Lets encrypt!

Some time ago i subscribed to the Lets Encrypt beta participation program. Let’s Encrypt is a free, automated, and open certificate authority (CA), run for the public’s benefit.
Yesterday i finally get an email, telling that my requested domains (smartmontools.org/www.smartmontools.org) are white-listed. So i decided to give it a try.

Installation

To use Lets Encrypt! service you will have to install ACME protocol client. ACME (Automatic Certificate Management Environment) is a protocol to automate all operations with PKI certificates. Current implementation is written on Python language and available on the github repository. You could find a lot of information about it usage in the online manual. I was trying to run it on the CentOS 6.7 and installation failed because of old (2.6) Python version. However, after some research, i been able to find a pull request with a patch for the 2.6 support. Hopefully it will go into mainline at some point, because py26 is still widely used. After this i been able to complete installation with letsencrypt-auto.

Usage

Lets Encrypt require you to verify that you own requested domain, as most other CA do. However with ACME this cold be done 100% automatically. There are different options on how to do this, initially i tried --standalone option. With it letsencrypt client creates standalone webserver for the authentication. However if you already have web server on port 80 you will have to stop it when client is running. It was working for me, but it requires short downtime, so i decided to look on other options. After all i found webroot authenticator, which allows to just create some files in the web root and later automatically removes them. To automate the process i created configuration file /etc/letsencrypt/cli.ini:

# This is an example of the kind of things you can do in a configuration file.
# All flags used by the client can be configured here. Run Let's Encrypt with
# "--help" to learn more about the available options.

# Use a 2048 or 4096 bit RSA key
rsa-key-size = 2048

# Use production server
server =  https://acme-v01.api.letsencrypt.org/directory

# Uncomment and update to register with the specified e-mail address
email = nospam@example.com

# Uncomment to use a text interface instead of ncurses
text = True

# Uncomment to use the standalone authenticator on port 443
# authenticator = standalone
# standalone-supported-challenges = dvsni

# Uncomment to use the webroot authenticator. Replace webroot-path with the
# path to the public_html / webroot folder being served by your web server.
authenticator = webroot
webroot-path = /var/www/html/smartmontools/static

# automatically agree with license
agree-dev-preview = True

# renew certificate if it is already exists
renew-by-default = True

I also had to make sure that nginx can provide required files to the remote, so i added such lines in my nginx site configuration:

    location /.well-known/acme-challenge/ {
        alias /var/www/html/smartmontools/static/.well-known/acme-challenge/;
    }

To use certificates in nginx i added path to the new certificates and key to the configuration:

    ssl_certificate /etc/letsencrypt/live/smartmontools.org/fullchain.pem;
    ssl_certificate_key /etc/letsencrypt/live/smartmontools.org/privkey.pem;

Now to re-new my certificates i just need to run

./letsencrypt-auto -d smartmontools.org -d www.smartmontools.org certonly

command and it will do the job. Also dont forget to reload nginx service if certificates are already configured.

Notes

LetsEncrypt certificates will expire in a 90 days, so it is recommended to renew them every 60 days. Also it is very recommended to setup nagios check to send an alert if expiration time is less then one week. In the feature i would also try to use ACME client on the OpenWRT box, but hopefully there will be some more suitable alternative for the embedded hardware. Finally i would recommend to test your web server SSL configuration with SSL Server Test from SSL Labs.

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