diff --git a/openbsc/include/openbsc/bsc_nat.h b/openbsc/include/openbsc/bsc_nat.h
index 412d9a8..068ef68 100644
--- a/openbsc/include/openbsc/bsc_nat.h
+++ b/openbsc/include/openbsc/bsc_nat.h
@@ -184,5 +184,6 @@
 int bsc_mgcp_init(struct bsc_nat *nat);
 
 struct bsc_connection *bsc_mgcp_find_con(struct bsc_nat *, int endpoint_number);
+struct msgb *bsc_mgcp_rewrite(struct msgb *msg, const char *ip, int port);
 
 #endif
diff --git a/openbsc/src/nat/bsc_mgcp_utils.c b/openbsc/src/nat/bsc_mgcp_utils.c
index 0f45a91..3639a7b 100644
--- a/openbsc/src/nat/bsc_mgcp_utils.c
+++ b/openbsc/src/nat/bsc_mgcp_utils.c
@@ -95,6 +95,67 @@
 	return NULL;
 }
 
+/* we need to replace some strings... */
+struct msgb *bsc_mgcp_rewrite(struct msgb *input, const char *ip, int port)
+{
+	static const char *ip_str = "c=IN IP4 ";
+	static const char *aud_str = "m=audio ";
+
+	char buf[128];
+	char *running, *token;
+	struct msgb *output;
+
+	if (msgb_l2len(input) > 4096 - 128) {
+		LOGP(DMGCP, LOGL_ERROR, "Input is too long.\n");
+		return NULL;
+	}
+
+	output = msgb_alloc_headroom(4096, 128, "MGCP rewritten");
+	if (!output) {
+		LOGP(DMGCP, LOGL_ERROR, "Failed to allocate new MGCP msg.\n");
+		return NULL;
+	}
+
+	running = (char *) input->l2h;
+	output->l2h = output->data;
+	for (token = strsep(&running, "\n"); token; token = strsep(&running, "\n")) {
+		int len = strlen(token);
+
+		/* ignore completely empty lines for now */
+		if (len == 0)
+			continue;
+
+		if (strncmp(ip_str, token, (sizeof ip_str) - 1) == 0) {
+			output->l3h = msgb_put(output, strlen(ip_str));
+			memcpy(output->l3h, ip_str, strlen(ip_str));
+			output->l3h = msgb_put(output, strlen(ip));
+			memcpy(output->l3h, ip, strlen(ip));
+			output->l3h = msgb_put(output, 2);
+			output->l3h[0] = '\r';
+			output->l3h[1] = '\n';
+		} else if (strncmp(aud_str, token, (sizeof aud_str) - 1) == 0) {
+			int payload;
+			if (sscanf(token, "m=audio %*d RTP/AVP %d", &payload) != 1) {
+				LOGP(DMGCP, LOGL_ERROR, "Could not parsed audio line.\n");
+				msgb_free(output);
+				return NULL;
+			}
+
+			snprintf(buf, sizeof(buf)-1, "m=audio %d RTP/AVP %d\r\n", port, payload);
+			buf[sizeof(buf)-1] = '\0';
+
+			output->l3h = msgb_put(output, strlen(buf));
+			memcpy(output->l3h, buf, strlen(buf));
+		} else {
+			output->l3h = msgb_put(output, len + 1);
+			memcpy(output->l3h, token, len);
+			output->l3h[len] = '\n';
+		}
+	}
+
+	return output;
+}
+
 static int mgcp_do_read(struct bsc_fd *fd)
 {
 	struct bsc_nat *nat;
diff --git a/openbsc/tests/bsc-nat/bsc_data.c b/openbsc/tests/bsc-nat/bsc_data.c
index ca170e2..28d5064 100644
--- a/openbsc/tests/bsc-nat/bsc_data.c
+++ b/openbsc/tests/bsc-nat/bsc_data.c
@@ -87,3 +87,58 @@
 0x00, 0x00, 0x49, 0x00, 0x01, 0x0b, 0x00, 0x09,
 0x01, 0x0b, 0x03, 0x01, 0x0a, 0x11, 0x01, 0x00,
 0x15 };
+
+/*
+ * MGCP messages
+ */
+
+/* nothing to patch */
+static const char crcx[] = "CRCX 23265295 8@mgw MGCP 1.0\r\nC: 394b0439fb\r\nL: p:20, a:AMR, nt:IN\r\nM: recvonly\r\n";
+static const char crcx_patched[] = "CRCX 23265295 8@mgw MGCP 1.0\r\nC: 394b0439fb\r\nL: p:20, a:AMR, nt:IN\r\nM: recvonly\r\n";
+
+
+/* patch the ip and port */
+static const char crcx_resp[] = "200 23265295\r\nI: 1\r\n\r\nv=0\r\nc=IN IP4 172.16.18.2\r\nm=audio 4002 RTP/AVP 98\r\na=rtpmap:98 AMR/8000\r\n";
+static const char crcx_resp_patched[] = "200 23265295\r\nI: 1\r\n\r\nv=0\r\nc=IN IP4 10.0.0.1\r\nm=audio 999 RTP/AVP 98\r\na=rtpmap:98 AMR/8000\r\n";
+
+/* patch the ip and port */
+static const char mdcx[] = " MDCX 23330829 8@mgw MGCP 1.0\r\nC: 394b0439fb\r\nI: 1\r\nL: p:20, a:AMR, nt:IN\r\nM: recvonly\r\n\r\nv=0\r\no=- 1049380491 0 IN IP4 172.16.18.2\r\ns=-\r\nc=IN IP4 172.16.18.2\r\nt=0 0\r\nm=audio 4410 RTP/AVP 126\r\na=rtpmap:126 AMR/8000/1\r\na=fmtp:126 mode-set=2;start-mode=0\r\na=ptime:20\r\na=recvonly\r\nm=image 4412 udptl t38\r\na=T38FaxVersion:0\r\na=T38MaxBitRate:14400\r\n";
+static const char mdcx_patched[] = " MDCX 23330829 8@mgw MGCP 1.0\r\nC: 394b0439fb\r\nI: 1\r\nL: p:20, a:AMR, nt:IN\r\nM: recvonly\r\n\r\nv=0\r\no=- 1049380491 0 IN IP4 172.16.18.2\r\ns=-\r\nc=IN IP4 10.0.0.23\r\nt=0 0\r\nm=audio 6666 RTP/AVP 126\r\na=rtpmap:126 AMR/8000/1\r\na=fmtp:126 mode-set=2;start-mode=0\r\na=ptime:20\r\na=recvonly\r\nm=image 4412 udptl t38\r\na=T38FaxVersion:0\r\na=T38MaxBitRate:14400\r\n";
+
+
+static const char mdcx_resp[] = "200 23330829\r\n\r\nv=0\r\nc=IN IP4 172.16.18.2\r\nm=audio 4002 RTP/AVP 98\r\na=rtpmap:98 AMR/8000\r\n";
+static const char mdcx_resp_patched[] = "200 23330829\r\n\r\nv=0\r\nc=IN IP4 10.0.0.23\r\nm=audio 5555 RTP/AVP 98\r\na=rtpmap:98 AMR/8000\r\n";
+
+struct mgcp_patch_test {
+	const char *orig;
+	const char *patch;
+	const char *ip;
+	const int port;
+};
+
+static const struct mgcp_patch_test mgcp_messages[] = {
+	{
+		.orig = crcx,
+		.patch = crcx_patched,
+		.ip = "0.0.0.0",
+		.port = 2323,
+	},
+	{
+		.orig = crcx_resp,
+		.patch = crcx_resp_patched,
+		.ip = "10.0.0.1",
+		.port = 999,
+	},
+	{
+		.orig = mdcx,
+		.patch = mdcx_patched,
+		.ip = "10.0.0.23",
+		.port = 6666,
+	},
+	{
+		.orig = mdcx_resp,
+		.patch = mdcx_resp_patched,
+		.ip = "10.0.0.23",
+		.port = 5555,
+	},
+};
diff --git a/openbsc/tests/bsc-nat/bsc_nat_test.c b/openbsc/tests/bsc-nat/bsc_nat_test.c
index 30f1490..217c4f4 100644
--- a/openbsc/tests/bsc-nat/bsc_nat_test.c
+++ b/openbsc/tests/bsc-nat/bsc_nat_test.c
@@ -410,6 +410,40 @@
 	talloc_free(nat);
 }
 
+static void test_mgcp_rewrite(void)
+{
+	int i;
+	struct msgb *input, *output;
+	fprintf(stderr, "Test rewriting MGCP messages.\n");
+
+	input = msgb_alloc(4096, "input");
+
+	for (i = 0; i < ARRAY_SIZE(mgcp_messages); ++i) {
+		const char *orig = mgcp_messages[i].orig;
+		const char *patc = mgcp_messages[i].patch;
+		const char *ip = mgcp_messages[i].ip;
+		const int port = mgcp_messages[i].port;
+
+		copy_to_msg(input, (const u_int8_t *) orig, strlen(orig) + 1);
+
+		output = bsc_mgcp_rewrite(input, ip, port);
+		if (msgb_l2len(output) != strlen(patc)) {
+			fprintf(stderr, "Wrong sizes for test: %d  %d != %d != %d\n", i, msgb_l2len(output), strlen(patc), strlen(orig));
+			fprintf(stderr, "String '%s' vs '%s'\n", (const char *) output->l2h, patc);
+			abort();
+		}
+
+		if (memcmp(output->l2h, patc, msgb_l2len(output)) != 0) {
+			fprintf(stderr, "Broken on %d msg: '%s'\n", i, (const char *) output->l2h);
+			abort();
+		}
+
+		msgb_free(output);
+	}
+
+	msgb_free(input);
+}
+
 int main(int argc, char **argv)
 {
 	struct debug_target *stderr_target;
@@ -423,6 +457,7 @@
 	test_paging();
 	test_mgcp_ass_tracking();
 	test_mgcp_find();
+	test_mgcp_rewrite();
 	return 0;
 }