diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat.c b/openbsc/src/osmo-bsc_nat/bsc_nat.c
index cdab406..cacb919 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat.c
@@ -670,8 +670,8 @@
 		if (!hdr48)
 			return;
 
-		proto = hdr48->proto_discr & 0x0f;
-		msg_type = hdr48->msg_type & 0xbf;
+		proto = gsm48_hdr_pdisc(hdr48);
+		msg_type = gsm48_hdr_msg_type(hdr48);
 		if (proto == GSM48_PDISC_MM &&
 		    msg_type == GSM48_MT_MM_CM_SERV_ACC)
 			con->authorized = 1;
diff --git a/openbsc/src/osmo-bsc_nat/bsc_nat_rewrite.c b/openbsc/src/osmo-bsc_nat/bsc_nat_rewrite.c
index ca5670c..58667fe 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_rewrite.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_rewrite.c
@@ -594,8 +594,8 @@
 		return msg;
 
 	link_id = msg->l3h[1];
-	proto = hdr48->proto_discr & 0x0f;
-	msg_type = hdr48->msg_type & 0xbf;
+	proto = gsm48_hdr_pdisc(hdr48);
+	msg_type = gsm48_hdr_msg_type(hdr48);
 
 	if (proto == GSM48_PDISC_CC && msg_type == GSM48_MT_CC_SETUP)
 		new_msg = rewrite_setup(nat, msg, parsed, imsi, hdr48, len);
diff --git a/openbsc/src/osmo-bsc_nat/bsc_ussd.c b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
index 1082414..2905c85 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_ussd.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_ussd.c
@@ -407,8 +407,8 @@
 	if (!hdr48)
 		return 0;
 
-	proto = hdr48->proto_discr & 0x0f;
-	msg_type = hdr48->msg_type & 0xbf;
+	proto = gsm48_hdr_pdisc(hdr48);
+	msg_type = gsm48_hdr_msg_type(hdr48);
 	ti = (hdr48->proto_discr & 0x70) >> 4;
 	if (proto != GSM48_PDISC_NC_SS)
 		return 0;