gistfile1.txt

#!/usr/bin/ruby

require 'openssl'

pathglob = File.join(ARGV[0], "*.crt")

sans = []

Dir.glob(pathglob).each do |cert_filename|

  cert = OpenSSL::X509::Certificate.new File.read(cert_filename)

  cert.extensions.each do |ext|
    next if ext.oid != "subjectAltName"
    ostr = OpenSSL::ASN1.decode(ext.to_der).value.last
    sequence = OpenSSL::ASN1.decode(ostr.value)
    sequence.value.each{|san|
      case san.tag
      when 2 # dNSName in GeneralName (RFC5280)
        sans << san.value
      end
    }
  end
end

puts sans.join("\n")