Developer Guide

Getting Started

Running a local copy of the client

Running the client in developer mode from your local tree is a little different than running Certbot as a user. To get set up, clone our git repository by running:

git clone

If you’re on macOS, we recommend you skip the rest of this section and instead run Certbot in Docker. You can find instructions for how to do this here. If you’re running on Linux, you can run the following commands to install dependencies and set up a virtual environment where you can run Certbot. You only need to do this once.

cd certbot
./certbot-auto --os-packages-only

Then in each shell where you’re working on the client, do:

source ./venv/bin/activate

After that, your shell will be using the virtual environment, and you run the client by typing:


Activating a shell in this way makes it easier to run unit tests with tox and integration tests, as described below. To reverse this, you can type deactivate. More information can be found in the virtualenv docs.

Find issues to work on

You can find the open issues in the github issue tracker. Comparatively easy ones are marked Good Volunteer Task. If you’re starting work on something, post a comment to let others know and seek feedback on your plan where appropriate.

Once you’ve got a working branch, you can open a pull request. All changes in your pull request must have thorough unit test coverage, pass our tests, and be compliant with the coding style.


When you are working in a file, there should also be a file either in the same directory as or in the tests subdirectory (if there isn’t, make one). While you are working on your code and tests, run python to run the relevant tests.

For debugging, we recommend putting import ipdb; ipdb.set_trace() statements inside the source code.

Once you are done with your code changes, and the tests in pass, run all of the unittests for Certbot with tox -e py27 (this uses Python 2.7).

Once all the unittests pass, check for sufficient test coverage using tox -e cover, and then check for code style with tox -e lint (all files) or pylint --rcfile=.pylintrc path/to/ (single file at a time).

Once all of the above is successful, you may run the full test suite, including integration tests, using tox. We recommend running the commands above first, because running all tests with tox is very slow, and the large amount of tox output can make it hard to find specific failures when they happen. Also note that the full test suite will attempt to modify your system’s Apache config if your user has sudo permissions, so it should not be run on a production Apache server.

If you have trouble getting the full tox suite to run locally, it is generally sufficient to open a pull request and let Github and Travis run integration tests for you.

Integration testing with the Boulder CA

To run integration tests locally, you need Docker and docker-compose installed and working. Fetch and start Boulder using:


If you have problems with Docker, you may want to try removing all containers and volumes and making sure you have at least 1GB of memory.

Run the integration tests using:


Code components and layout

contains all protocol specific code
main client code
certbot-apache and certbot-nginx
client code to configure specific web servers
configuration for packaging Certbot


Certbot has a plugin architecture to facilitate support for different webservers, other TLS servers, and operating systems. The interfaces available for plugins to implement are defined in and plugins/

The main two plugin interfaces are IAuthenticator, which implements various ways of proving domain control to a certificate authority, and IInstaller, which configures a server to use a certificate once it is issued. Some plugins, like the built-in Apache and Nginx plugins, implement both interfaces and perform both tasks. Others, like the built-in Standalone authenticator, implement just one interface.

There are also IDisplay plugins, which can change how prompts are displayed to a user.


Authenticators are plugins that prove control of a domain name by solving a challenge provided by the ACME server. ACME currently defines three types of challenges: HTTP, TLS-SNI, and DNS, represented by classes in acme.challenges. An authenticator plugin should implement support for at least one challenge type.

An Authenticator indicates which challenges it supports by implementing get_chall_pref(domain) to return a sorted list of challenge types in preference order.

An Authenticator must also implement perform(achalls), which “performs” a list of challenges by, for instance, provisioning a file on an HTTP server, or setting a TXT record in DNS. Once all challenges have succeeded or failed, Certbot will call the plugin’s cleanup(achalls) method to remove any files or DNS records that were needed only during authentication.


Installers plugins exist to actually setup the certificate in a server, possibly tweak the security configuration to make it more correct and secure (Fix some mixed content problems, turn on HSTS, redirect to HTTPS, etc). Installer plugins tell the main client about their abilities to do the latter via the supported_enhancements() call. We currently have two Installers in the tree, the ApacheConfigurator. and the NginxConfigurator. External projects have made some progress toward support for IIS, Icecast and Plesk.

Installers and Authenticators will oftentimes be the same class/object (because for instance both tasks can be performed by a webserver like nginx) though this is not always the case (the standalone plugin is an authenticator that listens on port 443, but it cannot install certs; a postfix plugin would be an installer but not an authenticator).

Installers and Authenticators are kept separate because it should be possible to use the StandaloneAuthenticator (it sets up its own Python server to perform challenges) with a program that cannot solve challenges itself (Such as MTA installers).

Installer Development

There are a few existing classes that may be beneficial while developing a new IInstaller. Installers aimed to reconfigure UNIX servers may use Augeas for configuration parsing and can inherit from AugeasConfigurator class to handle much of the interface. Installers that are unable to use Augeas may still find the Reverter class helpful in handling configuration checkpoints and rollback.

Writing your own plugin

Certbot client supports dynamic discovery of plugins through the setuptools entry points. This way you can, for example, create a custom implementation of IAuthenticator or the IInstaller without having to merge it with the core upstream source code. An example is provided in examples/plugins/ directory.

While developing, you can install your plugin into a Certbot development virtualenv like this:

Your plugin should show up in the output of the last command. If not, it was not installed properly.

Once you’ve finished your plugin and published it, you can have your users install it system-wide with pip install. Note that this will only work for users who have Certbot installed from OS packages or via pip. Users who run certbot-auto are currently unable to use third-party plugins. It’s technically possible to install third-party plugins into the virtualenv used by certbot-auto, but they will be wiped away when certbot-auto upgrades.


Please be aware though that as this client is still in a developer-preview stage, the API may undergo a few changes. If you believe the plugin will be beneficial to the community, please consider submitting a pull request to the repo and we will update it with any necessary API changes.

Coding style


  1. Be consistent with the rest of the code.

  2. Read PEP 8 - Style Guide for Python Code.

  3. Follow the Google Python Style Guide, with the exception that we use Sphinx-style documentation:

    def foo(arg):
        """Short description.
        :param int arg: Some number.
        :returns: Argument
        :rtype: int
        return arg
  4. Remember to use pylint.

Submitting a pull request


  1. Write your code!
  2. Make sure your environment is set up properly and that you’re in your virtualenv. You can do this by running ./tools/ (this is a very important step)
  3. Run tox -e lint to check for pylint errors. Fix any errors.
  4. Run tox --skip-missing-interpreters to run the entire test suite including coverage. The --skip-missing-interpreters argument ignores missing versions of Python needed for running the tests. Fix any errors.
  5. If your code touches communication with an ACME server/Boulder, you should run the integration tests, see integration.
  6. Submit the PR.
  7. Did your tests pass on Travis? If they didn’t, fix any errors.

Updating certbot-auto and letsencrypt-auto

Updating the scripts

Developers should not modify the certbot-auto and letsencrypt-auto files in the root directory of the repository. Rather, modify the letsencrypt-auto.template and associated platform-specific shell scripts in the letsencrypt-auto-source and letsencrypt-auto-source/pieces/bootstrappers directory, respectively.

Building letsencrypt-auto-source/letsencrypt-auto

Once changes to any of the aforementioned files have been made, the letsencrypt-auto-source/letsencrypt-auto script should be updated. In lieu of manually updating this script, run the build script, which lives at letsencrypt-auto-source/

python letsencrypt-auto-source/

Running will update the letsencrypt-auto-source/letsencrypt-auto script. Note that the certbot-auto and letsencrypt-auto scripts in the root directory of the repository will remain unchanged after this script is run. Your changes will be propagated to these files during the next release of Certbot.

Opening a PR

When opening a PR, ensure that the following files are committed:

  1. letsencrypt-auto-source/letsencrypt-auto.template and letsencrypt-auto-source/pieces/bootstrappers/*
  2. letsencrypt-auto-source/letsencrypt-auto (generated by

It might also be a good idea to double check that no changes were inadvertently made to the certbot-auto or letsencrypt-auto scripts in the root of the repository. These scripts will be updated by the core developers during the next release.

Updating the documentation

In order to generate the Sphinx documentation, run the following commands:

make -C docs clean html man

This should generate documentation in the docs/_build/html directory.

Running the client with Docker

You can use Docker Compose to quickly set up an environment for running and testing Certbot. This is especially useful for macOS users. To install Docker Compose, follow the instructions at


Linux users can simply run pip install docker-compose to get Docker Compose after installing Docker Engine and activating your shell as described in the Getting Started section.

Now you can develop on your host machine, but run Certbot and test your changes in Docker. When using docker-compose make sure you are inside your clone of the Certbot repository. As an example, you can run the following command to check for linting errors:

docker-compose run --rm --service-ports development bash -c 'tox -e lint'

You can also leave a terminal open running a shell in the Docker container and modify Certbot code in another window. The Certbot repo on your host machine is mounted inside of the container so any changes you make immediately take effect. To do this, run:

docker-compose run --rm --service-ports development bash

Now running the check for linting errors described above is as easy as:

tox -e lint

Notes on OS dependencies

OS-level dependencies can be installed like so:

letsencrypt-auto-source/letsencrypt-auto --os-packages-only

In general...

  • sudo is required as a suggested way of running privileged process
  • Python 2.6/2.7 is required
  • Augeas is required for the Python bindings
  • virtualenv and pip are used for managing other python library dependencies


For squeeze you will need to:

  • Use virtualenv --no-site-packages -p python instead of -p python2.


Package installation for FreeBSD uses pkg, not ports.

FreeBSD by default uses tcsh. In order to activate virtualenv (see below), you will need a compatible shell, e.g. pkg install bash && bash.