Certbot

Certbot is Electronic Frontier Foundation's ACME client, which is written in Python and provides conveniences like automatic web server configuration and a built-in webserver for the HTTP challenge. Certbot is recommended by Let's Encrypt.

Installation

Install the certbot package.

Plugins are available for automated configuration and installation of the issued certificates in web servers:

• The Nginx plugin can be installed with the certbot-nginx package.
• The Apache HTTP Server plugin can be installed with the certbot-apache package.

Configuration

Consult the Certbot documentation for more information about creation and usage of certificates.

Plugins

Warning: Configuration files may be rewritten to add settings and paths for Certbot certificates when using a plugin. Creating a backup first is recommended.

Nginx

The plugin certbot-nginx provides an automatic configuration for nginx. This plugin will try to detect the configuration setup for each domain. The plugin adds extra configuration recommended for security, settings for certificate use, and paths to Certbot certificates. See #Managing Nginx server blocks for examples.

First time setup of server-blocks:

# certbot --nginx

To renew certificates:

# certbot renew

To change certificates without modifying nginx configuration files:

# certbot --nginx certonly

See Certbot-Nginx on Arch Linux for more information and #Automatic renewal to keep installed certificates valid.

Managing Nginx server blocks

The following example may be used in all server blocks when managing these files manually:

/etc/nginx/sites-available/example

server {
  listen 443 ssl http2;
  listen [::]:443 ssl http2; # Listen on IPv6
  ssl_certificate /etc/letsencrypt/live/domain/fullchain.pem; # managed by Certbot
  ssl_certificate_key /etc/letsencrypt/live/domain/privkey.pem; # managed by Certbot
  include /etc/letsencrypt/options-ssl-nginx.conf;
  ..
}

See nginx#TLS for more information.

It is also possible to create a separated configuration file and include it in each server block:

/etc/nginx/conf/001-certbot.conf

ssl_certificate /etc/letsencrypt/live/domain/fullchain.pem; # managed by Certbot
ssl_certificate_key /etc/letsencrypt/live/domain/privkey.pem; # managed by Certbot
include /etc/letsencrypt/options-ssl-nginx.conf;

/etc/nginx/sites-available/example

server {
  listen 443 ssl http2;
  listen [::]:443 ssl http2; # Listen on IPv6
  include conf/001-certbot.conf;
  ..
}

Apache

The plugin certbot-apache provides an automatic configuration for the Apache HTTP Server. This plugin will try to detect the configuration setup for each domain. The plugin adds extra configuration recommended for security, settings for certificate use, and paths to Certbot certificates. See #Managing Apache virtual hosts for examples.

First time setup of virtual hosts:

# certbot --apache

To renew certificates:

# certbot renew

To change certificates without modifying Apache configuration files:

# certbot --apache certonly

See Certbot-Apache on Arch Linux for more information and #Automatic renewal to keep installed certificates valid.

Managing Apache virtual hosts

The following example may be used in all virtual hosts when managing these files manually:

/etc/httpd/conf/extra/001-certbot.conf

<IfModule mod_ssl.c>
<VirtualHost *:443>

Include /etc/letsencrypt/options-ssl-apache.conf
SSLCertificateFile /etc/letsencrypt/live/'domain'/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/'domain'/privkey.pem

</VirtualHost>
</IfModule>

When the VirtualHost above is automatically generated by certbot-apache, it looks like sometimes the 'Include' line may appear after the two 'SSLCertificate' lines. When this is the case, accessing the virtualHost from a browser using http wont work with an error SSL_PROTOCOL_ERROR. If this happens, simply put the Include line above the two SSLCertificates lines and restart httpd.

/etc/httpd/conf/httpd.conf

  <IfModule mod_ssl.c>
  Listen 443
  </IfModule>

  Include conf/extra/001-certbot.conf
  ..

See Apache HTTP Server#TLS for more information.

Webroot

Note:

• The Webroot method requires HTTP on port 80 for Certbot to validate.
• The Server Name must match that of its corresponding DNS.
• Permissions may need to be altered on the host to allow read-access to http://domain.tld/.well-known.

When using the webroot method the Certbot client places a challenge response inside /path/to/domain.tld/html/.well-known/acme-challenge/ which is used for validation.

The use of this method is recommended over a manual install; it offers automatic renewal and easier certificate management. However the usage of #Plugins may be the preferred since it allows automatic configuration and installation.

Mapping ACME-challenge requests

Management of can be made easier by mapping all HTTP-requests for .well-known/acme-challenge to a single folder, e.g. /var/lib/letsencrypt.

The path has then to be writable for Cerbot and the web server (e.g. nginx or Apache HTTP Server running as user http):

# mkdir -p /var/lib/letsencrypt/.well-known
# chgrp http /var/lib/letsencrypt
# chmod g+s /var/lib/letsencrypt

nginx

Create a file containing the location block and include this inside a server block:

/etc/nginx/conf.d/letsencrypt.conf

location ^~ /.well-known/acme-challenge/ {
  allow all;
  root /var/lib/letsencrypt/;
  default_type "text/plain";
  try_files $uri =404;
}

Example of a server configuration:

/etc/nginx/servers-available/domain.conf

server {
  server_name domain.tld
   ..
  include conf.d/letsencrypt.conf;
}

Apache

Create the file /etc/httpd/conf/extra/httpd-acme.conf:

/etc/httpd/conf/extra/httpd-acme.conf

Alias /.well-known/acme-challenge/ "/var/lib/letsencrypt/.well-known/acme-challenge/"
<Directory "/var/lib/letsencrypt/">
    AllowOverride None
    Options MultiViews SymLinksIfOwnerMatch IncludesNoExec
    Require method GET POST OPTIONS
</Directory>

Including this in /etc/httpd/conf/httpd.conf:

/etc/httpd/conf/httpd.conf

Include conf/extra/httpd-acme.conf

Obtain certificate(s)

Request a certificate for domain.tld using /var/lib/letsencrypt/ as public accessible path:

# certbot certonly --email email@example.com --webroot -w /var/lib/letsencrypt/ -d domain.tld

To add a (sub)domain, include all registered domains used on the current setup:

# certbot certonly --email email@example.com --webroot -w /var/lib/letsencrypt/ -d domain.tld,sub.domain.tld

To renew (all) the current certificate(s):

# certbot renew

Note: Do not forget to include the Let's Encrypt configuration file inside all related server blocks, those handling HTTP traffic as well as those handling HTTPS traffic, especially when using - as recommended - HSTS (HTTP Strict Transport Security). This can lead one to obtain certificate(s) smoothly, and once the setup is done, to be blocked for the renewal.

See #Automatic renewal as alternative approach.

Manual

If there is no plugin for your web server, use the following command:

# certbot certonly --manual

When preferring to use DNS challenge (TXT record) use:

# certbot certonly --manual --preferred-challenges dns

This will automatically verify your domain and create a private key and certificate pair. These are placed in /etc/letsencrypt/archive/your.domain/ and symlinked from /etc/letsencrypt/live/your.domain/.

You can then manually configure your web server to reference the private key, certificate and full certificate chain in the symlinked directory.

Note: Running this command multiple times, or renewing certificates will create multiple sets of files with a trailing number in /etc/letsencrypt/archive/your.domain/. Certbot automatically updates the symlinks in /etc/letsencrypt/live/your.domain/ to point to the latest instances of files so there is no need to update your webserver to point to the new key material.

DNS challenge

For servers which are not exposed to public internet, DNS-01 challenge can be used to verify domain ownership

Install the certbot plugin for your dns provider certbot-dns-*. Below example shows for cloudflare using certbot-dns-cloudflare.

Create a Credential file

/etc/certbot-cloudflare.creds.ini

dns_cloudflare_api_token = XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Get certificate.

# chmod 600 /etc/certbot-cloudflare.creds.ini
# certbot certonly --dns-cloudflare --dns-cloudflare-credentials /etc/certbot-cloudflare.creds.ini -d domain.tld

certbot-renew.service will renew automatically using the above credential file #Automatic renewal. See docs for obtaining api token and other options.

Advanced configuration

Automatic renewal

systemd

certbot comes with a systemd certbot-renew.service, which attempts to renew certificates that expire in less than 30 days. If all certificates are not due for renewal, this service does nothing.

If you do not use a plugin to manage the web server configuration automatically, the web server has to be reloaded manually to reload the certificates each time they are renewed. This can be done by adding --post-hook "systemctl reload nginx.service" to the ExecStart command [1]. Of course use httpd.service instead of nginx.service if appropriate. Additional services can be added to the systemctl command as needed, e.g. systemctl reload httpd dovecot postfix.

Note: Before enabling the certbot-renew.timer, check that the service is working correctly and is not trying to prompt anything. Note that the service may take up to 480 seconds to complete since a delay is added to calling certbot non-interactively since v0.29.0.

Enable and start certbot-renew.timer to check for certificate renewal twice a day, including a randomized delay so that everyone's requests for renewal will be spread over the day to lighten the Let's Encrypt server load [2].

Automatic renewal for wildcard certificates

The process is fairly simple. To issue a wildcard certificate, you have to do it via a DNS challenge request, using the ACMEv2 protocol.

While issuing a certificate manually is easy, it is not straight forward for automation. The DNS challenge represents a TXT record, given by certbot, which has to be set manually in the domain zone file.

You will need to update the zone file upon every renew. To avoid doing that manually, you may use RFC 2136 for which certbot has a plugin packaged in certbot-dns-rfc2136. You will also need to configure your DNS server to allow dynamic updates for TXT records.

Configure BIND for rfc2136

Generate a TSIG secret key:

$ tsig-keygen -a HMAC-SHA512 example-key

and add it in the configuration file:

/etc/named.conf

...
zone "domain.ltd" IN {
        ...
        // this is for certbot
        update-policy {
                grant example-key name _acme-challenge.domain.ltd. txt;
        };
        ...
};

key "example-key" {
        algorithm hmac-sha512;
        secret "a_secret_key";
};
...

Restart named.service.

Configure certbot for rfc2136

Install certbot-dns-rfc2136, then create a configuration file for it.

/etc/letsencrypt/rfc2136.ini

dns_rfc2136_server = IP.ADD.RE.SS
dns_rfc2136_name = example-key
dns_rfc2136_secret = INSERT_KEY_WITHOUT_QUOTES
dns_rfc2136_algorithm = HMAC-SHA512

Since the file contains a copy of the secret key, secure it with chmod by removing the group and others permissions.

Test what we did:

# certbot certonly --dns-rfc2136 --force-renewal --dns-rfc2136-credentials /etc/letsencrypt/rfc2136.ini --server https://acme-v02.api.letsencrypt.org/directory --email example@domain.ltd --agree-tos --no-eff-email -d 'domain.ltd' -d '*.domain.ltd'

If you pass the validation successfully and receive certificates, then you are good to go with automating certbot. Otherwise, something went wrong and you need to debug your setup. It basically boils down to running certbot renew from now on, see #Automatic renewal.

Deploying certificates to applications

certbot saves the certificates under /etc/letsencrypt/live. This directory can only be read by root user. A deploy hook script can be used to copy the certificates to app directory when they are renewed.

Example script to copy certs to app's state directory with correct permissions and restart app service. Create below script with executable permissions.

/etc/letsencrypt/renewal-hooks/deploy/myapp_cert_copy.sh

#!/bin/sh
set -eu

if [ "$RENEWED_DOMAINS" = "myapp.mydomain.com" ]
then
	app=myapp
	appuser=$app
	certpath="/var/lib/$app/certs"

	mkdir -p "$certpath"
	chmod 750 "$certpath"

	chown $appuser:$appuser "$certpath"
	install -o "$appuser" -g "$appuser" -m 444 "$RENEWED_LINEAGE/fullchain.pem" -t "$certpath"
	install -o "$appuser" -g "$appuser" -m 400 "$RENEWED_LINEAGE/privkey.pem" -t "$certpath"

	systemctl restart $app.service
	echo "$(date) Renewed and deployed certificates for $app" >> /var/log/cert-renew.log
fi

If certificate was already generated, the deploy script can be run using this command to copy the certs without regenerating new certificates:

$ certbot certonly -d myapp.mydomain.com --run-deploy-hooks --dry-run

See also

• Transport Layer Security#ACME clients
• EFF's Certbot documentation
• Let's Encrypt – List of ACME clients