diff --git a/udp.c b/udp.c
index 38bb8d8..eb32dda 100644
--- a/udp.c
+++ b/udp.c
@@ -109,6 +109,7 @@
 #include <sys/uio.h>
 #include <unistd.h>
 #include <time.h>
+#include <assert.h>
 
 #include "checksum.h"
 #include "util.h"
@@ -266,7 +267,6 @@ static struct mmsghdr	udp_mmh_sendto		[UDP_SPLICE_FRAMES];
 void udp_remap_to_tap(struct ctx *c, in_port_t port, in_port_t delta)
 {
 	c->udp.fwd_in.f.delta[port] = delta;
-	c->udp.fwd_in.rdelta[port + delta] = USHRT_MAX - delta;
 }
 
 /**
@@ -278,7 +278,23 @@ void udp_remap_to_tap(struct ctx *c, in_port_t port, in_port_t delta)
 void udp_remap_to_init(struct ctx *c, in_port_t port, in_port_t delta)
 {
 	c->udp.fwd_out.f.delta[port] = delta;
-	c->udp.fwd_out.rdelta[port + delta] = USHRT_MAX - delta;
+}
+
+/**
+ * udp_invert_portmap() - Compute reverse port translations for return packets
+ * @fwd:	Port forwarding configuration to compute reverse map for
+ */
+static void udp_invert_portmap(struct udp_port_fwd *fwd)
+{
+	int i;
+
+	assert(ARRAY_SIZE(fwd->f.delta) == ARRAY_SIZE(fwd->rdelta));
+	for (i = 0; i < ARRAY_SIZE(fwd->f.delta); i++) {
+		in_port_t delta = fwd->f.delta[i];
+
+		if (delta)
+			fwd->rdelta[(in_port_t)i + delta] = USHRT_MAX - delta;
+	}
 }
 
 /**
@@ -1267,7 +1283,7 @@ static void udp_splice_iov_init(void)
  *
  * Return: 0
  */
-int udp_init(const struct ctx *c)
+int udp_init(struct ctx *c)
 {
 	if (c->ifi4)
 		udp_sock4_iov_init();
@@ -1275,6 +1291,9 @@ int udp_init(const struct ctx *c)
 	if (c->ifi6)
 		udp_sock6_iov_init();
 
+	udp_invert_portmap(&c->udp.fwd_in);
+	udp_invert_portmap(&c->udp.fwd_out);
+
 	if (c->mode == MODE_PASTA) {
 		udp_splice_iov_init();
 		NS_CALL(udp_sock_init_ns, c);
diff --git a/udp.h b/udp.h
index cfd1a97..25422b6 100644
--- a/udp.h
+++ b/udp.h
@@ -14,7 +14,7 @@ int udp_tap_handler(struct ctx *c, int af, const void *addr,
 		    const struct pool *p, const struct timespec *now);
 void udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 		   const void *addr, in_port_t port);
-int udp_init(const struct ctx *c);
+int udp_init(struct ctx *c);
 void udp_timer(struct ctx *c, const struct timespec *ts);
 void udp_update_l2_buf(const unsigned char *eth_d, const unsigned char *eth_s,
 		       const uint32_t *ip_da);