Running GitLab Runner behind a proxy

This guide aims specifically to making GitLab Runner with Docker executor work behind a proxy.

Before continuing, ensure that you’ve already installed Docker and GitLab Runner on the same machine.

Configuring CNTLM

noteIf you already use a proxy without authentication, this section is optional and you can skip straight to configuring Docker. Configuring CNTLM is only needed if you are behind a proxy with authentication, but it’s recommended to use in any case.

CNTLM is a Linux proxy which can be used as a local proxy and has 2 major advantages compared to adding the proxy details everywhere manually:

  • One single source where you need to change your credentials
  • The credentials can not be accessed from the Docker runners

Assuming you have installed CNTLM, you need to first configure it.

Make CNTLM listen to the docker0 interface

For extra security, and to protect your server from the outside world, you can bind CNTLM to listen on the docker0 interface which has an IP that is reachable from inside the containers. If you tell CNTLM on the Docker host to bind only to this address, Docker containers are be able to reach it, but the outside world can’t.

  1. Find the IP that Docker is using:

    ip -4 -oneline addr show dev docker0
    

    This is usually 172.17.0.1, let’s call it docker0_interface_ip.

  2. Open the configuration file for CNTLM (/etc/cntlm.conf). Enter your username, password, domain and proxy hosts, and configure the Listen IP address which you found from the previous step. It should look like this:

    Username     testuser
    Domain       corp-uk
    Password     password
    Proxy        10.0.0.41:8080
    Proxy        10.0.0.42:8080
    Listen       172.17.0.1:3128 # Change to your docker0 interface IP
    
  3. Save the changes and restart its service:

    sudo systemctl restart cntlm
    

Configuring Docker for downloading images

noteThe following apply to OSes with systemd support.

Follow Docker’s documentation how to use a proxy.

The service file should look like this:

[Service]
Environment="HTTP_PROXY=http://docker0_interface_ip:3128/"
Environment="HTTPS_PROXY=http://docker0_interface_ip:3128/"

Adding Proxy variables to the GitLab Runner configuration

The proxy variables need to also be added to the GitLab Runner configuration, so that it can get builds assigned from GitLab behind the proxy.

This is basically the same as adding the proxy to the Docker service above:

  1. Create a systemd drop-in directory for the gitlab-runner service:

    mkdir /etc/systemd/system/gitlab-runner.service.d
    
  2. Create a file called /etc/systemd/system/gitlab-runner.service.d/http-proxy.conf that adds the HTTP_PROXY environment variable(s):

    [Service]
    Environment="HTTP_PROXY=http://docker0_interface_ip:3128/"
    Environment="HTTPS_PROXY=http://docker0_interface_ip:3128/"
    
  3. Save the file and flush changes:

    systemctl daemon-reload
    
  4. Restart GitLab Runner:

    sudo systemctl restart gitlab-runner
    
  5. Verify that the configuration has been loaded:

    systemctl show --property=Environment gitlab-runner
    

    You should see:

    Environment=HTTP_PROXY=http://docker0_interface_ip:3128/ HTTPS_PROXY=http://docker0_interface_ip:3128/
    

Adding the Proxy to the Docker containers

After you register your runner, you may want to propagate your proxy settings to the Docker containers (for example, for git clone).

To do this, you need to edit /etc/gitlab-runner/config.toml and add the following to the [[runners]] section:

pre_clone_script = "git config --global http.proxy $HTTP_PROXY; git config --global https.proxy $HTTPS_PROXY"
environment = ["https_proxy=http://docker0_interface_ip:3128", "http_proxy=http://docker0_interface_ip:3128", "HTTPS_PROXY=docker0_interface_ip:3128", "HTTP_PROXY=docker0_interface_ip:3128"]

Where docker0_interface_ip is the IP address of the docker0 interface.

noteIn our examples, we are setting both lower case and upper case variables because certain programs expect HTTP_PROXY and others http_proxy. Unfortunately, there is no standard on these kinds of environment variables.

Proxy settings when using dind service

When using the Docker-in-Docker executor (dind), it may be necessary to specify docker:2375,docker:2376 in the NO_PROXY environment variable. The ports are required, otherwise docker push is blocked.

Communication between dockerd from dind and the local docker client (as described here: https://hub.docker.com/_/docker/) uses proxy variables held in root’s Docker config.

To configure this, you need to edit /root/.docker/config.json to include your complete proxy configuration, for example:

{
    "proxies": {
        "default": {
            "httpProxy": "http://proxy:8080",
            "httpsProxy": "http://proxy:8080",
            "noProxy": "docker:2375,docker:2376"
        }
    }
}

In order to pass on the settings to the container of the Docker executor, a $HOME/.docker/config.json also needs to be created inside the container. This may be scripted as a before_script in the .gitlab-ci.yml, for example:

before_script:
  - mkdir -p $HOME/.docker/
  - 'echo "{ \"proxies\": { \"default\": { \"httpProxy\": \"$HTTP_PROXY\", \"httpsProxy\": \"$HTTPS_PROXY\", \"noProxy\": \"$NO_PROXY\" } } }" > $HOME/.docker/config.json'

Or alternatively, in the configuration of the gitlab-runner (/etc/gitlab-runner/config.toml) that is affected:

[[runners]]
  pre_build_script = "mkdir -p $HOME/.docker/ && echo \"{ \\\"proxies\\\": { \\\"default\\\": { \\\"httpProxy\\\": \\\"$HTTP_PROXY\\\", \\\"httpsProxy\\\": \\\"$HTTPS_PROXY\\\", \\\"noProxy\\\": \\\"$NO_PROXY\\\" } } }\" > $HOME/.docker/config.json"
noteAn additional level of escaping " is needed here because this is the creation of a JSON file with a shell specified as a single string inside a TOML file. Because this is not YAML, do not escape the :.

Note that if the NO_PROXY list needs to be extended, wildcards * only work for suffixes, but not for prefixes or CIDR notation. For more information, see https://github.com/moby/moby/issues/9145 and https://unix.stackexchange.com/questions/23452/set-a-network-range-in-the-no-proxy-environment-variable.

Handling rate limited requests

A GitLab instance may be behind a reverse proxy that has rate-limiting on API requests to prevent abuse. GitLab Runner sends multiple requests to the API and could go over these rate limits.

As a result, GitLab Runner handles rate limited scenarios with the following logic:

  1. A response code of 429 - TooManyRequests is received.
  2. The response headers are checked for a RateLimit-ResetTime header. The RateLimit-ResetTime header should have a value which is a valid HTTP Date (RFC1123), like Wed, 21 Oct 2015 07:28:00 GMT.
    • If the header is present and has a valid value the runner waits until the specified time and issues another request.
    • If the header is present, but isn’t a valid date, a fallback of 1 minute is used.
    • If the header is not present, no additional actions are taken, the response error is returned.
  3. The process above is repeated 5 times, then a gave up due to rate limit error is returned.
noteThe header RateLimit-ResetTime is case insensitive since all header keys are run through the http.CanonicalHeaderKey function.