Connecting to docker container via telnet

Question

I have a NET Core Console Application which has a socket binded to a port.
I have dockerized this application and i am trying to expose the binded port.
The program shows its output correctly when i use docker attach [instance] .
However when trying to connect with Telnet i get the error:

Error

$ telnet 127.0.0.1 20001
Connecting To 127.0.0.1...Could not open connection to the host, on port 8806: Connect failed

.NET Server

class Program {

    public const int BUFFER_SIZE = 1024;
    public const int EXPOSED_PORT = 20001;
    public const string ADDRESS = "127.0.0.1";
    public const int MAX_CONNECTIONS = 1;

    public static ReadOnlyMemory<byte> CreateMessage(ReadOnlyMemory<byte>request) {
        Memory<byte> response= Encoding.UTF8.GetBytes($"Response from Daemon:{DateTime.Now.ToString()}");
        ///------code------//
        return response;
    }

    static async Task Main(string[] args) {

        Memory<byte> buffer = ArrayPool<byte>.Shared.Rent(BUFFER_SIZE);

        using (Socket serverSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp)) {
            serverSocket.Bind(new IPEndPoint(IPAddress.Parse(ADDRESS), EXPOSED_PORT));
            Console.WriteLine($"Server started at {DateTime.Now.ToShortTimeString()} \r\nListening on port{EXPOSED_PORT}");
            serverSocket.Listen(MAX_CONNECTIONS);

            var client =await serverSocket.AcceptAsync();
            while (true) {
                int rxCount =await  client.ReceiveAsync(buffer, SocketFlags.None);
                if ((char) buffer.Span[0] == 'x'){
                    break;
                }
                await client.SendAsync(CreateMessage(buffer.Slice(0, rxCount)), SocketFlags.None);
            }
        }
        ArrayPool<byte>.Shared.Return(buffer.ToArray());

    }
}

Dockerfile

FROM microsoft/dotnet:latest
WORKDIR /app
COPY ./bin/Release/netcoreapp2.1/publish .
ENTRYPOINT [ "dotnet","DockerContainerDaemon.dll" ]
EXPOSE 20001

docker and build command script

dotnet build -c Release
dotnet publish
docker build --build-arg Port=20001 -t  daemon  .
docker stop inst1
docker rm inst1
docker run --name inst1 -p 20001:20001 -d daemon 

Running docker -ps -a

CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS                      NAMES
f675c3e502ad        daemon              "dotnet DockerContai…"   5 seconds ago       Up 3 seconds        0.0.0.0:20001->20001/tcp   inst1

P.S As someone pointed out i tried exposing with both 127.0.0.1 and 0.0.0.0 from the .NET Application. to no avail.

Answer 1

Try to bind the application to 0.0.0.0 instead of 127.0.0.1. This will solve your problem.