cert.rb

#!/usr/bin/env ruby

domain = "localhost"
organisation = "example"
country = "AT"
subjectAltDomains = [
  "DNS:#{domain}",
  "DNS:localhost.localdomain",
  "IP:127.0.0.1",
  "IP:10.0.2.2"
]

require 'openssl'
puts "Generating public and private keys..."
key = OpenSSL::PKey::RSA.new(4096)

subject = "/C=#{country}/O=#{organisation}/CN=#{domain}"

cert = OpenSSL::X509::Certificate.new
cert.subject = cert.issuer = OpenSSL::X509::Name.parse(subject)

cert.not_before = Time.now
cert.not_after = Time.now + 365*24*60*60
cert.public_key = key.public_key
cert.serial = 0x0
cert.version = 2

puts "Signing certificate..."
ef = OpenSSL::X509::ExtensionFactory.new
ef.subject_certificate = ef.issuer_certificate = cert

cert.extensions = [
  ef.create_extension("basicConstraints","CA:FALSE", true),
  ef.create_extension("subjectKeyIdentifier", "hash")
]
cert.add_extension ef.create_extension("authorityKeyIdentifier",
                                 "keyid:always,issuer:always")
cert.add_extension ef.create_extension("subjectAltName", subjectAltDomains.join(','))


cert.sign key, OpenSSL::Digest::SHA256.new

File.open("cert.pem", "w") { |f| f.write(cert.to_pem) }
File.open("key.pem", "w") { |f| f.write(key.to_s) }