Yo,
I had big problems with this myself, but now as far as I can tell, veth0
(renamed to eth0 in the v recent releases) is the virtual nic for dom0
which corresponds to the virtual eth0 nics in the domU's (the physical nic
itself has been renamed to peth0).
The vifU.N's are the dom0 links to the virtual nics ethN in domU, and
aren't to be treated as proper network interfaces in thier own right. If
you assign an IP address to, say, vif1.0 and attempt to communicate with
eth0 in dom1, you can ping between them and such, but ssh nor vnc wont
work, complaining for the most part about bad checksums.
The vif0.0 interface in dom0 is linked to dom0's virtual nic, veth0 (or
eth0 in the recent builds).
Now... The correct way to interface the cards and provide connectivity
between the domains and the outside world is to attach all the vifU.N
interfaces to a bridge in dom0 (normally xen-br0) together with the
physical nic, eth0 (or peth0 recently). IP addresses are then assigned to
the eth0 links and inter-domain sshing, vncing and communication with the
outside world all result.
Here's a badly drawn diagram to illustrate...
 ____________________________ _____________________
|dom0 | |dom1 |
| _______ | | ________ |
| |eth0 | | | |eth0 | |
| |IP_____| +--------------+-------+------|IP______| |
| | | | | |
| ____|_______|_____________ | | |
|| ___|___ ___|___ _______ || | |
|||vif0.0 |vif1.0 |peth0 | || | |
|||_______|_______|_______| || | |
||xen-br0_____________|_____|| | |
|_____________________|______| |_____________________|
 |
______________________|_____________________>>LAN>>>
 | |
I've used eth0 as the virtual nics and peth0 as the physical nic.
IP has been used to denote devices with an IP address. All the interfaces
there are automatically generated by Xen, and I used ifconfig to
statically set IP addresses (because I was having trouble with our lab
DHCP server, although it was assigning addresses to the correct cards),
and brctl addif xen-br0 vif1.0 etc. to add the vifs to the bridge, because
for some reason Xen wasn't...
Well thats my contribution for this evening.
John
P.S. Official documentation/Howto/wiki is a little on the sparse side for
this Ian.
Quoting Anthony.Golia@xxxxxxxxxxxxxxxxx:

On Thu, 8 Sep 2005, Ian Pratt wrote:

> Hi. what creates veth0 and vifu.0, when? what's the
> underlying architecture behind them? Just trying to
> understand the internals.
netback/loopback.c creates them.
It's effectively a point to point link, allowing domain 0 to connect on
to the bridge in the same manner that other domains do. (before, packets
to domain 0 were short-cutted and came off the bridge directly, which
could lead to traffic from other domains being stalled if a lot of
traffic was destined to user space in dom0.

thx. what's the diff betwn the two (veth0 and vifu.0) ?

Best,
Ian
)

Cheers,
Anthony
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