Solved: Top 3 Nginx 405 Errors on Docker & ASP.NET 2025

Introduction: The Dreaded 405

You’ve meticulously containerized your ASP.NET application, set up Nginx as a sleek reverse proxy in Docker, and everything seems perfect. You push your latest feature, and suddenly, your API calls start failing with a cryptic 405 Method Not Allowed error. This is one of the most frustrating errors in a modern web stack because it’s not a simple “Not Found” (404) or “Server Error” (500). It implies the server is there, the URL is correct, but the way you're trying to communicate (e.g., with a POST or PUT request) is being rejected.

In the world of Nginx, Docker, and ASP.NET, this error almost always points to a misconfiguration in the communication chain between the client, your Nginx proxy, and your ASP.NET application running on the Kestrel server. This 2025 guide will walk you through the three most common culprits and provide clear, actionable solutions to get your application back on track.

Understanding the 405 Method Not Allowed Error

Before diving into solutions, let's clarify what a 405 error truly means. The HTTP/1.1 specification defines the 405 status code as:

The method specified in the Request-Line is not allowed for the resource identified by the Request-URI. The response MUST include an Allow header containing a list of valid methods for the requested resource.

In simpler terms: You sent a request (e.g., `POST /api/users`) to a server. The server understands the `/api/users` part but does not permit the `POST` method for that specific URL. This is different from a 404, where the server wouldn't even know what `/api/users` is.

In our Docker stack, the request travels like this: Client -> Nginx Container -> ASP.NET Container. A breakdown at any point in this chain can trigger a 405. The key is to determine which component is rejecting the method: Nginx or the ASP.NET application itself.

The Top 3 Causes and Solutions

Let's systematically break down the most frequent reasons for seeing a 405 error and how to fix them. We'll start from the outside (Nginx) and work our way in (ASP.NET).

Cause 1: Incorrect `proxy_pass` Configuration

This is the number one suspect. Nginx acts as a gatekeeper. If it doesn't know where to properly forward a request, it might handle it itself. Since Nginx's default server isn't configured to handle POST requests to dynamic endpoints, it rightfully returns a 405.

The Problem: Your `proxy_pass` directive in `nginx.conf` is pointing to the wrong location. This often happens due to a misunderstanding of Docker's internal networking and DNS.

• Using `localhost` or `127.0.0.1` inside the Nginx container, which refers to the Nginx container itself, not your ASP.NET container.
• Using an incorrect Docker service name or port.

Diagnosis: Check your Nginx container's logs. You'll likely see an error message indicating it couldn't connect to the upstream server defined in `proxy_pass`.

docker logs your-nginx-container-name

Solution: Use the Docker service name of your ASP.NET application as the hostname in the `proxy_pass` directive. If you are using `docker-compose`, Docker provides a built-in DNS that resolves service names to their respective container IP addresses.

Consider this `docker-compose.yml`:

version: '3.8'
services:
  my-webapp:
    image: my-aspnet-app
    # No ports exposed externally, only to the Docker network

  nginx-proxy:
    image: nginx:latest
    ports:
      - "8080:80"
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf

Your `nginx.conf` should use the service name `my-webapp`:

http {
    server {
        listen 80;
        location / {
            # Correct: Use the Docker service name and the port Kestrel is listening on inside the container.
            proxy_pass http://my-webapp:8080; # Assuming your app listens on port 8080

            # Essential headers for ASP.NET Core
            proxy_set_header Host $host;
            proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
            proxy_set_header X-Forwarded-Proto $scheme;
        }
    }
}

Cause 2: Improper HTTP Method Forwarding

While a standard `proxy_pass` forwards all methods, complex Nginx configurations can inadvertently block them. This occurs if you have `if` statements or `limit_except` blocks that are too restrictive.

The Problem: Your Nginx configuration has logic that explicitly denies or fails to handle methods other than GET or HEAD. For example, a `limit_except` block meant to secure a location might be misconfigured to only allow GET.

Diagnosis: Use a tool like `curl` or Postman to test the exact same URL with different methods. If `GET` works but `POST` or `PUT` returns a 405, this is a strong indicator that Nginx is the culprit.

# This might work (200 OK)
curl http://your-domain.com/api/items

# This might fail (405 Method Not Allowed)
curl -X POST http://your-domain.com/api/items -d '{}' -H "Content-Type: application/json"

Solution: Review your Nginx `location` blocks. Remove any overly restrictive directives. A clean, simple `proxy_pass` block like the one in the first solution is almost always sufficient and correct. It transparently forwards all methods and headers to the backend application.

Avoid complex `if` logic for routing if possible. If you must restrict methods, ensure your `limit_except` block includes all necessary verbs:

# Example of a potentially problematic configuration
location /api/ {
    # This block only allows GET, HEAD, and POST. PUT/DELETE would fail.
    limit_except GET HEAD POST {
        deny all;
    }
    proxy_pass http://my-webapp:8080;
    # ... other proxy settings
}

Cause 3: ASP.NET Core Endpoint and Verb Mismatch

If you've confirmed your Nginx configuration is perfect, it's time to look inward at the ASP.NET application itself. The 405 error could be coming directly from the Kestrel server because your controller action isn't set up to handle the incoming request's HTTP method.

The Problem: The endpoint you are calling exists, but the controller action method is decorated with the wrong HTTP verb attribute. For example, you are sending a `POST` request to an action marked with `[HttpGet]`.

Diagnosis: The ultimate test is to bypass Nginx entirely. Temporarily expose the port of your ASP.NET container in your `docker-compose.yml` and hit it directly.

# In docker-compose.yml, temporarily change:
services:
  my-webapp:
    image: my-aspnet-app
    ports:
      - "5000:8080" # Map container port 8080 to host port 5000

Then, run `docker-compose up -d` and test with `curl`:

curl -X POST http://localhost:5000/api/users -d '{}' -H "Content-Type: application/json"

If you still get a 405 error, the problem is 100% within your C# code.

Solution: Carefully inspect your ASP.NET controller. Ensure the route template and the HTTP verb attribute (`[HttpPost]`, `[HttpPut]`, `[HttpDelete]`, etc.) match the request you are making.

Incorrect Code (will cause a 405 on POST):

[ApiController]
[Route("api/[controller]")]
public class UsersController : ControllerBase
{
    [HttpGet] // <-- WRONG! Should be [HttpPost]
    public IActionResult CreateUser([FromBody] NewUserDto user)
    {
        // ... logic to create a user
        return CreatedAtAction(nameof(GetUserById), new { id = 1 }, user);
    }
}

Correct Code:

[ApiController]
[Route("api/[controller]")]
public class UsersController : ControllerBase
{
    [HttpPost] // <-- CORRECT! Matches the POST request.
    public IActionResult CreateUser([FromBody] NewUserDto user)
    {
        // ... logic to create a user
        return CreatedAtAction(nameof(GetUserById), new { id = 1 }, user);
    }
}

Error Diagnosis: Quick Reference Table

Use this table to quickly narrow down the source of your 405 error.

Advanced Troubleshooting and Best Practices

Verify Docker Networking

Ensure both your Nginx and ASP.NET containers are on the same user-defined Docker network. While `docker-compose` handles this automatically, in manual `docker run` scenarios, you must connect them explicitly. Use `docker network inspect <network_name>` to see which containers are attached.

Enable Detailed ASP.NET Core Logging

In your `appsettings.Development.json`, increase the logging level to see exactly what Kestrel is receiving. This can reveal issues with routing or model binding.

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Debug" // Use Debug or Trace for max detail
    }
  }
}

Then check your application logs with `docker logs your-aspnet-container-name`.

Always Validate Your Nginx Config

Before restarting or reloading your Nginx container, always test your configuration syntax from within the container:

docker exec -it your-nginx-container-name nginx -t

This simple command can save you from downtime caused by a typo in your config files.

Conclusion

The Nginx 405 Method Not Allowed error in a Docker and ASP.NET stack can be a headache, but it's almost always a solvable configuration issue. By methodically checking your `proxy_pass` directive, ensuring all HTTP verbs are forwarded correctly, and finally verifying your ASP.NET controller attributes, you can systematically eliminate the potential causes. Remember the core diagnostic principle: isolate the problem. Test Nginx and your application separately to pinpoint the source of the failure quickly and efficiently.