ssl_allow_cname

ssl_allow_cname adds a parameter to Ruby's OpenSSL library: allow_cname.

This is for cases when you don't care about the host matching the CommonName or a SubjectAlternateName of a certificate (e.g., you've got other security measures), but surely don't want to turn off all peer verification.

Here's an example:

# This actually works, using the Ruby redis client's SSL support:

redis = Redis.new(
  # Say the other side of this connection has a certificate you've signed, but
  # you don't care what VPS instance it ends on with an arbitrary IP.
  url: 'rediss://198.199.120.202/',  
  ssl_params: {
    # And you only trust yourself as a CA.
    ca_file: '/etc/ssl/metermd/metermd-ca.crt',

    # And the following cert has been signed by your CA, and the key's valid...
    cert: OpenSSL::X509::Certificate.new(File.read('/etc/ssl/metermd/redis-client.crt')),
    key: OpenSSL::PKey::RSA.new(File.read('/etc/ssl/metermd/redis-client.keydh')),

    # You just care that you've authorized this certificate for this purpose,
    # and rely on the security of your CA being legit.
    allow_cname: 'redis-server'
  }
)

Using the allow_cname option disables host verification, but specifying allow_cname: :match will give you the same behavior as peer verification. When you don't specify allow_cname, everything works the same as out-of-the- box.

The value passed to allow_cname can take a few forms:

• A String, which means the CommonName presented must exactly match what you've specified.
• A Regexp, which will pass if it matches the CommonName of the peer certificate.
• A Proc, which can accept either (common_name) or (common_name, host) argument lists. Return true if you like it, false otherwise.
• The symbol :match, which accepts anything OpenSSL would've considered valid.
• An Array of any of the above, which operates in an OR, not AND, fashion.

For simplicity, and to make it easier to not get wrong, ssl_allow_cname does not consider SubjectAlternateNames, just the first CommonName. If you're running your own CA, you'll be able to arrange this.