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

[Daniel P. Berrange]

On Tue, May 04, 2010 at 03:49:50PM +0200, 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 .
> 
> 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.

This seems to be something we overlooked in the initial impl. I don't
see any alternative but to make QEMU listen on multiple sockets in
the scenario you describe. An even clear example is to consider binding
QEMU to a machine with multiple non-localhost addresses, eg

   myserver.com resolving to 192.168.122.41 + 2a00:123:456::1

because there's no way that the kernel can know/decide to automatically
listen on 192.168.122.41, when given the address 2a00:123:456::1 and if
the machine has many NICs, you can't assume the wildcard address is 
suitable either.

> The attached patch against QEMU 0.11.0 extends inet_listen() to create 
> sockets for as many addresses from the address list as possible and 
> adapts its callers and their data structures to deal with a linked 
> list of socket FDs rather than a single file descriptor.

The approach looks reasonable, though the patch is somewhat mangled
by the mix of tabs + spaces

Regards,
Daniel
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