Let’s Encrypt is an open, free, and completely automated Certificate Authority from the non-profit Internet Security Research Group (ISRG). The goal of Let’s Encrypt is to encrypt the web by removing the cost barrier and some of the technical barriers that discourage server administrators and organizations from obtaining certificates for use on Internet servers, primarily web servers. Most browsers trust certificates from Let’s Encrypt. These certificates can be used for web servers (HTTPS), SMTP servers, IMAP/POP3 servers, and other similar roles which utilize the same type of certificates.
The ACME Package for pfSense interfaces with Let’s Encrypt to handle the certificate generation, validation, and renewal processes.
Certificates from Let’s Encrypt are domain validated, and this validation ensures that the system requesting the certificate has authority over the domain in question. This validation can be performed in a number of ways, such as by proving ownership of the domain’s DNS records or hosting a file on a web server for the domain.
By using a certificate from Let’s Encrypt for a web server, including a firewall running pfSense software, the browser will trust the certificate and show a green check mark, padlock, or similar indication. The connection will be encrypted without the need for manually trusting an invalid certificate.
Let’s Encrypt certificates are valid for a period of 90 days, so they must be renewed periodically. The ACME package automates this renewal by using a cron job to check once per day to see if a certificate needs to be renewed.