'Describes index fields in the search domain, optionally
limited to a single ``IndexField``.
:type domain_name: string
:param domain_name: A string that represents the name of a
domain. Domain names must be unique across the domains
owned by an account within an AWS region. Domain names
must start with a letter or number and can contain the
following characters: a-z (lowercase), 0-9, and -
(hyphen). Uppercase letters and underscores are not
allowed.
:type field_names: list
:param field_names: Limits the response to the specified fields.
:raises: BaseException, InternalException, ResourceNotFoundException'

def describe_index_fields(self, domain_name, field_names=None):

    doc_path = ('describe_index_fields_response', 'describe_index_fields_result', 'index_fields')
    params = {'DomainName': domain_name}
    if field_names:
        for (i, field_name) in enumerate(field_names, 1):
            params[('FieldNames.member.%d' % i)] = field_name
    return self.get_response(doc_path, 'DescribeIndexFields', params, verb='POST', list_marker='IndexFields')
