IE11 trying to resolve proxy for DUMMY 00 host on first new tab

Question

Internet Explorer Version 11.0.9600.17420
IE 11.0.14 (KB3003057)
Windows 7 SP1 32bit
While testing IE 11 in offices with slow connections our test users reported strange application hangs when opening the first new tab. The initial load of the home page and all subsequent tabs after the first new one are working just fine.
The hangs will persist for nearly 2 minutes and then IE will load the desired page just fine without any user interaction. It's as if the application hangs somewhere in a loop.
While trying to solve the issue we were able to pinpoint it to our proxy.pac in which we're using the "IsInNet" function to determine if a certain host is in a specific subnet. After a few Wireshark traces we found that when opening the first new
tab (and only then) IE 11 will try to resolve the proxy for a host named "DUMMY<00>". Of course we don't have any DNS records for that name so IE seems to fall back to using NBNS. We only have one central NBNS server, so this
will cause the delay our remote users noticed (it takes them about five seconds for one request to be handled).
As a workaround we've adjusted our proxy.pac to save the resolved IP of the host to a variable, therefore reducing our name resolution queries from previously 35 to a single one. This doesn't get rid of the hang completely but it helps a lot in that
the delay is now more like five seconds than two minutes.
I'm now trying to figure out why IE is querying "DUMMY<00>" in the first place but so far I'm out of ideas. Does anyone have some insight as to why that's happening? Is it a certain setting we need to adjust or is this by design? We've never
noticed anything similar with IE 9.

Answer 1

Hi,
Untill now, based on my research, the only way to bypass this error is just workaround method, there is no fixed method to resolve the problem. You can try to use these two method as workaround method:
1. Add to the proxy pac file:
                function FindProxyForURL(url, host)
                // Workaround for IE11
                if (shExpMatch(url, "http://dummy/"))
                   return "DIRECT";
2. Add the “dummy” in the DNS or hosts files.
Roger Lu
TechNet Community Support