Troubleshooting WSL2 Mirrored Networking LAN IP Inaccessibility

Aug 10, 2025 by Henrik Larsen 64 views

Troubleshooting WSL2 Mirrored Networking Mode Inaccessibility via LAN IP

Hey guys! Today, we're diving deep into a tricky issue with WSL2's mirrored networking mode. Specifically, we're tackling the problem where you can't access services running in WSL2 from your host machine using the host's LAN IP. This can be a real head-scratcher, especially when everything else seems to be working fine. We'll break down the problem, look at the diagnostic steps, and analyze the packet-level data to understand exactly what's going on. So, buckle up, and let's get started!

The Problem: Hairpin Routing Failure in WSL2 Mirrored Networking

The core issue here is that when WSL2 is configured to use networkingMode=mirrored, a specific type of network traffic—hairpin traffic—fails. Hairpin traffic is when a request originates from within WSL2, is destined for the host's LAN IP, and needs to loop back to the host. Let's break this down with a scenario:

Imagine you're running a web server inside your WSL2 instance. You start it up, and it's listening on port 8000. Now, you want to access this server from your host machine using your host's LAN IP (e.g., 192.168.31.33). You fire up your browser or use curl, but... nothing. The connection times out. This is the problem we're addressing. The hairpin routing failure prevents this connection from happening, even though other types of network connections work perfectly fine. To really understand this problem, we need to dive deep into how mirrored networking is supposed to work and where things might be going wrong.

The key symptom of this issue is the inability to connect to services on the host machine from WSL2 using the host's LAN IP address, while connections via localhost work flawlessly. This discrepancy points to a specific problem in the network routing path when packets need to loop back to the same machine they originated from. The frustration arises because basic network connectivity seems fine – WSL2 can access the internet, and the host machine can access services within WSL2 using localhost. It's this particular scenario of using the LAN IP for a loopback that exposes the underlying issue. Understanding this distinction is crucial for effective troubleshooting, as it narrows down the potential causes and guides the diagnostic steps we need to take. So, let's move on to the specific steps taken to reproduce and diagnose this perplexing problem.

Reproducing the Issue: A Step-by-Step Guide

To effectively troubleshoot this, we need a consistent way to reproduce the problem. Here's a step-by-step guide to recreate the issue and confirm you're facing the same challenge. This process involves configuring WSL2 for mirrored networking, starting a simple web server within WSL2, and then making specific network requests to test different connectivity paths. By following these steps, you can isolate the hairpin routing failure and gather the necessary evidence to diagnose the root cause. Let's walk through the process:

Configure .wslconfig for Mirrored Networking:

First, you need to ensure that WSL2 is running in mirrored networking mode. Open your .wslconfig file (typically located in your user profile directory, like C:\Users\YourUsername\.wslconfig) and add or modify the following lines:
```
[wsl2]
networkingMode=mirrored
```
This setting tells WSL2 to use the mirrored networking mode, which aims to provide a more seamless networking experience by sharing the host's network interface. This is a crucial first step, as the issue we're troubleshooting is specific to this networking mode. Mirrored networking is designed to simplify network configurations, but as we'll see, it can sometimes introduce complexities that require careful diagnosis.
Restart WSL Fully:

For the configuration change to take effect, you need to fully restart WSL. Open PowerShell as an administrator and run the following command:

wsl --shutdown ```

This command stops all running WSL distributions and the WSL2 virtual machine. After running this, wait a few seconds to ensure everything is fully shut down. Then, start your WSL distro again by simply opening it (e.g., Ubuntu). This step is essential because WSL can sometimes cache network configurations, and a full shutdown ensures that the new settings are applied correctly. Failing to restart WSL properly can lead to inconsistent behavior and make troubleshooting more difficult. So, make sure you give it a full restart.

Start a Simple Web Server in WSL:

Inside your WSL distro, start a basic web server. Python's built-in http.server is perfect for this. Open a terminal in your WSL distro and run:
```
python3 -m http.server 8000
```
This command starts a web server on port 8000, serving files from the current directory. This server will act as our test endpoint. It's a simple and reliable way to have a service running within WSL that we can try to connect to. By using a well-known tool like Python's http.server, we eliminate the possibility of issues with custom server configurations and focus solely on the network connectivity aspects. Now that we have a server running, we can proceed to test the various network paths.
Test Network Connectivity with curl:

Now, we'll use curl to test different network paths and expose the issue. Open another terminal in your WSL distro and run the following commands, one at a time:
- Test 1: Localhost Loopback
```
curl http://localhost:8000
```
  This tests connectivity to the web server using the localhost address. This should succeed, confirming that the server is running and listening on the loopback interface. A successful response here indicates that the server within WSL is functioning correctly and can handle local connections. This serves as a baseline test to ensure that the basic server setup is working before we move on to more complex scenarios.
- Test 2: Outbound to LAN (Router)
```
# Replace 192.168.31.1 with your actual router's IP address
curl http://192.168.31.1
```
  Replace 192.168.31.1 with your router's IP address. This tests outbound connectivity to your local network. This should also succeed, demonstrating that WSL can communicate with other devices on your LAN. A successful connection here confirms that WSL can send and receive packets on the network, ruling out basic network misconfigurations. This test is crucial for isolating the problem to the specific hairpin routing scenario.
- Test 3: Hairpin Loopback to Host's LAN IP
```
# Replace 192.168.31.33 with your actual host's LAN IP
curl http://192.168.31.33:8000
```
  Replace 192.168.31.33 with your host's LAN IP address. This is the critical test that will fail. The curl command will likely hang until it times out, demonstrating the hairpin routing failure. This test specifically targets the scenario where traffic originating from WSL needs to loop back to the host's LAN IP. The failure here confirms that the issue is not a general network connectivity problem but a specific routing problem related to hairpin traffic in mirrored networking mode.

By performing these tests, you should be able to consistently reproduce the issue where hairpin traffic fails in WSL2's mirrored networking mode. Now that we can reliably reproduce the problem, we can move on to examining the expected and actual behaviors to further understand the scope of the issue.

Expected vs. Actual Behavior: What Should Happen, and What Does

Understanding the expected behavior versus the actual behavior is crucial for pinpointing the exact nature of a problem. In this case, we're dealing with network connectivity, so we need to be clear about how packets should flow and where the process breaks down. Let's walk through what we expect to happen with each curl command and then contrast that with what actually occurs when the hairpin routing issue manifests. This comparison will highlight the specific failure point and guide our troubleshooting efforts.

Expected Behavior

Ideally, all three curl commands should succeed without any issues. Let's break down why:

Test 1 (localhost): When you run curl http://localhost:8000, the request should stay entirely within the WSL2 instance. It's a loopback connection, meaning the traffic doesn't need to leave WSL2 to reach the web server. The expected behavior is a quick response from the server, displaying the directory listing or any other content served by the Python web server. This test verifies that the server is running correctly and that the loopback interface within WSL is functioning as expected.
Test 2 (Router): When you run curl to your router's IP address (e.g., curl http://192.168.31.1), the request should go from WSL2, through the Windows network stack, out to your local network, and then to your router. The expected behavior is a response from your router, which might be a login page or some other default page. This test confirms that WSL2 can send traffic to the external network and receive responses, indicating that the basic network setup is working correctly. This step is crucial for ensuring that the problem is not a general connectivity issue but rather something specific to internal routing.
Test 3 (Host LAN IP): This is where the problem lies. When you run curl to your host's LAN IP address and port 8000 (e.g., curl http://192.168.31.33:8000), the request should, in theory, go from WSL2, through the Windows network stack, recognize that the destination is the host machine itself, and loop back to the web server running on the host. The expected behavior is the same as Test 1: a response from the Python web server, displaying the directory listing. This is known as hairpin routing or loopback, where traffic loops back to the originating machine through its external IP address. This scenario is essential for many development and testing setups where services running in WSL need to be accessed as if they were on a separate machine.

Actual Behavior

Now, let's look at what actually happens when the issue is present:

Test 1 (localhost): As expected, this succeeds. The curl command returns the directory listing or content from the web server. This confirms the server is running and accessible locally within WSL2.
Test 2 (Router): This also succeeds. The curl command receives a response from the router, indicating that outbound connectivity is working as expected. This rules out general network connectivity problems.
Test 3 (Host LAN IP): This is where the failure occurs. The curl command hangs indefinitely, eventually timing out without receiving any response. This demonstrates the specific failure in the hairpin routing path. The request is not making it back to the host machine, even though it should. This is the crux of the problem we're trying to solve.

The discrepancy between the expected and actual behavior highlights that the issue is not a general network problem but a specific failure in how WSL2 handles hairpin traffic when using mirrored networking. The fact that localhost and outbound connections work fine narrows down the possible causes and focuses our attention on the routing mechanisms involved in looping traffic back to the host machine. With this clear understanding of the problem, we can move on to examining the diagnostic logs and data to uncover the root cause.

Diagnostic Logs and Evidence: Tracing the Packet Path

When troubleshooting network issues, diagnostic logs and packet-level data are your best friends. They provide a detailed view of what's happening under the hood, allowing you to trace the path of packets and identify where they're being dropped or misrouted. In this case, we've gathered several pieces of evidence, including the results of network resets, firewall configurations, Wireshark captures, and, most importantly, pktmon logs. Let's dive into each of these to understand how they help us diagnose the WSL2 hairpin routing problem.

Initial Troubleshooting Steps

Before diving into advanced diagnostics, several standard troubleshooting steps were taken:

Full OS Reinstall: To rule out any system-level corruption or misconfiguration, a complete reinstallation of Windows 11 was performed. The issue persisted even after the reinstall, indicating that the problem is not related to a specific system state but rather a more fundamental configuration or bug.
Network Resets: Standard (netsh winsock reset, etc.) and advanced (netsh advfirewall reset) network resets were performed. These resets aim to restore the network configuration to a default state, clearing any potential conflicts or misconfigurations. However, these steps did not resolve the issue, suggesting that the problem lies deeper within the networking stack.
Third-party Software: All third-party VPNs, proxies, and security software were uninstalled to eliminate any interference they might be causing. The issue persisted on a clean system, confirming that it's not related to conflicts with other applications.

These initial steps are essential for any systematic troubleshooting process. By eliminating common causes, we narrow down the scope of the problem and focus our attention on the more specific aspects of WSL2's mirrored networking mode.

Wireshark Analysis

Wireshark is a powerful network protocol analyzer that captures and analyzes network traffic. Capturing traffic on the host's physical NIC (Network Interface Card) during the failing connection attempt (curl http://192.168.31.33:8000) revealed a crucial piece of information: no packets were observed. This indicates that the packets are being dropped before they even reach the network interface, suggesting a problem within the Windows networking stack itself. Wireshark's inability to see the packets eliminates the possibility of the issue being related to network congestion, routing problems outside the host, or firewall rules blocking the traffic at the network interface level.

Firewall Rules

To ensure that the Windows Firewall was not blocking the traffic, a Hyper-V firewall rule was added to allow all inbound traffic to WSL in the private network. This was done using the following PowerShell command:

New-NetFirewallHyperVRule -DisplayName 'Allow All Inbound Traffic to WSL in Private Network' -Name 'WSL Rule' -Direction Inbound -Action Allow -VMCreatorId '{40E0AC32-46A5-438A-A0B2-2B479E8F2E90}' -Enabled True

This rule explicitly allows all inbound traffic destined for WSL, effectively ruling out the firewall as the cause of the problem. Despite adding this rule, the hairpin routing issue persisted, further suggesting that the problem lies elsewhere in the networking stack.

Key Evidence from `pktmon`

The most definitive evidence comes from pktmon, Windows' built-in kernel-level packet monitor. pktmon captures network traffic at a very low level, providing a detailed view of packet flow within the Windows kernel. A pktmon trace revealed the following:

The TCP SYN packet from WSL to 192.168.31.33:8000 is successfully generated.
The packet travels down the Tx (Transmit) path of the Windows networking stack.
Crucially, the packet successfully performs a hairpin turn, and its direction changes from Tx to Rx (Receive) within the kernel.
The packet then travels all the way up the Rx path, ready for delivery to the listening application.
There are no Drop events recorded in the entire trace for this packet.

This is a critical finding. The pktmon log shows that the packet makes it all the way through the networking stack, including the hairpin turn, without being dropped. This strongly suggests that the issue is not a routing or filtering problem within the kernel but rather something preventing the packet from being delivered to the listening application after it has been received by the networking stack.

Analyzing the `pktmon` Log

Let's look at a relevant excerpt from the pktmon log:

	00-15-5D-23-23-30 > BC-EC-A0-4A-FE-08, ethertype IPv4 (0x0800), length 66: 192.168.31.33.10455 > 192.168.31.33.8000: Flags [S], seq 3857641395, win 65535, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0
[02]0000.0000::2025-08-10 12:14:09.843709700 [Microsoft-Windows-PktMon] PktGroupId 562949953421432, PktNumber 1, Appearance 0, Direction Tx , Type Ethernet , Component 49, Edge 1, Filter 0, OriginalSize 66, LoggedSize 66
	00-15-5D-23-23-30 > BC-EC-A0-4A-FE-08, ethertype IPv4 (0x0800), length 66: 192.168.31.33.10455 > 192.168.31.33.8000: Flags [S], seq 3857641395, win 65535, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0
[02]0000.0000::2025-08-10 12:14:09.843712200 [Microsoft-Windows-PktMon] PktGroupId 562949953421433, PktNumber 1, Appearance 0, Direction Tx , Type Ethernet , Component 49, Edge 2, Filter 0, OriginalSize 66, LoggedSize 66
...
[02]0000.0000::2025-08-10 12:14:09.843766000 [Microsoft-Windows-PktMon] PktGroupId 562949953421445, PktNumber 1, Appearance 0, Direction Rx , Type Ethernet , Component 12, Edge 1, Filter 0, OriginalSize 66, LoggedSize 66
	00-15-5D-23-23-30 > BC-EC-A0-4A-FE-08, ethertype IPv4 (0x0800), length 66: 192.168.31.33.10455 > 192.168.31.33.8000: Flags [S], seq 3857641395, win 65535, options [mss 1460,nop,wscale 8,nop,nop,sackOK], length 0
[02]0000.0000::2025-08-10 12:14:09.843767100 [Microsoft-Windows-PktMon] PktGroupId 562949953421446, PktNumber 1, Appearance 0, Direction Rx , Type Ethernet , Component 53, Edge 2, Filter 0, OriginalSize 66, LoggedSize 66
...

This excerpt shows the TCP SYN packet being transmitted (Tx) and then received (Rx) within the Windows networking stack. The absence of any Drop events confirms that the packet is not being filtered or blocked. This detailed packet tracing is invaluable in isolating the problem to a specific stage in the network communication process. The fact that packets are making it through the networking stack but not reaching the application suggests a potential issue with how Windows is handling the delivery of these looped-back packets. Now, let's put all the evidence together and try to figure out what's really going on.

Conclusion: The Mystery of the Missing Hairpin Packets

Alright, guys, we've journeyed through the depths of WSL2 mirrored networking, faced the challenge of hairpin routing failure, and meticulously examined the diagnostic evidence. So, what have we learned? The core issue is that when WSL2 is configured in mirrored networking mode, connections from within WSL2 to the host's LAN IP address on a specific port fail, even though connections to localhost and outbound connections work perfectly. This indicates a problem with hairpin routing, where traffic needs to loop back to the host machine.

We've seen that standard troubleshooting steps like OS reinstalls, network resets, and disabling third-party software don't resolve the problem. Wireshark captures show that no packets are even reaching the network interface during the failing connection attempts, suggesting the issue is within the Windows networking stack itself. Firewall rules are explicitly configured to allow traffic to WSL, so that's not the culprit either.

The most compelling evidence comes from pktmon, which reveals that the packets successfully traverse the transmit (Tx) and receive (Rx) paths within the Windows kernel. Crucially, no packets are dropped. This tells us that the packets make it through the networking stack, perform the hairpin turn, and are ready for delivery to the application. However, they never arrive.

So, what's the final conclusion? The mystery of the missing hairpin packets points to a potential bug or misconfiguration in how Windows delivers these looped-back packets to the listening application when using WSL2's mirrored networking mode. The packets are making it through the network stack, but something is preventing them from reaching their destination. Further investigation and potentially a bug report to Microsoft might be necessary to fully resolve this issue. We've armed ourselves with detailed diagnostic data and a clear understanding of the problem, which is the first step towards a solution. Keep digging, and let's hope this gets resolved soon!