certsling

An opinionated script to sign tls keys via letsencrypt on your local computer by forwarding the HTTP/DNS challenge via ssh.

Installation

Best installed via pipsi:

% pipsi install certsling

Or some other way to install a python package with included scripts.

Requirements

You need an openssl executable in your path for key generation and signing.

Testing with staging server

With the -s option you can use the staging server of letsencrypt. This is advised, so you don't run into quota limits or similar until your setup works. The resulting certificate won't validate, but otherwise has the same content as a regular certificate.

Basic usage

Create a directory with the email address as the name, which you want to use for authentication with letsencrypt. For example webmaster@example.com:

% mkdir webmaster@example.com

Create a ssh connection to your server which forwards a remote port to the local port 8080:

% ssh root@example.com -R 8080:localhost:8080

On your server the webserver needs to proxy requests to example.com:80/.well-known/acme-challenge/* to that forwarded port. An example for nginx:

location /.well-known/acme-challenge/ {
    proxy_pass http://localhost:8080;
}

From the directory you created earlier, invoke the certsling script with for example:

% cd webmaster@example.com
% certsling example.com www.example.com

On first run, you are asked whether to create a user.key for authorization with letsencrypt.

After that, challenges for the selected domains are created and a server is started on port 8080 to provide responses. Your remote web server proxies them through the ssh connection to the locally running server.

If all went well, you get a server key and certificate in a new example.com folder:

% ls example.com
...
example.com-chained.crt
example.com.crt
example.com.key

The example.com-chained.crt file contains the full chain of you certificate together with the letsencrypt certificate.

Advanced usage

To use DNS based authentication, you need to have socat on your server. Additionally you need to setup your DNS, so it delegates _acme-challenge requests to your server. For that you can add something similar to this to your zone file or equivalent:

_acme-challenge IN NS www
_acme-challenge.www IN NS www

For the forwarding, you need to add port 8053:: Create a ssh connection to your server which forwards a remote port to the local port 8080:

% ssh root@example.com -R 8080:localhost:8080 -R 8053:localhost:8053

Then in that ssh session, run the following to forward UDP port 53 to TCP on port 8053:

# socat -T15 udp4-recvfrom:53,reuseaddr,fork tcp:localhost:8053

For certsling you need to add the --dns` option:

% certsling --dns example.com www.example.com

It will then first try the HTTP challenge and if that fails it will try the DNS challenge.