diff --git a/src/libcharon/plugins/kernel_netlink/kernel_netlink_shared.c b/src/libcharon/plugins/kernel_netlink/kernel_netlink_shared.c
index 72c13a697..339d0b9e6 100644
--- a/src/libcharon/plugins/kernel_netlink/kernel_netlink_shared.c
+++ b/src/libcharon/plugins/kernel_netlink/kernel_netlink_shared.c
@@ -514,7 +514,7 @@ METHOD(netlink_socket_t, netlink_send, status_t,
 	uintptr_t seq;
 	u_int try;
 
-	seq = ref_get(&this->seq);
+	seq = ref_get_nonzero(&this->seq);
 
 	for (try = 0; try <= this->retries; ++try)
 	{
@@ -694,7 +694,6 @@ netlink_socket_t *netlink_socket_create(int protocol, enum_name_t *names,
 			.send_ack = _netlink_send_ack,
 			.destroy = _destroy,
 		},
-		.seq = 200,
 		.mutex = mutex_create(MUTEX_TYPE_RECURSIVE),
 		.socket = socket(AF_NETLINK, SOCK_RAW, protocol),
 		.entries = hashtable_create(hashtable_hash_ptr, hashtable_equals_ptr, 4),
diff --git a/src/libcharon/sa/child_sa.c b/src/libcharon/sa/child_sa.c
index 494c36b2e..97ee88acb 100644
--- a/src/libcharon/sa/child_sa.c
+++ b/src/libcharon/sa/child_sa.c
@@ -2038,7 +2038,7 @@ child_sa_t *child_sa_create(host_t *me, host_t *other, child_cfg_t *config,
 							child_sa_create_t *data)
 {
 	private_child_sa_t *this;
-	static refcount_t unique_id = 0, unique_mark = 0;
+	static refcount_t unique_id = 0;
 
 	INIT(this,
 		.public = {
@@ -2096,7 +2096,7 @@ child_sa_t *child_sa_create(host_t *me, host_t *other, child_cfg_t *config,
 		.close_action = config->get_close_action(config),
 		.dpd_action = config->get_dpd_action(config),
 		.reqid = config->get_reqid(config),
-		.unique_id = ref_get(&unique_id),
+		.unique_id = ref_get_nonzero(&unique_id),
 		.mark_in = config->get_mark(config, TRUE),
 		.mark_out = config->get_mark(config, FALSE),
 		.if_id_in = config->get_if_id(config, TRUE) ?: data->if_id_in_def,
@@ -2127,27 +2127,7 @@ child_sa_t *child_sa_create(host_t *me, host_t *other, child_cfg_t *config,
 	}
 
 	allocate_unique_if_ids(&this->if_id_in, &this->if_id_out);
-
-	if (MARK_IS_UNIQUE(this->mark_in.value) ||
-		MARK_IS_UNIQUE(this->mark_out.value))
-	{
-		refcount_t mark = 0;
-		bool unique_dir = this->mark_in.value == MARK_UNIQUE_DIR ||
-						  this->mark_out.value == MARK_UNIQUE_DIR;
-
-		if (!unique_dir)
-		{
-			mark = ref_get(&unique_mark);
-		}
-		if (MARK_IS_UNIQUE(this->mark_in.value))
-		{
-			this->mark_in.value = unique_dir ? ref_get(&unique_mark) : mark;
-		}
-		if (MARK_IS_UNIQUE(this->mark_out.value))
-		{
-			this->mark_out.value = unique_dir ? ref_get(&unique_mark) : mark;
-		}
-	}
+	allocate_unique_marks(&this->mark_in.value, &this->mark_out.value);
 
 	if (!this->reqid)
 	{
diff --git a/src/libcharon/sa/ike_sa.c b/src/libcharon/sa/ike_sa.c
index 654508e4b..758a4cdba 100644
--- a/src/libcharon/sa/ike_sa.c
+++ b/src/libcharon/sa/ike_sa.c
@@ -3229,7 +3229,7 @@ ike_sa_t * ike_sa_create(ike_sa_id_t *ike_sa_id, bool initiator,
 		.my_auths = array_create(0, 0),
 		.other_auths = array_create(0, 0),
 		.attributes = array_create(sizeof(attribute_entry_t), 0),
-		.unique_id = ref_get(&unique_id),
+		.unique_id = ref_get_nonzero(&unique_id),
 		.keepalive_interval = lib->settings->get_time(lib->settings,
 								"%s.keep_alive", KEEPALIVE_INTERVAL, lib->ns),
 		.keepalive_dpd_margin = lib->settings->get_time(lib->settings,
diff --git a/src/libstrongswan/ipsec/ipsec_types.c b/src/libstrongswan/ipsec/ipsec_types.c
index bd8a4a52c..7d93f275a 100644
--- a/src/libstrongswan/ipsec/ipsec_types.c
+++ b/src/libstrongswan/ipsec/ipsec_types.c
@@ -150,6 +150,48 @@ bool mark_from_string(const char *value, mark_op_t ops, mark_t *mark)
 	return TRUE;
 }
 
+/**
+ * Allocate a unique mark that is non-zero and not one of the special values.
+ */
+static uint32_t allocate_unique_mark()
+{
+	static refcount_t unique_mark = 0;
+	uint32_t m;
+
+	m = ref_get_nonzero(&unique_mark);
+	while (MARK_IS_UNIQUE(m))
+	{
+		m = ref_get_nonzero(&unique_mark);
+	}
+	return m;
+}
+
+/*
+ * Described in header
+ */
+void allocate_unique_marks(uint32_t *in, uint32_t *out)
+{
+	if (MARK_IS_UNIQUE(*in) || MARK_IS_UNIQUE(*out))
+	{
+		uint32_t mark = 0;
+		bool unique_dir = *in == MARK_UNIQUE_DIR ||
+						  *out == MARK_UNIQUE_DIR;
+
+		if (!unique_dir)
+		{
+			mark = allocate_unique_mark();
+		}
+		if (MARK_IS_UNIQUE(*in))
+		{
+			*in = unique_dir ? allocate_unique_mark() : mark;
+		}
+		if (MARK_IS_UNIQUE(*out))
+		{
+			*out = unique_dir ? allocate_unique_mark() : mark;
+		}
+	}
+}
+
 /*
  * Described in header
  */
@@ -191,30 +233,46 @@ bool if_id_from_string(const char *value, uint32_t *if_id)
 	return TRUE;
 }
 
+/**
+ * Allocate a unique interface ID that is non-zero and not one of the special
+ * values.
+ */
+static uint32_t allocate_unique_if_id()
+{
+	static refcount_t unique_if_id = 0;
+	uint32_t if_id;
+
+	if_id = ref_get_nonzero(&unique_if_id);
+	while (IF_ID_IS_UNIQUE(if_id))
+	{
+		if_id = ref_get_nonzero(&unique_if_id);
+	}
+	return if_id;
+}
+
 /*
  * Described in header
  */
 void allocate_unique_if_ids(uint32_t *in, uint32_t *out)
 {
-	static refcount_t unique_if_id = 0;
 
 	if (IF_ID_IS_UNIQUE(*in) || IF_ID_IS_UNIQUE(*out))
 	{
-		refcount_t if_id = 0;
+		uint32_t if_id = 0;
 		bool unique_dir = *in == IF_ID_UNIQUE_DIR ||
 						  *out == IF_ID_UNIQUE_DIR;
 
 		if (!unique_dir)
 		{
-			if_id = ref_get(&unique_if_id);
+			if_id = allocate_unique_if_id();
 		}
 		if (IF_ID_IS_UNIQUE(*in))
 		{
-			*in = unique_dir ? ref_get(&unique_if_id) : if_id;
+			*in = unique_dir ? allocate_unique_if_id() : if_id;
 		}
 		if (IF_ID_IS_UNIQUE(*out))
 		{
-			*out = unique_dir ? ref_get(&unique_if_id) : if_id;
+			*out = unique_dir ? allocate_unique_if_id() : if_id;
 		}
 	}
 }
diff --git a/src/libstrongswan/ipsec/ipsec_types.h b/src/libstrongswan/ipsec/ipsec_types.h
index 5320c46ab..b3a26ce8e 100644
--- a/src/libstrongswan/ipsec/ipsec_types.h
+++ b/src/libstrongswan/ipsec/ipsec_types.h
@@ -242,6 +242,18 @@ enum mark_op_t {
  */
 bool mark_from_string(const char *value, mark_op_t ops, mark_t *mark);
 
+/**
+ * Allocate up to two unique marks depending on the given values.
+ *
+ * If the given values are MARK_UNIQUE, the values get replaced by a single
+ * unique mark. If the given values are MARK_UNIQUE_DIR, the values get
+ * replaced by a single unique mark for each direction.
+ *
+ * @param[out] in	inbound interface ID
+ * @param[out] out	outbound interface ID
+ */
+void allocate_unique_marks(uint32_t *in, uint32_t *out);
+
 /**
  * Special interface ID values to allocate a unique ID for each CHILD_SA/dir
  */
diff --git a/src/libstrongswan/tests/suites/test_utils.c b/src/libstrongswan/tests/suites/test_utils.c
index e4463f2d0..bbaca6deb 100644
--- a/src/libstrongswan/tests/suites/test_utils.c
+++ b/src/libstrongswan/tests/suites/test_utils.c
@@ -256,6 +256,26 @@ START_TEST(test_round)
 }
 END_TEST
 
+/*******************************************************************************
+ * ref_get/put
+ */
+
+START_TEST(test_refs)
+{
+	refcount_t r = 0xfffffffe;
+
+	ck_assert_int_eq(ref_cur(&r), 0xfffffffe);
+	ck_assert_int_eq(ref_get(&r), 0xffffffff);
+	ck_assert_int_eq(ref_get_nonzero(&r), 1);
+	ck_assert_int_eq(ref_get_nonzero(&r), 2);
+	ck_assert_int_eq(ref_cur(&r), 2);
+	ck_assert(!ref_put(&r));
+	ck_assert_int_eq(ref_cur(&r), 1);
+	ck_assert(ref_put(&r));
+	ck_assert_int_eq(ref_cur(&r), 0);
+}
+END_TEST
+
 /*******************************************************************************
  * streq
  */
@@ -1104,6 +1124,41 @@ START_TEST(test_mark_from_string)
 }
 END_TEST
 
+/*******************************************************************************
+ * allocate_unique_marks
+ */
+
+static struct {
+	uint32_t in;
+	uint32_t out;
+	uint32_t exp_in;
+	uint32_t exp_out;
+} unique_mark_data[] = {
+	{0, 0, 0, 0 },
+	{42, 42, 42, 42 },
+	{42, 1337, 42, 1337 },
+	/* each call increases the internal counter by 1 or 2*/
+	{MARK_UNIQUE, 42, 1, 42 },
+	{42, MARK_UNIQUE, 42, 2 },
+	{MARK_UNIQUE_DIR, 42, 3, 42 },
+	{42, MARK_UNIQUE_DIR, 42, 4 },
+	{MARK_UNIQUE, MARK_UNIQUE, 5, 5 },
+	{MARK_UNIQUE_DIR, MARK_UNIQUE, 6, 7 },
+	{MARK_UNIQUE, MARK_UNIQUE_DIR, 8, 9 },
+	{MARK_UNIQUE_DIR, MARK_UNIQUE_DIR, 10, 11 },
+};
+
+START_TEST(test_allocate_unique_marks)
+{
+	uint32_t mark_in = unique_mark_data[_i].in,
+			 mark_out = unique_mark_data[_i].out;
+
+	allocate_unique_marks(&mark_in, &mark_out);
+	ck_assert_int_eq(mark_in, unique_mark_data[_i].exp_in);
+	ck_assert_int_eq(mark_out, unique_mark_data[_i].exp_out);
+}
+END_TEST
+
 /*******************************************************************************
  * if_id_from_string
  */
@@ -1272,6 +1327,10 @@ Suite *utils_suite_create()
 	tcase_add_test(tc, test_round);
 	suite_add_tcase(s, tc);
 
+	tc = tcase_create("refcount");
+	tcase_add_test(tc, test_refs);
+	suite_add_tcase(s, tc);
+
 	tc = tcase_create("string helper");
 	tcase_add_loop_test(tc, test_streq, 0, countof(streq_data));
 	tcase_add_loop_test(tc, test_strneq, 0, countof(strneq_data));
@@ -1340,6 +1399,10 @@ Suite *utils_suite_create()
 	tcase_add_loop_test(tc, test_mark_from_string, 0, countof(mark_data));
 	suite_add_tcase(s, tc);
 
+	tc = tcase_create("allocate_unique_marks");
+	tcase_add_loop_test(tc, test_allocate_unique_marks, 0, countof(unique_mark_data));
+	suite_add_tcase(s, tc);
+
 	tc = tcase_create("if_id_from_string");
 	tcase_add_loop_test(tc, test_if_id_from_string, 0, countof(if_id_data));
 	suite_add_tcase(s, tc);
diff --git a/src/libstrongswan/utils/utils/atomics.h b/src/libstrongswan/utils/utils/atomics.h
index 0fcd99e18..58f1cd92e 100644
--- a/src/libstrongswan/utils/utils/atomics.h
+++ b/src/libstrongswan/utils/utils/atomics.h
@@ -124,6 +124,33 @@ bool cas_ptr(void **ptr, void *oldval, void *newval);
 
 #endif /* HAVE_GCC_ATOMIC_OPERATIONS */
 
+/**
+ * Get a new reference, but skip zero on overflow.
+ *
+ * If a reference counter is used to allocate unique identifiers, the
+ * refcount value may overflow if it is never decremented. The 0 identifier
+ * may have special semantics, hence returning can be problematic for some
+ * users.
+ *
+ * This call does an additional ref_get() if ref_get() overflows and returns
+ * zero. This ensures that zero is never returned, in the assumption that it
+ * has special meaning.
+ *
+ * @param ref	pointer to ref counter
+ * @return		new value of ref
+ */
+static inline refcount_t ref_get_nonzero(refcount_t *ref)
+{
+	refcount_t v;
+
+	v = ref_get(ref);
+	if (v == 0)
+	{
+		v = ref_get(ref);
+	}
+	return v;
+}
+
 /**
  * Initialize atomics utility functions
  */