[Qemu-devel] Re: [PATCH] Add pcap-based host network bridge

[Jan Kiszka <jan.kiszka@siemens.com>]

Anthony Liguori wrote:
> Jan Kiszka wrote:
>> This introduces bridged networking via pcap. Both Unix and Windows
>> platforms are supported.
>>
>> While tap-based bridging provides basically the same support pcap does,
>> the latter is often more handy to set up. Under Linux, it doesn't
>> require to configure a bridge and it is able to bypass the ebtables if
>> this is desired. Also under Windows, the bridge setup can be lengthy
>> procedure compared to using the pcap interface.
>>
>> Signed-off-by: Klaus Wenninger <klaus.wenninger@siemens-enterprise.com>
>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>   
> 
> The previous version of this had a fundamental flaw wrt guest->host
> communication on Linux.  My understanding was that this was an intrinsic
> limitation of pcap.

To my understanding too. We will try to look closer at this, maybe there
are smarter alternatives at least on some platforms (PF_PACKET under
Linux...?).

Klaus explained to me that there is some switch in winpcap to support
this, but it caused troubles due to weird packet loops. I don't have a
Windows platform to test, but I will see if we can clarify this in some
other way.

> 
> That's a blocker IMHO because it will invariably lead to many folks
> asking why ping to the host doesn't work (just like slirp).  Since it
> still requires root privileges, I don't think it improves a lot on tap.
> 

Well, in case we do not find a workaround for host<->guest
communication, would the patch remain unacceptable even with proper,
prominent documentation of this shortcoming?

Jan

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