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How to reach docker container `localhost` from Mac?

(Cookieisaac) #1

[How to reach docker container localhost from Mac?]

This is a problem specific for Docker for Mac. I have posted it on Stackoverflow as well. My Docker for Mac version is 18.06.0-ce.

I have a TCP server binding to localhost:5005 running inside Docker for Mac. (For security reason, I must not bind to

I have a TCP client sending request to this server from my Mac (not inside the docker container).

My question is, how do I make it work?

In Linux Docker, I would simply use --net=host so the server binds to my host lo interface, but it seems that Docker for Mac runs on a managed VM, so the host network behavior is different behavior.

To illustrate my point:

On MacBook

It simply would not work

[me@MacBook App]$ docker run -v `pwd`:/App -p nitincypher/docker-ubuntu-python-pip /App/
[me@MacBook App]$ ./ 
Client received data: 

On Linux

In comparison, it would be trivial to do on Linux by using host network mode. Since I’m using my Linux’s lo interface as my container lo interface.

[me@Linux App]$ docker run -v `pwd`:/App --net=host nitincypher/docker-ubuntu-python-pip /App/
Server Connection address: ('', 52172)
Server received data: Hello, World!
[me@Linux App]$ ./ 
Client received data: Hello, World!

My Simulated Server Code

Requirement: It MUST bind to localhost, and nothing else. So I cannot change it to

#!/usr/bin/env python

import socket

TCP_IP = 'localhost'
TCP_PORT = 5005
BUFFER_SIZE = 20  # Normally 1024, but we want fast response

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((TCP_IP, TCP_PORT))

conn, addr = s.accept()
print 'Server Connection address:', addr
while 1:
    data = conn.recv(BUFFER_SIZE)
    if not data: break
    print "Server received data:", data
    conn.send(data)  # echo

My Simulated Client Code

Requirement: It MUST be ran on MacBook, since the real client is written in CPP and compiled to run only on MacBook.

#!/usr/bin/env python

import socket

TCP_IP = 'localhost'
TCP_PORT = 5005
MESSAGE = "Hello, World!"

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((TCP_IP, TCP_PORT))
data = s.recv(BUFFER_SIZE)

print "Client received data:", data

(Cookieisaac) #2

I have posted the following workaround to my original Stackoverflow post, and the master copy of my answer will be maintained there. Below is the copy/paste to the original reponse.

Here’s a working solution. The basic idea is to use SSH tunneling to do the port forwarding.

High Level Idea

  • You first need to build a docker image to support SSH access, because
  1. ubuntu image doesn’t have a sshd out of box, and also
  2. you will need to know the password of root of your running container.
  • Then you will spin up your container as what you would normally do except that you are doing that based on the new image you created.
  • You create a SSH tunneling session from your MacBook, then you run your client on MacBook as you would normally do.

For reference, the command for SSH tunneling can be found here, the process of creating a sshd docker image is explained here, and how to ssh into docker container is explained here.
(Docker forum doesn’t allow me to post more than two URLs, so please see the link in my original stackoverflow answer)


  1. Create a Docker file Dockerfile

     #Use whatever image you are using on Docker Linux , say "FROM ubuntu:16.04"
     FROM nitincypher/docker-ubuntu-python-pip
     RUN apt-get update && apt-get install -y openssh-server
     RUN mkdir /var/run/sshd
     RUN echo 'root:screencast' | chpasswd
     RUN sed -i 's/PermitRootLogin prohibit-password/PermitRootLogin yes/' /etc/ssh/sshd_config
     # SSH login fix. Otherwise user is kicked off after login
     RUN sed 's@session\s*required\s* optional' -i /etc/pam.d/sshd
     ENV NOTVISIBLE "in users profile"
     RUN echo "export VISIBLE=now" >> /etc/profile
     EXPOSE 22
     CMD ["/usr/sbin/sshd", "-D"]
  2. Create a Docker Image from the Dockerfile

     [me@MacBook App]$ docker build -t my_ssh_python .
  3. Spin up your server container

     [me@MacBook App]$ docker run -d -P -v `pwd`:/App --name myserver my_ssh_python
  4. Start your server inside the container

     [me@MacBook App]$ docker exec myserver /App/
  5. Create a SSH tunnel

     [me@MacBook App]$ ssh root@`hostname` -p `docker port myserver 22 | awk -F ":" '{print $2}'` -L 8000:localhost:8000 -N
     #Password is "screencast" as you built in Dockerfile

    Note that

    a. You have to use the IP address of your MacBook instead of your docker container’s IP address.

    b. You will use the port where the default container ssh port 22 is mapped to on host

    c. In tunneling -L 8000:localhost:8000, you are saying forward anything from your MacBook 8000 (the first 8000) to Docker container’s localhost at port 8000

  6. Now you can use you client locally

     [me@MacBook App]$ ./ 
     Client received data: Hello, World!

    And on server side, you can see

     Server Connection address: ('', 55396)
     Server received data: Hello, World!