Get default route example

[daniel-noland]

This example was created in response to issue #63

[cathay4t]

This example could be a perfect place to demonstrate both kernel space route filtering and use space route filtering.

The table ID could be done by kernel filtering.
The destination_prefix_length == 0 cannot be use for kernel filter(because kernel only filter non-zero property).

This is example of kernel filtering on route table:

// SPDX-License-Identifier: MIT

use futures::stream::TryStreamExt;
use netlink_sys::AsyncSocket;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let (mut connection, handle, _) = rtnetlink::new_connection()?;
    connection
        .socket_mut()
        .socket_mut()
        .set_netlink_get_strict_chk(true)?;
    tokio::spawn(connection);
    let mut rt_handle = handle.route().get(rtnetlink::IpVersion::V4);
    let rt_nlmsg = rt_handle.message_mut();
    rt_nlmsg.header.table = 254;
    rt_nlmsg.header.destination_prefix_length = 0;

    let mut route_getter = rt_handle.execute();

    while let Some(rt_msg) = route_getter.try_next().await? {
        println!("HAHA {:?}", rt_msg);
    }
    Ok(())
}