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: Ulrich Weber <uweber@astaro.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 4 deletions

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 1a5fe9ad1947..dabf108ad811 100644
--- a/net/ipv6/ip6_output.c
+++ b/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;