ssl_tst.rb

#!/usr/bin/env ruby

require 'openssl'


def subject_alt_name(domains)
  domains = domains.split(/,/)
  ef = OpenSSL::X509::ExtensionFactory.new
  ef.create_extension("subjectAltName",
    domains.map { |d| "DNS: #{d}" }.join(','))
end

domainlist = 'test.example.com,test2.example.com,example.com,www.example.com'

rsa_key = OpenSSL::PKey::RSA.new(2048)
csr = OpenSSL::X509::Request.new
csr.subject = OpenSSL::X509::Name.new([
    ["C", 'AU'],
    ["ST", "NSW"],
    ["O", 'org'],
    ["CN", 'www.example.com']
  ])
csr.public_key = rsa_key.public_key

extensions = OpenSSL::ASN1::Set([OpenSSL::ASN1::Sequence(
  [subject_alt_name(domainlist)])])
csr.add_attribute(OpenSSL::X509::Attribute.new('extReq', extensions))
csr.add_attribute(OpenSSL::X509::Attribute.new('msExtReq', extensions))

csr.sign rsa_key, OpenSSL::Digest::SHA256.new
puts csr.to_pem