netfilter: conntrack: fix ct-state for ICMPv6 Multicast Router Discovery

So far Multicast Router Advertisements and Multicast Router
Solicitations from the Multicast Router Discovery protocol (RFC4286)
would be marked as INVALID for IPv6, even if they are in fact intact
and adhering to RFC4286.

This broke MRA reception and by that multicast reception on
IPv6 multicast routers in a Proxmox managed setup, where Proxmox
would install a rule like "-m conntrack --ctstate INVALID -j DROP"
at the top of the FORWARD chain with br-nf-call-ip6tables enabled
by default.

Similar to as it's done for MLDv1, MLDv2 and IPv6 Neighbor Discovery
already, fix this issue by excluding MRD from connection tracking
handling as MRD always uses predefined multicast destinations
for its messages, too. This changes the ct-state for ICMPv6 MRD messages
from INVALID to UNTRACKED.

This issue was found and fixed with the help of the mrdisc tool
(https://github.com/troglobit/mrdisc).
Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/uapi/linux/icmpv6.h

@@ -112,6 +112,7 @@ struct icmp6hdr {
 #define ICMPV6_MOBILE_PREFIX_ADV	147
 
 #define ICMPV6_MRDISC_ADV		151
+#define ICMPV6_MRDISC_SOL		152
 
 #define ICMPV6_MSG_MAX          255
 

net/netfilter/nf_conntrack_proto_icmpv6.c

@@ -62,7 +62,9 @@ static const u_int8_t noct_valid_new[] = {
 	[NDISC_ROUTER_ADVERTISEMENT - 130] = 1,
 	[NDISC_NEIGHBOUR_SOLICITATION - 130] = 1,
 	[NDISC_NEIGHBOUR_ADVERTISEMENT - 130] = 1,
-	[ICMPV6_MLD2_REPORT - 130] = 1
+	[ICMPV6_MLD2_REPORT - 130] = 1,
+	[ICMPV6_MRDISC_ADV - 130] = 1,
+	[ICMPV6_MRDISC_SOL - 130] = 1
 };
 
 bool nf_conntrack_invert_icmpv6_tuple(struct nf_conntrack_tuple *tuple,