Hi all!
I have spent hours debugging network issues between domU and dom0 - without success. I hope you have some ideas what could be the cause of the problem, or you can give me some hints how to debug this issue.
Situation: dom0 and PV domU are Ubuntu22.04 with XEN
4.16.0-1~ubuntu2.1(I had the same issues with Ubuntu 20.04 and XEN
4.11.3+24-g14b62ab3e5-1ubuntu2.3). The dom0 was setup by my provider
where I rent this baremetal server. We have 20+ other servers with other providers
with more or less identical setup and without issues.
dom0 vif1.0 <---------------> eth0 domU
192.168.0.1/24 192.168.0.2/24
No nftables, iptables, ebtables.
Packets pass from dom0 to domU, but almost not from domU to dom0.
"Almost not" means, that once in a while a packet pass through from the
domU to the dom0.
For example, dom0 pings 102.168.0.2. I use tcpdump in dom0 with "-i
vif1.0" and in domU with "-i eth0".
In the dom0 I see the ARP request for 192.168.0.2.
In the domU I see the ARP request for 192.168.0.2 arriving and I see the
ARP response back.
But the ARP response does not arrive in the dom0 (I do not see it in
tcpdump).
Once in a while the ARP response passes through to the domU. Then the
dom0 send ICMP ping request, which arrives in the domU, and the domU
responds with an ICMP response which does not arrive in the domU. Again,
once in a while a ping response arrives in the domU.
RX/TX statistics in the domU show plenty of packets coming in and sent
out.
RX/TX statistics in the dom0 show plenty of packets sent out, but only a
few packets coming in. For example TX=5000 packets, RX=20 packets.
No "dropped" or "error" indications.
As said before I have no idea where or why the packets do not pass
through to the dom0. I have plenty of identical setups on other servers.
I have enabled log_martians, checked verious sysctl setttings.
Everything looks fine. Also no indications in the kernel logs.
Questions: First, I would like to find out where the packets get dropped/filtered -
in the vif eth0 of the domU, in the vif vif1.0 of the dom0, or somewhere
inbetween?
Who forwards packets between the two vifs - the hypervisor or the dom0?
Can I somehow debug if a packet enters the vif but get silenty dropped?
Thanks
Klaus
--
Klaus Darilion, Head of Operations
nic.at GmbH, Jakob-Haringer-Straße 8/V
5020 Salzburg, Austria
