diff --git a/src/api/sockets.c b/src/api/sockets.c
index f67162ab..27d8bc76 100644
--- a/src/api/sockets.c
+++ b/src/api/sockets.c
@@ -2020,7 +2020,7 @@ lwip_getsockopt_impl(int s, int level, int optname, void *optval, socklen_t *opt
       LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_getsockopt(%d, IPPROTO_IP, IP_TOS) = %d\n",
                   s, *(int *)optval));
       break;
-#if LWIP_MULTICAST_TX_OPTIONS
+#if LWIP_IPV4 && LWIP_MULTICAST_TX_OPTIONS
     case IP_MULTICAST_TTL:
       LWIP_SOCKOPT_CHECK_OPTLEN_CONN_PCB(sock, *optlen, u8_t);
       if (NETCONNTYPE_GROUP(netconn_type(sock->conn)) != NETCONN_UDP) {
@@ -2049,7 +2049,7 @@ lwip_getsockopt_impl(int s, int level, int optname, void *optval, socklen_t *opt
       LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_getsockopt(%d, IPPROTO_IP, IP_MULTICAST_LOOP) = %d\n",
                   s, *(int *)optval));
       break;
-#endif /* LWIP_MULTICAST_TX_OPTIONS */
+#endif /* LWIP_IPV4 && LWIP_MULTICAST_TX_OPTIONS */
     default:
       LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_getsockopt(%d, IPPROTO_IP, UNIMPL: optname=0x%x, ..)\n",
                   s, optname));
@@ -2389,7 +2389,7 @@ lwip_setsockopt_impl(int s, int level, int optname, const void *optval, socklen_
       LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_setsockopt(%d, IPPROTO_IP, IP_TOS, ..)-> %d\n",
                   s, sock->conn->pcb.ip->tos));
       break;
-#if LWIP_MULTICAST_TX_OPTIONS
+#if LWIP_IPV4 && LWIP_MULTICAST_TX_OPTIONS
     case IP_MULTICAST_TTL:
       LWIP_SOCKOPT_CHECK_OPTLEN_CONN_PCB_TYPE(sock, optlen, u8_t, NETCONN_UDP);
       udp_set_multicast_ttl(sock->conn->pcb.udp, (u8_t)(*(const u8_t*)optval));
@@ -2410,7 +2410,7 @@ lwip_setsockopt_impl(int s, int level, int optname, const void *optval, socklen_
         udp_setflags(sock->conn->pcb.udp, udp_flags(sock->conn->pcb.udp) & ~UDP_FLAGS_MULTICAST_LOOP);
       }
       break;
-#endif /* LWIP_MULTICAST_TX_OPTIONS */
+#endif /* LWIP_IPV4 && LWIP_MULTICAST_TX_OPTIONS */
 #if LWIP_IGMP
     case IP_ADD_MEMBERSHIP:
     case IP_DROP_MEMBERSHIP:
diff --git a/src/core/init.c b/src/core/init.c
index ee415a97..1f89997e 100644
--- a/src/core/init.c
+++ b/src/core/init.c
@@ -96,7 +96,7 @@ PACK_STRUCT_END
   #error "If you want to use DHCP, you have to define LWIP_UDP=1 in your lwipopts.h"
 #endif
 #if (!LWIP_UDP && LWIP_MULTICAST_TX_OPTIONS)
-  #error "If you want to use IGMP/LWIP_MULTICAST_TX_OPTIONS, you have to define LWIP_UDP=1 in your lwipopts.h"
+  #error "If you want to use LWIP_MULTICAST_TX_OPTIONS, you have to define LWIP_UDP=1 in your lwipopts.h"
 #endif
 #if (!LWIP_UDP && LWIP_DNS)
   #error "If you want to use DNS, you have to define LWIP_UDP=1 in your lwipopts.h"
@@ -123,9 +123,6 @@ PACK_STRUCT_END
 #if (LWIP_IGMP && !LWIP_IPV4)
   #error "IGMP needs LWIP_IPV4 enabled in your lwipopts.h"
 #endif
-#if (LWIP_MULTICAST_TX_OPTIONS && !LWIP_IPV4)
-  #error "LWIP_MULTICAST_TX_OPTIONS needs LWIP_IPV4 enabled in your lwipopts.h"
-#endif
 #if ((LWIP_NETCONN || LWIP_SOCKET) && (MEMP_NUM_TCPIP_MSG_API<=0))
   #error "If you want to use Sequential API, you have to define MEMP_NUM_TCPIP_MSG_API>=1 in your lwipopts.h"
 #endif
diff --git a/src/core/ipv6/ip6.c b/src/core/ipv6/ip6.c
index 55567073..59d112d9 100644
--- a/src/core/ipv6/ip6.c
+++ b/src/core/ipv6/ip6.c
@@ -1028,6 +1028,11 @@ ip6_output_if_src(struct pbuf *p, const ip6_addr_t *src, const ip6_addr_t *dest,
       }
     }
   }
+#if LWIP_MULTICAST_TX_OPTIONS
+  if ((p->flags & PBUF_FLAG_MCASTLOOP) != 0) {
+    netif_loop_output(netif, p);
+  }
+#endif /* LWIP_MULTICAST_TX_OPTIONS */
 #endif /* ENABLE_LOOPBACK */
 #if LWIP_IPV6_FRAG
   /* don't fragment if interface has mtu set to 0 [loopif] */
diff --git a/src/core/udp.c b/src/core/udp.c
index 5bdb3cd1..25a04ade 100644
--- a/src/core/udp.c
+++ b/src/core/udp.c
@@ -514,7 +514,7 @@ udp_sendto_chksum(struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *dst_ip,
 {
 #endif /* LWIP_CHECKSUM_ON_COPY && CHECKSUM_GEN_UDP */
   struct netif *netif;
-  const ip_addr_t *dst_ip_route = dst_ip;
+  const ip_addr_t *src_ip_route;
 
   if ((pcb == NULL) || (dst_ip == NULL) || !IP_ADDR_PCB_VERSION_MATCH(pcb, dst_ip)) {
     return ERR_VAL;
@@ -522,8 +522,31 @@ udp_sendto_chksum(struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *dst_ip,
 
   LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE, ("udp_send\n"));
 
-#if LWIP_IPV4 && LWIP_MULTICAST_TX_OPTIONS
-  if (ip_addr_ismulticast(dst_ip_route)) {
+  if (IP_IS_ANY_TYPE_VAL(pcb->local_ip)) {
+    /* Don't call ip_route() with IP_ANY_TYPE */
+    src_ip_route = IP46_ADDR_ANY(IP_GET_TYPE(dst_ip));
+  } else {
+    src_ip_route = &pcb->local_ip;
+  }
+
+#if LWIP_MULTICAST_TX_OPTIONS
+  netif = NULL;
+  if (ip_addr_ismulticast(dst_ip)) {
+    /* For IPv6, the interface to use for packets with a multicast destination
+     * is specified using an interface index. The same approach may be used for
+     * IPv4 as well, in which case it overrides the IPv4 multicast override
+     * address below. Here we have to look up the netif by going through the
+     * list, but by doing so we skip a route lookup. If the interface index has
+     * gone stale, we fall through and do the regular route lookup after all. */
+    if (pcb->mcast_ifindex != NETIF_NO_INDEX) {
+      for (netif = netif_list; netif != NULL; netif = netif->next) {
+        if (pcb->mcast_ifindex == netif_num_to_index(netif)) {
+          break; /* found! */
+        }
+      }
+    }
+#if LWIP_IPV4
+    else
 #if LWIP_IPV6
     if (IP_IS_V4(dst_ip))
 #endif /* LWIP_IPV6 */
@@ -531,21 +554,21 @@ udp_sendto_chksum(struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *dst_ip,
       /* IPv4 does not use source-based routing by default, so we use an
          administratively selected interface for multicast by default.
          However, this can be overridden by setting an interface address
-         in pcb->multicast_ip that is used for routing. */
-      if (!ip_addr_isany_val(pcb->multicast_ip) &&
-          !ip4_addr_cmp(ip_2_ip4(&pcb->multicast_ip), IP4_ADDR_BROADCAST)) {
-        dst_ip_route = &pcb->multicast_ip;
+         in pcb->mcast_ip4 that is used for routing. If this routing lookup
+         fails, we try regular routing as though no override was set. */
+      if (!ip4_addr_isany_val(pcb->mcast_ip4) &&
+          !ip4_addr_cmp(&pcb->mcast_ip4, IP4_ADDR_BROADCAST)) {
+        netif = ip4_route_src(ip_2_ip4(src_ip_route), &pcb->mcast_ip4);
       }
     }
+#endif /* LWIP_IPV4 */
   }
-#endif /* LWIP_IPV4 && LWIP_MULTICAST_TX_OPTIONS */
 
-  /* find the outgoing network interface for this packet */
-  if(IP_IS_ANY_TYPE_VAL(pcb->local_ip)) {
-    /* Don't call ip_route() with IP_ANY_TYPE */
-    netif = ip_route(IP46_ADDR_ANY(IP_GET_TYPE(dst_ip_route)), dst_ip_route);
-  } else {
-    netif = ip_route(&pcb->local_ip, dst_ip_route);
+  if (netif == NULL)
+#endif /* LWIP_MULTICAST_TX_OPTIONS */
+  {
+    /* find the outgoing network interface for this packet */
+    netif = ip_route(src_ip_route, dst_ip);
   }
 
   /* no outgoing network interface could be found? */
@@ -604,10 +627,11 @@ udp_sendto_if_chksum(struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *dst_i
     return ERR_VAL;
   }
 
-  /* PCB local address is IP_ANY_ADDR? */
+  /* PCB local address is IP_ANY_ADDR or multicast? */
 #if LWIP_IPV6
   if (IP_IS_V6(dst_ip)) {
-    if (ip6_addr_isany(ip_2_ip6(&pcb->local_ip))) {
+    if (ip6_addr_isany(ip_2_ip6(&pcb->local_ip)) ||
+        ip6_addr_ismulticast(ip_2_ip6(&pcb->local_ip))) {
       src_ip = ip6_select_source_address(netif, ip_2_ip6(dst_ip));
       if (src_ip == NULL) {
         /* No suitable source address was found. */
@@ -737,11 +761,11 @@ udp_sendto_if_src_chksum(struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *d
   udphdr->chksum = 0x0000;
 
   /* Multicast Loop? */
-#if (LWIP_IPV4 && LWIP_MULTICAST_TX_OPTIONS) || (LWIP_IPV6 && LWIP_IPV6_MLD)
+#if LWIP_MULTICAST_TX_OPTIONS
   if (((pcb->flags & UDP_FLAGS_MULTICAST_LOOP) != 0) && ip_addr_ismulticast(dst_ip)) {
     q->flags |= PBUF_FLAG_MCASTLOOP;
   }
-#endif /* (LWIP_IPV4 && LWIP_MULTICAST_TX_OPTIONS) || (LWIP_IPV6 && LWIP_IPV6_MLD) */
+#endif /* LWIP_MULTICAST_TX_OPTIONS */
 
   LWIP_DEBUGF(UDP_DEBUG, ("udp_send: sending datagram of length %"U16_F"\n", q->tot_len));
 
@@ -917,7 +941,7 @@ udp_bind(struct udp_pcb *pcb, const ip_addr_t *ipaddr, u16_t port)
    * This is legacy support: scope-aware callers should always provide properly
    * zoned source addresses. Do the zone selection before the address-in-use
    * check below; as such we have to make a temporary copy of the address. */
-  if (IP_IS_V6(ipaddr) && ip6_addr_lacks_zone(ip_2_ip6(ipaddr), IP6_UNICAST)) {
+  if (IP_IS_V6(ipaddr) && ip6_addr_lacks_zone(ip_2_ip6(ipaddr), IP6_UNKNOWN)) {
     ip_addr_copy(zoned_ipaddr, *ipaddr);
     ip6_addr_select_zone(ip_2_ip6(&zoned_ipaddr), ip_2_ip6(&zoned_ipaddr));
     ipaddr = &zoned_ipaddr;
diff --git a/src/include/lwip/netif.h b/src/include/lwip/netif.h
index 7241b42a..0b47ec06 100644
--- a/src/include/lwip/netif.h
+++ b/src/include/lwip/netif.h
@@ -504,6 +504,7 @@ char * netif_index_to_name(u8_t idx, char *name);
 /* Interface indexes always start at 1 per RFC 3493, section 4, num starts at 0 */
 #define netif_num_to_index(netif)   ((netif)->num + 1)
 #define netif_index_to_num(index)   ((index) - 1)
+#define NETIF_NO_INDEX              (0)
 
 #ifdef __cplusplus
 }
diff --git a/src/include/lwip/opt.h b/src/include/lwip/opt.h
index eb911e92..1c4f2919 100644
--- a/src/include/lwip/opt.h
+++ b/src/include/lwip/opt.h
@@ -972,7 +972,29 @@
 
 /*
    ----------------------------------
-   ----- Multicast/IGMP options -----
+   -------- Multicast options -------
+   ----------------------------------
+*/
+/**
+ * @defgroup lwip_opts_multicast Multicast
+ * @ingroup lwip_opts_infrastructure
+ * @{
+ */
+/**
+ * LWIP_MULTICAST_TX_OPTIONS==1: Enable multicast TX support like the socket options
+ * IP_MULTICAST_TTL/IP_MULTICAST_IF/IP_MULTICAST_LOOP, as well as (currently only)
+ * core support for the corresponding IPv6 options.
+ */
+#if !defined LWIP_MULTICAST_TX_OPTIONS || defined __DOXYGEN__
+#define LWIP_MULTICAST_TX_OPTIONS       ((LWIP_IGMP || LWIP_IPV6_MLD) && LWIP_UDP)
+#endif
+/**
+ * @}
+ */
+
+/*
+   ----------------------------------
+   ---------- IGMP options ----------
    ----------------------------------
 */
 /**
@@ -990,14 +1012,6 @@
 #undef LWIP_IGMP
 #define LWIP_IGMP                       0
 #endif
-
-/**
- * LWIP_MULTICAST_TX_OPTIONS==1: Enable multicast TX support like the socket options
- * IP_MULTICAST_TTL/IP_MULTICAST_IF/IP_MULTICAST_LOOP
- */
-#if !defined LWIP_MULTICAST_TX_OPTIONS || defined __DOXYGEN__
-#define LWIP_MULTICAST_TX_OPTIONS       (LWIP_IGMP && LWIP_UDP)
-#endif
 /**
  * @}
  */
diff --git a/src/include/lwip/udp.h b/src/include/lwip/udp.h
index b9299073..30cfb332 100644
--- a/src/include/lwip/udp.h
+++ b/src/include/lwip/udp.h
@@ -91,8 +91,12 @@ struct udp_pcb {
   u16_t local_port, remote_port;
 
 #if LWIP_MULTICAST_TX_OPTIONS
-  /** outgoing network interface for multicast packets */
-  ip_addr_t multicast_ip;
+#if LWIP_IPV4
+  /** outgoing network interface for multicast packets, by IPv4 address (if not 'any') */
+  ip4_addr_t mcast_ip4;
+#endif /* LWIP_IPV4 */
+  /** outgoing network interface for multicast packets, by interface index (if nonzero) */
+  u8_t mcast_ifindex;
   /** TTL for outgoing multicast packets */
   u8_t mcast_ttl;
 #endif /* LWIP_MULTICAST_TX_OPTIONS */
@@ -159,9 +163,13 @@ void             udp_init       (void);
 #define udp_new_ip6() udp_new_ip_type(IPADDR_TYPE_V6)
 
 #if LWIP_MULTICAST_TX_OPTIONS
-#define udp_set_multicast_netif_addr(pcb, ip4addr) ip_addr_copy_from_ip4((pcb)->multicast_ip, *(ip4addr))
-#define udp_get_multicast_netif_addr(pcb)          ip_2_ip4(&(pcb)->multicast_ip)
-#define udp_set_multicast_ttl(pcb, value)      do { (pcb)->mcast_ttl = value; } while(0)
+#if LWIP_IPV4
+#define udp_set_multicast_netif_addr(pcb, ip4addr) ip4_addr_copy((pcb)->mcast_ip4, *(ip4addr))
+#define udp_get_multicast_netif_addr(pcb)          (&(pcb)->mcast_ip4)
+#endif /* LWIP_IPV4 */
+#define udp_set_multicast_netif_index(pcb, idx)    ((pcb)->mcast_ifindex = (idx))
+#define udp_get_multicast_netif_index(pcb)         ((pcb)->mcast_ifindex)
+#define udp_set_multicast_ttl(pcb, value)          ((pcb)->mcast_ttl = (value))
 #define udp_get_multicast_ttl(pcb)                 ((pcb)->mcast_ttl)
 #endif /* LWIP_MULTICAST_TX_OPTIONS */