This article details the commands used to administratively control the rekey and renewal behaviors of Let's Encrypt's certbot.
Detailed will be how to create the public/private key pair and certificate signing request manually and then request the Let's Encrypt certificate with certbot's certonly
command line option. This essentially avoids the automation of the renew
command requiring that we perform the various steps manually.
Certbot's renew
command by default generates a new public/private key pair and certificate signing request for each renewal. This means cerbot is 'rekeying' the certificate with each renewal. This is perfectly acceptable solution for most use cases of Let's Encrypt certificates. However, rekeying can optimally be done less frequently such as once a year or longer. Certbot's certonly
command line option allows you to specify your own certificate signing request instead of accepting one automatically generated by Certbot's renew
command. At each renewal interval, the same unmodified CSR is used to request a renewed certificate.
This method faciliates the option to continue using the same CSR for ongoing certificate renewal requests. Then rekey the certificate signing request at your desired long-term interval by generating a new key pair and CSR just prior to the next renewal request.
One specific reason one might choose this method, would be to maintain the same unchanging public key of a certificate. DANE TLSA DNS resource records are a hash of a certificate's public key and would need to be updated every time the public key changes. Let's Encrypt certificates are valid for only 3 months which would require updating the respective TLSA DNS RRs just as frequently. Manual rekeying can be scheduled to occur over longer time frames, respective DNS RRs would only need to be updated at that time.
Manually Create a Key Pair, CSR and Request a New Certificate
Start by creating then cd into a directory to work in, then generate a private/public key pair.
openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out rsa-privkey.pem
openssl pkey -in rsa-privkey.pem-out rsa-pubkey.pem -pubout
Next create a stripped down version of openssl.cnf
based on the following template and save it as csr.cnf
[ req ] prompt = no default_bits = 2048 distinguished_name = req_distinguished_name req_extensions = req_ext [ req_distinguished_name ] CN = example.com [ req_ext ] basicConstraints = CA:FALSE keyUsage = nonRepudiation, digitalSignature, keyEncipherment subjectAltName = @alt_names [ alt_names ] DNS.1 = example.com DNS.2 = www.example.com
In section [ req_distinguished_name ] change Common Name CN = example.com to the FQDN of your server.
In section [ alt_names ] add a DNS.1 = example.com statement with the FQDN of your server.
Optionally add any additional FQDN DNS entries that the server uses and you want included in the certificate. Only one DNS entry is required, the rest is optional based on your specific requirements.
Generate the certificate signing request with the custom csr.cnf
.openssl req -new -sha256 -key rsa-privkey.pem -nodes -config csr.cnf -outform pem -out rsa-csr.pem
Optionally to view and confirm the contents of the CSR in plain text.openssl req -in rsa-csr.pem -text
Request a new certificate with the new CSR.certbot certonly --standalone --csr rsa-csr.pem
Upon successful execution, certbot will drop three certificates in the current working directory.0000_cert.pem
equivalent to cert.pem
0000_chain.pem
equivalent to chain.pem
0001_chain.pem
equivalent to fullchain.pem
To view the contents of a certificate in plain text.openssl x509 -in 0000_cert.pem -text
A Scripted Solution For Automation
I am providing below the two scripts that I am personally using in my production environment.
Several important things to note:
- The scripts are written in
shells/bash
making this a run-time dependency. - Please comb through and edit the scripts to suit your environment.
- The scripts include the DANE TLSA RR management using
nsupdate
to update a local BIND name server. This is optional and can be edited out. - The
cerbot
command line in these scripts specifies--standalone
authentication. Substitute this with your preferred authentication method. - When initially testing the scripts, insert the
--dry-run
directive into each of thecertbot
command lines. Remove the directive when everything executes error free, to begin receiving your live certificates.
The purpose of rekey-cert
script is to generate a new public/private key pair and subsequent new certificate signing request, saving them in a rekey/
subdirectory. This script can be run at some extended interval of time such as once a year or longer.
The renew-cert
script renews the certificate using the existing CSR. This script will be run every two months.
If however the rekey/
subdirectory exists, then the script first moves the new key pair and CSR files from rekey/
to the main script directory, and deletes the now empty rekey/
subdirectory. Then the script makes the usual renewal request using the new certificate signing request.
To begin, create a directory at any desired location and copy into it csr.cnf
that was created in the previous section, as well as the following two scripts.
rekey-cert
#! /usr/local/bin/bash # rekey-cert v1.06 set -euo pipefail mydir=$(dirname $(readlink -f $0)) cd $mydir # Create rekey/ directory if it does not exist, exit if it does exist. if [ ! -d rekey/ ]; then mkdir rekey cd rekey/ else printf "\nThe rekey/ directory already exists!\n" printf "Either execute renew-cert next, or delete the rekey/ directory and rerun this script.\n\n" exit 1 fi # Generate an RSA private key printf "Generating New...\n" openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:2048 -out rsa-privkey.pem printf "✔ Private Key\n" # Generate the corresponding public key openssl pkey -in rsa-privkey.pem -out rsa-pubkey.pem -pubout printf "✔ Public Key\n" # Generate the certificate signing request openssl req -new -sha256 -key rsa-privkey.pem -nodes -config ../csr.cnf -outform pem -out rsa-csr.pem printf "✔ Certificate Signing Request\n" # Save the new TLSA hash openssl req -noout -pubkey -in rsa-csr.pem | openssl rsa -pubin -outform DER | sha256 > tlsa311 printf "✔ TLSA Hash\n" # Add the new TLSA 3 1 1 DNS resource records NEWTLSA=`cat tlsa311` printf "server zone-master.example.com\n" > nsupdate printf "update add _443._tcp.example.com. 86400 IN TLSA 3 1 1 $NEWTLSA\n" >> nsupdate printf "update add _443._tcp.www.example.com. 86400 IN TLSA 3 1 1 $NEWTLSA\n" >> nsupdate printf "send\n" >> nsupdate printf "update add _443._tcp.example.org. 86400 IN TLSA 3 1 1 $NEWTLSA\n" >> nsupdate printf "update add _443._tcp.www.example.org. 86400 IN TLSA 3 1 1 $NEWTLSA\n" >> nsupdate printf "send\n" >> nsupdate printf "quit\n" >> nsupdate nsupdate -k ../ddns-key nsupdate rm -f nsupdate printf "✔ TLSA Resource Record Added to DNS\n" printf "\nAll Rekey Tasks Completed Successfully!\n\n"
renew-cert
#! /usr/local/bin/bash # renew-cert v1.06 set -euo pipefail mydir=$(dirname $(readlink -f $0)) cd $mydir if [ ! -f rsa-privkey.pem ] && [ ! -f rekey/rsa-privkey.pem ]; then # A private key doesn't exist yet therefore this is the first-run # Executing rekey-cert to create initial key pair and CSR ./rekey-cert fi if [ ! -d rekey/ ]; then # If the rekey/ directory doesn't exist then simply # renew the certificate with the existing CSR certbot certonly --standalone --csr rsa-csr.pem -n else # If the rekey/ directory does exist then perform additional rekey related # tasks and renew the certificate with the new CSR if [ -f rsa-privkey.pem ]; then # Backup existing CSR and key files mv -f rsa-privkey.pem rsa-privkey.bak mv -f rsa-pubkey.pem rsa-pubkey.bak mv -f rsa-csr.pem rsa-csr.bak mv -f tlsa311 tlsa311.bak fi # Move all files from rekey/ to current directory then remove rekey/ mv -f rekey/* ./ rm -rf rekey/ # Renew Let's Encrypt Certificate With New CSR certbot certonly --standalone --csr rsa-csr.pem -n if [ -f tlsa311.bak ] && [ -f 0000_cert.pem ]; then # Remove Prior TLSA 3 1 1 DNS Resource Records OLDTLSA=`cat tlsa311.bak` printf "server zone-master.example.com\n" > nsupdate printf "update delete _443._tcp.example.com. 86400 IN TLSA 3 1 1 $OLDTLSA\n" >> nsupdate printf "update delete _443._tcp.www.example.com. 86400 IN TLSA 3 1 1 $OLDTLSA\n" >> nsupdate printf "send\n" >> nsupdate printf "update delete _443._tcp.example.org. 86400 IN TLSA 3 1 1 $OLDTLSA\n" >> nsupdate printf "update delete _443._tcp.www.example.org. 86400 IN TLSA 3 1 1 $OLDTLSA\n" >> nsupdate printf "send\n" >> nsupdate printf "quit\n" >> nsupdate nsupdate -k ddns-key nsupdate rm -f nsupdate fi fi if [ -f 0000_cert.pem ]; then if [ -f cert.pem ]; then # Backup existing certificates mv -f cert.pem cert.bak mv -f chain.pem chain.bak mv -f fullchain.pem fullchain.bak fi # Rename the newly acquired certificates mv -f 0000_cert.pem cert.pem mv -f 0000_chain.pem chain.pem mv -f 0001_chain.pem fullchain.pem printf "Certificate Renewal Successful!\n\n" else printf "Certificate Renewal Failed!\nAdditional steps have been skipped, and no changes have been made.\n\n" fi
Schedule Execution of Renew and Rekey Scripts
Each script can be scheduled in cron at your desired intervals. My personal preference is to renew every 2 months and to rekey once a year.
0 0 27 11 * ( /root/scripts/letsencrypt/rekey-cert ) 0 0 1 2,4,6,8,10,12 * ( /root/scripts/letsencrypt/renew-cert )
Every Nov 27 at midnight 00:00 rekey-cert
is run.
On the first day of every other month (Feb Apr June Aug Oct Dec) at midnight 00:00 renew-cert
is run.
Additional Certbot Information
The certbot user guide can be found at: https://eff-certbot.readthedocs.io/en/stable/using.html
Cerbot command-line options: https://eff-certbot.readthedocs.io/en/stable/using.html#certbot-command-line-options