GitLab Shell development guidelines

GitLab Shell handles Git SSH sessions for GitLab and modifies the list of authorized keys. GitLab Shell is not a Unix shell nor a replacement for Bash or Zsh.

GitLab supports Git LFS authentication through SSH.

Requirements

GitLab Shell is written in Go, and needs a Go compiler to build. It still requires Ruby to build and test, but not to run.

GitLab Shell runs on port 22 on an Omnibus installation. To use a regular SSH service, configure it on an alternative port.

Download and install the current version of Go. We follow the Go Release Policy and support:

The current stable version.
The previous two major versions.

How GitLab Shell works

When you access the GitLab server over SSH, GitLab Shell then:

Limits you to predefined Git commands (git push, git pull, git fetch).
Calls the GitLab Rails API to check if you are authorized, and what Gitaly server your repository is on.
Copies data back and forth between the SSH client and the Gitaly server.

If you access a GitLab server over HTTP(S) you end up in gitlab-workhorse.

`git pull` over SSH

graph LR
    A[Git pull] --> |via SSH| B[gitlab-shell]
    B -->|API call| C[gitlab-rails<br>authorization]
    C -->|accept or decline| D[Gitaly session]

`git push` over SSH

The git push command is not performed until after gitlab-rails accepts the push:

graph LR
subgraph User initiates
    A[Git push] -->|via SSH| B[gitlab-shell]
end
subgraph Gitaly
    B -->|establish Gitaly session| C[gitlab-shell pre-receive hook]
    C -->|API auth call| D[Gitlab-rails]
    D --> E[accept or decline push]
end

Full feature list

Modifies `authorized_keys`

GitLab Shell modifies the authorized_keys file on the client machine.

Contribute to GitLab Shell

To contribute to GitLab Shell:

Check if GitLab API access, and Redis via the internal API, can be reached: make check
Compile the gitlab-shell binaries, placing them into bin/: make compile
Run make install to build the gitlab-shell binaries and install. them onto the file system. The default location is /usr/local. To change it, set the PREFIX and DESTDIR environment variables.
To install GitLab from source on a single machine, run make setup. It compiles the GitLab Shell binaries, and ensures that various paths on the file system exist with the correct permissions. Do not run this command unless your installation method documentation instructs you to.

For more information, see CONTRIBUTING.md.

Run tests

When contributing, run tests:

Run tests with bundle install and make test.
Run Gofmt: make verify
Run both test and verify (the default Makefile target):
```
bundle install
make validate
```
If needed, configure Gitaly.

Configure Gitaly for local testing

Some tests need a Gitaly server. The docker-compose.yml file runs Gitaly on port 8075. To tell the tests where Gitaly is, set GITALY_CONNECTION_INFO:

export GITALY_CONNECTION_INFO='{"address": "tcp://localhost:8075", "storage": "default"}'
make test

If no GITALY_CONNECTION_INFO is set, the test suite still runs, but any tests requiring Gitaly are skipped. The tests always run in the CI environment.

Rate limiting

GitLab Shell performs rate-limiting by user account and project for Git operations. GitLab Shell accepts Git operation requests and then makes a call to the Rails rate-limiter, backed by Redis. If the user + project exceeds the rate limit, then GitLab Shell then drop further connection requests for that user + project.

The rate-limiter is applied at the Git command (plumbing) level. Each command has a rate limit of 600 per minute. For example, git push has 600 per minute, and git pull has another 600 per minute.

Because they are using the same plumbing command, git-upload-pack, git pull, and git clone are in effect the same command for the purposes of rate-limiting.

Gitaly also has a rate-limiter in place, but calls are never made to Gitaly if the rate limit is exceeded in GitLab Shell (Rails).

Logs in GitLab Shell

In general, you can determine the structure, but not content, of a GitLab Shell or gitlab-sshd session by inspecting the logs. Some guidelines:

We use gitlab.com/gitlab-org/labkit/log for logging.
Always include a correlation ID.
Log messages should be invariant and unique. Include accessory information in fields, using log.WithField, log.WithFields, or log.WithError.
Log both success cases and error cases.
Logging too much is better than not logging enough. If a message seems too verbose, consider reducing the log level before removing the message.

GitLab SaaS

A diagram of the flow of gitlab-shell on GitLab.com:

graph LR
    a2 --> b2
    a2  --> b3
    a2 --> b4
    b2 --> c1
    b3 --> c1
    b4 --> c1
    c2 --> d1
    c2 --> d2
    c2 --> d3
    d1 --> e1
    d2 --> e1
    d3 --> e1
    a1[Cloudflare] --> a2[TCP<br/> load balancer]
    e1[Git]

    subgraph HAProxy Fleet
    b2[HAProxy]
    b3[HAProxy]
    b4[HAProxy]
    end

    subgraph GKE
    c1[Internal TCP<br/> load balancer<br/>port 2222] --> c2[GitLab-shell<br/> pods]
    end

    subgraph Gitaly
    d1[Gitaly]
    d2[Gitaly]
    d3[Gitaly]
    end

GitLab Shell architecture

sequenceDiagram
    participant Git on client
    participant SSH server
    participant AuthorizedKeysCommand
    participant GitLab Shell
    participant Rails
    participant Gitaly
    participant Git on server

    Note left of Git on client: git fetch
    Git on client->>+SSH server: ssh git fetch-pack request
    SSH server->>+AuthorizedKeysCommand: gitlab-shell-authorized-keys-check git AAAA...
    AuthorizedKeysCommand->>+Rails: GET /internal/api/authorized_keys?key=AAAA...
    Note right of Rails: Lookup key ID
    Rails-->>-AuthorizedKeysCommand: 200 OK, command="gitlab-shell upload-pack key_id=1"
    AuthorizedKeysCommand-->>-SSH server: command="gitlab-shell upload-pack key_id=1"
    SSH server->>+GitLab Shell: gitlab-shell upload-pack key_id=1
    GitLab Shell->>+Rails: GET /internal/api/allowed?action=upload_pack&key_id=1
    Note right of Rails: Auth check
    Rails-->>-GitLab Shell: 200 OK, { gitaly: ... }
    GitLab Shell->>+Gitaly: SSHService.SSHUploadPack request
    Gitaly->>+Git on server: git upload-pack request
    Note over Git on client,Git on server: Bidirectional communication between Git client and server
    Git on server-->>-Gitaly: git upload-pack response
    Gitaly -->>-GitLab Shell: SSHService.SSHUploadPack response
    GitLab Shell-->>-SSH server: gitlab-shell upload-pack response
    SSH server-->>-Git on client: ssh git fetch-pack response