diff --git a/openbsc/include/openbsc/bsc_msg_filter.h b/openbsc/include/openbsc/bsc_msg_filter.h
index 7e66517..a974195 100644
--- a/openbsc/include/openbsc/bsc_msg_filter.h
+++ b/openbsc/include/openbsc/bsc_msg_filter.h
@@ -8,6 +8,7 @@
 #include <regex.h>
 
 struct vty;
+struct gsm48_hdr;
 
 /* TODO: remove */
 struct bsc_nat_parsed;
@@ -64,11 +65,13 @@
 /**
  * Content filtering.
  */
-int bsc_nat_filter_sccp_cr(struct bsc_connection *bsc, struct msgb *msg,
-			struct bsc_nat_parsed *, int *con_type, char **imsi,
+int bsc_msg_filter_initial(struct gsm48_hdr *hdr, size_t size,
+			struct bsc_connection *bsc,
+			int *con_type, char **imsi,
 			struct bsc_filter_reject_cause *cause);
-int bsc_nat_filter_dt(struct bsc_connection *bsc, struct msgb *msg,
-			struct nat_sccp_connection *con, struct bsc_nat_parsed *parsed,
+int bsc_msg_filter_data(struct gsm48_hdr *hdr, size_t size,
+			struct bsc_connection *bsc,
+			struct nat_sccp_connection *con,
 			struct bsc_filter_reject_cause *cause);
 
 /* IMSI allow/deny handling */
diff --git a/openbsc/include/openbsc/bsc_nat.h b/openbsc/include/openbsc/bsc_nat.h
index b9f1e56..10f11a6 100644
--- a/openbsc/include/openbsc/bsc_nat.h
+++ b/openbsc/include/openbsc/bsc_nat.h
@@ -22,6 +22,7 @@
 #define BSC_NAT_H
 
 #include "mgcp.h"
+#include "bsc_msg_filter.h"
 
 
 #include <osmocom/core/select.h>
@@ -435,6 +436,13 @@
 int bsc_nat_extract_lac(struct bsc_connection *bsc, struct nat_sccp_connection *con,
 				struct bsc_nat_parsed *parsed, struct msgb *msg);
 
+int bsc_nat_filter_sccp_cr(struct bsc_connection *bsc, struct msgb *msg,
+			struct bsc_nat_parsed *, int *con_type, char **imsi,
+			struct bsc_filter_reject_cause *cause);
+int bsc_nat_filter_dt(struct bsc_connection *bsc, struct msgb *msg,
+			struct nat_sccp_connection *con, struct bsc_nat_parsed *parsed,
+			struct bsc_filter_reject_cause *cause);
+
 /**
  * CTRL interface helper
  */
diff --git a/openbsc/src/libfilter/bsc_msg_filter.c b/openbsc/src/libfilter/bsc_msg_filter.c
index 5f8bff0..a1271a0 100644
--- a/openbsc/src/libfilter/bsc_msg_filter.c
+++ b/openbsc/src/libfilter/bsc_msg_filter.c
@@ -328,14 +328,12 @@
 
 
 /* Filter out CR data... */
-int bsc_nat_filter_sccp_cr(struct bsc_connection *bsc, struct msgb *msg,
-			struct bsc_nat_parsed *parsed, int *con_type,
+int bsc_msg_filter_initial(struct gsm48_hdr *hdr48, size_t hdr48_len,
+			struct bsc_connection *bsc,
+			int *con_type,
 			char **imsi, struct bsc_filter_reject_cause *cause)
 {
-	struct tlv_parsed tp;
-	struct gsm48_hdr *hdr48;
-	int hdr48_len;
-	int len, ret = 0;
+	int ret = 0;
 	uint8_t msg_type, proto;
 
 	*con_type = NAT_CON_TYPE_NONE;
@@ -343,39 +341,6 @@
 	cause->lu_reject_cause = GSM48_REJECT_PLMN_NOT_ALLOWED;
 	*imsi = NULL;
 
-	if (parsed->gsm_type != BSS_MAP_MSG_COMPLETE_LAYER_3) {
-		LOGP(DNAT, LOGL_ERROR,
-		     "Rejecting CR message due wrong GSM Type %d\n", parsed->gsm_type);
-		return -1;
-	}
-
-	/* the parsed has had some basic l3 length check */
-	len = msg->l3h[1];
-	if (msgb_l3len(msg) - 3 < len) {
-		LOGP(DNAT, LOGL_ERROR,
-		     "The CR Data has not enough space...\n");
-		return -1;
-	}
-
-	msg->l4h = &msg->l3h[3];
-	len -= 1;
-
-	tlv_parse(&tp, gsm0808_att_tlvdef(), msg->l4h, len, 0, 0);
-
-	if (!TLVP_PRESENT(&tp, GSM0808_IE_LAYER_3_INFORMATION)) {
-		LOGP(DNAT, LOGL_ERROR, "CR Data does not contain layer3 information.\n");
-		return -1;
-	}
-
-	hdr48_len = TLVP_LEN(&tp, GSM0808_IE_LAYER_3_INFORMATION);
-
-	if (hdr48_len < sizeof(*hdr48)) {
-		LOGP(DNAT, LOGL_ERROR, "GSM48 header does not fit.\n");
-		return -1;
-	}
-
-	hdr48 = (struct gsm48_hdr *) TLVP_VAL(&tp, GSM0808_IE_LAYER_3_INFORMATION);
-
 	proto = hdr48->proto_discr & 0x0f;
 	msg_type = hdr48->msg_type & 0xbf;
 	if (proto == GSM48_PDISC_MM &&
@@ -412,13 +377,12 @@
 	return auth_imsi(bsc, *imsi, cause);
 }
 
-int bsc_nat_filter_dt(struct bsc_connection *bsc, struct msgb *msg,
-		struct nat_sccp_connection *con, struct bsc_nat_parsed *parsed,
+int bsc_msg_filter_data(struct gsm48_hdr *hdr48, size_t len,
+		struct bsc_connection *bsc,
+		struct nat_sccp_connection *con,
 		struct bsc_filter_reject_cause *cause)
 {
-	uint32_t len;
 	uint8_t msg_type, proto;
-	struct gsm48_hdr *hdr48;
 
 	cause->cm_reject_cause = GSM48_REJECT_PLMN_NOT_ALLOWED;
 	cause->lu_reject_cause = GSM48_REJECT_PLMN_NOT_ALLOWED;
@@ -426,14 +390,6 @@
 	if (con->imsi_checked)
 		return 0;
 
-	/* only care about DTAP messages */
-	if (parsed->bssap != BSSAP_MSG_DTAP)
-		return 0;
-
-	hdr48 = bsc_unpack_dtap(parsed, msg, &len);
-	if (!hdr48)
-		return -1;
-
 	proto = hdr48->proto_discr & 0x0f;
 	msg_type = hdr48->msg_type & 0xbf;
 	if (proto != GSM48_PDISC_MM || msg_type != GSM48_MT_MM_ID_RESP)
diff --git a/openbsc/src/osmo-bsc_nat/Makefile.am b/openbsc/src/osmo-bsc_nat/Makefile.am
index 7a38430..d96a391 100644
--- a/openbsc/src/osmo-bsc_nat/Makefile.am
+++ b/openbsc/src/osmo-bsc_nat/Makefile.am
@@ -7,7 +7,7 @@
 
 osmo_bsc_nat_SOURCES = bsc_filter.c bsc_mgcp_utils.c bsc_nat.c bsc_nat_utils.c \
 		  bsc_nat_vty.c bsc_sccp.c bsc_ussd.c bsc_nat_ctrl.c \
-		  bsc_nat_rewrite.c bsc_nat_rewrite_trie.c
+		  bsc_nat_rewrite.c bsc_nat_rewrite_trie.c bsc_nat_filter.c
 osmo_bsc_nat_LDADD = \
 		$(top_builddir)/src/libmgcp/libmgcp.a \
 		$(top_builddir)/src/libbsc/libbsc.a \
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat_filter.c b/openbsc/src/osmo-bsc_nat/bsc_nat_filter.c
new file mode 100644
index 0000000..11d370c
--- /dev/null
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_filter.c
@@ -0,0 +1,105 @@
+/*
+ * (C) 2010-2015 by Holger Hans Peter Freyther <zecke@selfish.org>
+ * (C) 2010-2012 by On-Waves
+ * All Rights Reserved
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include <openbsc/bsc_nat.h>
+#include <openbsc/bsc_nat_sccp.h>
+#include <openbsc/bsc_msg_filter.h>
+#include <openbsc/debug.h>
+
+#include <osmocom/gsm/gsm0808.h>
+
+#include <osmocom/gsm/protocol/gsm_08_08.h>
+#include <osmocom/gsm/protocol/gsm_04_11.h>
+
+#include <osmocom/sccp/sccp.h>
+
+/* Filter out CR data... */
+int bsc_nat_filter_sccp_cr(struct bsc_connection *bsc, struct msgb *msg,
+			struct bsc_nat_parsed *parsed, int *con_type,
+			char **imsi, struct bsc_filter_reject_cause *cause)
+{
+	struct tlv_parsed tp;
+	struct gsm48_hdr *hdr48;
+	int hdr48_len;
+	int len;
+
+	*con_type = NAT_CON_TYPE_NONE;
+	cause->cm_reject_cause = GSM48_REJECT_PLMN_NOT_ALLOWED;
+	cause->lu_reject_cause = GSM48_REJECT_PLMN_NOT_ALLOWED;
+	*imsi = NULL;
+
+	if (parsed->gsm_type != BSS_MAP_MSG_COMPLETE_LAYER_3) {
+		LOGP(DNAT, LOGL_ERROR,
+		     "Rejecting CR message due wrong GSM Type %d\n", parsed->gsm_type);
+		return -1;
+	}
+
+	/* the parsed has had some basic l3 length check */
+	len = msg->l3h[1];
+	if (msgb_l3len(msg) - 3 < len) {
+		LOGP(DNAT, LOGL_ERROR,
+		     "The CR Data has not enough space...\n");
+		return -1;
+	}
+
+	msg->l4h = &msg->l3h[3];
+	len -= 1;
+
+	tlv_parse(&tp, gsm0808_att_tlvdef(), msg->l4h, len, 0, 0);
+
+	if (!TLVP_PRESENT(&tp, GSM0808_IE_LAYER_3_INFORMATION)) {
+		LOGP(DNAT, LOGL_ERROR, "CR Data does not contain layer3 information.\n");
+		return -1;
+	}
+
+	hdr48_len = TLVP_LEN(&tp, GSM0808_IE_LAYER_3_INFORMATION);
+
+	if (hdr48_len < sizeof(*hdr48)) {
+		LOGP(DNAT, LOGL_ERROR, "GSM48 header does not fit.\n");
+		return -1;
+	}
+
+	hdr48 = (struct gsm48_hdr *) TLVP_VAL(&tp, GSM0808_IE_LAYER_3_INFORMATION);
+	return bsc_msg_filter_initial(hdr48, hdr48_len, bsc, con_type, imsi, cause);
+}
+
+int bsc_nat_filter_dt(struct bsc_connection *bsc, struct msgb *msg,
+		struct nat_sccp_connection *con, struct bsc_nat_parsed *parsed,
+		struct bsc_filter_reject_cause *cause)
+{
+	uint32_t len;
+	struct gsm48_hdr *hdr48;
+
+	cause->cm_reject_cause = GSM48_REJECT_PLMN_NOT_ALLOWED;
+	cause->lu_reject_cause = GSM48_REJECT_PLMN_NOT_ALLOWED;
+
+	if (con->imsi_checked)
+		return 0;
+
+	/* only care about DTAP messages */
+	if (parsed->bssap != BSSAP_MSG_DTAP)
+		return 0;
+
+	hdr48 = bsc_unpack_dtap(parsed, msg, &len);
+	if (!hdr48)
+		return -1;
+
+	return bsc_msg_filter_data(hdr48, len, bsc, con, cause);
+}
diff --git a/openbsc/tests/bsc-nat/Makefile.am b/openbsc/tests/bsc-nat/Makefile.am
index 64cd9ac..26e5500 100644
--- a/openbsc/tests/bsc-nat/Makefile.am
+++ b/openbsc/tests/bsc-nat/Makefile.am
@@ -12,7 +12,8 @@
 			$(top_srcdir)/src/osmo-bsc_nat/bsc_nat_utils.c \
 			$(top_srcdir)/src/osmo-bsc_nat/bsc_nat_rewrite.c \
 			$(top_srcdir)/src/osmo-bsc_nat/bsc_nat_rewrite_trie.c \
-			$(top_srcdir)/src/osmo-bsc_nat/bsc_mgcp_utils.c
+			$(top_srcdir)/src/osmo-bsc_nat/bsc_mgcp_utils.c \
+			$(top_srcdir)/src/osmo-bsc_nat/bsc_nat_filter.c
 bsc_nat_test_LDADD = \
 			$(top_builddir)/src/libfilter/libfilter.a \
 			$(top_builddir)/src/libbsc/libbsc.a \