Question 83623 in Docker Question posted in
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TCP server started but connection refused inside a container – Docker

I have a golang application which starts a TCP server on port 8080. Every thing works fine when I run the app native.

However when I run it as a container, I am unable to even telnet to the port from within the container itself.

docker ps 

9bb08785b728   dp_local   "/bin/dragonpit-linux"   8 seconds ago   Up 7 seconds   8080-8081/tcp   youthful_villani


docker exec -it youthful_villani sh

/ # telnet localhost 8080
telnet: can't connect to remote host (127.0.0.1): Connection refused

Note: used 0.0.0.0 as well as 127.0.0.1 in place of localhost

TCP Server starting code

        var err error
        var lc net.ListenConfig
        th.listener, err = lc.Listen(ctx, "tcp", "0.0.0.0:8080")
        if err != nil {
            return err
        }
        clog.Info(ctx, "tcp protocol listening", "listenAddr", th.addr)

I hard-coded the address to see whats the issue.

My Dockerfile

FROM golang:1.18.0 as builder
RUN mkdir -p /build/
ADD .   /build
WORKDIR /build
RUN CGO_ENABLED=0 GOOS=linux go build -o tcp_server


FROM alpine:latest
EXPOSE 8080 8081
RUN mkdir -p /server/config
ADD config  /server/config/
ENV SMC_PATH /
COPY --from=builder /build/tcp_server /bin/
RUN apk update
RUN apk add busybox-extras 
ENTRYPOINT ["/bin/tcp_server"]

Output of docker-inspect

 "NetworkSettings": {
            "Bridge": "",
            "SandboxID": "a70f476a8a7376b1e5a935b67170145f2222e059c5b2a1a63da50519a491babf",
            "HairpinMode": false,
            "LinkLocalIPv6Address": "",
            "LinkLocalIPv6PrefixLen": 0,
            "Ports": {
                "8080/tcp": null,
                "8081/tcp": null
            },
            "SandboxKey": "/var/run/docker/netns/a70f476a8a73",
            "SecondaryIPAddresses": null,
            "SecondaryIPv6Addresses": null,
            "EndpointID": "7b6b9d0d5f83b8136919ac0f765167f6d380a8d836f460a0243bedeb3489a013",
            "Gateway": "172.17.0.1",
            "GlobalIPv6Address": "",
            "GlobalIPv6PrefixLen": 0,
            "IPAddress": "172.17.0.2",
            "IPPrefixLen": 16,
            "IPv6Gateway": "",
            "MacAddress": "02:42:ac:11:00:02",
            "Networks": {
                "bridge": {
                    "IPAMConfig": null,
                    "Links": null,
                    "Aliases": null,
                    "NetworkID": "16563fdda1d5059bb6e2800455f2e87ac8d02e040386eae595a215692e849d76",
                    "EndpointID": "7b6b9d0d5f83b8136919ac0f765167f6d380a8d836f460a0243bedeb3489a013",
                    "Gateway": "172.17.0.1",
                    "IPAddress": "172.17.0.2",
                    "IPPrefixLen": 16,
                    "IPv6Gateway": "",
                    "GlobalIPv6Address": "",
                    "GlobalIPv6PrefixLen": 0,
                    "MacAddress": "02:42:ac:11:00:02",
                    "DriverOpts": null
                }
            }
        }
    }

Docker running command and starting logs

docker run dp_local  
                       
25T14:59:46.367075600Z","caller":"build/main.go:24","msg":"Starting listening","listenAddr":"0.0.0.0:8080"

Edit:
I just printed out the Addr().String() of TCP Listener, I got this

{"level":"info","ts":"2022-04-25T15:44:28.952095700Z","caller":"server/tcp.go:65","msg":"[::]:8080"}
Tags:

2

Answers


  1. Chosen as BEST ANSWER
    0

    Answering my question here. It was actually a TCP parameter which was set incorrectly and caused issues. The culprit was MTU. For reference: https://www.baeldung.com/cs/tcp-max-packet-size#:~:text=The%20maximum%20size%20of%20a,size%20should%20never%20exceed%20MTU.


  2. 0

    Two Assumptions based on your question

    1. You want to expose ports 8080 and 8081

    EXPOSE 8080 8081
    2. You are running the docker image and the client on same machine

    In that case you need to bind the Docker container ports to ports on an interface on the local machine. Let’s use localhost. You docker run command should look like

    docker run -p 127.0.0.1:8080:8080/tcp -p 127.0.0.1:8081:8081/tcp dp_local

    This will bind container ports 8080 to 8080 and 8081 to 8081 on localhost of local machine.
    Read more here

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