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Subject[PATCH 3.10 05/18] netfilter: ipv4: defrag: set local_df flag on defragmented skb
3.10-stable review patch.  If anyone has any objections, please let me know.


From: Florian Westphal <>

commit 895162b1101b3ea5db08ca6822ae9672717efec0 upstream.

else we may fail to forward skb even if original fragments do fit
outgoing link mtu:

1. remote sends 2k packets in two 1000 byte frags, DF set
2. we want to forward but only see '2k > mtu and DF set'
3. we then send icmp error saying that outgoing link is 1500

But original sender never sent a packet that would not fit
the outgoing link.

Setting local_df makes outgoing path test size vs.
IPCB(skb)->frag_max_size, so we will still send the correct
error in case the largest original size did not fit
outgoing link mtu.

Reported-by: Maxime Bizon <>
Suggested-by: Maxime Bizon <>
Fixes: 5f2d04f1f9 (ipv4: fix path MTU discovery with connection tracking)
Signed-off-by: Florian Westphal <>
Signed-off-by: Pablo Neira Ayuso <>
Cc: Jiri Slaby <>
Signed-off-by: Greg Kroah-Hartman <>

net/ipv4/netfilter/nf_defrag_ipv4.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

--- a/net/ipv4/netfilter/nf_defrag_ipv4.c
+++ b/net/ipv4/netfilter/nf_defrag_ipv4.c
@@ -22,7 +22,6 @@
#include <net/netfilter/nf_conntrack_zones.h>

-/* Returns new sk_buff, or NULL */
static int nf_ct_ipv4_gather_frags(struct sk_buff *skb, u_int32_t user)
int err;
@@ -33,8 +32,10 @@ static int nf_ct_ipv4_gather_frags(struc
err = ip_defrag(skb, user);

- if (!err)
+ if (!err) {
+ skb->local_df = 1;
+ }

return err;

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