Duplicate http header: Connection

[doghappy]

I have a nodejs service which hosted on IIS, I got duplicate http header Connection when websocket handshake

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Server: Microsoft-IIS/10.0
Connection: Upgrade
Sec-WebSocket-Accept: NFMfylbwIygrHXmqMg4BFjGQITw=
Connection: Upgrade
X-Powered-By: ASP.NET
Date: Fri, 31 Mar 2023 14:58:21 GMT
EndTime: 22:58:21.838
ReceivedBytes: 109
SentBytes: 0

I also have a .NET client, it will connect to that service by websocket. but the implementation of ClientWebSocket of Microsoft will throw an exception after received that response:

        ValidateHeader(response.Headers, HttpKnownHeaderNames.Connection, "Upgrade");

        private static void ValidateHeader(HttpHeaders headers, string name, string expectedValue)
        {
            if (headers.NonValidated.TryGetValues(name, out HeaderStringValues hsv))
            {
                if (hsv.Count == 1)
                {
                    foreach (string value in hsv)
                    {
                        if (string.Equals(value, expectedValue, StringComparison.OrdinalIgnoreCase))
                        {
                            return;
                        }
                        break;
                    }
                }

                throw new WebSocketException(WebSocketError.HeaderError, SR.Format(SR.net_WebSockets_InvalidResponseHeader, name, hsv));
            }

            throw new WebSocketException(WebSocketError.Faulted, SR.Format(SR.net_WebSockets_MissingResponseHeader, name));
        }

The source code for the above can be found here: https://source.dot.net/#System.Net.WebSockets.Client/System/Net/WebSockets/WebSocketHandle.Managed.cs,9d3cf4c1f519f3a3,references

The expected Connection: Upgrade should be only one. Is this caused by iisnode?

web.config

<configuration>
    <system.webServer>
        <handlers>
            <add name="iisnode" path="v4-ws.js" verb="*" modules="iisnode" />
        </handlers>

        <rewrite>
            <rules>
                <rule name="DynamicContent">
                    <conditions>
                        <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="True"/>
                    </conditions>
                    <action type="Rewrite" url="v4-ws.js"/>
                </rule>

                <rule name="SocketIO" patternSyntax="ECMAScript">
                    <match url="socket.io.+"/>
                    <action type="Rewrite" url="v4-ws.js"/>
                </rule>
            </rules>
        </rewrite>

        <iisnode nodeProcessCommandLine="&quot;%programfiles%\nodejs\node.exe&quot;" />

        <webSocket enabled="true" receiveBufferLimit="4194304" pingInterval="00:00:10"/>
    </system.webServer>
</configuration>

Env:

• iisnode: v0.2.21
• socket.io: v4
• IIS: v10