Host header transform with invalid value results in duplicate host headers

[samsp-msft]

Bug found by customer with Yarp. See dotnet/yarp#334 for full thread.

Bug Description

When using route transforms, if the RequestHeader is set to Host and the set option is set to an invalid value, such as baz,bar, the resulting request forwarded to the backend can have multiple Host headers. However, if a valid value is specified, the behavior is correct (i.e. only one Host header is forwarded, with the value specified in the transform).

Reproducing

Create a config file with a transform specifying an invalid value for the Host header. My transform looks like:

"Transforms": [
  {
    "RequestHeader": "Host",
    "Set": "site,baz"
  },
]

The forwarded request will look like:

[ghost]

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[wfurt]

This may be related to #35126. I think this may not show up in enumeration and then YARP adds them even if the (invalid) header already exist.

[scalablecory]

We think this may be fixed by non-validated enumeration. This should be a pretty rare bug, though, so moving to Future seems reasonable -- @samsp-msft can you comment on how important this is for YARP, and we can move into 5.0 if needed.

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[MihaZupan]

The issue still exists since we check for the Host existing with validation.

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnection.cs

Line 347 in c9af66c

if (!request.HasHeaders || request.Headers.Host is null)

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http2Connection.cs

Line 1500 in c9af66c

if (request.HasHeaders && request.Headers.Host is string host)

runtime/src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/Http3RequestStream.cs

Line 634 in c9af66c

if (request.HasHeaders && request.Headers.Host is string host)