Securing SFTPGo with a free Let's Encrypt TLS Certificate

This tutorial shows how to create and configure a free Let's encrypt TLS certificate for the SFTPGo Web UI and REST API, the WebDAV service and the FTP service.

Obtaining a Let's Encrypt certificate involves solving a domain validation challenge issued by an ACME (Automatic Certificate Management Environment) server. This challenge verifies your ownership of the domain(s) you're trying to obtain a certificate for. Different challenge types exist, the most commonly used being HTTP-01. As its name suggests, it uses the HTTP protocol. While HTTP servers can be configured to use any TCP port, this challenge will only work on port 80 due to security measures.

More info about the supported challenge types can be found here.

There are several tools that allow you to obtain a Let's encrypt TLS certificate, in this tutorial we'll use the lego CLI.

The lego CLI supports all the Let's encrypt challenge types, in this tutorial we'll focus on HTTP-01 challenge type and make the following assumptions:

we are running SFTPGo on Linux
we need a TLS certificate for the sftpgo.com domain
we have an existing web server already running on port 80 for the sftpgo.com domain and the web root path is /var/www/sftpgo.com

Obtaining a certificate

Download the latest lego release and extract the lego binary in /usr/local/bin, then verify that it works.

lego -v
lego version 4.4.0 linux/amd64

We'll store the certificates in /var/lib/lego so create this directory.

sudo mkdir -p /var/lib/lego

Now get a certificate. The HTTP based challenge will be created in a file in /var/www/sftpgo.com/.well-known/acme-challenge. This directory must be publicly served by your web server.

sudo lego --accept-tos --path="/var/lib/lego" --email="<you email address here>" --domains="sftpgo.com" --http.webroot="/var/www/sftpgo.com" --http run

You should be now able to list your certificate.

sudo lego --path="/var/lib/lego" list
Found the following certs:
  Certificate Name: sftpgo.com
    Domains: sftpgo.com
    Expiry Date: 2021-09-09 19:41:51 +0000 UTC
    Certificate Path: /var/lib/lego/certificates/sftpgo.com.crt

Now copy the certificate inside a private path to the SFTPGo service.

sudo mkdir -p /etc/sftpgo/certs
sudo cp /var/lib/lego/certificates/sftpgo.com.{crt,key} /etc/sftpgo/certs
sudo chown -R sftpgo:sftpgo /etc/sftpgo/certs

Enable HTTPS for SFTPGo Web UI and REST API

Open the SFTPGo configuration file, search for the httpd section and change it as follow.

  "httpd": {
    "bindings": [
      {
        "port": 9443,
        "address": "",
        "enable_web_admin": true,
        "enable_web_client": true,
        "enable_https": true,
        "client_auth_type": 0,
        "tls_cipher_suites": [],
        "proxy_allowed": []
      }
    ],
    "templates_path": "/usr/share/sftpgo/templates",
    "static_files_path": "/usr/share/sftpgo/static",
    "backups_path": "/srv/sftpgo/backups",
    "web_root": "",
    "certificate_file": "/etc/sftpgo/certs/sftpgo.com.crt",
    "certificate_key_file": "/etc/sftpgo/certs/sftpgo.com.key",
    "ca_certificates": [],
    "ca_revocation_lists": []
  }

Restart SFTPGo to apply the changes. The HTTPS service is now available on port 9443.

Enable HTTPS for WebDAV service

Open the SFTPGo configuration file, search for the webdavd section and change it as follow.

  "webdavd": {
    "bindings": [
      {
        "port": 10443,
        "address": "",
        "enable_https": true,
        "client_auth_type": 0,
        "tls_cipher_suites": [],
        "prefix": "",
        "proxy_allowed": []
      }
    ],
    "certificate_file": "/etc/sftpgo/certs/sftpgo.com.crt",
    "certificate_key_file": "/etc/sftpgo/certs/sftpgo.com.key",
    ...

Restart SFTPGo to apply the changes. WebDAV is now availble over HTTPS on port 10443.

Enable explicit FTP over TLS

Open the SFTPGo configuration file, search for the ftpd section and change it as follow.

  "ftpd": {
    "bindings": [
      {
        "port": 2121,
        "address": "",
        "apply_proxy_config": true,
        "tls_mode": 1,
        "force_passive_ip": "",
        "client_auth_type": 0,
        "tls_cipher_suites": []
      }
    ],
    "banner": "",
    "banner_file": "",
    "active_transfers_port_non_20": true,
    "passive_port_range": {
      "start": 50000,
      "end": 50100
    },
    "disable_active_mode": false,
    "enable_site": false,
    "hash_support": 0,
    "combine_support": 0,
    "certificate_file": "/etc/sftpgo/certs/sftpgo.com.crt",
    "certificate_key_file": "/etc/sftpgo/certs/sftpgo.com.key",
    "ca_certificates": [],
    "ca_revocation_lists": []
  }

Restart SFTPGo to apply the changes. FTPES service is now available on port 2121 and TLS is required for both control and data connection (tls_mode is 1).

Automatic certificate renewal

SFTPGo can reload TLS certificates without service interruption, so we'll create a small bash script that copies the certificates inside the SFTPGo private directory and instructs SFTPGo to load them. We then configure lego to run this script when the certificates are renewed.

Create the file /usr/local/bin/sftpgo_lego_hook with the following contents.

#!/bin/bash

PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

CERTS_DIR=/etc/sftpgo/certs
mkdir -p ${CERTS_DIR}

cp ${LEGO_CERT_PATH} ${LEGO_CERT_KEY_PATH} ${CERTS_DIR}

chown -R sftpgo:sftpgo ${CERTS_DIR}
systemctl reload sftpgo

Ensure that this script is executable.

sudo chmod 755 /usr/local/bin/sftpgo_lego_hook

Now create a daily cron job to check the certificate expiration and renew it if necessary. For example create the file /etc/cron.daily/lego with the following contents.

#!/bin/bash

lego --accept-tos --path="/var/lib/lego" --email="<you email address here>" --domains="sftpgo.com" --http-timeout 60 --http.webroot="/var/www/sftpgo.com" --http renew --renew-hook="/usr/local/bin/sftpgo_lego_hook"

Ensure that this cron script is executable.

sudo chmod 755 /etc/cron.daily/lego

When the certificate is renewed you should see SFTPGo logs like the following to confirm that the new certificate was successfully loaded.

{"level":"debug","time":"2021-06-14T20:05:15.785","sender":"service","message":"Received reload request"}
{"level":"debug","time":"2021-06-14T20:05:15.785","sender":"httpd","message":"TLS certificate \"/etc/sftpgo/certs/sftpgo.com.crt\" successfully loaded"}
{"level":"debug","time":"2021-06-14T20:05:15.785","sender":"ftpd","message":"TLS certificate \"/etc/sftpgo/certs/sftpgo.com.crt\" successfully loaded"}
{"level":"debug","time":"2021-06-14T20:05:15.786","sender":"webdavd","message":"TLS certificate \"/etc/sftpgo/certs/sftpgo.com.crt\" successfully loaded"}

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