Configuration file generator for OpenVPN which also sets up a ca and generates keys.

vpngen.rb

#!/usr/bin/env ruby

# Just call this without arguments. It will show a friendly help text.
# For xterm-256color, it will even use colors for some commands!

class AppConfig
  @@default_remote   = 'vpn.example.com'
  @@default_networks = '10.0.0.0/24>192.168.1.0/24'
  @@default_subject  = '/C=US/ST=CA/L=San Francisco/O=Example/OU=/CN={{name}}'

  # If you want to use eliptic curve crypto, change the code in Crypto below.
  # As is, it only uses RSA, AES in CBC mode and SHA

  # lookup with: openvpn --show-ciphers
  @@default_cipher = 'AES-256-CBC'
  # lookup with: openvpn --show-tls
  @@default_tls_ciphers = %w(
    TLS-DHE-RSA-WITH-AES-256-GCM-SHA384
    TLS-DHE-RSA-WITH-AES-256-CBC-SHA256
    TLS-DHE-RSA-WITH-AES-128-GCM-SHA256
    TLS-DHE-RSA-WITH-AES-128-CBC-SHA256
    TLS-DHE-RSA-WITH-AES-256-CBC-SHA
    TLS-DHE-RSA-WITH-AES-128-CBC-SHA
  ).join(':')

  ### REQUIRED CLASS VARIABLES

  # environment variable prefix for this app
  @@prefix = 'VPN_'
  # [field, default, description]
  # Cases for default:
  #              nil  - not set, optional
  #      empty Array  - not set, mandatory
  #           Symbol  - default value of another field
  #   String, Fixnum  - that value
  @@parameters = [
    # values mainly used in config file generation
    [:devName,    'tun',     'name of the server\'s tunnel device'],
    [:devNode,    nil,       'name of the device node for Windows TUN/TAP driver'],
    [:protocol,   'udp',     'openvpn protocol'],
    [:port,       1194,      'external openvpn port on server'],
    [:portIntra,  :port,     'internal openvpn port on server'],
    [:serverDev,  'eth0',    'server network device name'],
    [:user,       'openvpn', 'openvpn user for privilege separation'],
    [:group,      :user,     'openvpn group for privilege separation'],
    [:remote,     @@default_remote,      'reachable address of vpn server'],
    [:networks,   @@default_networks,    'from network-ip/range to network-ip/range'],
    [:cipher,     @@default_cipher,      'used encryption cipher'],
    [:tlsCipher,  @@default_tls_ciphers, 'encryption ciphers used for tls'],
    [:maxClients, 16,    'maximum number of simultaneous clients (server)'],
    # values used in creation of certificates
    [:bits,       2048,  'bits per key or parameter'],
    [:bitsDh,     :bits, 'diffie hellman parameter bits'],
    [:bitsCa,     :bits, 'certificate authority rsa key bits'],
    [:bitsServer, :bits, 'server rsa key bits'],
    [:bitsClient, :bits, 'client rsa key bits'],
    [:certSubject, @@default_subject, 'certificate subject, "{{name}}" can use argument'],
    [:certValid,   3650, 'validity of certificate in days'],
    [:certValidCa, :certValid, 'validity of ca certificate in days'],
    [:fileSerial,  nil,  'file to add serial and subject to (needed for crl)']
  ]

  # all options defined above are injected with ConfigReader
  # but are read-only
  attr_reader *(@@parameters.collect{|a| a[0]})

  # unparsed arguments
  attr_reader :unparsed

  attr_reader :logVerbosity,
        :fromNet,
        :toNet

  attr_reader :fileTa,
        :fileDh,
        :fileCrl,
        :fileCaKey,
        :fileCaCert,
        :fileKey,
        :fileCert

  attr_accessor :mode, :name

  attr_accessor :blobTa,
        :blobDh,
        :blobCa,
        :blobServer,
        :blobKey,
        :blobCert

  def build()
    # fill in synthetic values
    @logVerbosity    = 3
    @fromNet, @toNet = @networks.split('>')
    @fileTa          = 'ta.key'
    @fileDh          = "dh#{@bitsDh}.pem"
    @fileCrl         = 'crl.pem'
    @fileCaKey       = 'ca.key'
    @fileCaCert      = 'ca.pem'
    return self
  end

  def use()
    # @mode is set after the call to build()
    @fileKey  = "#{@name || @mode}.key"
    @fileCert = "#{@name || @name}.pem"
    return self
  end

  def server?
    return @mode == 'server'
  end

  def net_format(what, format)
    return case format
    when :first
      what.sub(/\.[^\.]*$/, '.1/32')
    when :netmask
      ip, range = what.split('/')
      nm = 0xffffffff ^ (0xffffffff >> range.to_i)
      netmask = (0..3).collect{ |i| ((nm >> (3-i)*8) & 0xff) }.join('.')
      "#{ip} #{netmask}"
    else
      what
    end
  end

  def linkPem(pemType)
    key, file, blob = {
      ta:   ['tls-auth', @fileTa,     @blobTa],
      dh:   ['dh',       @fileDh,     @blobDh],
      ca:   ['ca',       @fileCaCert, @blobCa],
      cert: ['cert',     @fileCert,   @blobCert],
      key:  ['key',      @fileKey,    @blobKey]
    }[pemType]
    if blob
      blob = blob.gsub(/^[^a-zA-Z0-9=+\/\-].*/, '')
      blob.strip!()
      "<#{key}>\n#{blob}\n</#{key}>\n"
    elsif file
      "#{key} #{file}\n"
    else
      ""
    end
  end

  def config()
    # everything after __END__ marker is the template
    template = DATA.read
    if server?
      # is in server mode, remove server annotations
      template.gsub!(/##S:[ \t]*/, '')
    else
      # is in client mode, remove client annotations
      template.gsub!(/##C:[ \t]*/, '')
    end
    # remove remaining annotations including their content
    template.gsub!(/##[SC]:[^\n]*\n?/, '')
    # evaluate the template in context of this instance
    out = instance_eval("<<CONFIG_END\n" + template + "\nCONFIG_END")
    # fix whitespace: none at the front, just \n at the end, no more than
    # one blank line inside
    out.strip!
    out.gsub!(/([\t ]*\n){3,}/, "\n\n")
    return out + "\n"
  end
end



module ConfigReader
  # ARGV as a map including all parameters in the form
  # -KEY=VALUE
  # and the rest in an Array with the key :unparsed
  @@argMap = ARGV.inject({unparsed: []}) do |a, v|
    begin
      key, val = v.scan(/^(-[^=]*)=?(.*)$/)[0]
      a[key] = val.empty? ? true : val
    rescue
      a[:unparsed] << v
    end
    a
  end

  # read order for configuration parameters.
  # Per default:
  # 1. lookup in ARGV (command line arguments)
  # 2. lookup in ENV  (environment variables)
  # 3. use default value
  @@readOrder = [
    [:arg, @@argMap],
    [:env, ENV],
    :default
  ]

  # take the array of usable parameters in the format
  # [[NAME:symbol, DEFAULT, DESCRIPTION], ...]
  # and the environment variable prefix and
  # return a map {NAME => CONFIGURATION_PARAMETER, ...}
  # where CONFIGURATION_PARAMETER is
  # {
  #   symbol: NAME,
  #   arg: COMMAND_LINE_VARIABLE_PREFIX,
  #   env: ENVIRONMENT_VARIABLE_NAME,
  #   default: DEFAULT_VALUE,
  #   desc: DESCRIPTION
    # }
  def self.seed(parameters, envPrefix)
    envPrefix ||= ''
    data = {}
    parameters.each do |p|
      symbol, defval, description = p
      # key in command line arguments:
      # capital letters in symbol are replaced by '-' and lower case
      key = symbol.to_s().gsub(/([A-Z])/, '-\1').downcase()
      argkey = '-' + key
      # key in environment variables:
      # add envPrefix in front and use upper casse version of
      # command line key with '-' replaced with '_'
      envkey = envPrefix + key.gsub('-', '_').upcase()
      data[symbol] = {
        symbol:  symbol,
        arg:     argkey,
        env:     envkey,
        default: defval,
        desc:    description
      }
    end
    data
  end

  # fetch a configuration parameter with its description from seed.
  # returns [value, is_set]
  def self.value(parameter)
    @@readOrder.each do |source|
      if source == :default
        return [parameter[:default], false]
      end
      pk, values = source
      key = parameter[pk]
      if values.has_key? key
        return [values[key], true]
      end
    end
  end

  def self.fill(config)
    missing = []
    config.each do |key, cfgparam|
      v, s = value(cfgparam)
      if s || v.class != Symbol
        cfgparam[:value] = v
        cfgparam[:is_set] = s
      else
        missing << [key, v]
      end
    end
    until missing.empty?
      still_missing = []
      missing.each do |link|
        key, ref = link
        reference = config[ref]
        if reference.has_key? :value
          cfgparam = config[key]
          cfgparam[:value] = reference[:value]
          cfgparam[:is_set] = false
        else
          still_missing.push([key, ref])
        end
      end
      if missing.length == still_missing.length
        raise Exception.new("unpaired config defaults for #{missing.inspect}")
      end
      missing = still_missing
    end
    return config
  end

  def self.create(cfgClass)
    envPrefix = cfgClass.class_variable_get(:@@prefix)
    parameters = cfgClass.class_variable_get(:@@parameters)
    config = seed(parameters, envPrefix)
    cfgClass.class_variable_set(:@@expandedParameters, config)
    instance = cfgClass.new
    instance.instance_variable_set("@unparsed", @@argMap[:unparsed])
    fill({}.merge(config)).each do |k, v|
      instance.instance_variable_set("@#{k}", v[:value])
    end
    instance.build()
  end
end



module CliHelp
  def self.colorizer()
    if ENV['TERM'] == 'xterm-256color'
      # See http://stackoverflow.com/questions/1489183/colorized-ruby-output
      # and http://ascii-table.com/ansi-escape-sequences.php
      return Proc.new do |str, col|
        case col
        when :key # bold gray
          "\033[37;1m#{str}\033[0m"
        when :argkey # bold gray, bg-cyan
          "\033[46;37;1m#{str}\033[0m"
        when :envkey # bold gray, bg-blue
          "\033[44;37;1m#{str}\033[0m"
        when :value # bold brown
          "\033[33;1m#{str}\033[0m"
        when :default # brown
          "\033[33m#{str}\033[0m"
        when :warn # red
          "\033[31m#{str}\033[0m"
        else
          str.to_s()
        end
      end
    end
    return Proc.new do |str, col|
      str.to_s()
    end
  end

  def self.help(app)
    color = colorizer()
    lines = []
    cfg = app.class.class_variable_get(:@@expandedParameters)
    cfg.each do |k, parameter|
      value = app.send(k)
      argkey = parameter[:arg]
      envkey = parameter[:env]
      defval = parameter[:default]
      desc = parameter[:desc]
      dvtxt = case defval
      when String
        ': "' + color[defval, :default] + '"'
      when Symbol
        " is the value of #{color[cfg[defval][:arg], :argkey]}"
      else
        if defval == nil
          " does not exist"
        else
          ": #{color[defval, :default]}"
        end
      end
      printval = case
      when !parameter[:is_set]
        if defval == []
          color["not set", :warn] + ','
        else
          'not set,'
        end
      when value == defval
        'the'
      else
        "\"#{color[value, :value]}\","
      end
      lines << [
        "#{color[argkey, :argkey]} / #{color[envkey, :envkey]}",
        "    #{desc};",
        "    Is #{printval} default value#{dvtxt}"
      ].join("\n")
    end
    return (
      [
        "Configuration parameters as #{color['argument', :argkey]} " +
        "/ #{color['environment', :envkey]} variables"
      ] + lines.sort() +
      [
        "unknown arguments:",
        app.unparsed
      ]
    ).join("\n")
  end

  def self.example(app, all)
    cfg = app.class.class_variable_get(:@@expandedParameters)
    vars = cfg.keys.sort.inject('') do |a, k|
      arg = cfg[k]
      value = app.send(k)
      if all || arg[:is_set] && value != arg[:default]
        a += "#{arg[:env]}='#{value}' "
      end
      a
    end
    return vars + __FILE__ + ' ...'
  end
end



module Crypto
  require 'openssl'

  @@DIGEST = OpenSSL::Digest::SHA256
  @@SERIAL_DIGEST = @@DIGEST
  @@SIGN_DIGEST = @@DIGEST

  def self.serial(*certs)
    hash = @@SERIAL_DIGEST.new
    certs.each { |cert| hash << cert }
    # serial number binary length is max. 20 bytes:
    # use last 20 bytes from hash
    return OpenSSL::BN.new(hash.digest[-20, 20], 2)
  end

  def self.asName(value, defaultValue)
    return case value
    when NilClass
      defaultValue
    when String
      OpenSSL::X509::Name.parse(value)
    when OpenSSL::X509::Name
      value
    when OpenSSL::X509::Certificate
      value.subject
    when Array
      OpenSSL::X509::Name.new(value)
    else
      raise Exception.new("asName: could not [#{value}], unhandled class #{value.class}")
    end
  end

  def self.getKey(filename, pkClass, *generate_args)
    if filename != nil && (File.exists? filename)
      return pkClass.new(File.read(filename))
    end
    return pkClass.generate(*generate_args)
  end

  def self.storePem(filename, pemable)
    return if File.exists? filename
    mode = 0644
    if pemable.respond_to?(:private?) && pemable.private?
      mode = 0600
    end
    File.open(filename, 'w', mode) do |file|
      file.write(pemable.to_pem)
    end
  end

  def self.addExt(issuer, extable, extensions)
    factory = OpenSSL::X509::ExtensionFactory
    ef = case extable
    when OpenSSL::X509::Certificate
      factory.new(issuer, extable)
    when OpenSSL::X509::Request
      factory.new(issuer, nil, extable)
    when OpenSSL::X509::CRL
      factory.new(issuer, nil, nil, extable)
    else
      raise Exception.new("type #{extable.class} can handle extensions")
    end
    extensions.each do |k, v|
      extable.add_extension(ef.create_extension(k, v))
    end
    return extable
  end

  def self.newCert(info)
    cert = OpenSSL::X509::Certificate.new()
    cert.version = 2
    serial = info[:serial]
    cert.serial = case serial
    when NilClass
      1
    when Fixnum, OpenSSL::BN
      serial
    when Array
      serial(*serial)
    else
      raise Exception.new("invalid serial [#{info[:serial]}]")
    end
    subject = asName(info[:subject], nil)
    if subject == nil
      raise Exception.new("invalid subject [#{info[:subject]}]")
    end
    cert.subject = subject
    cert.issuer = asName(info[:issuerCert], subject)
    key = info[:key]
    cert.public_key = key.public_key
    cert.not_before = info[:starts]
    cert.not_after  = info[:expires]
    cert = addExt(info[:issuerCert] || cert, cert, info[:extension])
    cert.sign(info[:signKey] || key, @@SIGN_DIGEST.new)
  end

  def self.loadAs(filename, cls)
    return nil unless filename != nil && (File.exists? filename)
    cert = File.read(filename)
    return cls.new(cert)
  end

  ##
  ## general use functions, the ones above are rough and internal
  ##

  def self.rsaPrivKey(filename, bits)
    # use public exponent of 3 (RSA_3) instead of 65537 (RSA_F4)
    # to speed up operations. Claims of security improvements by
    # using larger exponents are appearently misguided:
    # https://www.imperialviolet.org/2012/03/16/rsae.html (Three should be fine)
    getKey(filename, OpenSSL::PKey::RSA, bits, 3)
  end

  def self.loadCert(filename)
    return self.loadAs(filename, OpenSSL::X509::Certificate)
  end

  def self.loadRSAKey(filename)
    return self.loadAs(filename, OpenSSL::PKey::RSA)
  end

  def self.ta(app)
    return File.read(app.fileTa) if File.exists? app.fileTa
    # The code below is an alternative for
    # `openvpn --genkey --secret #{app.fileTa}`
    # but returns the ta key pem as a String
    header, footer = ['BEGIN', 'END'].collect do |s|
      "-----#{s} OpenVPN Static key V1-----"
    end
    num_bytes = 4 * 64 # 2 keys with 64 bytes for hmac and cipher
    body = OpenSSL::Random.
            random_bytes(num_bytes).
            unpack("h32" * (num_bytes / 16))
    return ([header] + body + [footer, '']).join("\n")
  end

  def self.dh(app)
    # use default generator 2 , but make it explicit
    getKey(app.fileDh, OpenSSL::PKey::DH, app.bitsDh, 2)
  end

  def self.ca(app, key)
    cert = loadCert(app.fileCaCert)
    if cert
      return cert
    end
    starts = Time.now
    expires = starts + app.certValidCa.to_i * 24 * 60 * 60
    subject = app.certSubject.gsub('{{name}}', app.name || app.mode)
    return newCert({
      key:       key,
      subject:   subject,
      starts:    starts,
      expires:   expires,
      extension: {
        'basicConstraints'       => 'CA:TRUE,pathlen:1',
        'subjectKeyIdentifier'   => 'hash',
        # NOTE:
        # Removing issuer from authorityKeyIdentifier would
        # allow for certificate regeneration from the same
        # key which keeps all signed certificates valid.
        # As is, everything is invalidated when the CA
        # certificate expires.
        'authorityKeyIdentifier' => 'keyid,issuer:always',
        'keyUsage'               => 'cRLSign,keyCertSign'
      }
    })
  end

  def self.cert(app, key, caKey, caCert)
    # NOTE:
    # For a CRL, the required info can be extracted from the
    # configuration by piping the configuration file input with
    # its embedded certificate into
    # awk 'BEGIN{PRINT=0}{if($0=="<cert>"){PRINT=1}else if($0=="</cert>"){PRINT=0}else if(PRINT==1){print $0}}'\
    # | openssl x509 -serial -subject -noout
    cert = loadCert(app.fileCert)
    if cert
      return cert
    end
    starts = Time.now
    expires = starts + app.certValidCa.to_i * 24 * 60 * 60
    subject = app.certSubject.gsub('{{name}}', app.name || app.mode)
    ext = {
      'basicConstraints'       => 'CA:FALSE',
      'subjectKeyIdentifier'   => 'hash',
      'authorityKeyIdentifier' => 'keyid,issuer:always',
    }
    if app.server?
      ext = ext.merge({
        'extendedKeyUsage'       => 'serverAuth',
        'keyUsage'               => 'digitalSignature,keyEncipherment'
      })
    else
      ext = ext.merge({
        'extendedKeyUsage'       => 'clientAuth',
        'keyUsage'               => 'digitalSignature'
      })
    end
    return newCert({
      key:        key,
      subject:    subject,
      issuerCert: caCert,
      starts:     starts,
      expires:    expires,
      signKey:    caKey,
      serial:     [caCert.to_pem(), key.public_key.to_pem()],
      extension:  ext
    })
  end

  def self.crl(lastCrl, caKey, caCert, *revokeSerials)
    crl = case lastCrl
    when OpenSSL::X509::CRL
      lastCrl
    when String, File
      OpenSSL::X509::CRL.new(lastCrl)
    when NilClass
      new_crl = OpenSSL::X509::CRL.new
      new_crl.issuer = caCert.subject
      new_crl.version = 0
      cert = addExt(caCert, new_crl, {
        'authorityKeyIdentifier' => 'keyid:always,issuer:always'
      })
      new_crl
    else
      raise Exception.new("invalid CRL")
    end
    now = Time.now
    crl.last_update = now
    crl.next_update = now + 60 * 60 # in one hour
    crl.version += 1
    revokeSerials.each do |serial|
      rev = OpenSSL::X509::Revoked.new
      rev.time = now
      rev.serial = case serial
      when OpenSSL::BN
        serial
      when String
        OpenSSL::BN.new(serial)
      else
        raise Exception.new("unsupported serial type #{serial.class} for '#{serial}'")
      end
      crl.add_revoked rev
    end
    crl.sign(caKey, @@SIGN_DIGEST.new)
    return crl
  end

  # TODO still missing encryption of keys
  # IDEA generate template of client string to get rid of ta.key and ca files
end



# TODO handling of key en-/decryption passwords
# TODO certificate generation with external private key - public key as argument instead
if __FILE__ == $0
  app = ConfigReader.create(AppConfig)
  mode = app.unparsed.shift
  case mode
  when 'parameters'
    puts CliHelp.help(app.use())
  when 'config', 'config-full'
    puts CliHelp.example(app.use(), mode == 'config-full')
  when 'setup'
    app.mode = mode
    app.name = app.unparsed[0] || 'CA'
    app.use()
    caKey = Crypto.rsaPrivKey(app.fileCaKey, app.bitsCa)
    Crypto.storePem(app.fileCaKey, caKey)
    cert = Crypto.ca(app, caKey)
    Crypto.storePem(app.fileCaCert, cert)
    crl = Crypto.crl(nil, caKey, cert)
    Crypto.storePem(app.fileCrl, crl)
  when 'server', 'client'
    app.mode = mode
    app.name = app.unparsed[0] || mode
    app.use()
    if app.server?
      app.blobTa = Crypto.ta(app)
      unless File.exists?(app.fileTa)
        File.open(app.fileTa, 'w', 0644) do |file|
          file.write(app.blobTa)
        end
      end
      # dh parameters are server only
      app.blobDh = Crypto.dh(app).to_pem()
    end
    app.blobTa ||= File.read(app.fileTa)
    caKey = Crypto.loadRSAKey(app.fileCaKey)
    caCert = Crypto.loadCert(app.fileCaCert)
    app.blobCa ||= caCert.to_pem()
    key = Crypto.rsaPrivKey(
      nil, # do not store on HDD
      app.server? ? app.bitsServer : app.bitsClient
    )
    app.blobKey = key.to_pem()
    cert = Crypto.cert(app, key, caKey, caCert)
    if app.fileSerial
      File.open(app.fileSerial, 'a', 0600) do |file|
        values = [:serial, :not_before, :subject].collect{|f| cert.send(f)}
        file.puts values.join("\t")
      end
    end
    app.blobCert = cert.to_pem()
    puts app.config()
  when 'revoke'
    puts Crypto.crl(
      File.read(app.fileCrl),
      Crypto.loadRSAKey(app.fileCaKey),
      Crypto.loadCert(app.fileCaCert),
      *app.unparsed
    ).to_pem
  else
    program = __FILE__
    puts <<HELP_END
#{program} sets up and manages OpenVPN.
It can generate a new certificate authority and create client and server
configuration. To run OpenVPN, please read the configuration files, the
comment at the beginning of the server configuration file provides
the instructions.
How to call #{program} (best start with parameters and config-full):
  $ #{program} help              this help text
  $ #{program} parameters        lists available configuration parameters
  $ #{program} config-full       example for a call with full current configuration
  $ #{program} config            like config-full without defaults
  $ #{program} setup [CANAME]    create a CA and a CRL
  $ #{program} server [SRVNAME]  print a server config file
  $ #{program} client NAME       print a client config file
  $ #{program} revoke SERIAL*    print crl with added revoked serials
HELP_END
  end
end



# TODO: reduce "hand-window"
__END__
##S: # Per installation, run these on the server (assuming Linux):
##S: # $: useradd -M '#{@user}'
##S: # Also run these and consider adding them to /etc/rc.local so they are preserved on reboot
##S: # $: sysctl -w net.ipv4.ip_forward=1
##S: # $: ip link set dev '#{@serverDev}' promisc on
##S: # $: iptables -A FORWARD -i '#{@serverDev}' -o '#{@devName == 'tun' ? 'tun0' : @devName}' -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
##S: # $: iptables -A FORWARD -s '#{@fromNet}' -o '#{@serverDev}' -j ACCEPT
##S: # $: iptables -t nat -A POSTROUTING -s '#{@fromNet}' '!' -d '#{net_format(@fromNet, :first)}' -o '#{@serverDev}' -j MASQUERADE
##S:
##S: # For a management console:
##S: # connect with
##S: #   telnet 127.0.0.1 20000
##S: # and uncomment below to enable
##S: # management 127.0.0.1 20000

##C: client
##S: server #{net_format(@fromNet, :netmask)}
proto udp
##C: remote #{@remote} #{@port}
##S: port #{@portIntra}
##S: push "route #{net_format(@toNet, :netmask)}"

dev-type tun
dev #{@devName || 'tun'}
#{@devNode ? "dev-node #{@devNode}"  : ""}

# remove this if there are Windows clients with OpenVPN < 2.1
topology subnet

comp-lzo adaptive

cipher #{@cipher}
##S: # see https://community.openvpn.net/openvpn/wiki/Hardening
##S: tls-cipher #{@tlsCipher}
##S: remote-cert-tls client
##C: remote-cert-tls server
##S:
##S: persist-key
##S: persist-tun
##S: persist-local-ip
##S: persist-remote-ip
##S: push "persist-key"
##S: push "persist-tun"
##C:
##C: nobind
##S: max-clients #{@maxClients || 32}
##S: keepalive 4 20
##S:
##S: #{@user  ? "user  #{@user}"  : ''}
##S: #{@group ? "group #{@group}" : ''}

verb #{@logVerbosity || 3}
##S: mute 5

##S: # status logging, ip pool persist
##S: ifconfig-pool-persist   ipp.txt
##S: status                  openvpn-status.log

##S: #{@fileCrl ? "crl-verify #{@fileCrl}" : ''}

key-direction #{server? ? '0' : '1'}
#{
((server? ? [:dh] : []) + [:ta, :ca, :cert, :key]).
  collect{ |pem| linkPem(pem) }.join('')
}

Extension Suggestions

See https://community.openvpn.net/openvpn/wiki/Openvpn23ManPage
consider adding this to the configuration template:

ping 5
hand-window 10

Search for TODO, MAYBE, IDEA and NOTE in the source file above for some additional remarks.

Troubleshooting

This script somehow fails when it's called from Windows. I didn't have Windows on hand myself and couldn't debug it. The configurations work, though.

When OpenVPN is installed on a Raspberry Pi with Raspbian, the tls cipher list is borked.
First, update OpenSSL so you are heartbleed safe: openssl version -a should show a build date of April 20th 2014 or more recent.
Call openvpn --show-tls and copy the list.
Remove all entries containing RC4, DES, 3DES, NULL and ECDH.
Proofread what's left, I don't know what OpenVPN can handle in which version on which hardware.
Put the rest into your configuration:

• ":" separated in the server configuration: tls-cipher
• in the generator: AppConfig.@@default_tls_ciphers