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[Xen-users] DomU PV network problems

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Subject: [Xen-users] DomU PV network problems
From: Sauro Saltini <saltini@xxxxxx>
Date: Tue, 07 Sep 2010 23:50:55 +0200
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Hi everybody.

I've got a strange problem with PV networking on Linux.

My current configuration is :
- Xen 4.0.0
- DOM0 kernel
- DOMU kernels either or 2.6.36-rc3 (vanilla)

one of my DomU's is intended to act as a router/firewall for all the other ones.

I have configured 2 distinct bridges in Dom0 :
br0 - connects the firewall DomU "external" nic to the external faced host NIC (phisical) br1 - connects the "internal" virtual nic of firewall DomU (eth1) with the other DomU's virtual nics.

Each "guest" DomU has defined the firewall DomU's "internal" address as default gateway, the firewall by now acts simply as a NAT gateway, with ip_forward active and a single NAT rule to SNAT outgoing packets with his own external IP.

I've first installed the fw DomU as an hvm domain (nic's = ioemu) with slackware 13.0 and tried the whole thing connecting from one of the other DomU's to the external network and all worked smoothly.

As soon as I've converted the fw DomU's to a PV domain (using either or 2.6.36-rc3 kernels with PV drivers) something changed in a weird way...

I can still ping the firewall DomU both from "internal" domU's network and from the external lan, but packets from a DomU can't reach the external network anymore !

Running "tcpdump -nvvi" on both firewall's NICs and pinging an external host from one of the other domU's reveals that packets arrive on the firewall, are correctly NATted and appear on the external connected interface, but then simply disappear !

on Dom0 "tcpdump -nvvi br0" (br0 = external bridge) never shows up any traffic !

I've already tried to configure tx checksum offloading = off (ethtool -K <nic> tx off) on all the involved interfaces without any success.

Please help...
Many thanks in advance.

Sauro Saltini.

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