[BUG] CORS Preflight OPTIONS Requests Fail with 405 from Platform's Nginx Proxy

[JoseArias00]

To Reproduce

1. Deploy a standard ASP.NET Core backend service using a minimal Dockerfile (e.g., from mcr.microsoft.com/dotnet/aspnet:8.0).
2. Deploy a frontend application on a different Dokploy-provided subdomain.
3. In the Dokploy UI, configure a Traefik middleware for the backend service to handle CORS, allowing the frontend's origin.
4. From the frontend app in the browser, make a POST request with Content-Type: application/json to the backend, which triggers a preflight OPTIONS request.

Current vs. Expected behavior

Current behavior:
The browser's preflight OPTIONS request fails with a 405 Not Allowed status. The Server response header is consistently nginx/1.29.0. The Traefik middleware for CORS appears to be completely bypassed, and the request never reaches the application container.

Expected behavior:
The Traefik middleware should intercept the OPTIONS request, respond with a 204 No Content status and the correct Access-Control-* headers. This should allow the browser's subsequent POST request to proceed to the backend service.

Provide environment information

- Operating System: Ubuntu 24.04 
- Dokploy version: 0.22.7
- Cloud Provider (VPS): Hostinger
- Services: ASP.NET Core 8 backend, Vite frontend.

Which area(s) are affected? (Select all that apply)

Traefik

Are you deploying the applications where Dokploy is installed or on a remote server?

Same server where Dokploy is installed

Additional context

Hello team. After extensive debugging, we can confirm the issue is not within the user-deployed application container but seems to be at the platform's ingress level.

Key Findings:

1. The issue is not an internal Nginx proxy. We initially used a custom Dockerfile with Nginx. We proved via runtime logs (cat /etc/nginx/conf.d/default.conf) that the correct nginx.conf file (with CORS rules) was present in the container at runtime, but its directives were being ignored. We have since switched to a minimal ASP.NET base image with no Nginx inside, and the problem persists.

2. The Traefik configuration is correct but is being bypassed. The Server: nginx/1.29.0 header is the smoking gun. Even with no Nginx in my stack, this header is present in the 405 response. This strongly suggests there is a mandatory, global Nginx proxy in front of the user-configured Traefik router that is blocking OPTIONS requests.

Configuration that Fails

Here is the simplified, correct configuration that still results in the error.

Dockerfile:

# Stage 1: Build
FROM [mcr.microsoft.com/dotnet/sdk:8.0](https://mcr.microsoft.com/dotnet/sdk:8.0) AS build
WORKDIR /src
COPY . .
RUN dotnet restore "KDDs.csproj"
RUN dotnet publish "KDDs.csproj" -c Release -o /app/publish --no-restore

# Stage 2: Final
FROM [mcr.microsoft.com/dotnet/aspnet:8.0](https://mcr.microsoft.com/dotnet/aspnet:8.0) AS final
WORKDIR /app
COPY --from=build /app/publish .
ENV ASPNETCORE_URLS=http://+:3000
EXPOSE 3000
ENTRYPOINT ["dotnet", "KDDs.dll"]

Traefik YAML Configuration:

http:
  routers:
    kddsproject-backend-amo83j-router-4: # Name checked during last deploy
      rule: Host(`your-backend-url.traefik.me`)
      service: kddsproject-backend-amo83j-service-4
      entryPoints:
        - web
      middlewares:
        - cors-headers
  
  services:
    # ... service config
    
  middlewares:
    cors-headers:
      headers:
        accessControlAllowCredentials: true
        accessControlAllowHeaders:
          - "Content-Type"
          - "Authorization"
        accessControlAllowMethods:
          - "GET"
          - "POST"
          - "PUT"
          - "DELETE"
          - "OPTIONS"
        accessControlAllowOriginList:
          - "[http://your-frontend-url.traefik.me](http://your-frontend-url.traefik.me)"

Could you please investigate the platform's ingress configuration? Thank you!

Will you send a PR to fix it?

Maybe, need help

[djsisson]

@JoseArias00 traefik is just a go binary, there is no nginx inside of it

[Siumauricio]

As you mentioned @djsisson, we don't really use nginx anywhere. I don't know if your domain is being intercepted by some other service (it's happened to me), but I advise you to check where the domain is and disable any proxies in your DNS manager.

I don't think there's anything to fix here other than checking your services thoroughly. I'll transfer this to the discussions section in case you need further help.

@JoseArias00