Docker: Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock

Question

I am new to docker. I just tried to use docker in my local machine(Ubuntu 16.04) with Jenkins.

I configured a new job with below pipeline script.

Answer 1

Simply adding docker as a supplementary group for the jenkins user

sudo usermod -a -G docker jenkins

is not always enough when using a Docker image as the Jenkins Agent. That is, if your Jenkinsfile starts with pipeline{agent{dockerfile or pipeline{agent{image:

pipeline {
    agent {
        dockerfile {
            filename 'Dockerfile.jenkinsAgent'
        }
    }
    stages {

This is because Jenkins performs a docker run command, which results in three problems.

• The Agent will (probably) not have the Docker programs installed.
• The Agent will not have access to the Docker daemon socket, and so will try to run Docker-in-Docker, which is not recommended.
• Jenkins gives the numeric user ID and numeric group ID that the Agent should use. The Agent will not have any supplementary groups, because docker run does not do a login to the container (it's more like a sudo).

Installing Docker for the Agent

Making the Docker programs available within the Docker image simply requires running the Docker installation steps in your Dockerfile:

# Dockerfile.jenkinsAgent
FROM debian:stretch-backports
# Install Docker in the image, which adds a docker group
RUN apt-get -y update && \
 apt-get -y install \
   apt-transport-https \
   ca-certificates \
   curl \
   gnupg \
   lsb-release \
   software-properties-common

RUN curl -fsSL https://download.docker.com/linux/debian/gpg | apt-key add -
RUN add-apt-repository \
   "deb [arch=amd64] https://download.docker.com/linux/debian \
   $(lsb_release -cs) \
   stable"

RUN apt-get -y update && \
 apt-get -y install \
   docker-ce \
   docker-ce-cli \
   containerd.io

...

Sharing the Docker daemon socket

As has been said before, fixing the second problem means running the Jenkins Docker container so it shares the Docker daemon socket with the Docker daemon that is outside the container. So you need to tell Jenkins to run the Docker container with that sharing, thus:

pipeline {
    agent {
        dockerfile {
            filename 'Dockerfile.jenkinsAgent'
            args '-v /var/run/docker.sock:/var/run/docker.sock'
        }
    }

Setting UIDs and GIDs

The ideal fix to the third problem would be set up supplementary groups for the Agent. That does not seem possible. The only fix I'm aware of is to run the Agent with the Jenkins UID and the Docker GID (the socket has group write permission and is owned by root.docker). But in general, you do not know what those IDs are (they were allocated when the useradd ... jenkins and groupadd ... docker ran when Jenkins and Docker were installed on the host). And you can not simply tell Jenkins to user user jenkins and group docker

args '-v /var/run/docker.sock:/var/run/docker.sock -u jenkins:docker'

because that tells Docker to use the user and group that are named jenkins and docker within the image, and your Docker image probably does not have the jenkins user and group, and even if it did there would be no guarantee it would have the same UID and GID as the host, and there is similarly no guarantee that the docker GID is the same

Fortunately, Jenkins runs the docker build command for your Dockerfile in a script, so you can do some shell-script magic to pass through that information as Docker build arguments:

pipeline {
    agent {
        dockerfile {
            filename 'Dockerfile.jenkinsAgent'
            additionalBuildArgs  '--build-arg JENKINSUID=`id -u jenkins` --build-arg JENKINSGID=`id -g jenkins` --build-arg DOCKERGID=`stat -c %g /var/run/docker.sock`'
            args '-v /var/run/docker.sock:/var/run/docker.sock -u jenkins:docker'
        }
    }

That uses the id command to get the UID and GID of the jenkins user and the stat command to get information about the Docker socket.

Your Dockerfile can use that information to setup a jenkins user and docker group for the Agent, using groupadd, groupmod and useradd:

# Dockerfile.jenkinsAgent
FROM debian:stretch-backports
ARG JENKINSUID
ARG JENKINSGID
ARG DOCKERGID
...
# Install Docker in the image, which adds a docker group
RUN apt-get -y update && \
 apt-get -y install \
   apt-transport-https \
   ca-certificates \
   curl \
   gnupg \
   lsb-release \
   software-properties-common

RUN curl -fsSL https://download.docker.com/linux/debian/gpg | apt-key add -
RUN add-apt-repository \
   "deb [arch=amd64] https://download.docker.com/linux/debian \
   $(lsb_release -cs) \
   stable"

RUN apt-get -y update && \
 apt-get -y install \
   docker-ce \
   docker-ce-cli \
   containerd.io

...
# Setup users and groups
RUN groupadd -g ${JENKINSGID} jenkins
RUN groupmod -g ${DOCKERGID} docker
RUN useradd -c "Jenkins user" -g ${JENKINSGID} -G ${DOCKERGID} -M -N -u ${JENKINSUID} jenkins