diff --git a/sys/netlink/netlink_message_writer.h b/sys/netlink/netlink_message_writer.h
--- a/sys/netlink/netlink_message_writer.h
+++ b/sys/netlink/netlink_message_writer.h
@@ -49,9 +49,12 @@
 	void			*_storage;	/* Underlying storage pointer */
 	nl_writer_cb		*cb;		/* Callback to flush data */
 	union {
-		void		*arg_ptr;	/* Callback argument as pointer */
-		uint64_t	arg_uint;	/* Callback argument as int */
-	};
+		void		*ptr;
+		struct {
+			uint16_t	proto;
+			uint16_t	id;
+		} group;
+	} arg;
 	int			num_messages;	/* Number of messages in the buffer */
 	int			malloc_flag;	/* M_WAITOK or M_NOWAIT */
 	uint8_t			writer_type;	/* NS_WRITER_TYPE_* */
diff --git a/sys/netlink/netlink_message_writer.c b/sys/netlink/netlink_message_writer.c
--- a/sys/netlink/netlink_message_writer.c
+++ b/sys/netlink/netlink_message_writer.c
@@ -116,7 +116,7 @@
 static bool
 nlmsg_write_socket_buf(struct nl_writer *nw, void *buf, int datalen, int cnt)
 {
-	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d arg: %p", buf, datalen, nw);
+	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d arg: %p", buf, datalen, nw->arg.ptr);
 	if (__predict_false(datalen == 0)) {
 		free(buf, M_NETLINK);
 		return (true);
@@ -132,13 +132,14 @@
 	free(buf, M_NETLINK);
 
 	int io_flags = (nw->ignore_limit) ? NL_IOF_IGNORE_LIMIT : 0;
-	return (nl_send_one(m, (struct nlpcb *)(nw->arg_ptr), cnt, io_flags));
+	return (nl_send_one(m, (struct nlpcb *)(nw->arg.ptr), cnt, io_flags));
 }
 
 static bool
 nlmsg_write_group_buf(struct nl_writer *nw, void *buf, int datalen, int cnt)
 {
-	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d arg: %p", buf, datalen, nw->arg_ptr);
+	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d proto: %d id: %d", buf, datalen,
+	    nw->arg.group.proto, nw->arg.group.id);
 	if (__predict_false(datalen == 0)) {
 		free(buf, M_NETLINK);
 		return (true);
@@ -155,15 +156,15 @@
 	if (!success)
 		return (false);
 
-	nl_send_group(m, cnt, nw->arg_uint >> 16, nw->arg_uint & 0xFFFF);
+	nl_send_group(m, cnt, nw->arg.group.proto, nw->arg.group.id);
 	return (true);
 }
 
 static bool
 nlmsg_write_chain_buf(struct nl_writer *nw, void *buf, int datalen, int cnt)
 {
-	struct mbuf **m0 = (struct mbuf **)(nw->arg_ptr);
-	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d arg: %p", buf, datalen, nw->arg_ptr);
+	struct mbuf **m0 = (struct mbuf **)(nw->arg.ptr);
+	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d arg: %p", buf, datalen, nw->arg.ptr);
 
 	if (__predict_false(datalen == 0)) {
 		free(buf, M_NETLINK);
@@ -227,7 +228,7 @@
 nlmsg_write_socket_mbuf(struct nl_writer *nw, void *buf, int datalen, int cnt)
 {
 	struct mbuf *m = (struct mbuf *)buf;
-	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d arg: %p", buf, datalen, nw->arg_ptr);
+	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d arg: %p", buf, datalen, nw->arg.ptr);
 
 	if (__predict_false(datalen == 0)) {
 		m_freem(m);
@@ -237,14 +238,14 @@
 	m->m_pkthdr.len = datalen;
 	m->m_len = datalen;
 	int io_flags = (nw->ignore_limit) ? NL_IOF_IGNORE_LIMIT : 0;
-	return (nl_send_one(m, (struct nlpcb *)(nw->arg_ptr), cnt, io_flags));
+	return (nl_send_one(m, (struct nlpcb *)(nw->arg.ptr), cnt, io_flags));
 }
 
 static bool
 nlmsg_write_group_mbuf(struct nl_writer *nw, void *buf, int datalen, int cnt)
 {
 	struct mbuf *m = (struct mbuf *)buf;
-	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d arg: %p", buf, datalen, nw->arg_ptr);
+	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d arg: %p", buf, datalen, nw->arg.ptr);
 
 	if (__predict_false(datalen == 0)) {
 		m_freem(m);
@@ -253,7 +254,7 @@
 
 	m->m_pkthdr.len = datalen;
 	m->m_len = datalen;
-	nl_send_group(m, cnt, nw->arg_uint >> 16, nw->arg_uint & 0xFFFF);
+	nl_send_group(m, cnt, nw->arg.group.proto, nw->arg.group.id);
 	return (true);
 }
 
@@ -261,9 +262,9 @@
 nlmsg_write_chain_mbuf(struct nl_writer *nw, void *buf, int datalen, int cnt)
 {
 	struct mbuf *m_new = (struct mbuf *)buf;
-	struct mbuf **m0 = (struct mbuf **)(nw->arg_ptr);
+	struct mbuf **m0 = (struct mbuf **)(nw->arg.ptr);
 
-	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d arg: %p", buf, datalen, nw->arg_ptr);
+	NL_LOG(LOG_DEBUG2, "IN: ptr: %p len: %d arg: %p", buf, datalen, nw->arg.ptr);
 
 	if (__predict_false(datalen == 0)) {
 		m_freem(m_new);
@@ -324,7 +325,7 @@
 {
 	struct linear_buffer *lb = (struct linear_buffer *)buf;
 	char *data = (char *)(lb + 1);
-	struct nlpcb *nlp = (struct nlpcb *)(nw->arg_ptr);
+	struct nlpcb *nlp = (struct nlpcb *)(nw->arg.ptr);
 
 	if (__predict_false(datalen == 0)) {
 		free(buf, M_NETLINK);
@@ -365,7 +366,7 @@
 	m_append(m, datalen, data);
 	free(buf, M_NETLINK);
 
-	nl_send_group(m, cnt, nw->arg_uint >> 16, nw->arg_uint & 0xFFFF);
+	nl_send_group(m, cnt, nw->arg.group.proto, nw->arg.group.id);
 	return (true);
 }
 
@@ -440,7 +441,7 @@
 {
 	if (!nlmsg_get_buf(nw, size, false, nlp->nl_linux))
 		return (false);
-	nw->arg_ptr = (void *)nlp;
+	nw->arg.ptr = (void *)nlp;
 	nw->writer_target = NS_WRITER_TARGET_SOCKET;
 	nlmsg_set_callback(nw);
 	return (true);
@@ -451,7 +452,8 @@
 {
 	if (!nlmsg_get_buf(nw, size, false, false))
 		return (false);
-	nw->arg_uint = (uint64_t)protocol << 16 | (uint64_t)group_id;
+	nw->arg.group.proto = protocol;
+	nw->arg.group.id = group_id;
 	nw->writer_target = NS_WRITER_TARGET_GROUP;
 	nlmsg_set_callback(nw);
 	return (true);
@@ -463,7 +465,7 @@
 	if (!nlmsg_get_buf(nw, size, false, false))
 		return (false);
 	*pm = NULL;
-	nw->arg_ptr = (void *)pm;
+	nw->arg.ptr = (void *)pm;
 	nw->writer_target = NS_WRITER_TARGET_CHAIN;
 	nlmsg_set_callback(nw);
 	NL_LOG(LOG_DEBUG3, "setup cb %p (need %p)", nw->cb, &nlmsg_write_chain_mbuf);
@@ -542,7 +544,7 @@
 	/* Update callback data */
 	ns_new.writer_target = nw->writer_target;
 	nlmsg_set_callback(&ns_new);
-	ns_new.arg_uint = nw->arg_uint;
+	ns_new.arg = nw->arg;
 
 	/* Copy last (unfinished) header to the new storage */
 	int last_len = nw->offset - completed_len;