Crate netlink_packet_core

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netlink-packet-core provides a generic netlink message NetlinkMessage<T> that is independant of the sub-protocol. Such messages are not very useful by themselves, since they are just used to carry protocol-dependant messages. That is what the T represent: T is the NetlinkMessage’s protocol-dependant message. This can be any type that implements NetlinkSerializable and NetlinkDeserializable.

For instance, the netlink-packet-route crate provides rtnetlink messages via netlink_packet_route::RtnlMessage, and netlink-packet-audit provides audit messages via netlink_packet_audit::AuditMessage.

By itself, the netlink-packet-core crate is not very useful. However, it is used in netlink-proto to provide an asynchronous implementation of the netlink protocol for any sub-protocol. Thus, a crate that defines messages for a given netlink sub-protocol could integrate with netlink-packet-core and would get an asynchronous implementation for free. See the second example below for such an integration, via the NetlinkSerializable and NetlinkDeserializable traits.

This example shows how to serialize and deserialize netlink packet for the rtnetlink sub-protocol. It requires netlink-packet-route.

use netlink_packet_core::{NLM_F_DUMP, NLM_F_REQUEST};
use netlink_packet_route::{LinkMessage, RtnlMessage, NetlinkMessage,
NetlinkHeader};

// Create the netlink message, that contains the rtnetlink
// message
let mut packet = NetlinkMessage {
    header: NetlinkHeader {
        sequence_number: 1,
        flags: NLM_F_DUMP | NLM_F_REQUEST,
        ..Default::default()
    },
    payload: RtnlMessage::GetLink(LinkMessage::default()).into(),
};

// Before serializing the packet, it is important to call
// finalize() to ensure the header of the message is consistent
// with its payload. Otherwise, a panic may occur when calling
// serialize()
packet.finalize();

// Prepare a buffer to serialize the packet. Note that we never
// set explicitely `packet.header.length` above. This was done
// automatically when we called `finalize()`
let mut buf = vec![0; packet.header.length as usize];
// Serialize the packet
packet.serialize(&mut buf[..]);

// Deserialize the packet
let deserialized_packet =
    NetlinkMessage::<RtnlMessage>::deserialize(&buf).expect("Failed to deserialize message");

// Normally, the deserialized packet should be exactly the same
// than the serialized one.
assert_eq!(deserialized_packet, packet);

println!("{:?}", packet);

Let’s assume we have a netlink protocol called “ping pong” that defines two types of messages: “ping” messages, which payload can be any sequence of bytes, and “pong” message, which payload is also a sequence of bytes. The protocol works as follow: when an enpoint receives a “ping” message, it answers with a “pong”, with the payload of the “ping” it’s answering to.

“ping” messages have type 18 and “pong” have type “20”. Here is what a “ping” message that would look like if its payload is [0, 1, 2, 3]:

0                8                16              24               32
+----------------+----------------+----------------+----------------+
|                 packet length (including header) = 16 + 4 = 20    |
+----------------+----------------+----------------+----------------+
|     message type = 18 (ping)    |              flags              |
+----------------+----------------+----------------+----------------+
|                           sequence number                         |
+----------------+----------------+----------------+----------------+
|                            port number                            |
+----------------+----------------+----------------+----------------+
|       0        |         1      |         2      |        3       |
+----------------+----------------+----------------+----------------+

And the “pong” response would be:

0                8                16              24               32
+----------------+----------------+----------------+----------------+
|                 packet length (including header) = 16 + 4 = 20    |
+----------------+----------------+----------------+----------------+
|     message type = 20 (pong)    |              flags              |
+----------------+----------------+----------------+----------------+
|                           sequence number                         |
+----------------+----------------+----------------+----------------+
|                            port number                            |
+----------------+----------------+----------------+----------------+
|       0        |         1      |         2      |        3       |
+----------------+----------------+----------------+----------------+

Here is how we could implement the messages for such a protocol and integrate this implementation with netlink-packet-core:

use netlink_packet_core::{
    NetlinkDeserializable, NetlinkHeader, NetlinkMessage, NetlinkPayload, NetlinkSerializable,
};
use std::error::Error;
use std::fmt;

// PingPongMessage represent the messages for the "ping-pong" netlink
// protocol. There are only two types of messages.
#[derive(Debug, Clone, Eq, PartialEq)]
pub enum PingPongMessage {
    Ping(Vec<u8>),
    Pong(Vec<u8>),
}

// The netlink header contains a "message type" field that identifies
// the message it carries. Some values are reserved, and we
// arbitrarily decided that "ping" type is 18 and "pong" type is 20.
pub const PING_MESSAGE: u16 = 18;
pub const PONG_MESSAGE: u16 = 20;

// A custom error type for when deserialization fails. This is
// required because `NetlinkDeserializable::Error` must implement
// `std::error::Error`, so a simple `String` won't cut it.
#[derive(Debug, Clone, Eq, PartialEq)]
pub struct DeserializeError(&'static str);

impl Error for DeserializeError {
    fn description(&self) -> &str {
        self.0
    }
    fn source(&self) -> Option<&(dyn Error + 'static)> {
        None
    }
}

impl fmt::Display for DeserializeError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.0)
    }
}

// NetlinkDeserializable implementation
impl NetlinkDeserializable for PingPongMessage {
    type Error = DeserializeError;

    fn deserialize(header: &NetlinkHeader, payload: &[u8]) -> Result<Self, Self::Error> {
        match header.message_type {
            PING_MESSAGE => Ok(PingPongMessage::Ping(payload.to_vec())),
            PONG_MESSAGE => Ok(PingPongMessage::Pong(payload.to_vec())),
            _ => Err(DeserializeError(
                "invalid ping-pong message: invalid message type",
            )),
        }
    }
}

// NetlinkSerializable implementation
impl NetlinkSerializable for PingPongMessage {
    fn message_type(&self) -> u16 {
        match self {
            PingPongMessage::Ping(_) => PING_MESSAGE,
            PingPongMessage::Pong(_) => PONG_MESSAGE,
        }
    }

    fn buffer_len(&self) -> usize {
        match self {
            PingPongMessage::Ping(vec) | PingPongMessage::Pong(vec) => vec.len(),
        }
    }

    fn serialize(&self, buffer: &mut [u8]) {
        match self {
            PingPongMessage::Ping(vec) | PingPongMessage::Pong(vec) => {
                buffer.copy_from_slice(&vec[..])
            }
        }
    }
}

// It can be convenient to be able to create a NetlinkMessage directly
// from a PingPongMessage. Since NetlinkMessage<T> already implements
// From<NetlinkPayload<T>>, we just need to implement
// From<NetlinkPayload<PingPongMessage>> for this to work.
impl From<PingPongMessage> for NetlinkPayload<PingPongMessage> {
    fn from(message: PingPongMessage) -> Self {
        NetlinkPayload::InnerMessage(message)
    }
}

fn main() {
    let ping_pong_message = PingPongMessage::Ping(vec![0, 1, 2, 3]);
    let mut packet = NetlinkMessage::from(ping_pong_message);

    // Before serializing the packet, it is very important to call
    // finalize() to ensure the header of the message is consistent
    // with its payload. Otherwise, a panic may occur when calling
    // `serialize()`
    packet.finalize();

    // Prepare a buffer to serialize the packet. Note that we never
    // set explicitely `packet.header.length` above. This was done
    // automatically when we called `finalize()`
    let mut buf = vec![0; packet.header.length as usize];
    // Serialize the packet
    packet.serialize(&mut buf[..]);

    // Deserialize the packet
    let deserialized_packet = NetlinkMessage::<PingPongMessage>::deserialize(&buf)
        .expect("Failed to deserialize message");

    // Normally, the deserialized packet should be exactly the same
    // than the serialized one.
    assert_eq!(deserialized_packet, packet);

    // This should print:
    // NetlinkMessage { header: NetlinkHeader { length: 20, message_type: 18, flags: 0, sequence_number: 0, port_number: 0 }, payload: InnerMessage(Ping([0, 1, 2, 3])) }
    println!("{:?}", packet);
}

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