Where does the string docker.example.com in my GitLab CI job come from?

Question

I am trying to have a Docker image deployed to my server by GitLab CI, but it only worked once¹ and then dies with the error message below. Here is the .gitlab-ci.yml snippet:

deploy-image:
  image: docker:latest
  services:
    - docker:dind
  variables:
    DOCKER_HOST: ssh://gitlab-ci@$DEPLOY_HOST
  before_script:
    - eval $(ssh-agent -s)
    - echo "${SSH_KEY}" | ssh-add -
    - mkdir -p ~/.ssh
    - '[[ -f /.dockerenv ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" > ~/.ssh/config'
  script:
    - echo $CI_JOB_TOKEN | docker login --username gitlab-ci-token --password-stdin $CI_REGISTRY;
    - docker pull "$CI_REGISTRY_IMAGE"
    - docker run --name mycontainer --detach $CI_REGISTRY_IMAGE
  after_script:
    - docker logout $CI_REGISTRY

The error in the job log looks like that:

Executing "step_script" stage of the job script 00:04
Using docker image sha256:51453dcdd9bd51e503f75e6d42a4071469ad2ba816321781985041b6bc7776db for docker:latest with digest docker@sha256:ddf0d732dcbc3e2087836e06e50cc97e21bfb002a49c7d0fe767f6c31e01d65f ...
$ eval $(ssh-agent -s)
Agent pid 18
$ echo "${SSH_KEY}" | ssh-add -
Identity added: (stdin) (gitlab-ci@jacks)
$ mkdir -p ~/.ssh
$ [[ -f /.dockerenv ]] && echo -e "Host *\n\tStrictHostKeyChecking no\n\n" > ~/.ssh/config
$ echo $CI_JOB_TOKEN | docker login --username gitlab-ci-token --password-stdin $CI_REGISTRY;
error during connect: Post "http://docker.example.com/v1.24/auth": command [ssh -l gitlab-ci -- xxx.xxx.xxx.xxx docker system dial-stdio] has exited with exit status 255, please make sure the URL is valid, and Docker 18.09 or later is installed on the remote host: stderr=ssh: connect to host xxx.xxx.xxx.xxx port 22: Connection refused

While I x-ed out the IP address of the destination server in this post (which is the value of the DEPLOY_HOST env var), it literally says docker.example.com in the error message.

So my question would be, where that example string comes from.

This is on a self-hosted GitLab instance and the runner is also self-hosted on a server that is neither the destination host, nor the one where GitLab runs on.

¹ The connection issue itself seems to be a problem with the SSH connection and not related to the CI/CD pipeline config.

Nick ODell · Accepted Answer · 2022-10-17T21:57:10.243

It's a placeholder domain name for connecting over SSH. It doesn't mean that it's connecting to docker.example.com.

I ran into a similar error when using a Docker client over SSH. The error message was:

ERROR: Cannot connect to the Docker daemon at http://docker.example.com. Is the docker daemon running?

Reading the Docker source code, it looks like "docker.example.com" is a placeholder host name when connecting over SSH, resulting in the confusing error message.

In my case, the error was caused by the user I was logging in as not being part of the docker group, and therefore not having permission to use docker.

Where does the string docker.example.com in my GitLab CI job come from?

1 Answers1