Trying to connect to a python socket inside a docker container from host

Question

I have to implement to my Distributed systems class the Berkeley Algorithm and I chose to do it in python with sockets. The master is supposed to run in the host and the slaves in docker containers.

The closest I got from connecting from host (as a master) to the container (as the slave) was exposing the ports with the -p 9000:9000 flag the running the container, the host connects successfully to the container but doesn't receive or send anything (the same thing for the container) with that I have came to the conclusion that the python socket inside the process simply is not receiving packets from the port. I have already tried using -net=host flag but the host simply can't find the container. One progress that I had was to instantiate two docker containers and pinging one from another using the hostname provided in /etc/hosts but this is not what I really want.

I have the whole code in github if you need the source. The code is commented in English, but the documentation is in Portuguese

Summarising all I want to do is to open a socket with python inside a docker container and be able to reach in the host machine, what kind of network configuration do I need to do be able to do that?

EDIT: More info

The following bash script is used to instantiate three docker containers then execute a command into each one of them to clone my repo, cd into it and into a test folder containing a bash to execute a slave and then start the master at host:

docker run -it -d -p 127.0.0.1:9000:9000/tcp --name slave1 python bash
docker run -it -d -p 127.0.0.1:9001:9001/tcp --name slave2 python bash
docker run -it -d -p 127.0.0.1:9002:9002/tcp --name slave3 python bash

docker exec -t -d slave1 bash -c 'git clone https://github.com/guilhermePaciulli/BerkeleyAlgorithm.git;cd BerkeleyAlgorithm;git pull;cd test;bash slave_1.sh'
sleep 1
docker exec -t -d slave2 bash -c 'git clone https://github.com/guilhermePaciulli/BerkeleyAlgorithm.git;cd BerkeleyAlgorithm;git pull;cd test;bash slave_2.sh'
sleep 1
docker exec -t -d slave3 bash -c 'git clone https://github.com/guilhermePaciulli/BerkeleyAlgorithm.git;cd BerkeleyAlgorithm;git pull;cd test;bash slave_3.sh'
sleep 1

bash test/master.sh

To start each instance I use another bash command

• To instantiate the slave I use:

python ../main.py -s 127.0.0.1:9000 175 logs/slave_log_1.txt

The -s is a flag to tell the main.py class that this is a slave, the 127.0.0.1:9000 are the ip and port that this slave is going to listen (and the master is going to connect) and the rest are just configurations (this example is used for the first slave).

• And to instantiate the master I use:

python ./main.py -m 127.0.0.1:8080 185 15 test/slaves.txt test/logs/master_log.txt

Just like the slave the -m tells main that this is a master, 127.0.0.1:8080 are the ip and port that the master is going to connect to the slave and the rest are just configurations.

Answer 1

When you run a server-type process inside a Docker container, it needs to be configured to listen on the special "all interfaces" address 0.0.0.0. Each container has its own notion of localhost or 127.0.0.1, and if you set a process to listen or bind to 127.0.0.1, it can only be reached from its own localhost which is different from all other containers' localhost and the host's localhost.

In the server command you show, you'd run something like

python ../main.py -s 0.0.0.0:9000 175 logs/slave_log_1.txt

(Stringly consider building a Dockerfile to describe how to build and start your image. Starting a bunch of empty containers, git clone into each, and then manually launching processes is a lot of manual work to be lost as soon as you docker rm the container.)

Answer 2

I looked through your code and I see you creating the server socket and binding it to a port and listening, but I could not find where you call socket.accept() method ?