udhcpc6: set hop limit to 1 in emitted raw packets

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

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
--- a/networking/udhcp/d6_packet.c
+++ b/networking/udhcp/d6_packet.c
@@ -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;
 
         d6_dump_packet(d6_pkt);