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Date:   Tue, 30 Jul 2019 14:35:42 +0200
From:   Florian Westphal <fw@strlen.de>
To:     Rundong Ge <rdong.ge@gmail.com>
Cc:     davem@davemloft.net, kuznet@ms2.inr.ac.ru, yoshfuji@linux-ipv6.org,
        netdev@vger.kernel.org, pablo@netfilter.org, kadlec@netfilter.org,
        fw@strlen.de, roopa@cumulusnetworks.com,
        netfilter-devel@vger.kernel.org, coreteam@netfilter.org,
        bridge@lists.linux-foundation.org, nikolay@cumulusnetworks.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] bridge:fragmented packets dropped by bridge
Message-ID: <20190730123542.zrsrfvcy7t2n3d4g@breakpoint.cc>
References: <20190730122534.30687-1-rdong.ge@gmail.com>
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Rundong Ge <rdong.ge@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.