Support route lookup

Quick and dirty patch to support looking up a route to a destination
in addition to dumping all routes

Author: Orycterope

src/route/get.rs

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: MIT
 
+use std::net::IpAddr;
+
 use futures::{
     future::{self, Either},
     stream::{StreamExt, TryStream},
@@ -8,8 +10,8 @@ use futures::{
 
 use netlink_packet_core::{NetlinkMessage, NLM_F_DUMP, NLM_F_REQUEST};
 use netlink_packet_route::{
-    RouteMessage, RtnlMessage, AF_INET, AF_INET6, RTN_UNSPEC, RTPROT_UNSPEC,
-    RT_SCOPE_UNIVERSE, RT_TABLE_UNSPEC,
+    route::Nla, RouteMessage, RtnlMessage, AF_INET, AF_INET6, RTN_UNSPEC,
+    RTPROT_UNSPEC, RT_SCOPE_UNIVERSE, RT_TABLE_UNSPEC,
 };
 
 use crate::{try_rtnl, Error, Handle};
@@ -65,14 +67,30 @@ impl RouteGetRequest {
         &mut self.message
     }
 
+    pub fn to(mut self, ip: IpAddr) -> Self {
+        match ip {
+            IpAddr::V4(v4) => {
+                self.message.nlas.push(Nla::Destination(v4.octets().into()));
+                self.message.header.destination_prefix_length = 32;
+            }
+            IpAddr::V6(v6) => {
+                self.message.nlas.push(Nla::Destination(v6.octets().into()));
+                self.message.header.destination_prefix_length = 128;
+            }
+        }
+        self
+    }
+
     pub fn execute(self) -> impl TryStream<Ok = RouteMessage, Error = Error> {
         let RouteGetRequest {
             mut handle,
             message,
         } = self;
 
+        let should_dump = message.nlas.is_empty();
         let mut req = NetlinkMessage::from(RtnlMessage::GetRoute(message));
-        req.header.flags = NLM_F_REQUEST | NLM_F_DUMP;
+        req.header.flags =
+            NLM_F_REQUEST | if should_dump { NLM_F_DUMP } else { 0 };
 
         match handle.request(req) {
             Ok(response) => Either::Left(