Linux impl of mDNS is not setting interfaceId on browse/resolve callback

[samadDotDev]

Problem

Linux platform implementation of chip::Mdns is not setting interfaceId for browse/resolve callbacks once retrieved from avahi service browse/resolve.

Proposed Solution

MdnsAvahi::HandleBrowse and MdnsAvahi::HandleResolve need to set interfaceId in MdnsService structure before returning the services to browse callback.

diff --git a/src/platform/Linux/MdnsImpl.cpp b/src/platform/Linux/MdnsImpl.cpp
index 205d840c83..be6b9df526 100644
--- a/src/platform/Linux/MdnsImpl.cpp
+++ b/src/platform/Linux/MdnsImpl.cpp
@@ -591,6 +591,11 @@ void MdnsAvahi::HandleBrowse(AvahiServiceBrowser * browser, AvahiIfIndex interfa
             CopyTypeWithoutProtocol(service.mType, type);
             service.mProtocol               = GetProtocolInType(type);
             service.mAddressType            = ToAddressType(protocol);
+            service.mInterface              = INET_NULL_INTERFACEID;
+            if (interface != AVAHI_IF_UNSPEC)
+            {
+                service.mInterface = static_cast<chip::Inet::InterfaceId>(interface);
+            }
             service.mType[kMdnsTypeMaxSize] = 0;
             context->mServices.push_back(service);
         }
@@ -670,6 +675,11 @@ void MdnsAvahi::HandleResolve(AvahiServiceResolver * resolver, AvahiIfIndex inte
         result.mProtocol    = GetProtocolInType(type);
         result.mPort        = port;
         result.mAddressType = ToAddressType(protocol);
+        result.mInterface   = INET_NULL_INTERFACEID;
+        if (interface != AVAHI_IF_UNSPEC)
+        {
+            result.mInterface = static_cast<chip::Inet::InterfaceId>(interface);
+        }
         Platform::CopyString(result.mHostName, host_name);
         // Returned value is full QName, want only host part.
         char * dot = strchr(result.mHostName, '.');

[bzbarsky-apple]

Does this end up breaking use of devices that have an ipv6 link local address with a Linux controller?

@cecille @andy31415

[samadDotDev]

Yes it does, the interfaceId is required for addressing link-local ipv6 addresses and callbacks don't return it for such addresses.

[cecille]

The issue is correct and this can break IPv6 in practice. Depending on the network stack, if you use a LL address and there's only one interface (the case for most of the devices implemented here if I'm not mistaken), it will probably work even without the scope set. But yeah, the interface should be set. If you throw that patch up as a PR, I'm sure it'll get approved.