/// <summary>
/// A callback that is invoked prior to sending the proxied request. All HttpRequestMessage fields are
/// initialized except RequestUri, which will be initialized after the callback if no value is provided.
/// See <see cref="RequestUtilities.MakeDestinationAddress(string, PathString, QueryString)"/> for constructing a custom request Uri.
/// The string parameter represents the destination URI prefix that should be used when constructing the RequestUri.
/// The headers are copied by the base implementation, excluding some protocol headers like HTTP/2 pseudo headers (":authority").
/// </summary>
/// <param name="httpContext">The incoming request.</param>
/// <param name="proxyRequest">The outgoing proxy request.</param>
/// <param name="destinationPrefix">The uri prefix for the selected destination server which can be used to create the RequestUri.</param>
public virtual ValueTask TransformRequestAsync(HttpContext httpContext, HttpRequestMessage proxyRequest, string destinationPrefix)
{
foreach (var header in httpContext.Request.Headers)
{
var headerName = header.Key;
var headerValue = header.Value;
if (RequestUtilities.ShouldSkipRequestHeader(headerName))
{
continue;
}
RequestUtilities.AddHeader(proxyRequest, headerName, headerValue);
}
// https://datatracker.ietf.org/doc/html/rfc7230#section-3.3.3
// If a message is received with both a Transfer-Encoding and a
// Content-Length header field, the Transfer-Encoding overrides the
// Content-Length. Such a message might indicate an attempt to
// perform request smuggling (Section 9.5) or response splitting
// (Section 9.4) and ought to be handled as an error. A sender MUST
// remove the received Content-Length field prior to forwarding such
// a message downstream.
if (httpContext.Request.Headers.ContainsKey(HeaderNames.TransferEncoding) &&
httpContext.Request.Headers.ContainsKey(HeaderNames.ContentLength))
{
proxyRequest.Content?.Headers.Remove(HeaderNames.ContentLength);
}
// https://datatracker.ietf.org/doc/html/rfc7540#section-8.1.2.2
// The only exception to this is the TE header field, which MAY be
// present in an HTTP/2 request; when it is, it MUST NOT contain any
// value other than "trailers".
if (ProtocolHelper.IsHttp2OrGreater(httpContext.Request.Protocol))
{
var te = httpContext.Request.Headers.GetCommaSeparatedValues(HeaderNames.TE);
if (te != null)
{
for (var i = 0; i < te.Length; i++)
{
if (string.Equals(te[i], "trailers", StringComparison.OrdinalIgnoreCase))
{
var added = proxyRequest.Headers.TryAddWithoutValidation(HeaderNames.TE, te[i]);
Debug.Assert(added);
break;
}
}
}
}
return(default);
