diff --git a/src/checksum.c b/src/checksum.c
index 6f17240..01b73b4 100644
--- a/src/checksum.c
+++ b/src/checksum.c
@@ -95,11 +95,11 @@ unsigned short checksum_ipv4(struct s_ipv4_addr ip_src,
 	header->ip_dest = ip_dest;
 	header->zeros	= 0x0;
 	header->proto	= proto;
-	header->len	= htonl((unsigned int) length);
+	header->len	= htons(length);
 
 	memcpy(buffer + sizeof(struct s_ipv4_pseudo), payload, (int) length);
 
-	sum = checksum(buffer, sizeof(struct s_ipv6_pseudo) + (int) length);
+	sum = checksum(buffer, sizeof(struct s_ipv4_pseudo) + (int) length);
 
 	free(buffer);
 
diff --git a/src/ipv4.h b/src/ipv4.h
index 03d4bb3..faee16a 100644
--- a/src/ipv4.h
+++ b/src/ipv4.h
@@ -53,7 +53,7 @@ struct s_ipv4_pseudo {
 	struct s_ipv4_addr	ip_dest;	/* 32 b; destination address */
 	unsigned char		zeros;		/*  8 b */
 	unsigned char		proto;		/*  8 b; protocol in payload */
-	unsigned int		len;		/* 32 b; payload length */
+	unsigned short		len;		/* 16 b; payload length */
 } __attribute__ ((__packed__));
 
 int ipv4(struct s_ethernet *eth, char *packet);
diff --git a/src/udp.c b/src/udp.c
index cc8f424..4d58889 100644
--- a/src/udp.c
+++ b/src/udp.c
@@ -58,7 +58,20 @@ int udp_ipv4(struct s_ethernet *eth, struct s_ipv4 *ip4, char *payload,
 	/* parse UDP header */
 	udp = (struct s_udp *) payload;
 
-	/* TODO: checksum recheck */
+	/* checksum recheck */
+	if (udp->checksum != 0x0000) {
+		orig_checksum = udp->checksum;
+		udp->checksum = 0;
+		udp->checksum = checksum_ipv4(ip4->ip_src, ip4->ip_dest,
+					      payload_size, IPPROTO_UDP,
+					      (unsigned char *) udp);
+
+		if (udp->checksum != orig_checksum) {
+			/* packet is corrupted and shouldn't be processed */
+			printf("[Debug] Wrong checksum\n");
+			return 1;
+		}
+	}
 
 	/* find connection in NAT */
 	connection = nat_in(nat4_udp, ip4->ip_src, udp->port_src, udp->port_dest);
@@ -185,16 +198,17 @@ int udp_ipv6(struct s_ethernet *eth, struct s_ipv6 *ip6, char *payload)
 	udp->port_src = connection->ipv4_port_src;
 
 	/* compute UDP checksum */
-	udp->checksum = 0;
-	/* TODO: checksum computation; in IPv4 it's optional in UDP */
+	udp->checksum = 0x0;
+	udp->checksum = checksum_ipv4(ip4->ip_src, ip4->ip_dest,
+				      htons(ip6->len), IPPROTO_UDP,
+				      (unsigned char *) payload);
 
 	/* copy the payload data (with new checksum) */
 	memcpy(packet + sizeof(struct s_ipv4), payload, htons(ip6->len));
 
 	/* compute IPv4 checksum */
-	ip4->checksum = checksum_ipv4(ip4->ip_src, ip4->ip_dest,
-				      htons(ip4->len), IPPROTO_UDP,
-				      (unsigned char *) udp);
+	ip4->checksum = 0x0;
+	ip4->checksum = checksum(ip4, sizeof(struct s_ipv4));
 
 	/* send translated packet */
 	printf("[Debug] transmitting\n");