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Fiddler Everywhere can capture localhost requests when the client actually sends them through its proxy. The most reliable fix is to replace localhost or 127.0.0.1 with a proxyable alias such as ipv4.fiddler, ipv6.fiddler, your machine name, or (when the Host header must remain localhost) localhost.fiddler. If the request still bypasses Fiddler, add <-loopback> to the Gateway bypass list, then generate the request and inspect Live Traffic.
Why Fiddler Everywhere misses localhost
Many applications deliberately bypass HTTP proxies for loopback destinations. Browsers and some libraries proxy ordinary hostnames, but framework networking stacks can recognize localhost and 127.0.0.1 as local addresses and connect directly. Progress Telerik’s official Capturing Localhost Traffic guidance states that “Some frameworks (like .NET) are not proxying the localhost traffic.”
This is a client behavior, not proof that Fiddler is broken. Fiddler Everywhere can only show a request that reaches its proxy (normally the configured HTTP(S) proxy). The workflow below handles ordinary HTTP, HTTPS, .NET Framework, Windows PowerShell, terminal-launched processes, IPv4, IPv6 and applications that cannot use a proxy at all.
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- Start capture. Open Fiddler Everywhere and turn on its capturing mode. Verify that the application or operating system is configured to use the Fiddler proxy.
- Change the local URL. Instead of
http://localhost:8081/mytestpage.aspxorhttp://127.0.0.1:8081/mytestpage.aspx, try a machine-name URL orhttp://ipv4.fiddler:8081/mytestpage.aspx. Useipv6.fiddlerwhen the service is bound to IPv6. - Preserve a localhost Host header when necessary. For IIS Express scenarios that require the Host header to be
localhost, uselocalhost.fiddleras the URL host. - Make the request. Refresh the page, run the API call, or start the client operation.
- Inspect Live Traffic. The grid commonly records the destination as the loopback host, usually
127.0.0.1, even though the client used an alias. Filter for that destination rather than only searching for the alias.
Choose the right hostname
| Client URL | Use it when | Important detail |
|---|---|---|
http://<machine-name>:port/path |
The local service is reachable through the computer’s normal name. | The Host header changes to the machine name. |
http://ipv4.fiddler:port/path |
The service listens on IPv4 and the framework bypasses literal loopback names. | Fiddler routes the alias to IPv4 loopback. |
http://ipv6.fiddler:port/path |
The service listens on IPv6. | Use this instead of the IPv4 alias when testing an IPv6 binding. |
http://localhost.fiddler:port/path |
IIS Express or another service needs localhost semantics in the Host header. | The URL is proxyable while retaining the localhost-style host value expected by the scenario. |
http://localhost.:port/path |
.NET Framework or Windows PowerShell has a hardcoded localhost proxy bypass. | The trailing dot creates a distinct host name while resolving to the local machine. |
Use the smallest change that makes the client proxy the request. A hostname or Fiddler alias usually changes less than a global Gateway setting; changing the URL can, however, affect virtual-host routing, cookie scope, certificate names and application allowlists. Test those behaviors explicitly.
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Remove the loopback bypass in Gateway settings
If aliases do not appear in Live Traffic, edit Fiddler Everywhere’s Gateway settings and add the special token <-loopback> to the bypass list. This removes the default bypass for 127.0.0.1 and localhost for configured proxy traffic.
- Open the Gateway settings in Fiddler Everywhere.
- Locate the proxy bypass list.
- Add
<-loopback>as its own entry. - Save the setting, restart the client if it cached proxy configuration, and repeat the request.
- Confirm the session in Live Traffic and then remove or document the setting if other applications should not be routed through Fiddler.
This is broader than changing one URL: it changes bypass behavior for traffic using that gateway configuration. It may therefore expose unrelated local requests to capture and can alter applications that rely on direct loopback connections.
Capture terminal applications and child processes
For command-line tools, use Fiddler Everywhere’s dedicated Fiddler terminal, available from Home or Traffic. Run the application in that terminal so Fiddler can capture the terminal’s traffic and the traffic of child processes it launches. This avoids guessing whether a shell, runtime, or child process inherited the proxy variables.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsFor .NET Framework and Windows PowerShell, prefer a Fiddler alias or the trailing-dot form, for example http://localhost.:8080/. These stacks may be hardcoded to bypass proxies for literal localhost. If a script still does not appear, run it from the dedicated Fiddler terminal and check that it is using the expected HTTP or HTTPS proxy settings.
HTTPS localhost capture
HTTP aliases are not enough for encrypted traffic. Install and trust the Fiddler root certificate before attempting to decrypt HTTPS sessions.
- Use Fiddler Everywhere’s certificate-installation workflow to install its root certificate for the current operating-system user.
- Trust the certificate in the relevant system or application trust store. Some runtimes maintain their own certificate store and will not automatically use the operating system’s trust decision.
- Enable HTTPS decryption in Fiddler Everywhere and include the local host or hosts you intend to inspect.
- Repeat the request and look for a decrypted HTTPS session in Live Traffic.
If the request appears but shows a certificate error, the proxy path is working; the client simply does not trust the Fiddler certificate. If no session appears at all, solve proxy bypass or process-scope issues first. Do not leave a debugging root certificate trusted on a production machine longer than necessary, and remove it when the investigation is complete.
When proxy capture is the wrong mode
Some applications use networking APIs that ignore system proxy settings, open raw sockets, or run under a process context Fiddler cannot configure. In those cases, switch to Network Capturing mode. It observes outgoing TCP traffic at a lower level and can capture traffic that does not cooperate with an HTTP proxy.
- Requirements: the appropriate platform extension or kernel driver and administrative privileges.
- Scope: broader than a dedicated terminal or one URL; unrelated processes may be included.
- Trade-off: lower-level capture can identify connections that proxy mode misses, but it may provide less application-level control and requires elevated access.
Use proxy mode first for the least invasive diagnosis. Choose Network Capturing only when the client cannot be made to proxy its traffic.
Diagnose a missing localhost session
| Symptom | Likely cause | Fix |
|---|---|---|
No session for localhost or 127.0.0.1 |
The client bypasses loopback. | Use ipv4.fiddler, ipv6.fiddler, a machine name, or localhost.fiddler; then consider <-loopback>. |
| Alias request works, but Live Traffic search finds nothing | The grid records the destination as loopback. | Filter for 127.0.0.1 or the IPv6 loopback address. |
| HTTP appears, HTTPS does not | Certificate is missing, untrusted, or HTTPS decryption is disabled. | Install and trust the Fiddler root certificate, enable decryption, and check the runtime’s separate trust store. |
| .NET Framework or PowerShell still bypasses | Literal localhost is excluded by the stack. | Use a Fiddler alias or append a dot: localhost.:port. Run from the Fiddler terminal. |
| IIS Express rejects the request or selects the wrong site | The Host header changed when replacing localhost. | Try localhost.fiddler, which is intended for scenarios requiring localhost in the Host header. |
| Only one terminal command is captured | The process was launched outside Fiddler’s terminal or did not inherit its settings. | Open the dedicated Fiddler terminal and launch the parent process there. |
| Proxy mode never sees the connection | The application ignores proxies or uses raw TCP. | Use Network Capturing mode with the required extension or driver and administrator rights. |
| Requests are captured unexpectedly from other applications | The Gateway bypass change or network capture has broad scope. | Remove <-loopback> when finished, narrow the capture, or return to per-process proxy capture. |
Testing checklist for reliable results
- Record the exact URL, port, address family and expected Host header before changing anything.
- Confirm the local service is listening on the selected IPv4 or IPv6 interface and port.
- Start Fiddler capture before launching the client or refreshing the page.
- Test both a plain HTTP request and the HTTPS variant if TLS is part of the bug.
- Search Live Traffic by loopback destination, not only by the alias typed into the client.
- Check whether the client runs as a different user, service account, container or elevated process.
- After debugging, restore the original URL, bypass list, certificate trust and capture mode.
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FAQ
Does changing localhost to an alias change the application?
It can. The URL host may affect Host-based routing, cookies, certificate validation and allowlists. Use localhost.fiddler when the scenario specifically requires a localhost Host header, and verify the application’s routing after the capture starts.
Which address should I use for an IPv6-only service?
Use ipv6.fiddler and confirm that the service is listening on IPv6. Searching Live Traffic by an IPv4 loopback address will not find an IPv6-only session.
Is Network Capturing required for every localhost request?
No. Alias or machine-name routing is normally less invasive. Network Capturing is the fallback for applications that ignore proxies or use lower-level connections, and it requires an extension or driver plus administrative privileges.
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