[docker] How to update /etc/hosts file in Docker image during "docker build"

I want to update my /etc/hosts file during "docker build".

I added below line in Dockerfile but it's neither updating /etc/hosts file nor giving any error.

RUN echo "192.168.33.11    mynginx" >> /etc/hosts

I need to update /etc/hosts. Can anyone suggest on this?

Complete Answer

1. Prepare your own hosts file you wish to add to docker container;

1.2.3.4 abc.tv
5.6.7.8 domain.xyz
1.3.5.7 odd.org
2.4.6.8 even.net

2. COPY your hosts file into the container by adding the following line in the Dockerfile

COPY hosts /etc/hosts_extra

3. If you know how to use ENTRYPOINT or CMD or CRON job then incorporate the following command line into it or at least run this inside the running container:

cat /etc/hosts_extra >> etc/hosts;

4. You cannot add the following in the Dockerfile because the modification will be lost:

RUN cat /etc/hosts_extra >> etc/hosts;

With a more recent version of docker, this could be done with docker-compose and its extra_hosts directive

Add hostname mappings.
Use the same values as the docker run client --add-host parameter (which should already be available for docker 1.8).

extra_hosts:
 - "somehost:162.242.195.82"
 - "otherhost:50.31.209.229"

In short: modify /etc/hosts of your container when running it, instead of when building it.

With Docker 17.x+, you have a docker build --add-host mentioned below, but, as commented in issue 34078 and in this answer:

The --add-host feature during build is designed to allow overriding a host during build, but not to persist that configuration in the image.

The solutions mentioned do refer the docker-compose I was suggesting above:

• Run an internal DNS; you can set the default DNS server to use in the daemon; that way every container started will automatically use the configured DNS by default

• Use docker compose and provide a docker-compose.yml to your developers.
  The docker compose file allows you to specify all the options that should be used when starting a container, so developers could just docker compose up to start the container with all the options they need to set.

Following worked for me by mounting the file during docker run instead of docker build

docker service create --name <name>  --mount type=bind,source=/etc/hosts,dst=/etc/hosts   <image>

If this is useful for anyone, the HOSTALIASES env variable worked for me:

echo "fakehost realhost" > /etc/host.aliases
export HOSTALIASES=/etc/host.aliases

You can do with the following command at the time of running docker

docker run [OPTIONS] --add-host example.com:127.0.0.1 <your-image-name>:<your tag>

Here I am mapping example.com to localhost 127.0.0.1 and its working.

I'm using AWS Elasticbeanstalk + Docker + Supervisord.

Quick answer

Just add some code in Dockerfile.

CMD echo 123.123.123.123 this_is_my.host >> /etc/hosts; supervisord -n;

I think docker recently added the --add-host flag to docker build which is really great.

[Edit] So this feature was updated on 17.04.0-ce

For more detail on how to use docker build with the --add-host flag please visit: https://docs.docker.com/edge/engine/reference/commandline/build/

You can use the --add-host option during docker run.

For your case use: docker run --add-host mynginx:192.168.33.11 [image_name]:[tag]

This will update your /etc/hosts

you can check it by using following commands:

• docker exec -it [container_id] sh

if sh doesnt work for you ,then you can try bash or /bin/sh or /bin/bash

• cd etc
• cat hosts

You can not modify the host file in the image using echo in RUN step because docker daemon will maintain the file(/etc/hosts) and its content(hosts entry) when you start a container from the image.

However following can be used to achieve the same:

ENTRYPOINT ["/bin/sh", "-c" , "echo 192.168.254.10   database-server >> /etc/hosts && echo 192.168.239.62   redis-ms-server >> /etc/hosts && exec java -jar ./botblocker.jar " ]

Key to notice here is the use of exec command as docker documentation suggests. Use of exec will make the java command as PID 1 for the container. Docker interrupts will only respond to that.

See https://docs.docker.com/engine/reference/builder/#entrypoint

Since this still comes up as a first answer in Google I'll contribute possible solution.

Command taken from here suprisingly worked for me (Docker 1.13.1, Ubuntu 16.04) :

docker exec -u 0 <container-name> /bin/sh -c "echo '<ip> <name> >> /etc/hosts"

Just a quick answer to run your container using:

docker exec -it <container name> /bin/bash

once the container is open:

cd ..

then

`cd etc`

and then you can

cat hosts

or:

apt-get update
apt-get vim

or any editor you like and open it in vim, here you can modify say your startup ip to 0.0.0.0

Tis is me Dockefile
FROM XXXXX
ENV DNS_1="10.0.0.1 TEST1.COM"
ENV DNS_1="10.0.0.1 TEST2.COM" 
CMD ["bash","change_hosts.sh"]`

#cat change_hosts.sh
su - root -c "env | grep DNS | akw -F "=" '{print $2}' >> /etc/hosts"

• info
• user must su