[Bridge] [PATCH] bridge:fragmented packets dropped by bridge

Rundong Ge rdong.ge at gmail.com
Tue Jul 30 12:25:34 UTC 2019

Given following setup:
-modprobe br_netfilter
-echo '1' > /proc/sys/net/bridge/bridge-nf-call-iptables
-brctl addbr br0
-brctl addif br0 enp2s0
-brctl addif br0 enp3s0
-brctl addif br0 enp6s0
-ifconfig enp2s0 mtu 1300
-ifconfig enp3s0 mtu 1500
-ifconfig enp6s0 mtu 1500
-ifconfig br0 up

mtu1500 - mtu1500|bridge|1500 - mtu1500
  A                  |            B

With netfilter defragmentation/conntrack enabled, fragmented
packets from A will be defragmented in prerouting, and refragmented
at postrouting.
But in this scenario the bridge found the frag_max_size(1500) is
larger than the dst mtu stored in the fake_rtable whitch is
always equal to the bridge's mtu 1300, then packets will be dopped.

This modifies ip_skb_dst_mtu to use the out dev's mtu instead
of bridge's mtu in bridge refragment.

Signed-off-by: Rundong Ge <rdong.ge at gmail.com>
 include/net/ip.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/include/net/ip.h b/include/net/ip.h
index 29d89de..0512de3 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h
@@ -450,6 +450,8 @@ static inline unsigned int ip_dst_mtu_maybe_forward(const struct dst_entry *dst,
 static inline unsigned int ip_skb_dst_mtu(struct sock *sk,
 					  const struct sk_buff *skb)
+	if ((skb_dst(skb)->flags & DST_FAKE_RTABLE) && skb->dev)
+		return min(skb->dev->mtu, IP_MAX_MTU);
 	if (!sk || !sk_fullsock(sk) || ip_sk_use_pmtu(sk)) {
 		bool forwarding = IPCB(skb)->flags & IPSKB_FORWARDED;

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