def authenticate(username, password):
    """Authenticate with a DC/OS cluster and return an ACS token.
    return: ACS token
    """
    url = _gen_url('acs/api/v1/auth/login')

    creds = {
        'uid': username,
        'password': password
    }

    response = dcos.http.request('post', url, json=creds)

    if response.status_code == 200:
        return response.json()['token']
    else:
        return None