--- a/src/extra/checksum.c
+++ b/src/extra/checksum.c
@@ -11,6 +11,7 @@
 
 #include <stdio.h>
 #include <stdbool.h>
+#include <endian.h>
 #include <arpa/inet.h>
 #include <netinet/ip.h>
 #include <netinet/ip6.h>
@@ -26,8 +27,13 @@ uint16_t nfq_checksum(uint32_t sum, uint16_t *buf, int size)
 		sum += *buf++;
 		size -= sizeof(uint16_t);
 	}
-	if (size)
-		sum += *(uint8_t *)buf;
+	if (size) {
+#if __BYTE_ORDER == __BIG_ENDIAN
+		sum += (uint16_t)*(uint8_t *)buf << 8;
+#else
+		sum += (uint16_t)*(uint8_t *)buf;
+#endif
+	}
 
 	sum = (sum >> 16) + (sum & 0xffff);
 	sum += (sum >>16);
@@ -35,7 +41,7 @@ uint16_t nfq_checksum(uint32_t sum, uint16_t *buf, int size)
 	return (uint16_t)(~sum);
 }
 
-uint16_t nfq_checksum_tcpudp_ipv4(struct iphdr *iph)
+uint16_t nfq_checksum_tcpudp_ipv4(struct iphdr *iph, uint16_t protocol_id)
 {
 	uint32_t sum = 0;
 	uint32_t iph_len = iph->ihl*4;
@@ -46,13 +52,13 @@ uint16_t nfq_checksum_tcpudp_ipv4(struct iphdr *iph)
 	sum += (iph->saddr) & 0xFFFF;
 	sum += (iph->daddr >> 16) & 0xFFFF;
 	sum += (iph->daddr) & 0xFFFF;
-	sum += htons(IPPROTO_TCP);
+	sum += htons(protocol_id);
 	sum += htons(len);
 
 	return nfq_checksum(sum, (uint16_t *)payload, len);
 }
 
-uint16_t nfq_checksum_tcpudp_ipv6(struct ip6_hdr *ip6h, void *transport_hdr)
+uint16_t nfq_checksum_tcpudp_ipv6(struct ip6_hdr *ip6h, void *transport_hdr, uint16_t protocol_id)
 {
 	uint32_t sum = 0;
 	uint32_t hdr_len = (uint32_t *)transport_hdr - (uint32_t *)ip6h;
@@ -68,7 +74,7 @@ uint16_t nfq_checksum_tcpudp_ipv6(struct ip6_hdr *ip6h, void *transport_hdr)
 		sum += (ip6h->ip6_dst.s6_addr16[i] >> 16) & 0xFFFF;
 		sum += (ip6h->ip6_dst.s6_addr16[i]) & 0xFFFF;
 	}
-	sum += htons(IPPROTO_TCP);
+	sum += htons(protocol_id);
 	sum += htons(ip6h->ip6_plen);
 
 	return nfq_checksum(sum, (uint16_t *)payload, len);
diff --git a/src/extra/tcp.c b/src/extra/tcp.c
index d1cd79d..a66f392 100644
--- a/src/extra/tcp.c
+++ b/src/extra/tcp.c
@@ -96,7 +96,7 @@ nfq_tcp_compute_checksum_ipv4(struct tcphdr *tcph, struct iphdr *iph)
 {
 	/* checksum field in header needs to be zero for calculation. */
 	tcph->check = 0;
-	tcph->check = nfq_checksum_tcpudp_ipv4(iph);
+	tcph->check = nfq_checksum_tcpudp_ipv4(iph, IPPROTO_TCP);
 }
 EXPORT_SYMBOL(nfq_tcp_compute_checksum_ipv4);
 
@@ -110,7 +110,7 @@ nfq_tcp_compute_checksum_ipv6(struct tcphdr *tcph, struct ip6_hdr *ip6h)
 {
 	/* checksum field in header needs to be zero for calculation. */
 	tcph->check = 0;
-	tcph->check = nfq_checksum_tcpudp_ipv6(ip6h, tcph);
+	tcph->check = nfq_checksum_tcpudp_ipv6(ip6h, tcph, IPPROTO_TCP);
 }
 EXPORT_SYMBOL(nfq_tcp_compute_checksum_ipv6);
 
--- a/src/extra/udp.c
+++ b/src/extra/udp.c
@@ -96,7 +96,7 @@ nfq_udp_compute_checksum_ipv4(struct udphdr *udph, struct iphdr *iph)
 {
 	/* checksum field in header needs to be zero for calculation. */
 	udph->check = 0;
-	udph->check = nfq_checksum_tcpudp_ipv4(iph);
+	udph->check = nfq_checksum_tcpudp_ipv4(iph, IPPROTO_UDP);
 }
 EXPORT_SYMBOL(nfq_udp_compute_checksum_ipv4);
 
@@ -115,7 +115,7 @@ nfq_udp_compute_checksum_ipv6(struct udphdr *udph, struct ip6_hdr *ip6h)
 {
 	/* checksum field in header needs to be zero for calculation. */
 	udph->check = 0;
-	udph->check = nfq_checksum_tcpudp_ipv6(ip6h, udph);
+	udph->check = nfq_checksum_tcpudp_ipv6(ip6h, udph, IPPROTO_UDP);
 }
 EXPORT_SYMBOL(nfq_udp_compute_checksum_ipv6);
 
--- a/src/internal.h
+++ b/src/internal.h
@@ -15,8 +15,8 @@ struct iphdr;
 struct ip6_hdr;
 
 uint16_t nfq_checksum(uint32_t sum, uint16_t *buf, int size);
-uint16_t nfq_checksum_tcpudp_ipv4(struct iphdr *iph);
-uint16_t nfq_checksum_tcpudp_ipv6(struct ip6_hdr *ip6h, void *transport_hdr);
+uint16_t nfq_checksum_tcpudp_ipv4(struct iphdr *iph, uint16_t protocol_id);
+uint16_t nfq_checksum_tcpudp_ipv6(struct ip6_hdr *ip6h, void *transport_hdr, uint16_t protocol_id);
 
 struct pkt_buff {
 	uint8_t *mac_header;
