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Fast lookup of authorized SSH keys in the database

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NOTE: This document describes a drop-in replacement for the authorized_keys file. For standard (non-deploy key) users, consider using SSH certificates. They are even faster, but are not a drop-in replacement.

Regular SSH operations become slow as the number of users grows because OpenSSH searches for a key to authorize a user via a linear search. In the worst case, such as when the user is not authorized to access GitLab, OpenSSH scans the entire file to search for a key. This can take significant time and disk I/O, which delays users attempting to push or pull to a repository. Making matters worse, if users add or remove keys frequently, the operating system may not be able to cache the authorized_keys file, which causes the disk to be accessed repeatedly.

GitLab Shell solves this by providing a way to authorize SSH users via a fast, indexed lookup in the GitLab database. This page describes how to enable the fast lookup of authorized SSH keys.

Fast lookup is required for Geo

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Unlike Cloud Native GitLab, by default Linux package installations manage an authorized_keys file that is located in the git user's home directory. For most installations, this file is located under /var/opt/gitlab/.ssh/authorized_keys, but you can use the following command to locate the authorized_keys on your system:

getent passwd git | cut -d: -f6 | awk '{print $1"/.ssh/authorized_keys"}'

The authorized_keys file contains all the public SSH keys for users allowed to access GitLab. However, to maintain a single source of truth, Geo must be configured to perform SSH fingerprint lookups via database lookup.

As part of setting up Geo, you are required to follow the steps outlined below for both the primary and secondary nodes, but Write to "authorized keys" file must be unchecked only on the primary node, because it is reflected automatically on the secondary if database replication is working.

Set up fast lookup

GitLab Shell provides a way to authorize SSH users via a fast, indexed lookup to the GitLab database. GitLab Shell uses the fingerprint of the SSH key to check whether the user is authorized to access GitLab.

Fast lookup can be enabled with the following SSH servers:

You can run both services simultaneously by using separate ports for each service.

With gitlab-sshd

To set up gitlab-sshd, see the gitlab-sshd documentation. After gitlab-sshd is enabled, GitLab Shell and gitlab-sshd are configured to use fast lookup automatically.

With OpenSSH

WARNING: OpenSSH version 6.9+ is required because AuthorizedKeysCommand must be able to accept a fingerprint. Check the version of OpenSSH on your server with sshd -V.

Add the following to your sshd_config file. This file is usually located at /etc/ssh/sshd_config, but it is at /assets/sshd_config if you're using Docker from a Linux package installation:

Match User git    # Apply the AuthorizedKeysCommands to the git user only
  AuthorizedKeysCommand /opt/gitlab/embedded/service/gitlab-shell/bin/gitlab-shell-authorized-keys-check git %u %k
  AuthorizedKeysCommandUser git
Match all    # End match, settings apply to all users again

Reload OpenSSH:

# Debian or Ubuntu installations
sudo service ssh reload

# CentOS installations
sudo service sshd reload

Confirm that SSH is working by commenting out your user's key in the authorized_keys file (start the line with a # to comment it), and from your local machine, attempt to pull a repository or run:

ssh -T

A successful pull or welcome message means that GitLab was able to find the key in the database, as it is not present in the file.

NOTE: For self-compiled installations, the command would be located at /home/git/gitlab-shell/bin/gitlab-shell-authorized-keys-check if the install from source instructions were followed. You might want to consider creating a wrapper script somewhere else, as this command must be owned by root and not be writable by group or others. You could also consider changing the ownership of this command as required, but that might require temporary ownership changes during gitlab-shell upgrades.

WARNING: Do not disable writes until SSH is confirmed to be working perfectly; otherwise, the file quickly becomes out-of-date.

In the case of lookup failures (which are common), the authorized_keys file is still scanned. So Git SSH performance would still be slow for many users as long as a large file exists.

To disable writes to the authorized_keys file:

  1. On the left sidebar, at the bottom, select Admin Area.
  2. Select Settings > Network.
  3. Expand Performance optimization.
  4. Clear the Use authorized_keys file to authenticate SSH keys checkbox.
  5. Select Save changes.

Again, confirm that SSH is working by removing your user's SSH key in the UI, adding a new one, and attempting to pull a repository.

Then you can backup and delete your authorized_keys file for best performance. The current users' keys are already present in the database, so there is no need for migration or for users to re-add their keys.

How to go back to using the authorized_keys file

This overview is brief. Refer to the above instructions for more context.

  1. Rebuild the authorized_keys file.
  2. Enable writes to the authorized_keys file.
    1. On the left sidebar, at the bottom, select Admin Area.
    2. On the left sidebar, select Settings > Network.
    3. Expand Performance optimization.
    4. Select the Use authorized_keys file to authenticate SSH keys checkbox.
  3. Remove the AuthorizedKeysCommand lines from /etc/ssh/sshd_config or from /assets/sshd_config if you are using Docker from a Linux package installation.
  4. Reload sshd: sudo service sshd reload.

SELinux support and limitations

GitLab supports authorized_keys database lookups with SELinux.

Because the SELinux policy is static, GitLab doesn't support the ability to change internal webserver ports at the moment. Administrators would have to create a special .te file for the environment, as it isn't generated dynamically.

Additional documentation

Additional technical documentation for gitlab-sshd may be found in the GitLab Shell documentation.


SSH traffic slow or high CPU load

If your SSH traffic is slow or causing high CPU load, be sure to check the size of /var/log/btmp, and ensure it is rotated on a regular basis or after reaching a certain size. If this file is very large, GitLab SSH fast lookup can cause the bottleneck to be hit more frequently, thus decreasing performance even further. If you are able to, you may consider disabling UsePAM in your sshd_config to avoid reading /var/log/btmp altogether.

Running strace and lsof on a running sshd: git process returns debugging information. To get an strace on an in-progress Git over SSH connection for IP x.x.x.x, run:

sudo strace -s 10000 -p $(sudo netstat -tp | grep x.x.x.x | egrep 'ssh.*: git' | sed -e 's/.*ESTABLISHED *//' -e 's#/.*##')

Or get an lsof for a running Git over SSH process:

sudo lsof -p $(sudo netstat -tp | egrep 'ssh.*: git' | head -1 | sed -e 's/.*ESTABLISHED *//' -e 's#/.*##')