Why does a Docker container running a server expose port to the outside world even though said port is blocked by iptables?

english
#1

I'm having a problem with MySQL running inside a Docker container. My testing image is built from the following Dockerfile:

# See: https://index.docker.io/u/brice/mysql/

FROM ubuntu:12.10
MAINTAINER Joni Kahara <joni.kahara@async.fi> 

# Because docker replaces /sbin/init: https://github.com/dotcloud/docker/issues/1024
RUN dpkg-divert --local --rename --add /sbin/initctl
RUN ln -s /bin/true /sbin/initctl

RUN apt-get update
RUN apt-get upgrade -y

RUN apt-get -y install mysql-server

RUN sed -i -e"s/^bind-address\s*=\s*127.0.0.1/bind-address = 0.0.0.0/" /etc/mysql/my.cnf

RUN /usr/bin/mysqld_safe & \
    sleep 10s && \
    mysql -e "GRANT ALL ON *.* to 'root'@'%'; FLUSH PRIVILEGES;"

EXPOSE 3306

VOLUME ["/var/lib/mysql", "/var/log/mysql"]

CMD ["mysqld_safe"]

After building an image from the file above, I run it with:

docker run -p 3306:3306 asyncfi/magento-mysql

After which everything is swell and I can log in to this instance of MySQL from the local machine. However, I can also log in from any other machine.

I have set up my firewall to filter everything except traffic coming in to specific ports ("hidden" SSH, HTTP, HTTPS), and this filtering does in fact seem to work; if I for example run a Django development server on port 1234 then I am able to connect from the local machine, but not from outside. So the firewall seems to be filtering packets when they're destined to a server that is running as a "plain" process, but not when the server is running inside a container.

iptables -L -v --line-numbers says the following:

Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
num   pkts bytes target     prot opt in     out     source               destination
1     2265  107K ACCEPT     all  --  lo     any     anywhere             anywhere
2     240K  319M ACCEPT     all  --  any    any     anywhere             anywhere             ctstate RELATED,ESTABLISHED
3       14  1040 ACCEPT     tcp  --  any    any     anywhere             anywhere             tcp dpt:<REDACTED>
4       21  1092 ACCEPT     tcp  --  any    any     anywhere             anywhere             tcp dpt:http
5        6   360 ACCEPT     tcp  --  any    any     anywhere             anywhere             tcp dpt:https
6      538 34656 LOG        all  --  any    any     anywhere             anywhere             limit: avg 5/min burst 5 LOG level debug prefix "iptables DROP: "
7      551 35424 DROP       all  --  any    any     anywhere             anywhere

Chain FORWARD (policy ACCEPT 5 packets, 296 bytes)
num   pkts bytes target     prot opt in     out     source               destination
1        0     0 ACCEPT     all  --  docker0 docker0  anywhere             anywhere
2     6752  396K ACCEPT     all  --  docker0 !docker0  anywhere             anywhere
3     125K  188M ACCEPT     all  --  any    docker0  anywhere             anywhere             ctstate RELATED,ESTABLISHED

Chain OUTPUT (policy ACCEPT 51148 packets, 14M bytes)
num   pkts bytes target     prot opt in     out     source               destination

Docker version is:

Client version: 0.7.3
Go version (client): go1.2
Git commit (client): 8502ad4
Server version: 0.7.3
Git commit (server): 8502ad4
Go version (server): go1.2
Last stable version: 0.7.3

Why is the MySQL port exposed to the outside world?

#2

Thanks to #docker IRC channel users Michael Crosby and Paul Czar I am now able to answer my own question. The problem lies in the fact that I ran the container like this:

docker run -p 3306:3306 asyncfi/magento-mysql

This publishes the container's port to all interfaces of the host machine, which is definitely not what I was looking for at this time. To bind only to localhost, it was necessary to run the container as follows:

docker run -p 127.0.0.1:3306:3306 asyncfi/magento-mysql

Also the EXPOSE line in Dockerfile is not necessary as the "expose" mechanism is used to link containers.