diff --git a/hio/lib/dhcp-svr.c b/hio/lib/dhcp-svr.c
index 2fe229b..d2207fd 100644
--- a/hio/lib/dhcp-svr.c
+++ b/hio/lib/dhcp-svr.c
@@ -34,6 +34,61 @@ struct hio_svc_dhcs_t
 };
 
 
+/*                         
+                   binding address         link address for relay.
+lo                 ip address of lo        unspec
+lo/::1,            [::1]:547               
+lo/[::1]:544       [::1]:544                           
+lo/[::1]:544/xxx   [::1]:544               xxx
+
+join multicast group??
+*/
+
+struct hio_svc_dhcs_cfg_t
+{
+	hio_skad_t bind_addr; /* if this is link local? */
+	hio_skad_t link_addr; /* want to use a different link address for relay? << if the peer address is link local, need to set this*/
+	const hio_bch_t* ifce_id; /* want to set Interface-ID? */
+};
+
+typedef struct hio_svc_dhcs_cfg_t hio_svc_dhcs_cfg_t;
+
+static hio_dev_sck_t* open_socket (hio_t* hio, hio_svc_dhcs_cfg_t* cfg)
+{
+	hio_dev_sck_make_t m;
+	hio_dev_sck_t* sck = HIO_NULL;
+	int f;
+
+	f = hio_skad_family(&cfg->bind_addr);
+	if (f != HIO_AF_INET || f != HIO_AF_INET6) 
+	{
+		hio_seterrbfmt (hio, HIO_EINVAL, "invalid bind address family");
+		goto oops;
+	}
+
+	HIO_MEMSET (&m, 0, HIO_SIZEOF(m));
+	m.type = HIO_DEV_SCK_UDP6;
+	m.options = HIO_DEV_SCK_BIND_REUSEADDR | HIO_DEV_SCK_BIND_REUSEPORT | HIO_DEV_SCK_BIND_IGNERR;
+	//m.on_write = 
+	//m.on_read = ...
+	//m.on_connect = ...
+	//m.on_disconnect = ...
+	sck = hio_dev_sck_make(hio, 0, &m);
+	if (HIO_UNLIKELY(!sck)) goto oops;
+
+
+#if 0
+	if (hio_dev_sck_joingroup(sck, mcast_addr, ifindex) <= -1) goto oops;
+#endif
+
+
+	return sck;
+
+oops:
+	if (sck) hio_dev_sck_kill (sck);
+	return HIO_NULL;
+}
+
 hio_svc_dhcs_t* hio_svc_dhcs_start (hio_t* hio, const hio_skad_t* local_binds, hio_oow_t local_nbinds)
 {
 	hio_svc_dhcs_t* dhcs;
@@ -49,6 +104,7 @@ hio_svc_dhcs_t* hio_svc_dhcs_start (hio_t* hio, const hio_skad_t* local_binds, h
 
 	dhcs->hio = hio;
 
+
 	for (i = 0; i < local_nbinds; i++)
 	{
 		HIO_MEMSET (&info, 0, HIO_SIZEOF(info));
diff --git a/hio/lib/sck.c b/hio/lib/sck.c
index 55057e9..c358b89 100644
--- a/hio/lib/sck.c
+++ b/hio/lib/sck.c
@@ -2464,12 +2464,18 @@ int hio_dev_sck_timedwritev (hio_dev_sck_t* dev, hio_iovec_t* iov, hio_iolen_t i
 /* ========================================================================= */
 int hio_dev_sck_setsockopt (hio_dev_sck_t* dev, int level, int optname, void* optval, hio_scklen_t optlen)
 {
-	return setsockopt(dev->hnd, level, optname, optval, optlen);
+	int n;
+	n = setsockopt(dev->hnd, level, optname, optval, optlen);
+	if (n <= -1) hio_seterrwithsyserr (dev->hio, 0, errno);
+	return n;
 }
 
 int hio_dev_sck_getsockopt (hio_dev_sck_t* dev, int level, int optname, void* optval, hio_scklen_t* optlen)
 {
-	return getsockopt(dev->hnd, level, optname, optval, optlen);
+	int n;
+	n = getsockopt(dev->hnd, level, optname, optval, optlen);
+	if (n <= -1) hio_seterrwithsyserr (dev->hio, 0, errno);
+	return n;
 }
 
 int hio_dev_sck_getsockaddr (hio_dev_sck_t* dev, hio_skad_t* skad)
@@ -2502,6 +2508,42 @@ int hio_dev_sck_getpeeraddr (hio_dev_sck_t* dev, hio_skad_t* skad)
 	return 0;
 }
 
+int hio_dev_sck_joinmcastgroup (hio_dev_sck_t* dev, const hio_skad_t* mcast_skad, int ifindex)
+{
+	int f;
+
+	f = hio_skad_family(mcast_skad);
+	switch (f)
+	{
+		case HIO_AF_INET:
+		{
+			struct ip_mreqn mreq;
+			HIO_MEMSET (&mreq, 0, HIO_SIZEOF(mreq));
+			//mreq.imr_multiaddr
+			mreq.imr_ifindex = ifindex;
+			/*mreq.imr_address = */
+			if (hio_dev_sck_setsockopt(dev, IPPROTO_IP, IP_ADD_MEMBERSHIP, &mreq, HIO_SIZEOF(mreq)) <= -1) return -1;
+			return 0;
+		}
+
+		case HIO_AF_INET6:
+		{
+			struct ipv6_mreq mreq;
+			HIO_MEMSET (&mreq, 0, HIO_SIZEOF(mreq));
+			mreq.ipv6mr_interface = ifindex; /* TODO: can reuse the scope id? */
+			//mreq.ipv6mr_multiaddr = hio_skad_ip6ad();
+			if (hio_dev_sck_setsockopt(dev, IPPROTO_IP, IPV6_JOIN_GROUP, &mreq, HIO_SIZEOF(mreq)) <= -1) return -1;
+			return 0;
+		}
+	}
+	
+
+	hio_seterrbfmt (hio_dev_sck_gethio(dev), HIO_EINVAL, "invalid multicast address family");
+	return -1;
+}
+
+/* ========================================================================= */
+
 int hio_dev_sck_shutdown (hio_dev_sck_t* dev, int how)
 {
 	switch (how & (HIO_DEV_SCK_SHUTDOWN_READ | HIO_DEV_SCK_SHUTDOWN_WRITE))