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[libvirt] PXE boot issue with macvtap



Hello All,

I currently have an issue with using macvtap interfaces on KVM. 
I was wondering if this is a known issue?
Also may I request for any ideas on how I should go forward with this
issue?

Issue: 
When you try to PXE boot one KVM guest of another KVM guest over a macvtap interface
the client does not receive the tftp packets sent by the server and the
connection times out.

Setup:
Both the PXE client and the PXE server are VM's running on the same host,
using macvtap on top of a LOM interface for connectivity

Reproducible always.

Findings of the testing:
1) The issue is seen between guests on the same on the same host and not between 
guests on different hosts.

2) PXE booting works fine if we're using a linux bridge for connectivity rather
than macvtap

3) The initial DHCP stage works fine and the client acquires an IP address,
but then the TFTP attempt stalls.

4) The client sends an RRQ to which the server replies with option 
acknowledgement, which is exactly what should happen.

5) However the client appears not to receive the acknowledgement.

6) Both client and server keep retransmitting their packets for a while, until
the client times out and gives up.

7) Observing the ifconfig stats for the host macvtap interface for each guest,
the packet counters seemed to show that the packets were being received - so they
are dropped somewhere between macvtap on the host and the virtio device in the guest.

8) Tcpdump does not seem to work with macvtap so could not inspect further.  

-- 
Many Thanks,
Regards,
Shradha Shah


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