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..42cfb9e
--- /dev/null
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_filter.c
@@ -0,0 +1,320 @@
+/*
+ * Access filtering
+ */
+/*
+ * (C) 2010-2011 by Holger Hans Peter Freyther <zecke@selfish.org>
+ * (C) 2010-2011 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_msc.h>
+#include <openbsc/gsm_data.h>
+#include <openbsc/debug.h>
+#include <openbsc/ipaccess.h>
+
+#include <osmocom/core/linuxlist.h>
+#include <osmocom/core/talloc.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>
+
+static int lst_check_deny(struct bsc_nat_acc_lst *lst, const char *mi_string)
+{
+	struct bsc_nat_acc_lst_entry *entry;
+
+	llist_for_each_entry(entry, &lst->fltr_list, list) {
+		if (!entry->imsi_deny)
+			continue;
+		if (regexec(&entry->imsi_deny_re, mi_string, 0, NULL, 0) == 0)
+			return 0;
+	}
+
+	return 1;
+}
+
+/* apply white/black list */
+static int auth_imsi(struct bsc_connection *bsc, const char *mi_string)
+{
+	/*
+	 * Now apply blacklist/whitelist of the BSC and the NAT.
+	 * 1.) Allow directly if the IMSI is allowed at the BSC
+	 * 2.) Reject if the IMSI is not allowed at the BSC
+	 * 3.) Reject if the IMSI not allowed at the global level.
+	 * 4.) Allow directly if the IMSI is allowed at the global level
+	 */
+	struct bsc_nat_acc_lst *nat_lst = NULL;
+	struct bsc_nat_acc_lst *bsc_lst = NULL;
+
+	bsc_lst = bsc_nat_acc_lst_find(bsc->nat, bsc->cfg->acc_lst_name);
+	nat_lst = bsc_nat_acc_lst_find(bsc->nat, bsc->nat->acc_lst_name);
+
+
+	if (bsc_lst) {
+		/* 1. BSC allow */
+		if (bsc_nat_lst_check_allow(bsc_lst, mi_string) == 0)
+			return 1;
+
+		/* 2. BSC deny */
+		if (lst_check_deny(bsc_lst, mi_string) == 0) {
+			LOGP(DNAT, LOGL_ERROR,
+			     "Filtering %s by imsi_deny on bsc nr: %d.\n", mi_string, bsc->cfg->nr);
+			rate_ctr_inc(&bsc_lst->stats->ctr[ACC_LIST_BSC_FILTER]);
+			return -2;
+		}
+
+	}
+
+	/* 3. NAT deny */
+	if (nat_lst) {
+		if (lst_check_deny(nat_lst, mi_string) == 0) {
+			LOGP(DNAT, LOGL_ERROR,
+			     "Filtering %s by nat imsi_deny on bsc nr: %d.\n", mi_string, bsc->cfg->nr);
+			rate_ctr_inc(&nat_lst->stats->ctr[ACC_LIST_NAT_FILTER]);
+			return -3;
+		}
+	}
+
+	return 1;
+}
+
+static int _cr_check_loc_upd(struct bsc_connection *bsc,
+			     uint8_t *data, unsigned int length,
+			     char **imsi)
+{
+	uint8_t mi_type;
+	struct gsm48_loc_upd_req *lu;
+	char mi_string[GSM48_MI_SIZE];
+
+	if (length < sizeof(*lu)) {
+		LOGP(DNAT, LOGL_ERROR,
+		     "LU does not fit. Length is %d \n", length);
+		return -1;
+	}
+
+	lu = (struct gsm48_loc_upd_req *) data;
+	mi_type = lu->mi[0] & GSM_MI_TYPE_MASK;
+
+	/*
+	 * We can only deal with the IMSI. This will fail for a phone that
+	 * will send the TMSI of a previous network to us.
+	 */
+	if (mi_type != GSM_MI_TYPE_IMSI)
+		return 0;
+
+	gsm48_mi_to_string(mi_string, sizeof(mi_string), lu->mi, lu->mi_len);
+	*imsi = talloc_strdup(bsc, mi_string);
+	return auth_imsi(bsc, mi_string);
+}
+
+static int _cr_check_cm_serv_req(struct bsc_connection *bsc,
+				 uint8_t *data, unsigned int length,
+				 int *con_type, char **imsi)
+{
+	static const uint32_t classmark_offset =
+				offsetof(struct gsm48_service_request, classmark);
+
+	char mi_string[GSM48_MI_SIZE];
+	uint8_t mi_type;
+	int rc;
+	struct gsm48_service_request *req;
+
+	/* unfortunately in Phase1 the classmark2 length is variable */
+
+	if (length < sizeof(*req)) {
+		LOGP(DNAT, LOGL_ERROR,
+		     "CM Serv Req does not fit. Length is %d\n", length);
+		return -1;
+	}
+
+	req = (struct gsm48_service_request *) data;
+	if (req->cm_service_type == 0x8)
+		*con_type = NAT_CON_TYPE_SSA;
+	rc = gsm48_extract_mi((uint8_t *) &req->classmark,
+			      length - classmark_offset, mi_string, &mi_type);
+	if (rc < 0) {
+		LOGP(DNAT, LOGL_ERROR, "Failed to parse the classmark2/mi. error: %d\n", rc);
+		return -1;
+	}
+
+	/* we have to let the TMSI or such pass */
+	if (mi_type != GSM_MI_TYPE_IMSI)
+		return 0;
+
+	*imsi = talloc_strdup(bsc, mi_string);
+	return auth_imsi(bsc, mi_string);
+}
+
+static int _cr_check_pag_resp(struct bsc_connection *bsc,
+			      uint8_t *data, unsigned int length,
+			      char **imsi)
+{
+	struct gsm48_pag_resp *resp;
+	char mi_string[GSM48_MI_SIZE];
+	uint8_t mi_type;
+
+	if (length < sizeof(*resp)) {
+		LOGP(DNAT, LOGL_ERROR, "PAG RESP does not fit. Length was %d.\n", length);
+		return -1;
+	}
+
+	resp = (struct gsm48_pag_resp *) data;
+	if (gsm48_paging_extract_mi(resp, length, mi_string, &mi_type) < 0) {
+		LOGP(DNAT, LOGL_ERROR, "Failed to extract the MI.\n");
+		return -1;
+	}
+
+	/* we need to let it pass for now */
+	if (mi_type != GSM_MI_TYPE_IMSI)
+		return 0;
+
+	*imsi = talloc_strdup(bsc, mi_string);
+	return auth_imsi(bsc, mi_string);
+}
+
+static int _dt_check_id_resp(struct bsc_connection *bsc,
+			     uint8_t *data, unsigned int length,
+			     struct sccp_connections *con)
+{
+	char mi_string[GSM48_MI_SIZE];
+	uint8_t mi_type;
+	int ret;
+
+	if (length < 2) {
+		LOGP(DNAT, LOGL_ERROR, "mi does not fit.\n");
+		return -1;
+	}
+
+	if (data[0] < length - 1) {
+		LOGP(DNAT, LOGL_ERROR, "mi length too big.\n");
+		return -2;
+	}
+
+	mi_type = data[1] & GSM_MI_TYPE_MASK;
+	gsm48_mi_to_string(mi_string, sizeof(mi_string), &data[1], data[0]);
+
+	if (mi_type != GSM_MI_TYPE_IMSI)
+		return 0;
+
+	ret = auth_imsi(bsc, mi_string);
+	con->imsi_checked = 1;
+	con->imsi = talloc_strdup(con, mi_string);
+	return ret;
+}
+
+
+/* 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 tlv_parsed tp;
+	struct gsm48_hdr *hdr48;
+	int hdr48_len;
+	int len;
+	uint8_t msg_type, proto;
+
+	*con_type = NAT_CON_TYPE_NONE;
+	*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 &&
+	    msg_type == GSM48_MT_MM_LOC_UPD_REQUEST) {
+		*con_type = NAT_CON_TYPE_LU;
+		return _cr_check_loc_upd(bsc, &hdr48->data[0], hdr48_len - sizeof(*hdr48), imsi);
+	} else if (proto == GSM48_PDISC_MM &&
+		  msg_type == GSM48_MT_MM_CM_SERV_REQ) {
+		*con_type = NAT_CON_TYPE_CM_SERV_REQ;
+		return _cr_check_cm_serv_req(bsc, &hdr48->data[0],
+					     hdr48_len - sizeof(*hdr48),
+					     con_type, imsi);
+	} else if (proto == GSM48_PDISC_RR &&
+		   msg_type == GSM48_MT_RR_PAG_RESP) {
+		*con_type = NAT_CON_TYPE_PAG_RESP;
+		return _cr_check_pag_resp(bsc, &hdr48->data[0], hdr48_len - sizeof(*hdr48), imsi);
+	} else {
+		/* We only want to filter the above, let other things pass */
+		*con_type = NAT_CON_TYPE_OTHER;
+		return 0;
+	}
+}
+
+int bsc_nat_filter_dt(struct bsc_connection *bsc, struct msgb *msg,
+		      struct sccp_connections *con, struct bsc_nat_parsed *parsed)
+{
+	uint32_t len;
+	uint8_t msg_type, proto;
+	struct gsm48_hdr *hdr48;
+
+	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) {
+		return _dt_check_id_resp(bsc, &hdr48->data[0], len - sizeof(*hdr48), con);
+	} else {
+		return 0;
+	}
+}