The level 4 protocol is part of the UDP and TCP calculations. nfq_checksum_tcpudp_ipv4() was using IPPROTO_TCP in this calculation, which gave the wrong answer for UDP. --- src/extra/checksum.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/extra/checksum.c b/src/extra/checksum.c index f367f75..3ef288f 100644 --- a/src/extra/checksum.c +++ b/src/extra/checksum.c @@ -46,7 +46,7 @@ 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(iph->protocol); sum += htons(len); return nfq_checksum(sum, (uint16_t *)payload, len); -- 2.14.5
