Get default route example

This example was created in response to [issue rust-netlink#63](rust-netlink#63)

examples/get_route_default.rs

@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: MIT
+
+//! To find the default route for a given ip address family (i.e., IPv4 or IPv6),
+//!
+//! 1. pick a routing table (in Linux this is almost always table `254` unless
+//!    you are using VRFs or something)
+//! 1. find all routes in the routing table with
+//!    `.header.destination_prefix_length == 0` (that is, routes which are
+//!    either `0.0.0.0/0` for IPv4 or `::/0` for IPv6)
+//! 2. take the route with the _lowest_ priority among those.
+
+use std::collections::BTreeMap;
+
+use futures::stream::TryStreamExt;
+use netlink_packet_route::route::{RouteAttribute, RouteMessage, RouteType};
+
+use rtnetlink::{new_connection, IpVersion};
+
+type RouteTableId = u8;
+type RoutePriority = Option<u32>;
+
+#[tokio::main]
+async fn main() -> Result<(), ()> {
+    let (connection, handle, _) = new_connection().unwrap();
+    tokio::spawn(connection);
+
+    let mut default_routes: BTreeMap<
+        RouteTableId,
+        BTreeMap<RoutePriority, RouteMessage>,
+    > = BTreeMap::new();
+
+    // Change to `IpVersion::V6` and this will work for the IPv6 routing tables
+    let mut all_routes = handle.clone().route().get(IpVersion::V4).execute();
+
+    while let Ok(Some(route)) = all_routes.try_next().await {
+        if route.header.destination_prefix_length != 0 {
+            continue;
+        }
+        if route.header.kind != RouteType::Unicast {
+            continue;
+        }
+        let prio = route.attributes.iter().find_map(|attr| match attr {
+            RouteAttribute::Priority(prio) => Some(*prio),
+            _ => None,
+        });
+        if let Some(prio_map) = default_routes.get_mut(&route.header.table) {
+            prio_map.insert(prio, route);
+        } else {
+            let mut prio_map = BTreeMap::new();
+            let table_id = route.header.table;
+            prio_map.insert(prio, route);
+            default_routes.insert(table_id, prio_map);
+        }
+    }
+
+    for (table, prio_map) in default_routes.iter() {
+        println!("Default routes:");
+        println!("Table: {table}");
+        for (_, route) in prio_map.iter() {
+            println!("\t{route:?}");
+        }
+    }
+
+    Ok(())
+}