Next GitLab Runner Token Architecture

Summary

GitLab Runner is a core component of GitLab CI/CD that runs CI/CD jobs in a reliable and concurrent environment. Ever since the beginnings of the service as a Ruby program, runners are registered in a GitLab instance with a registration token - a randomly generated string of text. The registration token is unique for its given scope (instance, group, or project). The registration token proves that the party that registers the runner has administrative access to the instance, group, or project to which the runner is registered.

This approach has worked well in the initial years, but some major known issues started to become apparent as the target audience grew:

In light of these issues, it is important that we redesign the way in which we connect runners to the GitLab instance so that we can guarantee traceability, security, and performance.

We call this new mechanism the "next GitLab Runner Token architecture".

Proposal

The proposal addresses the issues of a single token per scope and token storage by eliminating the need for a registration token. Runner creation happens in the GitLab Runners settings page for the given scope, in the context of the authenticated user, which provides traceability. The page provides instructions to configure the newly-created runner in supported environments using the existing gitlab-runner register command.

The remaining concerns become non-issues due to the elimination of the registration token.

Comparison of current and new runner registration flow

graph TD
    subgraph new[<b>New registration flow</b>]
    A[<b>GitLab</b>: User creates a runner in GitLab UI and adds the runner configuration] -->|<b>GitLab</b>: creates ci_runners record and returns<br/>new 'glrt-' prefixed authentication token| B
    B(<b>Runner</b>: User runs 'gitlab-runner register' command with</br>authentication token to register new runner manager with<br/>the GitLab instance) --> C{<b>Runner</b>: Does a .runner_system_id file exist in<br/>the gitlab-runner configuration directory?}
    C -->|Yes| D[<b>Runner</b>: Reads existing system ID] --> F
    C -->|No| E[<b>Runner</b>: Generates and persists unique system ID] --> F
    F[<b>Runner</b>: Issues 'POST /runner/verify' request<br/>to verify authentication token validity] --> G{<b>GitLab</b>: Is the authentication token valid?}
    G -->|Yes| H[<b>GitLab</b>: Creates ci_runner_machine database record if missing] --> J[<b>Runner</b>: Store authentication token in .config.toml]
    G -->|No| I(<b>GitLab</b>: Returns '403 Forbidden' error) --> K(gitlab-runner register command fails)
    J --> Z(Runner and runner manager are ready for use)
    end

    subgraph current[<b>Current registration flow</b>]
    A'[<b>GitLab</b>: User retrieves runner registration token in GitLab UI] --> B'
    B'[<b>Runner</b>: User runs 'gitlab-runner register' command<br/>with registration token to register new runner] -->|<b>Runner</b>: Issues 'POST /runner request' to create<br/>new runner and obtain authentication token| C'{<b>GitLab</b>: Is the registration token valid?}
    C' -->|Yes| D'[<b>GitLab</b>: Create ci_runners database record] --> F'
    C' -->|No| E'(<b>GitLab</b>: Return '403 Forbidden' error) --> K'(gitlab-runner register command fails)
    F'[<b>Runner</b>: Store authentication token<br/>from response in .config.toml] --> Z'(Runner is ready for use)
    end

    style new fill:#f2ffe6

Using the authentication token in place of the registration token

In this proposal, runners created in the GitLab UI are assigned authentication tokens prefixed with glrt- (GitLab Runner Token).

The prefix allows the existing register command to use the authentication token in lieu of the current registration token (--registration-token), requiring minimal adjustments in existing workflows. The authentication token is shown to the user only once - after completing the creation flow - to discourage unintended reuse.

Given that the runner is pre-created through the GitLab UI, the register command fails if provided with arguments that are exposed in the runner creation form. Some examples are --tag-list, --run-untagged, --locked, or --access-level as these are sensitive parameters that should be decided at creation time by an administrator/owner. The runner configuration is generated through the existing register command, which can behave in two different ways depending on whether it is supplied a registration token or an authentication token in the --registration-token argument:

Transition period

During a transition period, legacy tokens ("registration tokens") continue to be shown on the GitLab Runners settings page and to be accepted by the gitlab-runner register command. The legacy workflow is nevertheless discouraged in the UI. Users are steered towards the new flow consisting of creating the runner in the UI and using the resulting authentication token with the gitlab-runner register command as they do today. This approach reduces disruption to users responsible for deploying runners.

Reusing the runner authentication token across many machines

In the existing autoscaling model, a new runner is created whenever a new job needs to be executed. This has led to many situations where runners are left behind and become stale.

In the proposed model, a ci_runners table entry describes a configuration that the user can reuse across multiple machines, and runner state from each machine (for example, IP address, platform, or architecture) is moved to a separate table (ci_runner_machines). A unique system identifier is generated automatically whenever the runner application starts up or the configuration is saved. This allows differentiating the machine in which the runner is being used.

The system_id value complements the short runner token that is used to identify a runner in command line output, CI job logs, and GitLab UI.

Given that the creation of runners involves user interaction, it should be possible to eventually lower the per-plan limit of CI runners that can be registered per scope.

Generating a system_id value

We ensure that a unique system identifier is assigned at all times to a gitlab-runner installation. The ID is derived from an existing machine identifier such as /etc/machine-id (on Linux) and hashed for privacy, in which case it is prefixed with s_. If an ID is not available, a random string is used instead, in which case it is prefixed with r_.

This unique ID identifies the gitlab-runner process and is sent on POST /api/v4/jobs requests for all runners in the config.toml file.

The ID is generated and saved both at gitlab-runner startup and whenever the configuration is saved to disk. Instead of saving the ID at the root of config.toml though, we save it to a new file that lives next to it - .runner_system_id. The goal for this new file is to make it less likely that IDs get reused due to manual copying of the config.toml file

s_cpwhDr7zFz4xBJujFeEM

Runner identification in CI jobs

For users to identify the machine where the job was executed, the unique identifier needs to be visible in CI job contexts. As a first iteration, GitLab Runner will include the unique system identifier in the build logs, wherever it publishes the short token SHA.

Given that the runner can potentially be reused with different unique system identifiers, we should store the unique system ID in the database. This ensures the unique system ID maps to a GitLab Runner's system_id value with the runner token. A new ci_runner_machines table holds information about each unique runner manager, with information regarding when the runner last connected, and what type of runner it was.

In the long term, the relevant fields are to be moved from the ci_runners into ci_runner_machines. Until the removal milestone though, they should be kept in the ci_runners as a fallback when a matching ci_runner_machines record does not exist. An expected scenario is the case when the table is created but the runner hasn't pinged the GitLab instance (for example if the runner is offline).

In addition, we should add the following columns to ci_runners:

• a creator_id column to keep track of who created a runner;
• a registration_type enum column to ci_runners to signal whether a runner has been created using the legacy register method, or the new UI-based method. Possible values are :registration_token and :authenticated_user. This allows the stale runner cleanup service to determine which runners to clean up, and allows future uses that may not be apparent.

CREATE TABLE ci_runners (
  ...
  creator_id bigint
  registration_type int8
)

A new p_ci_runner_machine_builds table joins the ci_runner_machines and ci_builds tables, to avoid adding more pressure to those tables. We might consider a more efficient way to store contacted_at than updating the existing record.

CREATE TABLE p_ci_runner_machine_builds (
    partition_id bigint DEFAULT 100 NOT NULL,
    build_id bigint NOT NULL,
    runner_machine_id bigint NOT NULL
)
PARTITION BY LIST (partition_id);

CREATE TABLE ci_runner_machines (
    id bigint NOT NULL,
    system_xid character varying UNIQUE NOT NULL,
    contacted_at timestamp without time zone,
    version character varying,
    revision character varying,
    platform character varying,
    architecture character varying,
    ip_address character varying,
    executor_type smallint,
    config jsonb DEFAULT '{}'::jsonb NOT NULL
);

Advantages

• Easier for users to wrap their minds around the concept: instead of two types of tokens, there is a single type of token - the per-runner authentication token. Having two types of tokens frequently results in misunderstandings when discussing issues;
• Runners can always be traced back to the user who created it, using the audit log;
• The claims of a CI runner are known at creation time, and cannot be changed from the runner (for example, changing the access_level/protected flag). Authenticated users may however still edit these settings through the GitLab UI;
• Easier cleanup of stale runners, which doesn't touch the ci_runner table.

Details

In the proposed approach, we create a distinct way to configure runners that is usable alongside the current registration token method during a transition period. The idea is to avoid having the Runner make API calls that allow it to leverage a single "god-like" token to register new runners.

The new workflow looks as follows:

1. The user opens the Runners settings page (instance, group, or project level);

2. The user fills in the details regarding the new desired runner, namely description, tags, protected, locked, etc.;

3. The user clicks Create. That results in the following:

   1. Creates a new runner in the ci_runners table (and corresponding glrt- prefixed authentication token);
   2. Presents the user with instructions on how to configure this new runner on a machine, with possibilities for different supported deployment scenarios (for example, shell, docker-compose, Helm chart, etc.) This information contains a token which is available to the user only once, and the UI makes it clear to the user that the value shall not be shown again, as registering the same runner multiple times is discouraged (though not impossible).

4. The user copies and pastes the instructions for the intended deployment scenario (a register command), leading to the following actions:

   1. Upon executing the new gitlab-runner register command in the instructions, gitlab-runner performs a call to the POST /api/v4/runners/verify with the given runner token;
   2. If the POST /api/v4/runners/verify GitLab endpoint validates the token, the config.toml file is populated with the configuration;
   3. Whenever a runner pings for a job, the respective ci_runner_machines record is "upserted" with the latest information about the runner (with Redis cache in front of it like we do for Runner heartbeats).

As part of the transition period, we provide admins and top-level group owners with an instance/group-level setting (allow_runner_registration_token) to disable the legacy registration token functionality and enforce using only the new workflow. Any attempt by a gitlab-runner register command to hit the POST /api/v4/runners endpoint to register a new runner with a registration token results in a HTTP 410 Gone status code.

The instance setting is inherited by the groups. This means that if the legacy registration method is disabled at the instance method, the descendant groups/projects mandatorily prevents the legacy registration method.

The registration token workflow is to be deprecated (with a deprecation notice printed by the gitlab-runner register command) and removed at a future major release after the concept is proven stable and customers have migrated to the new workflow.

Handling of legacy runners

Legacy versions of GitLab Runner do not send the unique system identifier in its requests, and we will not change logic in Workhorse to handle unique system IDs. This can be improved upon in the future after the legacy registration system is removed, and runners have been upgraded to newer versions.

Job pings from such legacy runners results in a ci_runner_machines record containing a <legacy> system_xid field value.

Not using the unique system ID means that all connected runners with the same token are notified, instead of just the runner matching the exact system identifier. While not ideal, this is not an issue per-se.

ci_runner_machines record lifetime

New records are created in 2 situations:

• when the runner calls the POST /api/v4/runners/verify endpoint as part of the gitlab-runner register command, if the specified runner token is prefixed with glrt-. This allows the frontend to determine whether the user has successfully completed the registration and take an appropriate action;
• when GitLab is pinged for new jobs and a record matching the token+system_id does not already exist.

Due to the time-decaying nature of the ci_runner_machines records, they are automatically cleaned after 7 days after the last contact from the respective runner.

Required adaptations

Migration to ci_runner_machines table

When details from ci_runner_machines are needed, we need to fall back to the existing fields in ci_runner if a match is not found in ci_runner_machines.

REST API

API endpoints receiving runner tokens should be changed to also take an optional system_id parameter, sent alongside with the runner token (most often as a JSON parameter on the request body).

GraphQL CiRunner type

The CiRunner type closely reflects the ci_runners model. This means that machine information such as ipAddress, architectureName, and executorName among others are no longer singular values in the proposed approach. We can live with that fact for the time being and start returning lists of unique values, separated by commas. The respective CiRunner fields must return the values for the ci_runner_machines entries (falling back to ci_runner record if non-existent).

Stale runner cleanup

The functionality to clean up stale runners needs to be adapted to clean up ci_runner_machines records instead of ci_runners records.

At some point after the removal of the registration token support, we'll want to create a background migration to clean up stale runners that have been created with a registration token (leveraging the enum column created in the ci_runners table.

Runner creation through API

Automated runner creation is possible through a new GraphQL mutation and the existing POST /runners REST API endpoint. The difference in the REST API endpoint is that it is modified to accept a request from an authorized user with a scope (instance, a group, or a project) instead of the registration token. These endpoints are only available to users that are allowed to create runners at the specified scope.

Implementation plan

Stage 1 - Deprecations

Stage 2 - Prepare gitlab-runner for system_id

Stage 2a - Prepare GitLab Runner Helm Chart and GitLab Runner Operator

Stage 3 - Database changes

Stage 4 - Create runners from the UI

Stage 5 - Optional disabling of registration token

Stage 6 - Enforcement

Stage 7 - Removals

FAQ

Please follow the user documentation.

Status

Status: RFC.

Who

Proposal:

DRIs:

Domain experts: