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Okay, this is killing me.

Debian Squeeze, Xen 4.0, brand new install. No iptables rules whatsoever except for the ones added by the default xen bridge script.

Dom0 can reach the entire world, no problems.

DomU can receive packets from some hosts, but not from others. For instance, if I ping Host A, it works fine. If I ping Host B, the DomU reports 100% packet loss.

The hosts are random, but consistent (even after reboots). I can see no pattern to why some work and others don't. In fact, in some cases, different virtual hosts on the same server (an other server at a different data center) are divided; some work and others do not. I can reboot (DomU or Dom0 too) and the same hosts will work or fail as before.

If I tcpdump on the Host B while pinging from the DomU, everything looks fine. It sees the echo request coming in and says it's sending one back.

However, if I tcpdump peth0 on the Dom0, it never sees the echo reply.

Any ideas what could be happening? I'm tearing my hair out here.


EDIT: Turns out my hosting provider (Rackspace) had a routing problem which was the cause of the issue. Xen and Linux are exonerated. Thanks to everyone for looking at the problem. Too bad I'll never get the hair back.

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Bridged or nat? –  Rilindo Jun 25 '11 at 3:24
    
Bridged. Both Dom0 and DomU have public IPs. –  Tun H S Lee Jun 25 '11 at 8:04
    
Welcome to Server Fault! Could you please post an answer with the solution and accept it for other people to benefit? –  Bart De Vos Jun 25 '11 at 20:26

2 Answers 2

Missing or incorrect netmask can cause stuff like this, which is easy to misconfigure with bridged networking. I'd look at the networking stuff before looking for weird xen behavior.

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It seems there was a routing problem at the hosting company. As you suspected, it was not a Xen issue. –  Tun H S Lee Jun 25 '11 at 20:19

Sounds very much like a networking bridge problem. Check that all the virtual interfaces are bridge to the same bridge and that their IP configurations are done correctly.

# brctl show 

Sometimes, if you bring down a bridge and bring it up again, you may need to re-add the interfaces to the bridge again.

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