code: func (mock *MockDNSClient) LookupHost(_ context.Context, hostname string) ([]net.IP, error) {
  	if hostname == "always.invalid" ||
  		hostname == "invalid.invalid" ||
  		hostname == "email.only" {
  		return []net.IP{}, nil
  	}
  	if hostname == "always.timeout" {
  		return []net.IP{}, &DNSError{dns.TypeA, "always.timeout", MockTimeoutError(), -1}
  	}
  	if hostname == "always.error" {
  		return []net.IP{}, &DNSError{dns.TypeA, "always.error", &net.OpError{
  			Err: errors.New("some net error"),
  		}, -1}
  	}
  	// dual-homed host with an IPv6 and an IPv4 address
  	if hostname == "ipv4.and.ipv6.localhost" {
  		return []net.IP{
  			net.ParseIP("::1"),
  			net.ParseIP("127.0.0.1"),
  		}, nil
  	}
  	if hostname == "ipv6.localhost" {
  		return []net.IP{
  			net.ParseIP("::1"),
  		}, nil
  	}
  	ip := net.ParseIP("127.0.0.1")
  	return []net.IP{ip}, nil
  }
docs: // LookupHost is a mock
  //
  // Note: see comments on LookupMX regarding email.only
  //