Office 365 PAC File

When we talk about Office 365 services with our customers, a lot of the discussion revolves around the networking components.  While we generally have a “good idea” of networking, we’re not experts in your technology and will frequently tell you to go talk to your vendors.  We have some guidance on how to configure your environments, but generally don’t provide configuration files or samples that can be used in products that are not our own (understandably).

One area that we typically have a lot of talks on is the proxy environment.  Typically, the security people want all of the organization’s egress traffic to flow through a proxy (presumably to categorize site visit frequency and bandwidth consumption patterns).  However, all Office 365 traffic is encrypted, so aside from seeing the request URL, none of the content will be visible.  As such, the proxies will terminate this traffic and then start a new session on the requesting client’s behalf (hence, “proxy.”)  This works fine for simple HTTP web requests from browsers.  Office applications (such as Outlook) were originally designed for LAN environments and consequently, still sometimes behave like that.  An Outlook client with a few open mailboxes and a GAL query may consume 10 or 15 sessions by itself on the proxy sever; at some point, with enough users doing this, you will exhaust the resources in your proxy environment for no discernable benefit.  Thus, we recommend customers bypass the proxy of Office 365 requests.

Typically, you can configure your proxy server environment (if its an implicit proxy) or configure desktops (if it’s explicit) with a PAC file, which contains instructions for handling URLs and IP addresses (such as “go to proxy” or “go direct”).  Successfully bypassing your proxy requires two parts:

  1. Configure a bypass list.  Possible options include a proxy automatic configuration file (PAC) or a Group Policy Object that explicitly defines which sites to not send to a proxy server.
  2. Configure an outbound firewall rule to allow access to the appropriate IPs and URLs.

Some modern firewalls include URL filtering, but many in the field do not.  For those customers with more traditional hardware, you will  need to configure those two line items separately.  The sticky wicket is that many administrators don’t know how to actually write a proxy PAC file.  A PAC file is a JavaScript construct that tells the browser where to send URL requests.  Once you deploy a Proxy PAC file, you need to keep it up to date (which can be tedious).  I have attempted to kill both of those birds with this script.  You can schedule it and set its output to a network location, and then point a GPO or WPAD discovery to this output.

For more information on how a PAC file works, you can head over to, which shows the structure of a PAC file and some of the instructions that can be used.  You can also download the PacParser tool from, which you can use to validate this (or any other) PAC file.


You can download the full script below over at the TN Gallery.  If you’d like some information on how to distribute the PAC file, please see my other blog post, “Deploying of Office 365 Proxy PAC to Manage Your Users.”

Check out for the latest version of this tool.

Comments (13)

  1. Dennis says:

    When a wildcard “*” is used should the function not be shExpMatch? Example below.

    if (shExpMatch(host, “*.local”))
    return “DIRECT”;

    I do not believe this will work:

    if (dnsDomainIs(host, “*.local”))
    return “DIRECT”;

    Someone correct me if I am wrong.

    1. It’s valid according to PacParser.

      pactester -p testpac.pac -u http://*.local

      I’ll check the FindProxyForUrl guidelines to see if I can dig up some more information.

      1. Darren says:

        autoprox,exe (which calls InternetGetProxyInfo inside IE) doesn’t seem to agree with the wildcard matching under dnsDomain like that

        you need to drop the *, so for example dnsDomainIs(host, “”)
        not dnsDomainIs(host, “*”)

        1. Darren,

          Can you give me some more information? I ran autoprox and got this result:

          C:\temp>autoprox.exe Test_PAC.pac
          The Winsock 2.2 dll was found okay
          autoproxy file path is : c:\temp\Test_PAC.pac
          Calling InternetInitializeAutoProxyDll with c:\temp\Test_PAC.pac
          Calling InternetGetProxyInfo with url and host
          Proxy returned for url is:
          PROXY; DIRECT

          How are you testing?

    2. While PacParser and AutoProxy didn’t return errors for me, I did update the Proxy configuration tool to use shExpMatch. I’ve also added a lot of new features, so check out the new tool!

    3. I’ve updated the tool, so you can try again.

  2. Dario says:

    Hi, I am having performance issues with this .pac file. Maybe a .pac file with 300 lines is too large… What do you think?

    1. I have customers with PAC files that are thousands of lines long. What type of problem are you seeing?

      1. Simon Bentley says:

        Hi, we have implemented the .pac file but we are also seeing slow web browsing from browsers using this PAC file. Any ideas?

  3. Johan Parlevliet says:

    There are many duplicates in the output.
    I see 5 times: dnsDomainIs(host, “”)|| in the proxy.pac
    I see 3 times shExpMatch(host, “*”)|| which makes “” unnecessary

    1. It’s because they’re listed as URLs for other services. I don’t filter them out.

  4. Colin says:

    This will only work if you have a DIRECT route from the clients to the Internet gateway. Many large enterprise environments do not have this for security reasons. The other challenge is configuring the IP list on the Firewall as Microsoft are constantly changing the list. As you say, URL filtering would be better but many Firewalls do not support this.

  5. Karen says:

    Is there something similar that can be used on a user workstation? Recently I updated my laptop. I have McAfee Security Center installed, and some how I can no longer access any site that has file sharing capabilities. I also connected this laptop to azure, so is fully managed via Office 365 Mobile capabilities. Since the updates, I cannot access Office 365 One Drive or SharePoint and I’m running out of items to test. Outlook and Skype for Business work great, and so are all other components (i.e., Word, PowerPoint, Excel, etc.).

    I looked into the firewall policies and even added some rules with the IPs provided, but no luck. Any help would be highly appreciate it. Granted, I could reset the whole computer once more, but I’m just hoping I don’t have to.

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