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

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Date:   Tue, 30 Jul 2019 21:50:05 +0800
From:   Rundong Ge <rdong.ge@...il.com>
To:     Florian Westphal <fw@...len.de>
Cc:     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@...ulusnetworks.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bridge:fragmented packets dropped by bridge

> How can a bridge forward a frame from A/B to mtu1300?
It is free for user to set different MTU for bridge ports. In our case
only tcp traffic between A/B and mtu 1300, and mss negotiation can
make packets less than 1300.

> What happens without netfilter or non-fragmented packets?
Without br_netfilter it works fine, there is no defragmentation and
refragmentation, fragmented packets will egress directly.

Florian Westphal <fw@...len.de> 于2019年7月30日周二 下午8:35写道：
>
> 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?
>
> > 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.