netdev - Re: PMTUD broken inside network namespace with multipath routing

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Date:   Wed, 12 Aug 2020 15:37:29 +0300
From:   mastertheknife <mastertheknife@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: PMTUD broken inside network namespace with multipath routing

Hello David,

I tried and it seems i can reproduce it:

# Create test NS
root@...t:~# ip netns add testns
# Create veth pair, veth0 in host, veth1 in NS
root@...t:~# ip link add veth0 type veth peer name veth1
root@...t:~# ip link set veth1 netns testns
# Configure veth1 (NS)
root@...t:~# ip netns exec testns ip addr add 192.168.252.209/24 dev veth1
root@...t:~# ip netns exec testns ip link set dev veth1 up
root@...t:~# ip netns exec testns ip route add default via 192.168.252.100
root@...t:~# ip netns exec testns ip route add 192.168.249.0/24
nexthop via 192.168.252.250 nexthop via 192.168.252.252
# Configure veth0 (host)
root@...t:~# brctl addif vmbr2 veth0
root@...t:~# ip link set veth0 up


# Tests
root@...t:~# ip netns exec testns ping -M do -s 1450 192.168.249.116
PING 192.168.249.116 (192.168.249.116) 1450(1478) bytes of data.
>From 192.168.252.252 icmp_seq=1 Frag needed and DF set (mtu = 1366)
ping: local error: Message too long, mtu=1366
ping: local error: Message too long, mtu=1366
ping: local error: Message too long, mtu=1366
^C
--- 192.168.249.116 ping statistics ---
4 packets transmitted, 0 received, +4 errors, 100% packet loss, time 81ms

root@...t:~# ip netns exec testns ping -M do -s 1450 192.168.249.134
PING 192.168.249.134 (192.168.249.134) 1450(1478) bytes of data.
>From 192.168.252.252 icmp_seq=1 Frag needed and DF set (mtu = 1366)
>From 192.168.252.252 icmp_seq=2 Frag needed and DF set (mtu = 1366)
>From 192.168.252.252 icmp_seq=3 Frag needed and DF set (mtu = 1366)
^C
--- 192.168.249.134 ping statistics ---
3 packets transmitted, 0 received, +3 errors, 100% packet loss, time 40ms

root@...t:~# ip netns exec testns ip route show cache
192.168.249.116 via 192.168.252.250 dev veth1
    cache expires 584sec mtu 1366
192.168.249.134 via 192.168.252.250 dev veth1
    cache expires 593sec mtu 1366
root@...t:~# ip netns exec testns ip route get 192.168.249.116
192.168.249.116 via 192.168.252.250 dev veth1 src 192.168.252.209 uid 0
    cache expires 578sec mtu 1366
root@...t:~# ip netns exec testns ip route get 192.168.249.134
192.168.249.134 via 192.168.252.252 dev veth1 src 192.168.252.209 uid 0
    cache


Please notice the above, 'ip route show cache' and 'ip route get'
return different nexthop for 192.168.249.134, i suspect that may be
part of the problem.

Thank you,
Kfir

On Tue, Aug 11, 2020 at 1:13 AM David Ahern <dsahern@...il.com> wrote:
>
> On 8/3/20 12:39 PM, mastertheknife wrote:
> > In summary: It seems that it doesn't matter who is the nexthop. If the
> > ICMP response isn't from the nexthop, it'll be rejected.
> > About why i couldn't reproduce this outside LXC, i don't know yet but
> > i will keep trying to figure this out.
>
> do you have a shell script that reproduces the problem?