fiddler

Previously, I’ve described how to capture a network traffic log from Microsoft Edge, Google Chrome, and applications based on Chromium or Electron.

In this post, I aim to catalog some guidance for looking at these logs to help find the root cause of captured problems and otherwise make sense of the data collected.

Last Update: April 24, 2020I expect to update this post over time as I continue to gain experience in analyzing network logs.

Choose A Viewer – Fiddler or Catapult

After you’ve collected the net-export-log.json file using the about:net-export page in the browser, you’ll need to decide how to analyze it.

The NetLog file format consists of a JSON-encoded stream of event objects that are logged as interesting things happen in the network layer. At the start of the file there are dictionaries mapping integer IDs to symbolic constants, followed by event objects that make use of those IDs. As a consequence, it’s very rare that a human will be able to read anything interesting from a NetLog.json file using just a plaintext editor or even a JSON parser.

The most common (by far) approach to reading NetLogs is to use the Catapult NetLog Viewer, a HTML/JavaScript application which loads the JSON file and parses it into a much more readable set of events.

An alternative approach is to use the NetLog Importer for Telerik Fiddler.

Importing NetLogs to Fiddler

For Windows users who are familiar with Fiddler, the NetLog Importer extension for Fiddler is easy-to-use and it enables you to quickly visualize HTTP/HTTPS requests and responses. The steps are easy:

  1. Install the NetLog Importer,
  2. Open Fiddler, ideally in Viewer mode fiddler.exe -viewer
  3. Click File > Import > NetLog JSON
  4. Select the JSON file to import

In seconds, all of the HTTP/HTTPS traffic found in the capture will be presented for your review. If the log was compressed before it was sent to you, the importer will automatically extract the first JSON file from a chosen .ZIP or .GZ file, saving you a step.

In addition to the requests and responses parsed from the log, there are a number of pseudo-Sessions with a fake host of NETLOG that represent metadata extracted from the log:

These pseudo-sessions include:

  • RAW_JSON contains the raw constants and event data. You probably will never want to examine this view.
  • CAPTURE_INFO contains basic data about the date/time of the capture, what browser and OS version were used, and the command line arguments to the browser.
  • ENABLED_EXTENSIONS contains the list of extensions that are enabled in this browser instance. This entry will be missing if the log was captured using the –log-net-log command line argument.
  • URL_REQUESTS contains a dictionary mapping every event related to URL_REQUEST back to the URL Requests to which it belongs. This provides a different view of the events that were used in the parsing of the Web Sessions added to the traffic list.
  • SECURE_SOCKETS contains a list of all of the HTTPS sockets that were established for network requests, including the certificates sent by the server and the parameters requested of any client certificates. The Server certificates can be viewed by saving the contents of a –BEGIN CERTIFICATE– entry to a file named something.cer. Alternatively, select the line, hit CTRL+C, click Edit > Paste As Sessions, select the Certificates Inspector and press its Content Certificates button.

You can then use Fiddler’s UI to examine each of the Web Sessions.

Limitations

The NetLog format currently does not store request body bytes, so those will always be missing (e.g. on POST requests).

Unless the Include Raw Bytes option was selected by the user collecting the capture, all of the response bytes will be missing as well. Fiddler will show a “dropped” notice when the body bytes are missing:

If the user did not select the Include Cookies and Credentials option, any Cookie or Authorization headers will be stripped down to help protect private data:

Scenario: Finding URLs

You can use Fiddler’s full text search feature to look for URLs of interest if the traffic capture includes raw bytes. Otherwise, you can search the Request URLs and headers alone.

On any session, you can use Fiddler’s “P” keystroke (or the Select > Parent Request context menu command) to attempt to walk back to the request’s creator (e.g. referring HTML page).

You can look for the traffic_annotation value that reflects why a resource was requested by looking for the X-Netlog-Traffic_Annotation Session Flag.

Scenario: Cookie Issues

If Fiddler sees that cookies were not set or sent due to features like SameSiteByDefault cookies, it will make a note of that in the Session using a psuedo $NETLOG-CookieNotSent or $NETLOG-CookieNotSet header on the request or response:

Closing Notes

If you’re interested in learning more about this extension, see announcement blog post and the open-source code.

While the Fiddler Importer is very convenient for analyzing many types of problems, for others, you need to go deeper and look at the raw events in the log using the Catapult Viewer.


Viewing NetLogs with the Catapult Viewer

Opening NetLogs with the Catapult NetLog Viewer is even simpler:

  1. Navigate to the web viewer
  2. Select the JSON file to view

If you find yourself opening NetLogs routinely, you might consider using a shortcut to launch the Viewer in an “App Mode” browser instance: msedge.exe --app=https://netlog-viewer.appspot.com/#import

The App Mode instance is a standalone window which doesn’t contain tabs or other UI:

Note that the Catapult Viewer is a standalone HTML application. If you like, you can save it as a .HTML file on your local computer and use it even when completely disconnected from the Internet. The only advantage to loading it from appspot.com is that the version hosted there is updated from time to time.

Along the left side of the window are tabs that offer different views of the data– most of the action takes place on the Events tab.

Tips

If the problem only exists on one browser instance, check the Command Line parameters and Active Field trials sections on the Import tab to see if there’s an experimental flag that may have caused the breakage. Similarly, check the Modules tab to see if there are any browser extensions that might explain the problem.

Each URL Request has a traffic_annotation value which is a hash you can look up in annotations.xml. That annotation will help you find what part of Chromium generated the network request:

Most requests generated by web content will be generated by the blink_resource_loader, navigations will have navigation_url_loader and requests from features running in the browser process are likely to have other sources.

Scenario: DNS Issues

Look at the DNS tab on the left, and HOST_RESOLVER_IMPL_JOB entries in the Events tab.

One interesting fact: The DNS Error page performs an asynchronous probe to see whether the configured DNS provider is working generally. The Error page also has automatic retry logic; you’ll see a duplicate URL_REQUEST sent shortly after the failed one, with the VALIDATE_CACHE load flag added to it. In this way, you might see a DNS_PROBE_FINISHED_NXDOMAIN error magically disappear if the user’s router’s DNS flakes.

Scenario: Cookie Issues

Look for COOKIE_INCLUSION_STATUS events for details about each candidate cookie that was considered for sending or setting on a given URL Request. In particular, watch for cookies that were excluded due to SameSite or similar problems.

Scenario: HTTPS Handshaking Issues

Look for SSL_CONNECT_JOB and CERT_VERIFIER_JOB entries. Look at the raw TLS messages on the SOCKET entries.

Scenario: HTTPS Certificate Issues

Note: While NetLogs are great for capturing certs, you can also get the site’s certificate from the browser’s certificate error page.

The NetLog includes the certificates used for each HTTPS connection in the base64-encoded SSL_CERTIFICATES_RECEIVED events.

You can just copy paste each certificate (including ---BEGIN to END---) out to a text file, name it log.cer, and use the OS certificate viewer to view it. Or you can use Fiddler’s Inspector, as noted above.

If you’ve got Chromium’s repo, you can instead use the script at \src\net\tools\print_certificates.py to decode the certificates. There’s also a cert_verify_tool in the Chromium source you might build and try. For Mac, using verify-cert to check the cert and dump-trust-settings to check the state of the Root Trust Store might be useful.

In some cases, running the certificate through an analyzer like https://crt.sh/lintcert can flag relevant problems.

Scenario: Authentication Issues

Look for HTTP_AUTH_CONTROLLER events, and responses with the status codes 401, 403, and 407.

For instance, you might find that the authentication fails with ERR_INVALID_AUTH_CREDENTIALS unless you enable the browser’s DisableAuthNegotiateCnameLookup policy (Kerberos has long been very tricky).

Scenario: Debugging Proxy Configuration Issues

See Debugging the behavior of Proxy Configuration Scripts.


Got a great NetLog debugging tip I should include here? Please leave a comment and teach me!

-Eric

In a world where software and systems seem to march inexorably toward complexity, I love it when things get simpler.

Years ago, Firefox required non-obvious configuration changes to even send traffic to Fiddler. Eventually, Mozilla changed their default behavior on Windows to adopt the system’s proxy, meaning that Firefox would automatically use Fiddler when it was attached, just like Chrome, IE, Edge and other major browsers.

Unlike other browsers, Firefox also has its own Trusted Root Certificates store, which means that if you attempt to load a HTTPS page through Firefox with Fiddler’s HTTPS Decryption Mode enabled, you’ll get an error page:

FirefoxMITMDetected
MOZILLA_PKIX_ERROR_MITM_DETECTED error page

To configure Firefox to trust Fiddler’s root certificate, you used to have to manually install it by opening the FiddlerRoot.cer file, ticking the “Trust this CA to identify websites” box, and clicking OK:

FirefoxCA
The old way: Manually trusting Fiddler’s certificate

Making matters more annoying, any time you reset Fiddler’s root certificates (e.g. using the Actions button inside Tools > Fiddler Options > HTTPS), you had to do the whole dance over again. If you wanted to remove the obsolete root certificates, you had to visit a buried configuration UI:

ManualTrustFF
The old way: Administering the Firefox Certificate Store

Yesterday, I was delighted to learn that Firefox added a better option way back in Firefox 49. Simply visit about:config in Firefox and toggle the security.enterprise_roots.enabled setting to true.

FirefoxEnterprise
Enable the new hotness in about:config

After you make this change, Firefox will automatically trust certificates chained to roots in the Windows Trusted Root Certificate store, just like Chrome, Edge, IE and other browsers. Unfortunately, Mozilla has declined to turn this on by default, but it’s still a great option.

 

-Eric

Recently, a developer asked me how to enable Brotli content-compression support in FiddlerCore applications, so that APIs like oSession.GetResponseBodyAsString() work properly when the entity body has been compressed using brotli.

Right now, support requires two steps:

  1. Put brotli.exe (installed by Fiddler or off Github) into a Tools subfolder of the folder containing your application’s executable.
  2. Ensure that the Environment.SpecialFolder.MyDocuments folder exists and contains a FiddlerCore subfolder (e.g. C:\users\username\documents\FiddlerCore).

Step #1 allows FiddlerCore to find brotli.exe. Alternatively, you can set the fiddler.config.path.Tools preference to override the folder.

Step #2 allows FiddlerCore to create necessary temporary files. Sadly, this folder cannot presently be overridden [Bug].

One day, Fiddler might not need brotli.exe any longer, as Brotli compression is making its way into the framework.

 

 

Since moving to Google at the beginning of 2016, I’ve gained some perspective about my work on Fiddler over the prior 12+ years. Mostly, I’m happy about what I accomplished, although I’m a bit awed about how much work I put into it, and how big my “little side project” turned out to be.

It’s been interesting to see where the team at Telerik has taken the tool since then. Some things I’m not so psyched about (running the code through an obfuscator has been a source of bugs and annoyance), but the one feature I think is super-cool is support for writing FiddlerScript in C#. That’s a feature I informally supported via an extension, but foolishly (in hindsight) never invested in baking into the tool itself. That’s despite the fact that JScript.NET is a bit of an abomination which is uncomfortable for both proper JavaScript developers and .NET developers. But I digress… C# FiddlerScript is really neat, and even though it may take a bit of effort to port the many existing example FiddlerScript snippets, I think many .NET developers will find it worthwhile.

I’ve long been hesitant about adopting the more fancy features of the modern .NET framework, LINQ key among them. For a while, I justified this as needing Fiddler to work on the bare .NET 2.0 framework, but that excuse is long gone. And I’ll confess, after using LINQ in FiddlerScript, it feels awkward and cumbersome not to.

To use LINQ in FiddlerScript, you must be using the C# scripting engine and you must add System.core.dll inside Tools > Fiddler Options > Scripting. Then, add using System.Linq; to the top of your C# script file.

After you make these changes, you can do things like:

    var arrSess = FiddlerApplication.UI.GetAllSessions();
    bool b = arrSess.Any(s=>s.HostnameIs("Example.com"));
    FiddlerApplication.UI.SetStatusText((b) ? "Found it!":"Didn't find it.");

-Eric Lawrence

If you’ve tried to use Fiddler with iOS10 beta or Android 7 Nougat, you have probably found that HTTPS decryption isn’t working, even if you use the latest Fiddler and the Fiddler Certificate Maker add-on. Unfortunately, at the moment both platforms are broken, but for different reasons. In both cases, the client will fail to receive responses for HTTPS requests, and Fiddler will only show a CONNECT tunnel.

iOS 10 Change

After installing the FiddlerRoot certificate, one also needs to go to Settings -> General -> About -> Certificate Trust Settings and manually enable full trust for the FiddlerRoot root certificate, including accepting a dialog that says that this will allow a third-party to eavesdrop on all your communications.

iOS 10 Beta Bug (Fixed for final version)

The beta of iOS 10 had a bug whereby, if the response to a HTTP CONNECT tunnel request contains a Connection: close response header, the client will close the connection instead of doing as it should and waiting until the TCP/IP connection closes. A few minor platforms have had the same bug over the years, but iOS is definitely the first important platform with this issue. At least two bugs have been filed with the Apple “Radar” bug reporter.

Working around this limitation is simple. In Fiddler, click Rules > Customize Rules. Scroll to the OnBeforeResponse function. Just inside that function, add the following lines:

  if (oSession.HTTPMethodIs("CONNECT")) {
oSession["ui-backcolor"] = "red";
oSession.ResponseHeaders.Remove("Connection");
}

Save the file and try connecting again.

Android 7 Feature

In contrast to the iOS regression, the change in Android 7 was intentional. The Android team has decided that, by default, HTTPS certificate validation for apps targeting API Level 24 and later will ignore all user-installed root certificates, meaning that your efforts to manually trust Fiddler’s root certificate will be fruitless. Individually application developers can temporarily override this change while debugging by updating the application’s configuration:

<network-security-config>  
      <debug-overrides>  
           <trust-anchors>  
                <!-- Trust user added CAs while debuggable only -->
                <certificates src="user" />  
           </trust-anchors>  
      </debug-overrides>  
 </network-security-config>

…or at all times…

<network-security-config>  
      <base-config>  
            <trust-anchors>  
                <!-- Trust preinstalled CAs -->  
                <certificates src="system" />  
                <!-- Additionally trust user added CAs -->  
                <certificates src="user" />  
           </trust-anchors>  
      </base-config>  
 </network-security-config>

…Unfortunately, these changes can only be undertaken by application developers and not end-users. End-users will probably need to jailbreak their devices, akin to what is required to circumvent certificate pinning.

Certificate Validity Length

By default, Fiddler-generated certificates are valid for five years (and backdated one year). However, this can cause an ERR_CERT_VALIDITY_TOO_LONG error in Chrome on Android.  To fix this for Fiddler’s default (CertEnroll) certificate generator,  run about:config in QuickExec to edit preferences. Set fiddler.certmaker.ValidDays to 820 and, if needed, reset your certificates in Fiddler using the Tools > Fiddler Options > HTTPS > Actions button.

-Eric

Enough malware researchers now depend upon Fiddler that some bad guys won’t even try to infect your system if you have Fiddler installed.

The Malware Bytes blog post has the details, but the gist of it is that the attackers use JavaScript to probe the would-be victim’s PC for a variety of software. Beyond Kaspersky, TrendMicro, and MBAM security software, the fingerprinting script also checks for VirtualBox, Parallels, VMWare, and Fiddler. If any of these programs are thought to be installed, the exploit attempt is abandoned and the would-be victim is skipped.

This isn’t the only malware we’ve seen hiding from Fiddler—earlier attempts use tricks to see whether Fiddler is actively running and intercepting traffic and only abandon the exploit if it is.

This behavior is, of course, pretty silly. But it makes me happy anyway.

Preventing Detection of Fiddler

Malware researchers who want to help ensure Fiddler cannot be detected by bad guys should take the following steps:

  1. Do not put Fiddler directly on the “victim” machine.
    – If you must, at least install it to a non-default path.
  2. Instead, run Fiddler as a proxy server external to the victim machine (either use a different physical machine or a VM).
    – Tick the Tools > Fiddler Options > Connections > Allow remote computers to connect checkbox. Restart Fiddler and ensure the machine’s firewall allows inbound traffic to port 8888.
    – Point the victim’s proxy settings at the remote Fiddler instance.
    – Visit http://fiddlerserverIP:8888/ from the victim and install the Fiddler root certificate
  3. Click Rules > Customize Rules and update the FiddlerScript so that the OnReturningError function wipes the response headers and body and replaces them with non-descript strings. Some fingerprinting JavaScript will generate bogus AJAX requests and then scan the response to see whether there are signs of Fiddler.

-Eric