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Subject[PATCH] ipv6: ip6mr: Recalc UDP checksum before forwarding
Currently, when forwarding from a Virtual Interface to a Physical
Interface, ip_summed is set to a value of CHECKSUM_UNNECESSARY and
the UDP checksum has not been calculated.

When the packet is then forwarded by a Multicast Router, the checksum
value is left as is and therefore rejected by the receiving

This patch ensures the checksum is recalculated before forwarding.

Signed-off-by: Brendan McGrath <>

It's a bit ugly putting UDP specific code in this spot - but I'm not
aware of any other protocols that are:
a) multicast;
b) forwarded; and
c) checksummed

net/ipv6/ip6mr.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index 890f9bda..ee4370a 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c
@@ -2077,6 +2077,13 @@ static int ip6mr_forward2(struct net *net, struct mr6_table *mrt,
ipv6h = ipv6_hdr(skb);

+ if (ipv6h->nexthdr == NEXTHDR_UDP &&
+ skb->ip_summed != CHECKSUM_PARTIAL) {
+ struct udphdr *uh = udp_hdr(skb);
+ udp6_set_csum(false, skb, &ipv6_hdr(skb)->saddr,
+ &ipv6_hdr(skb)->daddr, ntohs(uh->len));
+ }
IP6CB(skb)->flags |= IP6SKB_FORWARDED;

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