ipv6: Use 1280 as min MTU for ipv6 forwarding

Clients will set their MTU to 1280 if they receive a
ICMPV6_PKT_TOOBIG message with an MTU less than 1280.

To allow encapsulating of packets over a 1280 link
we should always accept packets with a size of 1280
for forwarding even if the path has a lower MTU and
fragment the encapsulated packets afterwards.

In case a forwarded packet is not going to be encapsulated
a ICMPV6_PKT_TOOBIG msg will still be send by ip6_fragment()
with the correct MTU.
Signed-off-by: NUlrich Weber <uweber@astaro.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -402,6 +402,7 @@ int ip6_forward(struct sk_buff *skb)
 	struct ipv6hdr *hdr = ipv6_hdr(skb);
 	struct inet6_skb_parm *opt = IP6CB(skb);
 	struct net *net = dev_net(dst->dev);
+	u32 mtu;
 
 	if (net->ipv6.devconf_all->forwarding == 0)
 		goto error;
@@ -508,10 +509,14 @@ int ip6_forward(struct sk_buff *skb)
 		}
 	}
 
-	if (skb->len > dst_mtu(dst)) {
+	mtu = dst_mtu(dst);
+	if (mtu < IPV6_MIN_MTU)
+		mtu = IPV6_MIN_MTU;
+
+	if (skb->len > mtu) {
 		/* Again, force OUTPUT device used as source address */
 		skb->dev = dst->dev;
-		icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, dst_mtu(dst));
+		icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
 		IP6_INC_STATS_BH(net,
 				 ip6_dst_idev(dst), IPSTATS_MIB_INTOOBIGERRORS);
 		IP6_INC_STATS_BH(net,
@@ -621,8 +626,7 @@ static int ip6_fragment(struct sk_buff *skb, int (*output)(struct sk_buff *))
 	mtu = ip6_skb_dst_mtu(skb);
 
 	/* We must not fragment if the socket is set to force MTU discovery
-	 * or if the skb it not generated by a local socket.  (This last
-	 * check should be redundant, but it's free.)
+	 * or if the skb it not generated by a local socket.
 	 */
 	if (!skb->local_df) {
 		skb->dev = skb_dst(skb)->dev;