11.9.1. General description¶

Via the lets-encrypt resource, SFTPPlus can act as an ACME client, in order to generate or renew certificates signed by Let's Encrypt's CA.

Let’s Encrypt is a free, automated, and open certificate authority (CA), run for the public’s benefit. It is a service provided by the Internet Security Research Group (ISRG). It offers everyone a convenient way to get fairly large numbers of SSL/TLS/X.509 certificates, in an automated way, completely for free.

You can find out more about Let's Encrypt by visiting the dedicated website.

The IETF-standardized ACME protocol, RFC 8555, is the ACME v2 protocol used by SFTPPlus when interacting with the Let's Encrypt service.

The ACME v1 protocol is deprecated as of November 2019, therefore no longer supported in the latest version of SFTPPlus.

You can still use SFTPPlus with ACME v1 until June 2021, assuming that your first request for your certificate was issued before November 2019 and you keep using SFTPPlus version 3.53.0 or older.

This page focuses on the implementation and operation of SFTPPlus' Let's Encrypt ACME client. For configuration information, refer to the dedicated Let's Encrypt configuration reference page.

SFTPPlus acts as an embedded certbot. Installing certbot is not required. SFTPPlus includes its own Let's Encrypt client that is independent of certbot.

11.9.2. Available Challenge¶

A challenge is a method/process used by SFTPPlus to prove that it has control over a specific domain. This is done in order to prevent other people from obtaining certificates for the domains you own.

HTTP-01 is the only challenge supported, and it uses port 80. SFTPPlus provides a self-contained implementation of the HTTP-01 challenge. There is no need for an external web server or a separate certbot.

Wildcard certificates are not supported by the HTTP-01 challenge, hence not supported in SFTPPlus. As a workaround, you can create a single certificate with up to 100 names.

TLS-SNI-01, which uses port 443 is not supported, as it was discontinued for security reasons.

DNS-01 requires configuration of a DNS server on port 53, and it's not supported. Most often, the DNS server doesn't run on the same machine as SFTPPlus, making the deployment more complicated and introducing dependencies on external services.

11.9.3. HTTP-01 Challenge Deployment¶

In order for SFTPPlus to prove that it is the legitimate owner of the requested domain, the Let's Encrypt server will ask SFTPPlus to host a file at a fixed URL.

For example, to get a certificate for domain ftp.example.com, the SFTPPlus server should be reachable at http://ftp.example.com:80/.

This means that port 80 should be open on the server hosting the SFTPPlus instance, or be forwarded to the SFTPPlus instance.

In order to successfully obtain a certificate for a domain like ftp.your-domain.com you need to configure your DNS so that the ftp.your-domain.com domain will resolve to an IP address used by SFTPPlus. The DNS record should be configured and active before making any certificate requests.

11.9.4. Running SFTPPlus behind a firewall or load balancer¶

When SFTPPlus is behind a firewall or a load balancer, it's important to make sure that port 80 is forwarded to SFTPPlus' Let's Encrypt resource.

In such a setup, SFTPPlus's Let's Encrypt resource can be configured with any port, as long as the public port 80 is forwarded to SFTPPlus.

11.9.5. Running SFTPPlus together with an external web server on port 80¶

If you already have another server on port 80 handling the http://ftp.your-domain.com/ URLs, you should configure a proxy or redirection for the .well-known/acme-challenge/ URL.

Any request made to http://ftp.your-domain.com/.well-known/acme-challenge/ should be forwarded to http://12.23.45.67:1234/.well-known/acme-challenge/ where 12.23.45.67 and 1234 is the IP address and port of the SFTPPlus instance.

11.9.6. Setting Let's Encrypt for a new service¶

SFTPPlus can automatically manage the SSL/TLS certificates for any file transfer service which uses it.

When adding a new service, a Let's Encrypt issued certificate might not be already available. The new service will start with a provisional self-signed certificate. SFTPPlus will automatically update the certificate as soon as a new certificate is obtained from Let's Encrypt.

A self-signed certificate is auto-generated with each new SFTPPlus installation, and used by default for services created via the Web Manager.

By starting with a self-signed certificate, you can validate that all the other configurations are set up and working according to your needs. Once the service works with a self-signed certificate, you can move to using Let's Encrypt certificates.

11.9.7. Certificate Generation / Renewal¶

When the SFTPPlus lets-encrypt resource starts, it will automatically generate certificates for all newly configured domains. At start, it will check all the previously generated certificates and automatically renew certificates which are older than 60 days.

Any certificate generated by Let's Encrypt is valid for 90 days.

While the SFTPPlus lets-encrypt resource is running, it will schedule automatic renewal for the configured domains, one per day.

Once a certificate is generated or renewed, SFTPPlus will automatically restart the file transfer services configured to use those certificates.

If a certificate is about to expire in less than 15 days, SFTPPlus will emit a dedicated failure event and will try again the next day.

11.9.8. Account Registration¶

No manual or external account creation is required.

SFTPPlus will automatically create an account and register it to the Let's Encrypt server.

A registered account is required before asking the Let's Encrypt server to issue new certificates.

If you already have a Let's Encrypt account and would like to use that account for the certificates managed by SFTPPlus, get in touch at support@proatria.com, and we will provide instructions for setting up an existing account.

11.9.9. Testing and Experimentation¶

You can check that SFTPPlus' Let's Encrypt resource is correctly configured by accessing the following URL from a remote computer: http://ftp.your-domain.com/.well-known/acme-challenge/test.txt.

You should see a page with the Let's Encrypt Ready text.

For testing or casually checking out the Let's Encrypt integration, we highly recommend testing against the Let's Encrypt staging environment before using the Let's Encrypt production environment.

This will allow you to get things right before issuing trusted certificates, and reduce the chance of hitting the request rate limits.

The acme_url configuration option is used to instruct SFTPPlus to use different Let's Encrypt environments.

For production, the configuration will look like:

[resources/17c97aa6-1c17-4485-878c-68b427b82f35]
type = lets-encrypt
name = lets-encrypt-public

address = 0.0.0.0
port = 80
acme_url = https://acme-v02.api.letsencrypt.org/directory
contact_email = admin-contact@your.domain.tld

For testing/staging, the configuration will look like:

[resources/17c97aa6-1c17-4485-878c-68b427b82f35]
type = lets-encrypt
name = lets-encrypt-testing

address = 0.0.0.0
port = 80
acme_url = https://acme-staging-v02.api.letsencrypt.org/directory

11.9.10. Let's Encrypt Request Limits¶

The main Let's Encrypt ACME server imposes a set of limits, in terms of how often you can request new certificates.

SFTPPlus does not impose any extra limit, and will rely on the limits defined on the remote Let's Encrypt / ACME server.

You will see an error message when SFTPPlus operations exceed a certain limit.

You can find up to date information about the certificate generation limits on the Let's Encrypt Rate Limit <https://letsencrypt.org/docs/rate-limits/> page.

When testing an SFTPPlus deployment, use the Let's Encrypt staging environment, as documented in previous sections.