TCP/Telnet from inside docker container - docker

Question

I have a 'mcr.microsoft.com/dotnet/aspnet:5.0-buster-slim' Docker container, with 2 things on it:
-a custom SDK running on it
-a dotnet5 application that connects to the SDK with TCP
If I connect the the Docker's bash, I can use telnet localhost 54321 to connect to my SDK successfully
If I run the Windows SDK version on my development computer (Windows), and I run my application IIS Express instead of Docker, I can successfully connect with a telnet library (host 'localhost', port '54321'), this works
However, I want to run both the SDK and my dotnet application in a Docker container, and when I try to connect from inside the container (the same thing that works on the IIS hosted version), this does not work. By running 'telnet localhost 54321' from the docker container commandline I can confirm that the SDK is running. What am I doing wrong?
Dockerfile:
FROM mcr.microsoft.com/dotnet/aspnet:5.0-buster-slim AS base
RUN apt-get update && apt-get install -y telnet
RUN apt-get update && apt-get install -y libssl1.1
RUN apt-get update && apt-get install -y libpulse0
RUN apt-get update && apt-get install -y libasound2
RUN apt-get update && apt-get install -y libicu63
RUN apt-get update && apt-get install -y libpcre2-16-0
RUN apt-get update && apt-get install -y libdouble-conversion1
RUN apt-get update && apt-get install -y libglib2.0-0
RUN mkdir /sdk
COPY ["Server/Sdk/SomeSDK*", "sdk/"]
WORKDIR /app
EXPOSE 80
EXPOSE 443
FROM mcr.microsoft.com/dotnet/sdk:5.0 AS build
WORKDIR /src
COPY ["Server/MyProject.Server.csproj", "Server/"]
COPY ["Shared/MyProject.Shared.csproj", "Shared/"]
COPY ["Client/MyProject.Client.csproj", "Client/"]
RUN dotnet restore "Server/MyProject.Server.csproj"
COPY . .
WORKDIR "/src/Server"
RUN dotnet build "MyProject.Server.csproj" -c Release -o /app/build
FROM build AS publish
RUN dotnet publish "MyProject.Server.csproj" -c Release -o /app/publish
FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "MyProject.Server.dll"]
Code (working on IIS, when SDK is running on Windows, but not when both SDK and code are running inside the same Docker container):
var telnetClient = new TelnetClient("localhost", 54321, TimeSpan.FromSeconds(1), new CancellationToken());
await telnetClient.Connect();
Thread.Sleep(2000);
await telnetClient.Send("init");
Command line (working BOTH from windows CLI, and from Docker bash (so also when the code is not working, this is working):
$telnet localhost 54321
$init
The issue might be (but I'm not sure, as I've received this result from using direct command line '$telnet localhost 54321' from within dotnet: 'telnet: connection refused by remote host'

Answer 1

Make sure you use in your docker run command also --network=host to use the same network if you want to reach out of the container or create a bridge with --network=bridge (for example with another container).
By default the Docker container is spawn on a separate, dedicated and private subnet on your machine (mostly 172.17.0.0/16) which is different from your machine's default/local subnet (127.0.0.0/8).
For connecting into the host's subnet in this case 127.0.0.0/8 you need --network=host. For communication within the same container though it's not necessary and works out of the box.
For accessing the service in container from the outside you need to make sure you have your application's port published either with --publish HOST_PORT:DOCKER_PORT or --publish-all (random port(s) on host are then assigned, check with docker ps).
Host to container (normal)
# host
telnet <container's IP> 8000 # connects, typing + return shows in netcat stdout
# container
docker run --rm -it --publish 8000:8000 alpine nc -v -l -p 8000
Container to host (normal)
# host
nc -v -l -p 8000
# container, docker run -it alpine
apk add busybox-extras
telnet localhost 8000 # hangs, is dead
Container to host (on host network)
# host
nc -v -l -p 8000
# container, docker run -it --network=host alpine
apk add busybox-extras
telnet localhost 8000 # connects, typing + return shows in netcat stdout
Within container
# start container
docker run -it --name alpine alpine
apk add busybox-extras
# exec into container to create service on 4000
docker exec -it alpine nc -v -l -p 4000
# exec into container to create service on 5000
docker exec -it alpine nc -v -l -p 5000
# telneting from the original terminal (the one with apk add)
telnet localhost 4000 # connects, works
telnet localhost 5000 # connects, works