client.rb

require 'drb/ssl'

DRb.start_service
DRb.config.merge!(
  SSLVerifyMode: OpenSSL::SSL::VERIFY_PEER,
  SSLCACertificateFile: 'pki/ca.crt',
  SSLPrivateKey: OpenSSL::PKey::RSA.new(File.read("client/pki/private/ManveruClient.key")),
)

DRb.config[:SSLVerifyCallback] = ->(ok, _){ ok }

obj = DRbObject.new_with_uri('drbssl://localhost:1234')
p obj.size

server.rb

require 'drb/ssl'

DRb.start_service(
  'drbssl://localhost:1234',
  {},
  SSLCertName: [["CN", DRb::DRbSSLSocket.getservername]],
  SSLCertificate: OpenSSL::X509::Certificate.new(File.read('pki/ca.crt')),
  SSLVerifyMode: OpenSSL::SSL::VERIFY_PEER,
  SSLPrivateKey: OpenSSL::PKey::RSA.new(File.read('pki/private/ca.key')),
)
DRb.thread.join