Testing for weak Diffie-Hellman HTTPS (Logjam) keys

To test a HTTP/S server for weak Diffie-Hellman (DH) SSL / TLS ciphers, you may use the following command (Linux):

$ openssl s_client -connect [target]:443 -cipher "EDH"

EDH requires use of weak DH keys. If it connects, you may GET / HTTP/1.0 to confirm.

A secure host should not connect, e.g.

$ openssl s_client -connect www.gmail.com:443 -cipher "EDH"
CONNECTED(00000003)
139671352862352:error:14077410:SSL routines:SSL23_GET_SERVER_HELLO:sslv3 alert handshake failure:s23_clnt.c:770:

https://weakdh.org/