[Development] QtNetwork: using system proxy by default

[shane.kearns at accenture.com]

> -----Original Message-----
> From: development-bounces+shane.kearns=accenture.com at qt-project.org
> [mailto:development-bounces+shane.kearns=accenture.com at qt-project.org]
> On Behalf Of Thiago Macieira
> Sent: 15 October 2012 18:41
> To: development at qt-project.org
> Subject: Re: [Development] QtNetwork: using system proxy by default
>
> On segunda-feira, 15 de outubro de 2012 16.33.39,
> shane.kearns at accenture.com
> wrote:
> > When using the windows API, we can enable either DNS, DHCP or both.
> > Using DNS only would be faster, but may not work for some users if
> > their office network used DHCP only deployment.
> >
> > Also, I checked today and it looks like Chromium have now implemented
> > DHCP autodetection by themselves as well. They are checking it on
> > startup & caching the results.
>
> IIRC, DHCP requires binding to port 67 in order to send the request,
> which requires root privileges on Unix systems. That means we can't do
> it.

Yes, that puts this outside our scope.
I'd rather not be implementing this only for windows.

> I'd create a (wall-clock) timer and enable the DHCP resolution on
> Windows only once every 10 minutes. I'd also use it as a fallback if
> and only if the DNS resolution failed.

That doesn't help with the 12 second freeze, since both methods fail if
you are on a network with no autoproxy.
We need to be calling WinHttpGetProxyForUrl or WinHttpDetectAutoProxyConfigUrl
in a thread, like we do for name resolutions.

There is also the non blocking WinHttpGetProxyForUrlEx, but that is new in windows 8.

> The only drawback is that we'll insist on a bad, cached result if the
> user moves from a network with DHCP and no DNS, to a network with no
> autoproxy. We could cache the host's IP addresses and discard the cache
> if they have changed too. Since we'll do that only once every 10
> minutes, the overhead will be minimal.
>

Caching the result and having some ways to invalidate the cache makes sense.
WinHttpDetectAutoProxyConfigUrl is the best thing to use, since it gives a pac file url,
which is the same thing as the static configuration would give us.

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