How to get rid of the weak cipher (TLS_RSA_WITH_RC4_128_SHA) and SSLv3 in Exim4 with GnuTLS (which is the default on Ubuntu 14.04) using the tls_require_ciphers instruction in the exim config.
A few weeks ago i fiddled around with letsencrypt and exim. Getting the certs working in exim took a while, but that is for another story)
After i got the Certs working i did some testing. For this i used ssl-tools.net (among others). It reported several issues. I had SSLv3 enabled and this weird TLS_RSA_WITH_RC4_128_SHA **was reported as a weak cipher. Now began the journey that lasted several hours. I quickly arrived at the gnutls manual with a list of “valid”config options. Which gave me the syntax to disable SSLv3 (just add -VERS-SSL3.0 to the available ciphers (the **- disables this cipher))
Now ss-tools reported only a single issue with the weak TLS_RSA_WITH_RC4_128_SHA cipher. After some more googling around, and talking to other sysadmins (who were using openssl instead of gnutls) i just tried our the several options from the gnutls manual. A lot of stuff didn’t work because my version is probably too old. Often when i got a working combination of enabled/disabled ciphers ssl-tools reported that my exim does not support PerfectForwardSecrecy(PFS), but at least all weak ciphers were gone. It was ping-pong between weak ciphers, no PFS or some random error. I just could not find the gnutls name that would disable the reported weak cipher. Then i found a name translation table for cipher names according to the standard and their corresponding names in gnutls and openssl. That finally gave me the idea. I had to disable ARCFOUR-128. **So my final working tls_require_ciphers looked something like that **“SECURE:+VERS-TLS-ALL:-VERS-SSL3.0:-ARCFOUR-128”.
In exim4 with GnuTLS on Ubuntu 14.04 to disable SSLv3 and TLS_RSA_WITH_RC4_128_SHA while keeping PFS, disable TLS_RSA_WITH_RC4_128_SHA with -ARCFOUR-128 **and SSLv3 with -VERS-SSLv3.0**