[OSX] ip_mreqn struct is not supported for IPv4 Multicast

src/libraries/System.Net.Sockets/tests/FunctionalTests/SocketOptionNameTest.cs

@@ -70,7 +70,6 @@ public void MulticastOption_CreateSocketSetGetOption_GroupAndInterfaceIndex_SetS
         }
 
         [ConditionalFact(nameof(CanRunMulticastTests))]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/113827", typeof(PlatformDetection), nameof(PlatformDetection.IsAppleMobile))]
         public async Task MulticastInterface_Set_AnyInterface_Succeeds()
         {
             // On all platforms, index 0 means "any interface"

src/native/libs/Common/pal_config.h.in

@@ -70,6 +70,7 @@
 #cmakedefine01 HAVE_SUPPORT_FOR_DUAL_MODE_IPV4_PACKET_INFO
 #cmakedefine01 HAVE_IN_PKTINFO
 #cmakedefine01 HAVE_IP_MREQN
+#cmakedefine01 HAVE_IP_MULTICAST_IFINDEX
 #cmakedefine01 HAVE_NETINET_TCP_VAR_H
 #cmakedefine01 HAVE_NETINET_UDP_VAR_H
 #cmakedefine01 HAVE_NETINET_IP_VAR_H

src/native/libs/System.Native/pal_networking.c

@@ -1186,6 +1186,28 @@ int32_t SystemNative_GetIPv4MulticastOption(intptr_t socket, int32_t multicastOp
     return Error_SUCCESS;
 }
 
+#if HAVE_IP_MULTICAST_IFINDEX
+static int32_t SetIPv4MulticastInterfaceByIndexOrIPAddress(int fd, IPv4MulticastOption* option)
+{
+    if (option->InterfaceIndex != 0)
+    {
+        // Use IP_MULTICAST_IFINDEX when interface index is specified
+        uint32_t ifindex = (uint32_t)option->InterfaceIndex;
+        int err = setsockopt(fd, IPPROTO_IP, IP_MULTICAST_IFINDEX, &ifindex, sizeof(ifindex));
+        return err == 0 ? Error_SUCCESS : SystemNative_ConvertErrorPlatformToPal(errno);
+    }
+    else
+    {
+        struct ip_mreq opt;
+        memset(&opt, 0, sizeof(struct ip_mreq));
+        opt.imr_multiaddr.s_addr = option->MulticastAddress;
+        opt.imr_interface.s_addr = option->LocalAddress;
+        int err = setsockopt(fd, IPPROTO_IP, IP_MULTICAST_IF, &opt, sizeof(opt));
+        return err == 0 ? Error_SUCCESS : SystemNative_ConvertErrorPlatformToPal(errno);
+    }
+}
+#endif
+
 int32_t SystemNative_SetIPv4MulticastOption(intptr_t socket, int32_t multicastOption, IPv4MulticastOption* option)
 {
     if (option == NULL)
@@ -1201,6 +1223,13 @@ int32_t SystemNative_SetIPv4MulticastOption(intptr_t socket, int32_t multicastOp
         return Error_EINVAL;
     }
 
+#if HAVE_IP_MULTICAST_IFINDEX
+    if (optionName == SocketOptionName_SO_IP_MULTICAST_IF)
+    {
+        return SetIPv4MulticastInterfaceByIndexOrIPAddress(fd, option);
+    }
+#endif
+
 #if HAVE_IP_MREQN
     struct ip_mreqn opt;
     memset(&opt, 0, sizeof(struct ip_mreqn));

src/native/libs/configure.cmake

@@ -82,6 +82,18 @@ check_c_source_compiles(
     "
     HAVE_IP_MREQN)
 
+check_c_source_compiles(
+    "
+    #include <sys/socket.h>
+    #include <${SOCKET_INCLUDES}>
+    int main(void)
+    {
+        int opt = IP_MULTICAST_IFINDEX;
+        return 0;
+    }
+    "
+    HAVE_IP_MULTICAST_IFINDEX)
+
 # /in_pktinfo
 
 check_c_source_compiles(