1 files changed, 11 insertions, 7 deletions
diff --git a/networking/udhcp/d6_packet.c b/networking/udhcp/d6_packet.c
index 3a1bb3df1..79b2946ef 100644
@@ -92,16 +92,20 @@ int FAST_FUNC d6_send_raw_packet(
/* size, excluding IP header: */
packet.udp.len = htons(sizeof(struct udphdr) + d6_pkt_size);
packet.ip6.ip6_plen = packet.udp.len;
- /* UDP checksum skips first four bytes of IP header.
- * IPv6 'hop limit' field should be 0.
- * 'next header' field should be summed as if it is in a different
- * position, therefore we write its value into ip6_hlim:
+ * Someone was smoking weed (at least) while inventing UDP checksumming:
+ * UDP checksum skips first four bytes of IPv6 header.
+ * 'next header' field should be summed as if it is one more byte
+ * to the right, therefore we write its value (IPPROTO_UDP)
+ * into ip6_hlim, and its 'real' location remains zero-filled for now.
packet.ip6.ip6_hlim = IPPROTO_UDP;
- packet.udp.check = inet_cksum((uint16_t *)&packet + 2,
- offsetof(struct ip6_udp_d6_packet, data) - 4 + d6_pkt_size);
+ packet.udp.check = inet_cksum(
+ (uint16_t *)&packet + 2,
+ offsetof(struct ip6_udp_d6_packet, data) - 4 + d6_pkt_size
/* fix 'hop limit' and 'next header' after UDP checksumming */
- packet.ip6.ip6_hlim = 8;
+ packet.ip6.ip6_hlim = 1; /* observed Windows machines to use hlim=1 */
packet.ip6.ip6_nxt = IPPROTO_UDP;