How do I enable perfect forward secrecy by default on Apache?

How about:

SSLProtocol all -SSLv2 -SSLv3SSLHonorCipherOrder OnSSLCipherSuite EECDH+AES:EDH+AES:-SHA1:EECDH+RC4:EDH+RC4:RC4-SHA:EECDH+AES256:EDH+AES256:AES256-SHA:!aNULL:!eNULL:!EXP:!LOW:!MD5

Note the addition of the -SSLv3 flag to disable SSLv3. This is added to protect against the POODLE attack.

This will prefer perfect forward secrecy, but not at the expense of being vulnerable to the BEAST attack. Since Apache lacks a way to configure cipher preference based on protocol version, I fake it by referring to ciphers only available in the newer protocols. Specifically, AES was only available with SHA1 hashing until TLSv1.2. Thus the list starts with the TLSv1.2 ephemeral Diffie-Hellman ciphers, then RC4 (first with ephemeral DH, then without), and finally a BEAST-vulnerable AES option. Excluding no auth / weak encryption / weak hashing at the end is just for good hygiene and could be omitted since no such ciphers were introduced. If performance is a concern, use EECDH only and omit EDH.

In combination with Apache 2.2 (thus no EECDH as @Bruno says), per https://www.ssllabs.com/ssltest/analyze.html, this achieves PFS for iOS Safari only. IE and Firefox are TLSv1.0 so they get RC4 to avoid BEAST. (Alas, there is no such thing as EDH RC4, so without EECDH, you give up PFS). This is, I believe, the best one could hope for with those browsers on Apache 2.2. Chrome is the only one poorly served, since it supports TLSv1.1 and could use EDH AES without being vulnerable to BEAST. Instead, it gets RC4-RSA like Firefox and IE. Upgrading Apache to enable EECDH RC4 should get PFS for Firefox, IE, and Chrome.

Update 2013-11-09:

I've found a few alternate recommendations around the web. They put less emphasis on BEAST protection (perhaps wise; BEAST is mostly mitigated client-side now) and more emphasis on perfect forward secrecy. To varying degrees they also have stronger preferences for GCM and greater reluctance to accept RC4.

Of particular note are, I think, the following recommendations:

• Mozilla OpSec
• Ivan Ristic (of Qualys)
• Geoffroy Gramaize

Personally, I'm going to go with Mozilla OpSec's. Their reasoning is well explained on their page. Of note, they prefer AES128 over AES256. In their words: "[AES128] provides good security, is really fast, and seems to be more resistant to timing attacks."

Noteworthy in Ivan Ristic's and Geoffroy Gramaize's recommendation is that SSLv3 is disabled. I think this mostly just breaks IE6, though some security related differences between SSLv3 and TLS v1.0 are mentioned on Wikipedia.

Also before I didn't talk about CRIME and BREACH. To protect against CRIME, disable SSL compression. This is included in the examples linked. To protected against BREACH, you need to disable compression at the HTTP level. For Apache 2.4, just do this once globally:

<Location />  SetEnvIfExpr "%{HTTPS} == 'on'" no-gzip</Location>

For older versions of Apache, place this in each VirtualHost where SSLEngine is on:

<Location />    SetEnv no-gzip</Location>

Update 2014-10-14:The Mozilla OpSec guide is now split into recommendations for old/intermediate/modern compatibility. With the settings from intermediate or modern, you end up with SSLv3 disabled. That will protect against the POODLE attack.

From my own understanding, you need to activate SSLHonorCipherOrder and to prepend SSLCipherSuite with ECDHE and DHE ciphers from openssl ciphers -v

From my /etc/apache2/mods-available/ssl.conf:

SSLHonorCipherOrder onSSLCipherSuite ECDHE-RSA-RC4-SHA:ECDHE-RSA-AES128-SHA:DHE-RSA-AES128-SHA:DHE-RSA-CAMELLIA128-SHA:AES128-SHA:RC4-SHA:HIGH:!aNULL:!MD5:!ADH

To test your website, you can use: https://www.ssllabs.com/ssltest

Note: Eliptic Curve DHE only seems to work with Apache 2.3.3 or higher (see source and Bruno's comment).

The cipher suites that provide Perfect Forward Secrecy are those that use an ephemeral form of the Diffie-Hellman key exchange. Their disadvantage is their overhead, which can be improved by using the elliptic curve variants (see Vincent Bernat's blog.)

The cipher suites in Apache Httpd (provided you're using mod_ssl compiled with OpenSSL) are configured using SSLCipherSuite, which takes a list as you would see when using the openssl ciphers command. If you look at the OpenSSL man page, you'll find kEDH is what you're looking for. (You can also list cipher suites individually.)