Re: PMTUD broken inside network namespace with multipath routing

Hello David.

I was able to solve it while troubleshooting some fragmentation issue.
The VTI interfaces had MTU of 1480 by default. I reduced to them to
the real PMTUD (1366) and now its all working just fine.
I am not sure how its related and why, but seems like it solved the issue.

P.S: while reading the relevant code in the kernel, i think i spotted
some mistake in net/ipv4/route.c, in function "update_or_create_fnhe".
It looks like it loops over all the exceptions for the nexthop entry,
but always overwriting the first (and only) entry, so effectively only
1 exception can exist per nexthop entry.
Line 678:
"if (fnhe) {"
Should probably be:
"if (fnhe && fnhe->fnhe_daddr == daddr) {"


Thank you for your efforts,
Kfir Itzhak

On Fri, Aug 14, 2020 at 10:08 AM mastertheknife
[off-list ref] wrote:

Hello David,

It's on a production system, vmbr2 is a bridge with eth.X VLAN
interface inside for the connectivity on that 252.0/24 network. vmbr2
has address 192.168.252.5 in that case
192.168.252.250 and 192.168.252.252 are CentOS8 LXCs on another host,
with libreswan inside for any/any IPSECs with VTi interfaces.

Everything is kernel 5.4.44 LTS

I wish i could fully reproduce all of it in a script, but i am not
sure how to create such hops that return this ICMP

Thank you,
Kfir


On Wed, Aug 12, 2020 at 10:21 PM David Ahern [off-list ref] wrote:

quoted

On 8/12/20 6:37 AM, mastertheknife wrote:

quoted

Hello David,

I tried and it seems i can reproduce it:

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

vmbr2's config is not defined.

ip li add vmbr2 type bridge
ip li set veth0 master vmbr2
ip link set veth0 up

anything else? e.g., address for vmbr2? What holds 192.168.252.250 and
192.168.252.252