"
-            border_routing: 1
-            internet: 0
-            dnssd_plat: 0
-            otbr_mdns: "avahi"
-            cert_scripts: ./tests/scripts/thread-cert/border_router/*.py
-            packet_verification: 2
           - name: "Border Router MATN (mDNSResponder)"
             otbr_options: "-DOT_DUA=ON -DOT_ECDSA=ON -DOT_MLR=ON -DOT_SERVICE=ON -DOT_SRP_SERVER=ON -DOTBR_COVERAGE=ON -DOTBR_DUA_ROUTING=ON -DOTBR_TREL=OFF -DOTBR_DNS_UPSTREAM_QUERY=ON"
             border_routing: 1
@@ -105,24 +97,16 @@ jobs:
             otbr_mdns: "mDNSResponder"
             cert_scripts: ./tests/scripts/thread-cert/backbone/*.py
             packet_verification: 1
-          - name: "Border Router TREL with FEATURE_FLAG (avahi)"
-            otbr_options: "-DOT_DUA=ON -DOT_ECDSA=ON -DOT_MLR=ON -DOT_SERVICE=ON -DOT_SRP_SERVER=ON -DOTBR_COVERAGE=ON -DOTBR_DUA_ROUTING=ON -DOTBR_FEATURE_FLAGS=ON -DOTBR_TELEMETRY_DATA_API=ON -DOTBR_TREL=ON -DOTBR_DNS_UPSTREAM_QUERY=ON"
-            border_routing: 1
-            internet: 0
-            dnssd_plat: 0
-            otbr_mdns: "avahi"
-            cert_scripts: ./tests/scripts/thread-cert/border_router/*.py
-            packet_verification: 2
           - name: "Border Router with OT Core Advertising Proxy (avahi)"
-            otbr_options: "-DOT_DUA=ON -DOT_ECDSA=ON -DOT_MLR=ON -DOT_SERVICE=ON -DOT_SRP_SERVER=ON -DOTBR_COVERAGE=ON -DOTBR_DUA_ROUTING=ON -DOTBR_TREL=ON -DOTBR_DNS_UPSTREAM_QUERY=ON"
+            otbr_options: "-DOT_DUA=ON -DOT_ECDSA=ON -DOT_MLR=ON -DOT_SERVICE=ON -DOT_SRP_SERVER=ON -DOTBR_COVERAGE=ON -DOTBR_DUA_ROUTING=ON -DOTBR_TREL=OFF -DOTBR_DNS_UPSTREAM_QUERY=ON"
             border_routing: 1
             internet: 0
             dnssd_plat: 1
             otbr_mdns: "avahi"
             cert_scripts: ./tests/scripts/thread-cert/border_router/*.py
             packet_verification: 1
           - name: "Border Router with OT Core Advertising Proxy (mDNSResponder)"
-            otbr_options: "-DOT_DUA=ON -DOT_ECDSA=ON -DOT_MLR=ON -DOT_SERVICE=ON -DOT_SRP_SERVER=ON -DOTBR_COVERAGE=ON -DOTBR_DUA_ROUTING=ON -DOTBR_TREL=ON -DOTBR_DNS_UPSTREAM_QUERY=ON"
+            otbr_options: "-DOT_DUA=ON -DOT_ECDSA=ON -DOT_MLR=ON -DOT_SERVICE=ON -DOT_SRP_SERVER=ON -DOTBR_COVERAGE=ON -DOTBR_DUA_ROUTING=ON -DOTBR_TREL=OFF -DOTBR_DNS_UPSTREAM_QUERY=ON"
             border_routing: 1
             internet: 0
             dnssd_plat: 1

src/trel_dnssd/trel_dnssd.cpp

@@ -301,25 +301,22 @@ void TrelDnssd::OnTrelServiceInstanceAdded(const Mdns::Publisher::DiscoveredInst
     {
         otbrLogDebug("Peer address: %s", addr.ToString().c_str());
 
-        // Skip anycast (Refer to https://datatracker.ietf.org/doc/html/rfc2373#section-2.6.1)
-        if (addr.m64[1] == 0)
+        // Require link-local. Thread requires TREL peers to advertise link-local via mDNS.
+        if (!addr.IsLinkLocal())
         {
             continue;
         }
 
-        // If there are multiple addresses, we prefer the address
-        // which is numerically smallest. This prefers GUA over ULA
-        // (`fc00::/7`) and then link-local (`fe80::/10`).
-
+        // Pick the smallest link-local to be robust to reorderings.
         if (selectedAddress.IsUnspecified() || (addr < selectedAddress))
         {
             selectedAddress = addr;
         }
     }
 
-    if (aInstanceInfo.mAddresses.empty())
+    if (selectedAddress.IsUnspecified())
     {
-        otbrLogWarning("Peer %s does not have any IPv6 address, ignored", aInstanceInfo.mName.c_str());
+        otbrLogWarning("Peer %s does not have any IPv6 link-local address, ignored", aInstanceInfo.mName.c_str());
         ExitNow();
     }