Netlink parser data flow

NetlinkSocketBase: receive the data

When NetlinkSocketBase receives the data from a netlink socket, it can do it in two ways:

1. get data directly with socket.recv() or socket.recv_into()

2. run a buffer thread that receives the data asap and leaves in the buffer_queue to be consumed later by recv() or recv_into()

NetlinkSocketBase implements these two receive methods, that choose the data source – directly from the socket or from buffer_queue – depending on the buffer_thread property:

pyroute2.netlink.nlsocket.NetlinkSocketBase

Marshal: get and run parsers

Marshal should choose a proper parser depending on the key, flags and sequence_number. By default it uses only nlmsg->type as the key and nlmsg->flags, and there are several ways to customize getting parsers.

1. Use custom key_format, key_offset and key_mask. The latter is used to partially match the key, while key_format and key_offset are used to struct.unpack() the key from the raw netlink data.

2. You can overload Marshal.get_parser() and implement your own way to get parsers. A parser should be a simple function that gets only data, offset and length as arguments, and returns one dict compatible message.

pyroute2.netlink.nlsocket.Marshal

def parse(self, data, seq=None, callback=None, skip_alien_seq=False):
    '''
    Parse string data.

    At this moment all transport, except of the native
    Netlink is deprecated in this library, so we should
    not support any defragmentation on that level
    '''
    offset = 0
    # there must be at least one header in the buffer,
    # 'IHHII' == 16 bytes
    while offset <= len(data) - 16:
        # pick type and length
        (length, key, flags, sequence_number) = struct.unpack_from(
            'IHHI', data, offset
        )
        if skip_alien_seq and sequence_number != seq:
            continue
        if not 0 < length <= len(data):
            break
        # support custom parser keys
        # see also: pyroute2.netlink.diag.MarshalDiag
        if self.key_format is not None:
            (key,) = struct.unpack_from(
                self.key_format, data, offset + self.key_offset
            )
            if self.key_mask is not None:
                key &= self.key_mask

        parser = self.get_parser(key, flags, sequence_number)
        msg = parser(data, offset, length)
        offset += length
        if msg is None:
            continue

        if callable(callback) and seq == sequence_number:
            try:
                if callback(msg):
                    continue
            except Exception:
                pass

        mtype = msg['header'].get('type', None)
        if mtype in (1, 2, 3, 4) and 'event' not in msg:
            msg['event'] = mtypes.get(mtype, 'none')
        self.fix_message(msg)
        yield msg

The message parser routine must accept data, offset, length as the arguments, and must return a valid nlmsg or dict, with the mandatory fields, see the spec below. The parser can also return None which tells the marshal to skip this message. The parser must parse data for one message.

Mandatory message fields, expected by NetlinkSocketBase methods:

{
    'header': {
        'type': int,
        'flags': int,
        'error': None or NetlinkError(),
        'sequence_number': int,
    }
}

Per-request parsers

Sometimes, it may be reasonable to handle a particular response with a specific parser rather than a generic one. An example is IPRoute.get_default_routes(), which could be slow on systems with huge amounts of routes.

Instead of parsing every route record as rtmsg, this method assigns a specific parser to its request. The custom parser doesn’t parse records blindly, but looks up only for default route records in the dump, and then parses only matched records with the standard routine:

pyroute2.iproute.linux.IPRoute

async def get_default_routes(self, family=AF_UNSPEC, table=DEFAULT_TABLE):
    '''
    Get default routes
    '''
    msg = rtmsg()
    msg['family'] = family
    dump_filter, _ = get_dump_filter(
        'route', 'dump', {'table': table} if table is not None else {}
    )
    request = NetlinkRequest(
        self,
        msg,
        msg_type=RTM_GETROUTE,
        msg_flags=NLM_F_DUMP | NLM_F_REQUEST,
        parser=default_routes,
    )
    await request.send()
    return request.response()

pyroute2.iproute.parsers

def default_routes(data, offset, length):
    '''
    Only for RTM_NEWROUTE.

    This parser returns:

    * rtmsg() -- only for default routes (no RTA_DST)
    * nlmsg() -- NLMSG_DONE
    * None for any other messages
    '''
    header = get_header(data, offset)
    if header['type'] == NLMSG_DONE:
        return msg_done(header)

    # skip to NLA: offset + nlmsg header + rtmsg data
    cursor = offset + 28

    # iterate NLA, if meet RTA_DST -- return None (not a default route)
    while cursor < offset + length:
        nla_length, nla_type = struct.unpack_from('HH', data, cursor)
        nla_length = (nla_length + 3) & ~3  # align, page size = 4
        cursor += nla_length
        if nla_type == 1:
            return

    # no RTA_DST, a default route -- spend time to decode using the
    # standard routine
    msg = rtmsg(data, offset=offset)
    msg.decode()
    msg['header']['error'] = None  # required
    return msg

To assign a custom parser to a request/response communication, you should know first sequence_number, be it allocated dynamically with NetlinkSocketBase.addr_pool.alloc() or assigned statically. Then you can create a record in NetlinkSocketBase.seq_map:

#
def my_parser(data, offset, length):
    ...
    return parsed_message

msg_seq = nlsocket.addr_pool.alloc()
msg = nlmsg()
msg['header'] = {
    'type': my_type,
    'flags': NLM_F_REQUEST | NLM_F_ACK,
    'sequence_number': msg_seq,
}
msg['data'] = my_data
msg.encode()
nlsocket.seq_map[msg_seq] = my_parser
nlsocket.sendto(msg.data, (0, 0))
for reponse_message in nlsocket.get(msg_seq=msg_seq):
    handle(response_message)

NetlinkSocketBase: pick correct messages

The netlink protocol is asynchronous, so responses to several requests may come simultaneously. Also the kernel may send broadcast messages that are not responses, and have sequence_number == 0. As the response may contain multiple messages, and may or may not be terminated by some specific type of message, the task of returning relevant messages from the flow is a bit complicated.

Let’s look at an example:

The message flow on the diagram features sequence_number == 0 broadcasts and sequence_number == 1 request and response packets. To complicate it even further you can run a request with sequence_number == 2 before the final response with sequence_number == 1 comes.

To handle that, NetlinkSocketBase.get() buffers all the irrelevant messages, returns ones with only the requested sequence_number, and uses locks to wait on the resource.

The current implementation is relatively complicated and will be changed in the future.

pyroute2.netlink.nlsocket.NetlinkSocketBase