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

Florian Westphal fw at strlen.de
Tue Jul 30 12:35:42 UTC 2019

Rundong Ge <rdong.ge at gmail.com> wrote:
> 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
>                  multi-port
> mtu1500 - mtu1500|bridge|1500 - mtu1500
>   A                  |            B
>                    mtu1300

How can a bridge forward a frame from A/B to mtu1300?

> With netfilter defragmentation/conntrack enabled, fragmented
> packets from A will be defragmented in prerouting, and refragmented
> at postrouting.

Yes, but I don't see how that relates to the problem at hand.

> 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.

What happens without netfilter or non-fragmented packets?

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

It seems quite a hack?  The above setup should use a router, not a bridge.

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