Re: [Qemu-devel] Patch to improve handling of server sockets

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/04/2010 04:47 PM, Gerd Hoffmann wrote:
> On 05/04/10 18:23, Anthony Liguori wrote:
>> On 05/04/2010 08:49 AM, Reinhard Max wrote:
>>> Hi,
>>>
>>> I am maintaining the tightvnc package for openSUSE and was recently
>>> confronted with an alleged vnc problem with QWMU that turned out to be
>>> a shortcoming in QEMU's code for handling TCP server sockets, which is
>>> used by the vnc and char modules.
>>>
>>> The problem occurs when the address to listen on is given as a name
>>> which resolves to multiple IP addresses the most prominent example
>>> being "localhost" resolving to 127.0.0.1 and ::1 .
>
> My tigervnc (tightvnc successor) has IPv6 support and handles this 
> just fine.  There is also the option to force qemu to listen on ipv4 
> (or ipv6) only.
>
>>> The existing code stopped walking the list of addresses returned by
>>> getaddrinfo() as soon as one socket was successfully opened and bound.
>>> The result was that a qemu instance started with "-vnc localhost:42"
>>> only listened on ::1, wasn't reachable through 127.0.0.1. The fact
>>> that the code set the IPV6_V6ONLY socket option didn't help, because
>>> that option only works when the socket gets bound to the IPv6 wildcard
>>> address (::), but is useless for explicit address bindings.
>
> Indeed.
>
>> But that said, I'm not sure we're doing the wrong thing right now. Gerd,
>> what do you think about this behavior?
>
> Reinhard is correct.  If a hostname resolves to multiple addresses 
> like this ...
>
>    zweiblum kraxel ~# host zweiblum
>    zweiblum.home.kraxel.org has address 192.168.2.101
>    zweiblum.home.kraxel.org has IPv6 address 
> 2001:6f8:1cb3:2:216:41ff:fee1:3d40
>
> ... qemu should listen on all addresses returned.  Which in turn 
> requires multiple listening sockets.
>
> Changing this is a big deal though, thats why I've taken the somewhat 
> unclean shortcut to listen on the first match only when implementing 
> this.  Clients are supposed to try to connect to all addresses 
> returned by the lookup (and they do if they got IPv6 support), thus 
> this usually doesn't cause trouble in practice.
>
> When going for multiple listening sockets in qemu we have to figure 
> how we'll handle this in a number of places as there is no single 
> listening address any more.  Reporting the vnc server address in QMP 
> is one.  I'm sure there are more.

Okay, that makes sense.  Personally, I'm inclined to agree that this is 
a client problem.

Regards,

Anthony Liguori

> cheers,
>   Gerd
>