Linux SSL/TLS

The main SSL tools on Linux to manage certificates are

• keytool
• openssl

Usefull OpenSSL commands

Renew a certificate based on the current certificate

openssl x509 -x509toreq -in current_certificate.crt -out new_certificate_request.csr -signkey the_current_certificate_private_key.key

Verify a certificate from a CLI

curl --insecure -v https://www.example.com/Managing-Your-Account 2>&1 | awk 'BEGIN { cert=0 } /^\* Server certificate:/ { cert=1 } /^\*/ { if (cert) print }'

If your webserver uses SNI scheme rathcer is IP based, specify the vhost server using -servername option, otherwise can be skipped

echo | openssl s_client -connect <web_server_IP>:443 -servername www.example.com 2>/dev/null | openssl x509 -inform pem -noout -text

                                     Apache_server        Hosted_website_in_virtual_host_directive
                                           |                               |
lc0hymgt01# openssl s_client -connect 10.0.0.1:443 -servername www.example.com

OpenSSL test modules

With Server Name Indication (SNI), a web server can have multiple SSL certificates installed on the same IP address. SNI-capable browsers will specify the hostname of the server they’re trying to reach during the initial handshake process. This allows the web server to determine the correct SSL certificate to use for the connection. If you try to connect to Small Sites server with s_client, you’ll find that you receive the default SSL certificate installed on my server and not the one for this site. Therefore we need to use the -servername argument and s_client will do the additional SNI negotiation step.

OpenSSL provides three modules that allow you to test SSL connections: s_client, s_server and s_time. The first two, as the names suggest, are for simulating a client and a server in an SSL connection. The third one is for connection timing tests. I’ll start with a closer look at the s_client module.

S_client is particularly useful for checking which protocols and which ciphers the server agrees to use. This information is useful in security and functionality audits. For example, you could use this protocol information to find servers that don’t accept a legitimate protocol or cipher, thus preventing a legitimate client from connecting. You could also locate servers that accept weak protocols or ciphers and could thus allow a malicious attack.

s_client - output interpretation - https://www.feistyduck.com/library/openssl-cookbook/online/ch-testing-with-openssl.html

Preview a certificate from CLI

openssl x509 -in /etc/httpd/conf/ssl.crt/certificate.crt -text -noout

Disabling Diffie-Hellman on Apache Servers

On each web server, in the ssl.conf file or, in some cases, the main Apache conf file, add the !DH: identifier to the start of the SSLCipherSuite config option string.

Procedure In Apache's conf directory, locate file: ssl.conf or httpd.conf

Look for the SSLCipherSuite keyword, whose string value must be similar to the following string:

"ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP"

Add !DH: after the ALL: list so that the line looks like the following string:

"ALL:!DH:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP"

Note: The !ADH: string in the above string is now redundant and can be removed. Repeat this edit in every SSL config section, if you are not using one global section. Save the file. Restart the web server for the changes to take effect.