netdev - Re: [PATCH] bridge:fragmented packets dropped by bridge

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Date:   Wed, 28 Aug 2019 17:21:46 +0800
From:   Rundong Ge <rdong.ge@...il.com>
To:     Jan Engelhardt <jengelh@...i.de>
Cc:     Florian Westphal <fw@...len.de>, davem@...emloft.net,
        kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        netdev@...r.kernel.org, Pablo Neira Ayuso <pablo@...filter.org>,
        kadlec@...filter.org, Roopa Prabhu <roopa@...ulusnetworks.com>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        bridge@...ts.linux-foundation.org,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bridge:fragmented packets dropped by bridge

Jan Engelhardt <jengelh@...i.de> 于2019年8月26日周一 下午3:59写道：
>
>
> On Tuesday 2019-07-30 14:35, Florian Westphal wrote:
> >Rundong Ge <rdong.ge@...il.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?
>
> There might be a misunderstanding here judging from the shortness of this
> thread.
>
> I understood it such that the bridge ports (eth0,eth1) have MTU 1500, yet br0
> (in essence the third bridge port if you so wish) itself has MTU 1300.
>
> Therefore, frame forwarding from eth0 to eth1 should succeed, since the
> 1300-byte MTU is only relevant if the bridge decides the packet needs to be
> locally delivered.

Under this setup when I do "ping B -l 2000" from A, the fragmented
packets will be dropped by bridge.
When the "/proc/sys/net/bridge/bridge-nf-call-iptables" is on, bridge
will do defragment at PREROUTING and re-fragment at POSTROUTING. At
the re-fragment bridge will check if the max frag size is larger than
the bridge's MTU in  br_nf_ip_fragment(), if it is true packets will
be dropped.
And this patch use the outdev's MTU instead of the bridge's MTU to do
the br_nf_ip_fragment.