Rundong Ge <[email protected]> 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.
> 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 <[email protected]> 于2019年7月30日周二 下午8:35写道：
>
> Rundong Ge <[email protected]> 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.
On Tuesday 2019-07-30 14:35, Florian Westphal wrote:
>Rundong Ge <[email protected]> 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.
Jan Engelhardt <[email protected]> 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 <[email protected]> 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.