• Supported options for self-signed certificates targeting the GitLab server
  • Git cloning
• Trusting TLS certificates for Docker and Kubernetes executors
  • Trusting the certificate for user scripts
  • Trusting the certificate for the other CI/CD stages
    • Docker
    • Kubernetes
• Troubleshooting

Self-signed certificates or custom Certification Authorities

GitLab Runner provides two options to configure certificates to be used to verify TLS peers:

• For connections to the GitLab server: the certificate file can be specified as detailed in the Supported options for self-signed certificates targeting the GitLab server section.

  This solves the x509: certificate signed by unknown authority problem when registering a runner.

  For existing Runners, the same error can be seen in Runner logs when trying to check the jobs:

    Couldn't execute POST against https://hostname.tld/api/v4/jobs/request:
    Post https://hostname.tld/api/v4/jobs/request: x509: certificate signed by unknown authority
  

• A more generic approach which also covers other scenarios such as user scripts, connecting to a cache server or an external Git LFS store: a certificate can be specified and installed on the container as detailed in the Trusting TLS certificates for Docker and Kubernetes executors section.

  An example job log error concerning a Git LFS operation that is missing a certificate:

    LFS: Get https://object.hostname.tld/lfs-dev/c8/95/a34909dce385b85cee1a943788044859d685e66c002dbf7b28e10abeef20?X-Amz-Expires=600&X-Amz-Date=20201006T043010Z&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=svcgitlabstoragedev%2F20201006%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-SignedHeaders=host&X-Amz-Signature=012211eb0ff0e374086e8c2d37556f2d8ca4cc948763e90896f8f5774a100b55: x509: certificate signed by unknown authority
  

Supported options for self-signed certificates targeting the GitLab server

This section refers to the situation where only the GitLab server requires a custom certificate. If other hosts (e.g. object storage service without proxy download enabled) also require a custom certificate authority (CA), please see the next section.

GitLab Runner supports the following options:

• Default - Read the system certificate: GitLab Runner reads the system certificate store and verifies the GitLab server against the certificate authorities (CA) stored in the system. Note that reading from the system certificate store is not supported in Windows.

• Specify a custom certificate file: GitLab Runner exposes the tls-ca-file option during registration (gitlab-runner register --tls-ca-file=/path), and in config.toml under the [[runners]] section. This allows you to specify a custom certificate file. This file will be read every time the Runner tries to access the GitLab server. If you are using GitLab Runner Helm chart, you will need to configure certificates as described in Providing a custom certificate for accessing GitLab.

• Read a PEM certificate: GitLab Runner reads the PEM certificate (DER format is not supported) from a predefined file:

  • /etc/gitlab-runner/certs/gitlab.example.com.crt on *nix systems when GitLab Runner is executed as root.

    If your server address is https://gitlab.example.com:8443/, create the certificate file at: /etc/gitlab-runner/certs/gitlab.example.com.crt.

    You can use the openssl client to download the GitLab instance’s certificate to /etc/gitlab-runner/certs:

    openssl s_client -showcerts -connect gitlab.example.com:443 -servername gitlab.example.com < /dev/null 2>/dev/null | openssl x509 -outform PEM > /etc/gitlab-runner/certs/gitlab.example.com.crt
    

    To verify that the file is correctly installed, you can use a tool like openssl. For example:

    echo | openssl s_client -CAfile /etc/gitlab-runner/certs/gitlab.example.com.crt -connect gitlab.example.com:443 -servername gitlab.example.com
    

  • ~/.gitlab-runner/certs/gitlab.example.com.crt on *nix systems when GitLab Runner is executed as non-root.
  • ./certs/gitlab.example.com.crt on other systems. If running GitLab Runner as a Windows service, this will not work. Specify a custom certificate file instead.

Notes:

• If your GitLab server certificate is signed by your CA, use your CA certificate (not your GitLab server signed certificate). You might need to add the intermediates to the chain as well. For example, if you have a primary, intermediate, and root certificate, you can put all of them into one file:

    -----BEGIN CERTIFICATE-----
    (Your primary SSL certificate: your_domain_name.crt)
    -----END CERTIFICATE-----
    -----BEGIN CERTIFICATE-----
    (Your intermediate certificate)
    -----END CERTIFICATE-----
    -----BEGIN CERTIFICATE-----
    (Your root certificate)
    -----END CERTIFICATE-----
  

• If you are updating the certificate for an existing Runner, restart it.
• If you already have a Runner configured through HTTP, update your instance path to the new HTTPS URL of your GitLab instance in your config.toml.
• As a temporary and insecure workaround, to skip the verification of certificates, in the variables: section of your .gitlab-ci.yml file, set the CI variable GIT_SSL_NO_VERIFY to true.

Git cloning

The Runner injects missing certificates to build the CA chain by using CI_SERVER_TLS_CA_FILE. This allows git clone and artifacts to work with servers that do not use publicly trusted certificates.

This approach is secure, but makes the Runner a single point of trust.

Trusting TLS certificates for Docker and Kubernetes executors

There are two contexts that need to be taken into account when we consider registering a certificate on a container:

• The user image, which is used to run the user script. In this scenario, the user must take ownership regarding how to install a certificate, since this is highly dependent on the image itself, and the Runner has no way of knowing how to install a certificate in each possible scenario.
• The Runner helper image, which is used to handle Git, artifacts, and cache operations. In this scenario, the user only needs to make a certificate file available at a specific location (for example, /etc/gitlab-runner/certs/ca.crt), and the Docker container will automatically install it for the user.

Trusting the certificate for user scripts

If your build script needs to communicate with peers through TLS and needs to rely on a self-signed certificate or custom Certificate Authority, you will need to perform the certificate installation in the build job, as the Docker container running the user scripts doesn’t have the certificate files installed by default. This might be required to use a custom cache host, perform a secondary git clone, or fetch a file through a tool like wget, for example.

To install the certificate:

1. Map the necessary files as a Docker volume so that the Docker container that will run the scripts can see them. Do this by adding a volume inside the respective key inside the [runners.docker] in the config.toml file, for example:

   • Linux:

       [[runners]]
        name = "docker"
        url = "https://example.com/"
        token = "TOKEN"
        executor = "docker"
     
        [runners.docker]
          image = "ubuntu:latest"
     
          # Add path to your ca.crt file in the volumes list
          volumes = ["/cache", "/path/to-ca-cert-dir/ca.crt:/etc/gitlab-runner/certs/ca.crt:ro"]
     

2. Linux-only: Use the mapped file (e.g ca.crt) in a pre_build_script that:

   1. Copies it to /usr/local/share/ca-certificates/ca.crt inside the Docker container.

   2. Installs it by running update-ca-certificates --fresh. For example (commands vary based on the distribution you’re using):

      • On Ubuntu:

          [[runners]]
            name = "docker"
            url = "https://example.com/"
            token = "TOKEN"
            executor = "docker"
        
            # Copy and install CA certificate before each job
            pre_build_script = """
            apt-get update -y > /dev/null
            apt-get install -y ca-certificates > /dev/null
        
            cp /etc/gitlab-runner/certs/ca.crt /usr/local/share/ca-certificates/ca.crt
            update-ca-certificates --fresh > /dev/null
            """
        

      • On Alpine:

          [[runners]]
            name = "docker"
            url = "https://example.com/"
            token = "TOKEN"
            executor = "docker"
        
            # Copy and install CA certificate before each job
            pre_build_script = """
            apk update >/dev/null
            apk add ca-certificates > /dev/null
            rm -rf /var/cache/apk/*
        
            cp /etc/gitlab-runner/certs/ca.crt /usr/local/share/ca-certificates/ca.crt
            update-ca-certificates --fresh > /dev/null
            """
        

If you just need the GitLab server CA cert that can be used, you can retrieve it from the file stored in the CI_SERVER_TLS_CA_FILE variable:

curl --cacert "${CI_SERVER_TLS_CA_FILE}"  ${URL} -o ${FILE}

Trusting the certificate for the other CI/CD stages

You can map a certificate file to /etc/gitlab-runner/certs/ca.crt on Linux, or C:\GitLab-Runner\certs\ca.crt on Windows. The Runner helper image installs this user-defined ca.crt file at start-up, and uses it when performing operations like cloning and uploading artifacts, for example.

Docker

• Linux:

    [[runners]]
     name = "docker"
     url = "https://example.com/"
     token = "TOKEN"
     executor = "docker"
  
     [runners.docker]
       image = "ubuntu:latest"
  
       # Add path to your ca.crt file in the volumes list
       volumes = ["/cache", "/path/to-ca-cert-dir/ca.crt:/etc/gitlab-runner/certs/ca.crt:ro"]
  

• Windows:

    [[runners]]
     name = "docker"
     url = "https://example.com/"
     token = "TOKEN"
     executor = "docker"
  
     [runners.docker]
       image = "mcr.microsoft.com/windows/servercore:2004"
  
       # Add directory holding your ca.crt file in the volumes list
       volumes = ["c:\\cache", "c:\\path\\to-ca-cert-dir:C:\\GitLab-Runner\\certs:ro"]
  

Kubernetes

To provide a certificate file to jobs running in Kubernetes:

1. Store the certificate as a Kubernetes secret in your namespace:

   kubectl create secret generic <SECRET_NAME> --namespace <NAMESPACE> --from-file=<CERT_FILE>
   

2. Mount the secret as a volume in your runner, replacing <SECRET_NAME> and <LOCATION> with appropriate values:

   config: |
     [[runners]]
       [runners.kubernetes]
         namespace = "{{.Release.Namespace}}"
         image = "ubuntu:latest"
       [[runners.kubernetes.volumes.secret]]
           name = "<SECRET_NAME>"
           mount_path = "<LOCATION>"
   

   The mount_path is the directory in the container where the certificate is stored. If you used /etc/gitlab-runner/certs/ as the mount_path and ca.crt as your certificate file, your certificate is available at /etc/gitlab-runner/certs/ca.crt inside your container.

3. As part of the job, install the mapped certificate file to the system certificate store. For example, in an Ubuntu container:

   script:
     - cp /etc/gitlab-runner/certs/ca.crt /usr/local/share/ca-certificates/
     - update-ca-certificates
   

Due to a known issue in the Kubernetes executor’s handling of the helper image’s ENTRYPOINT, the mapped certificate file isn’t automatically installed to the system certificate store.

Troubleshooting

Refer to the general SSL troubleshooting documentation.

In addition, you can use the tlsctl tool to debug GitLab certificates from the Runner’s end.