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Offline generation of Let’s Encrypt certificates

Sometimes we need to get a Let’s Encrypt SSL certificate for a system that might not be connected in the internet or where the certbot client is not able to be installed. There is an easy way to generate a SSL chain that we can use in our internal applications.

Install certbot

On a linux system (even a temporary one) install certbot. The example below is performed on a Ubuntu 18.04 box.

$ sudo apt install certbot
$ certbot –version
certbot 0.27.0

We are going to generate a certificate for our host with a host name hostname.domain.com. Let’s encrypt will allow an offline update through a DNS challenge so that means that during the certificate generation you should have an open screen of you DNS registrars/manager.

Initiate the request by the following command

$ sudo certbot certonly –manual –preferred-challenges dns -d hostname.domain.com
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator manual, Installer None
Obtaining a new certificate
Performing the following challenges:
dns-01 challenge for 
hostname.domain.com

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
NOTE: The IP of this machine will be publicly logged as having requested this
certificate. If you’re running certbot in manual mode on a machine that is not
your server, please ensure you’re okay with that.

Are you OK with your IP being logged?
– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
(Y)es/(N)o: Y

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –
Please deploy a DNS TXT record under the name
_acme-challenge.hostname.domain.com with the following value:

VUiRL_FOsDDlOFGYVhZCsIHVtfJ03usFLxkPfVvmOos

Before continuing, verify the record is deployed.

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –

Press Enter to Continue

Waiting for verification…

Now it is time to add the TXT record on your DNS server. As soon the record is there and you click enter the following will continue on your terminal

Cleaning up challenges

IMPORTANT NOTES:
– Congratulations! Your certificate and chain have been saved at:
/etc/letsencrypt/live/hostname.domain.com/fullchain.pem
Your key file has been saved at:
/etc/letsencrypt/live/
hostname.domain.com/privkey.pem
Your cert will expire on 2021-07-11. To obtain a new or tweaked
version of this certificate in the future, simply run certbot
again. To non-interactively renew *all* of your certificates, run
“certbot renew”
– If you like Certbot, please consider supporting our work by:

Donating to ISRG / Let’s Encrypt: https://letsencrypt.org/donate
Donating to EFF: https://eff.org/donate-le

An there you go the new private key and the certificate chain are in the default letsencrypt location at:
/etc/letsencrypt/live/hostname.domain.com/fullchain.pem
/etc/letsencrypt/live/hostname.domain.com/privkey.pem

In case you already have a CSR file from a device or server then just add the –csr to the above command with the csr file as argument:

$ sudo certbot certonly –manual –preferred-challenges dns -d hostname.domain.com –csr <csr_file.csr>

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