yarp host header override

Mon, 10 Feb 2025 00:00:00 +0000

About

I was curious to see how the yarp proxy handles the host header when proxying.

Learning

They copy over all the original request headers to a new http message. Afterwards, a set of transformers are applied to the http message.

One of these transformers is the RequestHeaderOriginalHostTransform which overrides the Host header with the destination host.

  else if (existingHost is null || string.Equals(originalHost, existingHost, StringComparison.Ordinal))
  {
      // Use the host from destination configuration (which may be null) if either:
      // * there is no host header set, or
      // * the original host header is being suppressed and has not been modified by the transform pipeline
      context.ProxyRequest.Headers.Host = destinationConfigHost;
  }

Allowed HTTP Request Header Types

Sun, 09 Feb 2025 00:00:00 +0000

About

I'm trying to build a reverse proxy but it seems that there are some constraints on the headers that we can add to a HttpRequestMessage.

open System.Net.Http

let client = new HttpClient()

let request = new HttpRequestMessage(HttpMethod.Post, "https://example.com")

let tryAddHeader (message : HttpRequestMessage) (k : string) (v : string) =
    match message.Headers.TryAddWithoutValidation(k,v) with
    | true -> ()
    | false -> eprintfn $"Failed to add header: {k}."

let addHeader (message : HttpRequestMessage) (k : string) (v : string) =
    message.Headers.Add(k,v)

tryAddHeader request "Content-Type" "text/plain" // XXX: Failed to add header: Content-Type.
tryAddHeader request "Content-Length" "7" // XXX: Failed to add header: Content-Length.

addHeader request "Content-Length" "7" // XXX: throws an exception.

let response = client.Send(request)

Allowed Headers

The TryAddWithoutValidation will verify whether the header is allowed before adding it.

Csharp on August Feng

yarp host header override

About

Learning

Allowed HTTP Request Header Types

About

Allowed Headers