I have installed Xen Dom0 on SL 5 and CentOS 5 before and various DomU
under them (SL5, CentOS 5, Ubuntu).    I have upgraded these Dom0 and
DomU to the latest 5.2. I didn't have any network problems at all.

Recently, I installed a new (fresh install) Dom0 in CentOS 5.2 and DomUs
under it, now the network won't work for these DomUs.  I can ping these
DomU from anywhere and I can ping from these DomU to anythere, but all
other network function won't work at all, such as update the DomUs. 
From Dom0, I can ssh into DomU and vice verse, but I can not ssh into
DomU from anywhere else nor ssh out from DomU to anywhere else. 
Apparently, the network bridging is not functioning properly (or
behaving as in the previous versions 5, 5,1).  Redhat did introduce
libvirtd and dnsmasq in 5.2 which were also running on my upgraded Dom0,
where I didn't encounter any network problem.

I did find a post somewhere about libvirtd which the author turned off
libvirt to solve his/her network problem, I tried the same thing, but I
couldn't ssh to my Dom0 after that.

I would appreciate if someone could help to solve this network bridging
problem.  Thank you.

-- 
Zhi-Wei Lu
Bioinformatics Core
Genome Center
University of California, Davis
(530) 752-2698