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Over the years, the most interesting class of support requests for Fiddler are of the form: My application or website is failing, but when I try to capture a repro with Fiddler, the problem goes away completely! How is Fiddler fixing it?

In general, Fiddler isn’t designed to automatically “fix” problems in web applications—it’s designed only to permit you to debug them. If you enable Fiddler’s Lint feature, it can flag problematic traffic to get your attention, but it still won’t fix the issues it finds.

However, in some cases the introduction of a proxy (e.g. Fiddler) into the client-server channel changes things just enough that intermittent errors become more rare, or even go away entirely. In today’s post, we’ll explore some of these situations.


The most common fault which is “fixed” by Fiddler concerns .NET applications which successfully complete a few HTTP or HTTPS requests but then “hang” and refuse to send any more, or begin to send requests very slowly and serially, even when requests are made on parallel threads.

When the developer starts using Fiddler to debug the traffic, the problem goes away entirely.

In these cases, the problem always turns out to be that the developer called the GetResponseStream() method but failed to subsequently call Close() on the resulting object. The consequence is that .NET keeps the object alive indefinitely, which causes subsequent requests to eventually block on the default connections-per-host limit. I haven’t stepped into the System.NET code myself, but my guess is that running Fiddler mitigates this problem because .NET enforces a higher limit for proxy connections.

This fix for this problem is simple: Ensure you always call Close() on the result of a call to GetResponseStream().

Slow Requests

Another common problem concerns applications which make many requests in a row but execute more slowly than the developer hopes. When Fiddler is started, the application suddenly accelerates and requests complete in much less time.

There are several possible explanations for this behavior.


In some cases, the time required to open a new network connection to the server is greater than the time required to send the request and download the response. Therefore, if the client opens a new connection for every request, the application’s performance is greatly degraded. The practice of reusing a single TCP/IP connection for multiple requests is called “keep-alive” and it’s the default behavior in HTTP/1.1. However, clients or servers may choose to disable keep-alive by either sending a Connection: close header or by abruptly closing the connection after each transaction.

Fiddler maintains a “connection pool” of idle keep-alive connections to the server. When the a client request comes in, this pool is first checked to determine if an existing connection is available on which the request can be sent. Even if the client specifies a Connection: close request header, that only causes Fiddler to close the client’s connection after the response is sent—the server connection is returned to the pool (unless it too disabled keep-alive).

What this means is that if your client isn’t using Keep-Alive connections, its performance can be severely impacted. However, when Fiddler is introduced, performance is improved because “expensive” server connections are reused. (Since Fiddler and the client are (typically) running on the same computer, establishing a new connection from the client to Fiddler is very fast.)

The fix for this problem is simple: Ensure that your client is using KeepAlive connections. That’s as simple as:

  1. Ensure that you’re using HTTP/1.1
  2. Ensure that you haven’t disabled Keep-Alive (e.g. set the KeepAlive property of the HTTPWebRequest object to true)
  3. Don’t send Connection: Close headers

Note that creating connections to servers can be even more expensive than the simple TCP/IP establishment cost. First, there’s TCP/IP Slow-Start, a congestion-management feature of the protocol that means that new connections have a slower transfer rate than longer-lived connections. Next, if you’re using HTTPS, there’s an expensive cryptographic handshake which must be performed on each new connection. Lastly, if your connections use either the NTLM or Negotiate authentication protocols, you may find that each new connection requires a 3-step handshake (e.g. the server sends a HTTP/401 challenge, the client resends the request, the server sends another HTTP/401 challenge, the client resends the request with a challenge-response, and the server finally sends a HTTP/200). Because these are “connection-oriented” authentication protocols, subsequent requests over an existing connection may be able to avoid these extra round-trips.

Buffer Sizes

In rare cases, users report that large file uploads occur much faster through Fiddler. In some cases, they find that their application’s uploads time-out without Fiddler but succeed when Fiddler is running.

Fiddler buffers the complete HTTP request in memory before sending it to the server; request data is sent to the server using efficiently-sized buffers. If the client is using an inefficient buffer size when writing to the network, Fiddler can help mask this problem because Fiddler is running locally and the client’s network inefficiency is less problematic when the traffic hasn’t even left the machine.

Internet Explorer 6 had this problem and it required a registry change to use efficient buffers; IE7 and later utilized better defaults to avoid this problem. Today, most client frameworks use properly-sized buffers, but older implementations may not.

Slow Proxy Determination

Some developers have found that the first network request sent by their application is delayed by as long as 30 seconds on some machines, or that every request sent suffers from such a delay. When Fiddler is started, they note that Fiddler itself spends a fair bit of time on the Looking up gateway… splashscreen startup step, but after that, both Fiddler and their application suffer no delays.

In these cases, the typical problem is that the system is configured to Automatically Detect Proxy Settings or is configured to use a Proxy Configuration script. This configuration is found inside Internet Explorer’s Tools > Internet Options > Connections > LAN Settings screen. (Be sure to check when Fiddler is not running):


The performance problem arises when the proxy determination process (WPAD) takes a long time to complete (either fail or succeed), or when the URL of the automatic configuration script is unreachable.

Fiddler helps resolve proxy determination performance problems because it changes the system’s proxy settings to point directly at Fiddler, such that other applications spend no time trying to figure out which proxy to use.

Fiddler itself attempts to find the system’s proxy during its “Looking up gateway…” startup step, but it caches the result of that process for all subsequent requests from all applications. Most applications will individually cache the proxy determination once per session, but some do not and thus pay the penalty repeatedly. For instance, on Windows Vista and Windows 7, the WebDAV framework attempts to re-detect the proxy on every single operation, which can lead to a huge performance problem.

Only a small percentage of users require a proxy server to reach the Internet, and therefore simply unchecking both of these boxes can usually resolve the performance problem. If the PC requires a proxy, you should speak to your IT Administrators about how to improve the proxy’s performance.

HTTPS Issues

Some users find that HTTPS traffic only works when Fiddler is running and decrypting secure traffic.

Certificate Errors

By default, Fiddler warns you about invalid certificates when connecting to HTTPS sites:


If you elect to ignore this warning, Fiddler will effectively “hide” the certificate error from the client application, such that it only sees the certificate Fiddler generated for HTTPS interception.

Most browsers show a meaningful error message if they encounter an invalid certificate:


…but many applications will fail silently or with a confusing error message. Even within the browser, sometimes no error message is shown (e.g. when using XmlHttpRequest).

The fix here is simple: Correct or replace the server’s certificate.

Protocol Versioning

Some users have found that their applications fail to establish HTTPS connections with certain servers unless Fiddler is running and decrypting traffic, in which case everything works fine. This problem typically arises when the application or user has enabled the TLS/1.1 or TLS/1.2 protocols, which can cause some servers to misbehave.

Running Fiddler resolves such problems because Fiddler, by default, only uses SSLv3.0 and TLSv1.0 when communicating with HTTPS servers, avoiding the compatibility problems seen on legacy servers. If your computer has the .NET Framework v4.5 installed, Fiddler v4 can be configured to attempt to use TLS/1.1+ which will lead to the same connection failure seen when Fiddler wasn’t present.

To resolve this issue, either disable TLS/1.1+ on the client, or better yet, nag the server operator to upgrade their software to support the TLS standard.

About the Author

Eric Lawrence

(@ericlaw) has built websites and web client software since the mid-1990s. After over a decade of working on the web for Microsoft, Eric joined Telerik in October 2012 to enhance the Fiddler Web Debugger on a full-time basis. With his recent move to Austin, Texas, Eric has now lived in the American South, North, West, and East.


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