net: dns: Join mDNS multicast group for resolving

If mDNS resolver is enabled but mDNS responder is not, then
mDNS multicast address group is not joined. This would prevent
the mDNS resolver to receive the responses. Fix this by
joining the mDNS multicast group if mDNS responder is not
enabled (because the responder will join the group itself).

Fixes #86477

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

Author: jukkar

subsys/net/lib/dns/resolve.c

@@ -25,6 +25,8 @@ LOG_MODULE_REGISTER(net_dns_resolve, CONFIG_DNS_RESOLVER_LOG_LEVEL);
 #include <zephyr/net/net_ip.h>
 #include <zephyr/net/net_pkt.h>
 #include <zephyr/net/net_mgmt.h>
+#include <zephyr/net/igmp.h>
+#include <zephyr/net/mld.h>
 #include <zephyr/net/dns_resolve.h>
 #include <zephyr/net/socket_service.h>
 #include "../../ip/net_private.h"
@@ -148,6 +150,34 @@ static bool server_is_llmnr(sa_family_t family, struct sockaddr *addr)
 	return false;
 }
 
+static void join_ipv4_mcast_group(struct net_if *iface, void *user_data)
+{
+	struct sockaddr *addr = user_data;
+	int ret;
+
+	ret = net_ipv4_igmp_join(iface, &net_sin(addr)->sin_addr, NULL);
+	if (ret < 0 && ret != -EALREADY) {
+		NET_DBG("Cannot join %s mDNS group (%d)", "IPv4", ret);
+	} else {
+		NET_DBG("Joined %s mDNS group %s", "IPv4",
+			net_sprint_ipv4_addr(&net_sin(addr)->sin_addr));
+	}
+}
+
+static void join_ipv6_mcast_group(struct net_if *iface, void *user_data)
+{
+	struct sockaddr *addr = user_data;
+	int ret;
+
+	ret = net_ipv6_mld_join(iface, &net_sin6(addr)->sin6_addr);
+	if (ret < 0 && ret != -EALREADY) {
+		NET_DBG("Cannot join %s mDNS group (%d)", "IPv6", ret);
+	} else {
+		NET_DBG("Joined %s mDNS group %s", "IPv6",
+			net_sprint_ipv6_addr(&net_sin6(addr)->sin6_addr));
+	}
+}
+
 static void dns_postprocess_server(struct dns_resolve_context *ctx, int idx)
 {
 	struct sockaddr *addr = &ctx->servers[idx].dns_server;
@@ -180,6 +210,26 @@ static void dns_postprocess_server(struct dns_resolve_context *ctx, int idx)
 				net_sin(addr)->sin_port = htons(53);
 			}
 		}
+
+		/* Join the mDNS multicast group if responder is not enabled,
+		 * because it will join the group itself.
+		 */
+		if (!IS_ENABLED(CONFIG_MDNS_RESPONDER) && ctx->servers[idx].is_mdns) {
+			struct in_addr mcast_addr = { { { 224, 0, 0, 251 } } };
+
+			if (net_sin(addr)->sin_addr.s_addr == mcast_addr.s_addr) {
+				struct net_if *iface;
+
+				iface = net_if_get_by_index(ctx->servers[idx].if_index);
+				if (iface == NULL) {
+					/* Join all interfaces */
+					net_if_foreach(join_ipv4_mcast_group, addr);
+				} else {
+					/* Join specific interface */
+					join_ipv4_mcast_group(iface, addr);
+				}
+			}
+		}
 	} else {
 		ctx->servers[idx].is_mdns = server_is_mdns(AF_INET6, addr);
 		if (!ctx->servers[idx].is_mdns) {
@@ -198,6 +248,25 @@ static void dns_postprocess_server(struct dns_resolve_context *ctx, int idx)
 				net_sin6(addr)->sin6_port = htons(53);
 			}
 		}
+
+		if (!IS_ENABLED(CONFIG_MDNS_RESPONDER) && ctx->servers[idx].is_mdns) {
+			struct in6_addr mcast_addr = { { { 0xff, 0x02, 0, 0, 0, 0, 0, 0,
+						0, 0, 0, 0, 0, 0, 0, 0xfb } } };
+
+			if (memcmp(&net_sin6(addr)->sin6_addr, &mcast_addr,
+				   sizeof(struct in6_addr)) == 0) {
+				struct net_if *iface;
+
+				iface = net_if_get_by_index(ctx->servers[idx].if_index);
+				if (iface == NULL) {
+					/* Join all interfaces */
+					net_if_foreach(join_ipv6_mcast_group, addr);
+				} else {
+					/* Join specific interface */
+					join_ipv6_mcast_group(iface, addr);
+				}
+			}
+		}
 	}
 }