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Excessive PAC file Requests...Skype? RRS feed

  • Question

  • Hi

    I work within a large environment with users utilizing O365. Everything is working fine, no error messages etc. However, my Network Team has noticed a massive amount of requests for the PAC file. The IIS log files back this up with around 300,000 requests per minute. Most requests show up in the IS logs as file not modified (304) or successful (200) and about 15% of requests are for the WPAD entry which suggests that the DHCP delivery method is failing for some client machines possibly due to contention due to the excessive requests. The PAC file has a timeout of 30 minutes.

    We use Windows 7 and Skype for Business 2016. Two IIS servers hosting the PAC file are behind an F5 Load Blancer.

    The article below made me begin to suspect Skype and it not caching the PAC file but the article relates to a previous version of Skype/Lync

    The download for PAC file is frequently requested from the Skype for Business or Lync 2013 client

    The process causing the most requests is svchost.exe, second is offliceclicktorun.exe and lync.exe is in 16th place in the heavy hitters.

    I'm just looking for some pointers as to where I can look to see if my suspicion is correct or whether something else is making the requests for the PAC. Does anyone know how I can see logs from Skype for PAC file requests?

    Any help much appreciated.

    Thanks

    Kev


    Wednesday, October 16, 2019 3:17 PM

Answers

  • Hi Kev,

    For your question, I found that there is not enough information to explain this. Considering the issue is about Office 365 network connectivity, I suggest you ask questions at Office 365 forum in Microsoft Tech Community. For more information on Office 365 network connectivity, read Set up your network for Office 365.

    As you are working with a complicated scenario, to help you troubleshoot this problem, it would be best to open a paid support ticket with Microsoft Support to help review your problem and help you test remotely.  

    Best Regards,

    Herb


    Please remember to mark the replies as answers if they helped. If you have feedback for TechNet Subscriber Support, contact tnsf@microsoft.com.

    Click here to learn more. Visit the dedicated forum to share, explore and talk to experts about Microsoft Office 2019.

    • Marked as answer by KevJackson Thursday, October 31, 2019 8:44 AM
    Wednesday, October 30, 2019 3:16 AM

All replies

  • Hi Kev,

    Based on your description, the issue is mainly related Skype (Skype for Business 2016). To help you better resolve the issue, I will move the thread to Skype for Business forum. You may get more helpful replies there.

    Thanks for your kind understanding.

    Best Regards,

    Herb


    Please remember to mark the replies as answers if they helped. If you have feedback for TechNet Subscriber Support, contact tnsf@microsoft.com.

    Click here to learn more. Visit the dedicated forum to share, explore and talk to experts about Microsoft Office 2019.

    Thursday, October 17, 2019 7:12 AM
  • Hi Kev,

    A Proxy Auto-Configuration (PAC) file is a JavaScript function definition that determines whether web browser requests (HTTP, HTTPS, and FTP) go direct to the destination or are forwarded to a web proxy server.

    According to your description, I found a document may be related to your problem.

    Please refer to the following link:

    https://www.undocumented-features.com/2015/11/16/office-365-pac-file/

    Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make sure that you completely understand the risk before retrieving any suggestions from the above link.


    Best Regards,
    Sharon Zhao


    Please remember to mark the replies as answers if they helped. If you have feedback for TechNet Subscriber Support, contact tnsf@microsoft.com.

    Thursday, October 17, 2019 8:09 AM
  • Hi Sharon

    Many thanks for your reply but I don't believe the link you sent is relevant. The content of the PAC file is fine, everything is working. The issue I have is that the PAC file is being requested excessively.

    My train of thought is directed towards how Skype for Business 2016 caches the PAC file and trying to discover if this is working correctly.

    I'm happy to consider other suggestions but the issue is not the contents of the PAC file.

    Thanks

    Kev

    Thursday, October 17, 2019 10:48 AM
  • Hi Kev,

    Up to now, I didn’t find any other related document about why the PAC file is being requested excessively.

    I will do some discussion with others. If there is any update, I will share with you.


    Best Regards,
    Sharon Zhao


    Please remember to mark the replies as answers if they helped. If you have feedback for TechNet Subscriber Support, contact tnsf@microsoft.com.

    Tuesday, October 22, 2019 8:09 AM
  • Hi Sharon

    Thanks for sticking with this. I have been able to get some tools into my environment and have a few more statistics that may help us (it looks like my suspicions surrounding Skype for Business were unfounded).

    Using Wireshark I've been able to monitor PAC file requests. As the PC is sitting idle (with or without Office apps open) there are more requests for the PAC file then I would expect but I'm not bothered by those particular requests at this time.

    However, when I launch Office apps I see lots of requests for the PAC file. The figures below do fluctuate but they do follow the trend on repeated testing.

     


    Application PAC File Requests as Application Launches
    Skype 21
    Word 6
    Outlook 55
    Excel 34
    Publisher 54
    Access 22
    IE 3
    Chrome

    1

    Is there some way I can check/configure how Office apps cache the PAC file?

    Thanks

    Kev

    Friday, October 25, 2019 12:07 PM
  • Hi Kev,

    For your question, I found that there is not enough information to explain this. Considering the issue is about Office 365 network connectivity, I suggest you ask questions at Office 365 forum in Microsoft Tech Community. For more information on Office 365 network connectivity, read Set up your network for Office 365.

    As you are working with a complicated scenario, to help you troubleshoot this problem, it would be best to open a paid support ticket with Microsoft Support to help review your problem and help you test remotely.  

    Best Regards,

    Herb


    Please remember to mark the replies as answers if they helped. If you have feedback for TechNet Subscriber Support, contact tnsf@microsoft.com.

    Click here to learn more. Visit the dedicated forum to share, explore and talk to experts about Microsoft Office 2019.

    • Marked as answer by KevJackson Thursday, October 31, 2019 8:44 AM
    Wednesday, October 30, 2019 3:16 AM
  • OK, understood.

    Herb, Sharon... thanks for your efforts. Hopefully I'll get to the bottom of it soon.

    Kev

    Thursday, October 31, 2019 8:44 AM