502 Errors Shown in Fiddler But Not In Microsoft Network Monitor

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rjsargeant

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Jun 20, 2011, 9:45:52 AM6/20/11
to Fiddler
Hi,

When I start Google Chrome, Fidder shows three consecutive 502 errors
immediately after the 'CONNECT www.google.com:443' command. The
supposed host in each of these 3 errors is a random sequence of
characters e.g. fnaxnfamfa. Chrome exhibits no problem and, the
Google page displays without any problems. When I check within
Microsoft Network Analyzer (with Fiddler closed), no 502 errors are
shown.

Any thoughts would be appreciated.

Regards

Richard

EricLaw

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Jun 20, 2011, 12:59:11 PM6/20/11
to Fiddler
That's a feature of Chrome.

In NetMon, you won't see the attempts to connect to the random
hostnames, instead you'll see three failed DNS lookups for three
random hostnames.

Chrome does this to attempt to determine whether the user is behind an
ISP/network environment that hijacks DNS lookup failures and returns
bogus records that point to the ISP's search pages (for which the ISP
gets advertising money). Chrome needs to do this in order for their
"Hey, this address doesn't exist so do a google search" feature to
work properly and not get confused by the bogus DNS records.

When you have a proxy in place (for example, Fiddler) Chrome does the
same sort of thing but with a GET (since it's the proxy's job to do
the DNS lookup, but the browser must actually make a HTTP request for
the DNS request to happen).

-Eric

On Jun 20, 6:45 am, rjsargeant <richard.sarge...@gmail.com> wrote:
> Hi,
>
> When I start Google Chrome, Fidder shows three consecutive 502 errors
> immediately after the 'CONNECTwww.google.com:443'command.  The
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