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TLS/SSL related stuff

Prerequisites

If needed, in Debian or Ubutnu gnutls-cli can be installed using

apt install gnutls-bin

Check certificate of a Web server

Using OpenSSL:

openssl s_client -verify_return_error -connect <HOST>:443

Using GnuTLS:

gnu-tls-cli <HOST>

Check certificate of an SMTP server

Using OpenSSL:

openssl s_client -verify_return_error -starttls smtp -connect <HOST>:25

Using GnuTLS 

gnutls-cli --starttls-proto smtp <HOST>

Check security of an existing TLS setup

This needs Docker be installed so you can run Docker images.

Create a file /usr/local/bin/testssl with the following content:

#!/bin/sh
docker run --rm -ti drwetter/testssl.sh $1

The run the following command to test the TLS setup for the web server running at domain.example:

testssl domain.example

You can also test other services by adding the port number - for example for IMAP with TLS at port 993:

testssl domain.example:993