Public bug reported:
Hi,
I have discovered that with the Ubuntu 5.4.44 kernel, PMTUD (Path MTU
discovery) is broken inside LXC (network namespace) if using multipath
routing. This breaks TCP, because it keeps trying to send oversized
packets.
Host test:
----------
root@host1:~# ip route add 192.168.247.100/32 dev vmbr2 nexthop via
192.168.252.250 dev vmbr2 nexthop via 192.168.252.252 dev vmbr2
root@host1:~# ip route | grep -A2 192.168.247.100
192.168.247.100
nexthop via 192.168.252.250 dev vmbr2 weight 1
nexthop via 192.168.252.252 dev vmbr2 weight 1
root@host1:~# ping -M do -s 1380 192.168.247.100
PING 192.168.247.100 (192.168.247.100) 1380(1408) bytes of data.
>From 192.168.252.252 icmp_seq=1 Frag needed and DF set (mtu = 1406)
ping: local error: Message too long, mtu=1406
ping: local error: Message too long, mtu=1406
ping: local error: Message too long, mtu=1406
ping: local error: Message too long, mtu=1406
^C
--- 192.168.247.100 ping statistics ---
5 packets transmitted, 0 received, +5 errors, 100% packet loss, time 80ms
root@host1:~# ip route get 192.168.247.100
192.168.247.100 via 192.168.252.250 dev vmbr2 src 192.168.252.15 uid 0
cache expires 583sec mtu 1406
LXC container inside that host:
-------------------------------
[root@lxctest ~]# ip route add 192.168.247.100/32 dev eth0 nexthop via
192.168.252.250 dev eth0 nexthop via 192.168.252.252 dev eth0
[root@lxctest ~]# ip route
default via 192.168.252.100 dev eth0 proto static metric 100
192.168.247.100
nexthop via 192.168.252.250 dev eth0 weight 1
nexthop via 192.168.252.252 dev eth0 weight 1
192.168.252.0/24 dev eth0 proto kernel scope link src 192.168.252.207 metric
100
[root@lxctest ~]# ping -M do -s 1380 192.168.247.100
PING 192.168.247.100 (192.168.247.100) 1380(1408) bytes of data.
>From 192.168.252.252 icmp_seq=1 Frag needed and DF set (mtu = 1406)
>From 192.168.252.252 icmp_seq=2 Frag needed and DF set (mtu = 1406)
>From 192.168.252.252 icmp_seq=3 Frag needed and DF set (mtu = 1406)
>From 192.168.252.252 icmp_seq=4 Frag needed and DF set (mtu = 1406)
[root@lxctest ~]# ip route get 192.168.247.100
192.168.247.100 via 192.168.252.252 dev eth0 src 192.168.252.207 uid 0
cache
What seems to be happening, is that when multipath routing is used inside LXC
(or any network namespace), the kernel doesn't generate a routing exception to
force the lower MTU.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: Incomplete
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https://bugs.launchpad.net/bugs/1890131
Title:
Kernel 5.4.44 PMTUD broken inside LXC with multipath routing
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