ferrix/crates/fib/src/plan.rs

//! What should be in the kernel's table, what is, and what to do about the
//! difference. No I/O here: this is told what the routing table's best
//! routes are and what the kernel said, and answers with the changes to
//! ask the kernel for.

use std::net::{IpAddr, Ipv6Addr};

use bgp_packet::message::NlriNextHop;
use bgp_primitives::constants::IpVersion;
use bgp_primitives::ip_prefix::IpPrefix;

use crate::netlink::KIND_UNICAST;
use crate::topology::Topology;

// Asked for every route: a quick hash matters here.
pub(crate) type HashMap<K, V> = std::collections::HashMap<K, V, foldhash::fast::RandomState>;
pub(crate) type HashSet<K> = std::collections::HashSet<K, foldhash::fast::RandomState>;

/// Where the kernel is to send packets for a prefix.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct Target {
    pub gateway: IpAddr,
    /// The index of the interface the gateway is on.
    pub interface: u32,
}

/// Why a route's next hop is not something the kernel can be given.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Unresolved {
    /// The route has no next hop.
    Missing,
    /// An IPv6 route with an IPv4 next hop.
    OtherFamily,
    /// The next hop is one of this machine's own addresses.
    Ourselves,
    /// The next hop is not in the network of any interface that is up,
    /// and none of the kernel's routes that count says the way to it
    /// (see `crate::routes`).
    NoRoute,
    /// The only route to the next hop is the default route, which is not
    /// taken as the way to a next hop unless asked for
    /// (`Topology::via_default`).
    OnlyDefault,
    /// The route to the next hop is one that discards packets: a
    /// blackhole, unreachable or prohibit route.
    Discarded,
    /// The route is for a network this machine is on itself, or for part
    /// of one. The kernel reaches that directly; a route from a peer that
    /// said otherwise would send the neighbors' traffic, and the sessions
    /// with them, wherever the peer liked.
    OwnNetwork,
}

impl Unresolved {
    pub const ALL: [Self; 7] = [
        Self::Missing,
        Self::OtherFamily,
        Self::Ourselves,
        Self::NoRoute,
        Self::OnlyDefault,
        Self::Discarded,
        Self::OwnNetwork,
    ];

    pub fn as_str(self) -> &'static str {
        match self {
            Self::Missing => "no next hop",
            Self::OtherFamily => "IPv4 next hop for an IPv6 route",
            Self::Ourselves => "next hop is this machine",
            Self::NoRoute => "no route to the next hop",
            Self::OnlyDefault => "next hop reachable only by the default route",
            Self::Discarded => "the route to the next hop discards packets",
            Self::OwnNetwork => "inside a network of this machine",
        }
    }
}

// A reason's place in `ALL` is its number as declared, which is what
// [`Plan`] counts them by.
const _: () = {
    let mut at = 0;
    while at < Unresolved::ALL.len() {
        assert!(Unresolved::ALL[at] as usize == at);
        at += 1;
    }
};

/// The peer a route was learned from, as far as its next hop is concerned.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct Neighbor {
    pub address: IpAddr,
    /// The interface the session with the peer is on, where the peer's
    /// address is a link-local one and says nothing without it; zero
    /// otherwise.
    pub interface: u32,
}

impl From<IpAddr> for Neighbor {
    fn from(address: IpAddr) -> Self {
        Self {
            address,
            interface: 0,
        }
    }
}

/// Works out where the kernel is to send packets for a route to `prefix`
/// with the next hop `next_hop`, learned from `peer`.
///
/// A next hop that is a neighbor on one of our interfaces is where the
/// packets go. One that is not is looked up in the kernel's own routes,
/// and the packets go where the route that covers it sends them: to its
/// gateway, or onto its interface. A link-local next hop means something
/// only on one link, and that is taken to be the link the peer itself is
/// on.
pub fn resolve(
    topology: &Topology,
    prefix: &IpPrefix,
    next_hop: Option<&NlriNextHop>,
    peer: Option<Neighbor>,
) -> Result<Target, Unresolved> {
    if topology.interfaces.has_network_around(prefix) {
        return Err(Unresolved::OwnNetwork);
    }
    resolve_next_hop(topology, prefix.address_family(), next_hop, peer)
}

/// The way to `gateway`, which is not a neighbor, by the kernel's routes
/// that count: where the route that covers it sends packets. One step
/// only: the kernel sees to it that a route's own gateway is a neighbor.
fn through_routes(
    topology: &Topology,
    routes: IpVersion,
    gateway: IpAddr,
) -> Result<Target, Unresolved> {
    let route = topology.routes.lookup(gateway).ok_or(Unresolved::NoRoute)?;
    if route.prefix.length() == 0 && !topology.via_default {
        return Err(Unresolved::OnlyDefault);
    }
    if route.kind != KIND_UNICAST {
        return Err(Unresolved::Discarded);
    }
    let (hop, interface) = match route.nexthop {
        Some(id) => {
            // A group of nexthops has no gateway or interface of its own;
            // which of its members packets take is the kernel's to say,
            // and nothing to give it as a route.
            let nexthop = topology.nexthops.get(&id).ok_or(Unresolved::NoRoute)?;
            (
                nexthop.gateway,
                nexthop.interface.ok_or(Unresolved::NoRoute)?,
            )
        }
        None => (route.gateway, route.interface.ok_or(Unresolved::NoRoute)?),
    };
    if !topology
        .interfaces
        .link(interface)
        .is_some_and(|link| link.up)
    {
        return Err(Unresolved::NoRoute);
    }
    // A route without a gateway puts the destination on its interface:
    // the next hop itself is the neighbor there.
    let gateway = hop.unwrap_or(gateway);
    // An IPv4 route may go by an IPv6 gateway (RFC 8950, and the kernel
    // allows it); an IPv6 route by an IPv4 gateway is nothing.
    if routes == IpVersion::Ipv6 && gateway.is_ipv4() {
        return Err(Unresolved::NoRoute);
    }
    Ok(Target { gateway, interface })
}

/// The part of [`resolve`] that is the same for every route of IP version
/// `routes` with one next hop from one peer: where the next hop is. For
/// those who ask it of many such routes, and ask of each only whether it
/// is for one of the machine's own networks.
pub fn resolve_next_hop(
    topology: &Topology,
    routes: IpVersion,
    next_hop: Option<&NlriNextHop>,
    peer: Option<Neighbor>,
) -> Result<Target, Unresolved> {
    let interfaces = &topology.interfaces;
    let next_hop = next_hop.ok_or(Unresolved::Missing)?;
    let version = match next_hop {
        NlriNextHop::Ipv4(_) => IpVersion::Ipv4,
        NlriNextHop::Ipv6(_) | NlriNextHop::Ipv6WithLl { .. } => IpVersion::Ipv6,
    };
    // An IPv4 route may have an IPv6 next hop: that is how routes go over
    // a link with only IPv6 addresses (RFC 8950), and the kernel takes
    // such a route. The other way about means nothing.
    if version != routes && version == IpVersion::Ipv4 {
        return Err(Unresolved::OtherFamily);
    }
    let direct = |gateway: IpAddr| {
        if interfaces.is_local(gateway) {
            return Err(Unresolved::Ourselves);
        }
        match interfaces.connected(gateway) {
            Some(connected) => Ok(Target {
                gateway,
                interface: connected.interface,
            }),
            None => through_routes(topology, routes, gateway),
        }
    };
    // The interface the peer is a neighbor on, for a link-local gateway.
    let on_peers_link = |gateway: Ipv6Addr| {
        let interface = match peer.ok_or(Unresolved::NoRoute)? {
            // The session is on a link-local address itself, and so says
            // which link: every interface has fe80::/64, and looking the
            // peer up there would find any of them.
            Neighbor { interface, .. } if interface != 0 => interfaces
                .link(interface)
                .filter(|link| link.up)
                .map(|link| link.index),
            Neighbor { address, .. } => interfaces
                .connected(address)
                .map(|connected| connected.interface),
        }
        .ok_or(Unresolved::NoRoute)?;
        if interfaces
            .addresses_of(interface)
            .any(|ours| ours.local == IpAddr::V6(gateway))
        {
            return Err(Unresolved::Ourselves);
        }
        Ok(Target {
            gateway: gateway.into(),
            interface,
        })
    };
    match *next_hop {
        NlriNextHop::Ipv4(gateway) => direct(gateway.into()),
        NlriNextHop::Ipv6(gateway) if gateway.is_unicast_link_local() => on_peers_link(gateway),
        NlriNextHop::Ipv6(gateway) => direct(gateway.into()),
        // The global address if it is a neighbor; else the link-local one,
        // which is there for exactly that case (RFC 2545 §3).
        // Some put the link-local address, or nothing, where the global
        // one goes, having no global one to give.
        NlriNextHop::Ipv6WithLl { global, link_local }
            if global.is_unspecified() || global.is_unicast_link_local() =>
        {
            on_peers_link(link_local)
        }
        // Where there is no way to the global one, the link-local one is
        // tried; a way that is there and is bad (ourselves, a blackhole)
        // is an answer as it stands.
        NlriNextHop::Ipv6WithLl { global, link_local } => match direct(global.into()) {
            Err(Unresolved::NoRoute | Unresolved::OnlyDefault) => on_peers_link(link_local),
            resolved => resolved,
        },
    }
}

/// A change to ask the kernel for.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Op {
    /// Put this route there, in place of ours for the prefix if there is
    /// one.
    Install {
        prefix: IpPrefix,
        target: Target,
    },
    Remove {
        prefix: IpPrefix,
    },
}

impl Op {
    pub fn prefix(&self) -> &IpPrefix {
        match self {
            Self::Install { prefix, .. } | Self::Remove { prefix } => prefix,
        }
    }
}

struct Entry {
    /// What the kernel has; `None` for a route found there at startup that
    /// is not of a shape we make.
    target: Option<Target>,
    /// Whether the prefix was heard of since the last reset.
    confirmed: bool,
    /// Whether the route is one an earlier run left that is being kept
    /// for now though not confirmed: see [`Plan::adopt`].
    kept: bool,
}

/// The routes of ours in the kernel, kept in step with the best routes.
///
/// It follows what a reader of the routing table is given: the whole
/// table between a reset and its completion, and changes after. Whatever
/// is in the kernel and was not confirmed by the end of a whole table is
/// removed: that is how routes left by an earlier run, or made stale by a
/// change of interfaces, are cleared out.
#[derive(Default)]
pub struct Plan {
    /// What the kernel has been asked to have, and has not refused.
    installed: HashMap<IpPrefix, Entry>,
    /// Prefixes whose best route is not installed, and why.
    unresolved: HashMap<IpPrefix, Unresolved>,
    /// How many of them for each reason, in the order of
    /// [`Unresolved::ALL`].
    unresolved_counts: [usize; Unresolved::ALL.len()],
    /// Prefixes whose route the kernel refused.
    refused: HashSet<IpPrefix>,
}

impl Plan {
    /// How many routes of ours the kernel has.
    pub fn installed(&self) -> usize {
        self.installed.len()
    }

    /// How many prefixes have a best route that could not be given to the
    /// kernel for want of a usable next hop.
    pub fn unresolved(&self) -> usize {
        self.unresolved.len()
    }

    /// The same, by reason, leaving out the reasons there are none of.
    pub fn unresolved_by_reason(&self) -> impl Iterator<Item = (Unresolved, usize)> {
        Unresolved::ALL
            .into_iter()
            .zip(self.unresolved_counts)
            .filter(|(_, count)| *count > 0)
    }

    /// Notes why the best route of `prefix` is not installed, or with
    /// `None` that this is no longer so.
    fn set_unresolved(&mut self, prefix: IpPrefix, reason: Option<Unresolved>) {
        let before = match reason {
            Some(reason) => self.unresolved.insert(prefix, reason),
            None => self.unresolved.remove(&prefix),
        };
        if let Some(before) = before {
            self.unresolved_counts[before as usize] -= 1;
        }
        if let Some(reason) = reason {
            self.unresolved_counts[reason as usize] += 1;
        }
    }

    /// Forgets which prefixes are unresolved or refused, and why.
    fn forget_unresolved_and_refused(&mut self) {
        self.unresolved.clear();
        self.unresolved_counts = [0; Unresolved::ALL.len()];
        self.refused.clear();
    }

    /// How many prefixes have a best route that the kernel refused.
    pub fn refused(&self) -> usize {
        self.refused.len()
    }

    /// Notes a route of ours that the kernel already has. It stays only if
    /// the routing table turns out to have the same.
    ///
    /// With `keep` it is not removed for being absent from a whole table,
    /// only by [`Plan::release`]. That is for the routes an earlier run
    /// left: when the daemon starts, its table is empty until its peers
    /// have sent theirs again, and to remove everything meanwhile would
    /// stop the machine forwarding for as long as that takes.
    pub fn adopt(&mut self, prefix: IpPrefix, target: Option<Target>, keep: bool) {
        self.installed.insert(
            prefix,
            Entry {
                target,
                confirmed: false,
                kept: keep,
            },
        );
    }

    /// How many routes are being kept from an earlier run without the
    /// routing table having confirmed them.
    pub fn kept(&self) -> usize {
        self.installed
            .values()
            .filter(|entry| entry.kept && !entry.confirmed)
            .count()
    }

    /// The time for keeping unconfirmed routes of an earlier run is over:
    /// those the routing table still does not have go.
    pub fn release(&mut self) -> Vec<Op> {
        let mut stale = Vec::new();
        self.installed.retain(|prefix, entry| {
            let goes = entry.kept && !entry.confirmed;
            entry.kept = false;
            if goes {
                stale.push(Op::Remove { prefix: *prefix });
            }
            !goes
        });
        stale
    }

    /// The whole table follows: nothing is known to be wanted until it is
    /// heard of again.
    pub fn reset(&mut self) {
        for entry in self.installed.values_mut() {
            entry.confirmed = false;
        }
        self.forget_unresolved_and_refused();
    }

    /// The best route for `prefix` is now one that resolves as `target`.
    pub fn best(&mut self, prefix: IpPrefix, target: Result<Target, Unresolved>) -> Option<Op> {
        // Nearly always there is nothing to forget.
        if !self.refused.is_empty() {
            self.refused.remove(&prefix);
        }
        match target {
            Ok(target) => {
                if !self.unresolved.is_empty() {
                    self.set_unresolved(prefix, None);
                }
                let entry = self.installed.entry(prefix).or_insert(Entry {
                    target: None,
                    confirmed: true,
                    kept: false,
                });
                entry.confirmed = true;
                if entry.target == Some(target) {
                    return None;
                }
                entry.target = Some(target);
                Some(Op::Install { prefix, target })
            }
            Err(reason) => {
                self.set_unresolved(prefix, Some(reason));
                // Packets must not go on following a route that is no
                // longer the best.
                self.installed
                    .remove(&prefix)
                    .map(|_| Op::Remove { prefix })
            }
        }
    }

    /// `prefix` has no route any more.
    pub fn withdrawn(&mut self, prefix: IpPrefix) -> Option<Op> {
        self.set_unresolved(prefix, None);
        self.refused.remove(&prefix);
        self.installed
            .remove(&prefix)
            .map(|_| Op::Remove { prefix })
    }

    /// The whole table has been given: what was not in it goes.
    pub fn synced(&mut self) -> Vec<Op> {
        let stale: Vec<IpPrefix> = self
            .installed
            .iter()
            .filter(|(_, entry)| !entry.confirmed && !entry.kept)
            .map(|(prefix, _)| *prefix)
            .collect();
        stale
            .into_iter()
            .map(|prefix| {
                self.installed.remove(&prefix);
                Op::Remove { prefix }
            })
            .collect()
    }

    /// The kernel refused `op`. Returns what to ask of it instead, if
    /// anything.
    pub fn refused_by_kernel(&mut self, op: &Op) -> Option<Op> {
        match op {
            Op::Install { prefix, target } => {
                // If the plan has moved on since, this is old news.
                if self.installed.get(prefix)?.target != Some(*target) {
                    return None;
                }
                self.installed.remove(prefix);
                self.refused.insert(*prefix);
                // What the kernel refused to replace, it still has.
                Some(Op::Remove { prefix: *prefix })
            }
            // It was not there to remove: as wanted.
            Op::Remove { .. } => None,
        }
    }

    /// Everything goes: the changes that empty the kernel of our routes.
    pub fn clear(&mut self) -> Vec<Op> {
        self.forget_unresolved_and_refused();
        self.installed
            .drain()
            .map(|(prefix, _)| Op::Remove { prefix })
            .collect()
    }
}

#[cfg(test)]
mod tests {
    use proptest::prelude::*;

    use super::*;
    use crate::netlink::{Address, Link, Message, Nexthop, PROTOCOL_BGP, Route, TABLE_MAIN};

    fn prefix(s: &str) -> IpPrefix {
        IpPrefix::try_from(s).unwrap()
    }

    fn target(gateway: &str, interface: u32) -> Target {
        Target {
            gateway: gateway.parse().unwrap(),
            interface,
        }
    }

    fn interfaces() -> Topology {
        let mut topology = Topology::new(TABLE_MAIN, false);
        for (index, local, network) in [
            (2, "192.0.2.1", "192.0.2.0/24"),
            (2, "2001:db8::1", "2001:db8::/64"),
            (3, "198.51.100.1", "198.51.100.0/24"),
        ] {
            topology.apply(&Message::Link {
                present: true,
                link: Link {
                    index,
                    name: format!("eth{index}"),
                    up: true,
                },
            });
            topology.apply(&Message::Address {
                present: true,
                address: Address {
                    interface: index,
                    local: local.parse().unwrap(),
                    network: prefix(network),
                },
            });
        }
        topology
    }

    /// A route of the kernel's main table, as a static route is.
    fn kernel_route(prefix_text: &str, gateway: Option<&str>, interface: Option<u32>) -> Route {
        Route {
            prefix: prefix(prefix_text),
            table: TABLE_MAIN,
            protocol: 4,
            metric: 0,
            gateway: gateway.map(|g| g.parse().unwrap()),
            interface,
            nexthop: None,
            kind: KIND_UNICAST,
        }
    }

    fn with_route(topology: &mut Topology, route: Route) {
        assert!(topology.apply(&Message::Route {
            present: true,
            route,
        }));
    }

    fn neighbor(address: &str) -> Neighbor {
        address.parse::<IpAddr>().unwrap().into()
    }

    #[test]
    fn a_next_hop_resolves_if_it_is_a_neighbor() {
        let interfaces = interfaces();
        let v4 = prefix("203.0.113.0/24");
        let v6 = prefix("2001:db8:100::/48");
        let peer = Some(neighbor("192.0.2.2"));
        let v4_hop = |s: &str| NlriNextHop::Ipv4(s.parse().unwrap());
        let v6_hop = |s: &str| NlriNextHop::Ipv6(s.parse().unwrap());

        assert_eq!(
            resolve(&interfaces, &v4, Some(&v4_hop("192.0.2.2")), peer),
            Ok(target("192.0.2.2", 2))
        );
        // A neighbor other than the peer: a route server's routes.
        assert_eq!(
            resolve(&interfaces, &v4, Some(&v4_hop("198.51.100.9")), peer),
            Ok(target("198.51.100.9", 3))
        );
        assert_eq!(
            resolve(&interfaces, &v6, Some(&v6_hop("2001:db8::2")), peer),
            Ok(target("2001:db8::2", 2))
        );
        for (hop, why) in [
            (Some(v4_hop("10.9.9.9")), Unresolved::NoRoute),
            (Some(v4_hop("192.0.2.1")), Unresolved::Ourselves),
            (None, Unresolved::Missing),
        ] {
            assert_eq!(resolve(&interfaces, &v4, hop.as_ref(), peer), Err(why));
        }
        assert_eq!(
            resolve(&interfaces, &v6, Some(&v4_hop("192.0.2.2")), peer),
            Err(Unresolved::OtherFamily)
        );
        // A route for part of a network we are on is not for the kernel,
        // however good its next hop; one for more than the network is.
        for ours in [
            "192.0.2.0/24",
            "192.0.2.128/25",
            "192.0.2.7/32",
            "2001:db8::/64",
        ] {
            let ours = prefix(ours);
            let hop = match ours.address_family() {
                IpVersion::Ipv4 => v4_hop("192.0.2.2"),
                IpVersion::Ipv6 => v6_hop("2001:db8::2"),
            };
            assert_eq!(
                resolve(&interfaces, &ours, Some(&hop), peer),
                Err(Unresolved::OwnNetwork),
                "{ours}"
            );
        }
        assert_eq!(
            resolve(
                &interfaces,
                &prefix("192.0.0.0/22"),
                Some(&v4_hop("192.0.2.2")),
                peer
            ),
            Ok(target("192.0.2.2", 2))
        );
        // An IPv4 route by way of an IPv6 neighbor: a link with only IPv6
        // addresses (RFC 8950).
        assert_eq!(
            resolve(&interfaces, &v4, Some(&v6_hop("2001:db8::2")), peer),
            Ok(target("2001:db8::2", 2))
        );
        assert_eq!(
            resolve(
                &interfaces,
                &v4,
                Some(&NlriNextHop::Ipv6WithLl {
                    global: "2001:db8:ffff::2".parse().unwrap(),
                    link_local: "fe80::2".parse().unwrap(),
                }),
                Some(neighbor("2001:db8::2")),
            ),
            Ok(target("fe80::2", 2))
        );
    }

    #[test]
    fn a_link_local_next_hop_is_on_the_peers_link() {
        let interfaces = interfaces();
        let v6 = prefix("2001:db8:100::/48");
        let peer = Some(neighbor("2001:db8::2"));
        let link_local: Ipv6Addr = "fe80::2".parse().unwrap();
        let both = |global: &str| NlriNextHop::Ipv6WithLl {
            global: global.parse().unwrap(),
            link_local,
        };
        // The global address is a neighbor: it is used.
        assert_eq!(
            resolve(&interfaces, &v6, Some(&both("2001:db8::2")), peer),
            Ok(target("2001:db8::2", 2))
        );
        // It is not: the link-local one, on the interface the peer is on.
        assert_eq!(
            resolve(&interfaces, &v6, Some(&both("2001:db8:ffff::2")), peer),
            Ok(target("fe80::2", 2))
        );
        assert_eq!(
            resolve(&interfaces, &v6, Some(&NlriNextHop::Ipv6(link_local)), peer),
            Ok(target("fe80::2", 2))
        );
        // A peer that is not a neighbor says nothing about the link.
        let far = Some(neighbor("2001:db8:ffff::9"));
        assert_eq!(
            resolve(&interfaces, &v6, Some(&NlriNextHop::Ipv6(link_local)), far),
            Err(Unresolved::NoRoute)
        );
        assert_eq!(
            resolve(&interfaces, &v6, Some(&both("2001:db8::1")), peer),
            Err(Unresolved::Ourselves)
        );
    }

    /// A session on link-local addresses: which link is known from the
    /// session and from nothing else, every interface having fe80::/64.
    #[test]
    fn a_link_local_peer_is_on_the_interface_of_its_session() {
        let mut interfaces = interfaces();
        for (index, local) in [(2, "fe80::a"), (3, "fe80::b")] {
            interfaces.apply(&Message::Address {
                present: true,
                address: Address {
                    interface: index,
                    local: local.parse().unwrap(),
                    network: prefix("fe80::/64"),
                },
            });
        }
        let v4 = prefix("203.0.113.0/24");
        let v6 = prefix("2001:db8:100::/48");
        let link_local: Ipv6Addr = "fe80::2".parse().unwrap();
        let on = |interface| {
            Some(Neighbor {
                address: link_local.into(),
                interface,
            })
        };
        // However the peer says it: the address alone, or twice, or after
        // nothing where a global one would go.
        for next_hop in [
            NlriNextHop::Ipv6(link_local),
            NlriNextHop::Ipv6WithLl {
                global: link_local,
                link_local,
            },
            NlriNextHop::Ipv6WithLl {
                global: Ipv6Addr::UNSPECIFIED,
                link_local,
            },
        ] {
            for interface in [2, 3] {
                for route in [&v4, &v6] {
                    assert_eq!(
                        resolve(&interfaces, route, Some(&next_hop), on(interface)),
                        Ok(target("fe80::2", interface)),
                        "{next_hop:?} on {interface}"
                    );
                }
            }
            // An interface that is not there, or is down.
            assert_eq!(
                resolve(&interfaces, &v6, Some(&next_hop), on(9)),
                Err(Unresolved::NoRoute)
            );
        }
        // Our own address on that link, which on another is a neighbor's.
        let ours = NlriNextHop::Ipv6("fe80::a".parse().unwrap());
        assert_eq!(
            resolve(&interfaces, &v6, Some(&ours), on(2)),
            Err(Unresolved::Ourselves)
        );
        assert_eq!(
            resolve(&interfaces, &v6, Some(&ours), on(3)),
            Ok(target("fe80::a", 3))
        );
        // A global next hop on such a session is looked up as any other.
        let global = NlriNextHop::Ipv6("2001:db8::2".parse().unwrap());
        assert_eq!(
            resolve(&interfaces, &v6, Some(&global), on(3)),
            Ok(target("2001:db8::2", 2))
        );
    }

    /// A next hop that is no neighbor is reached the way the kernel's
    /// own routes say, and the route goes into the kernel with that way's
    /// gateway and interface.
    #[test]
    fn a_far_next_hop_resolves_by_the_kernels_routes() {
        let mut topology = interfaces();
        let v4 = prefix("203.0.113.0/24");
        let v6 = prefix("2001:db8:100::/48");
        let peer = Some(neighbor("192.0.2.2"));
        let v4_hop = |s: &str| NlriNextHop::Ipv4(s.parse().unwrap());
        let v6_hop = |s: &str| NlriNextHop::Ipv6(s.parse().unwrap());
        let far = v4_hop("10.9.9.9");

        // Nothing says the way.
        assert_eq!(
            resolve(&topology, &v4, Some(&far), peer),
            Err(Unresolved::NoRoute)
        );
        // The default route does not count unless asked for.
        with_route(
            &mut topology,
            kernel_route("0.0.0.0/0", Some("192.0.2.254"), Some(2)),
        );
        assert_eq!(
            resolve(&topology, &v4, Some(&far), peer),
            Err(Unresolved::OnlyDefault)
        );
        topology.via_default = true;
        assert_eq!(
            resolve(&topology, &v4, Some(&far), peer),
            Ok(target("192.0.2.254", 2))
        );
        topology.via_default = false;
        // A static route with a gateway: the gateway is where packets go.
        with_route(
            &mut topology,
            kernel_route("10.9.0.0/16", Some("198.51.100.9"), Some(3)),
        );
        assert_eq!(
            resolve(&topology, &v4, Some(&far), peer),
            Ok(target("198.51.100.9", 3))
        );
        // A more specific route onto an interface, as a tunnel's is: the
        // next hop itself is the neighbor there.
        topology.apply(&Message::Link {
            present: true,
            link: Link {
                index: 7,
                name: "wg0".into(),
                up: true,
            },
        });
        with_route(&mut topology, kernel_route("10.9.9.0/24", None, Some(7)));
        assert_eq!(
            resolve(&topology, &v4, Some(&far), peer),
            Ok(target("10.9.9.9", 7))
        );
        // The tunnel goes down: the kernel keeps such routes, we do not
        // use them.
        topology.apply(&Message::Link {
            present: true,
            link: Link {
                index: 7,
                name: "wg0".into(),
                up: false,
            },
        });
        assert_eq!(
            resolve(&topology, &v4, Some(&far), peer),
            Err(Unresolved::NoRoute)
        );
        // A route that names a nexthop object goes where the object says.
        with_route(
            &mut topology,
            Route {
                nexthop: Some(77),
                ..kernel_route("10.9.9.9/32", None, None)
            },
        );
        assert_eq!(
            resolve(&topology, &v4, Some(&far), peer),
            Err(Unresolved::NoRoute),
            "an object the kernel has not told of"
        );
        topology.apply(&Message::Nexthop {
            present: true,
            nexthop: Nexthop {
                id: 77,
                protocol: 4,
                gateway: Some("192.0.2.77".parse().unwrap()),
                interface: Some(2),
            },
        });
        assert_eq!(
            resolve(&topology, &v4, Some(&far), peer),
            Ok(target("192.0.2.77", 2))
        );
        // A route that discards packets is a way to nowhere.
        with_route(
            &mut topology,
            Route {
                kind: 6, // blackhole
                ..kernel_route("10.9.9.9/32", None, None)
            },
        );
        assert_eq!(
            resolve(&topology, &v4, Some(&far), peer),
            Err(Unresolved::Discarded)
        );
        // Routes of BGP's never say the way, be the prefix ever so long.
        assert!(!topology.apply(&Message::Route {
            present: true,
            route: Route {
                protocol: PROTOCOL_BGP,
                metric: 20,
                ..kernel_route("10.9.9.9/32", Some("198.51.100.9"), Some(3))
            },
        }));
        // An IPv6 next hop by an IPv4 gateway is no way for an IPv6 route,
        // but is one for an IPv4 route (RFC 8950 the other way about is
        // what the kernel allows).
        with_route(
            &mut topology,
            kernel_route("2001:db8:9::/48", Some("198.51.100.9"), Some(3)),
        );
        assert_eq!(
            resolve(&topology, &v6, Some(&v6_hop("2001:db8:9::9")), peer),
            Err(Unresolved::NoRoute)
        );
        with_route(
            &mut topology,
            kernel_route("2001:db8:9::/48", Some("2001:db8::9"), Some(2)),
        );
        assert_eq!(
            resolve(&topology, &v6, Some(&v6_hop("2001:db8:9::9")), peer),
            Ok(target("2001:db8::9", 2))
        );
        assert_eq!(
            resolve(&topology, &v4, Some(&v6_hop("2001:db8:9::9")), peer),
            Ok(target("2001:db8::9", 2))
        );
        // A neighbor is a neighbor, whatever the routes say.
        with_route(
            &mut topology,
            kernel_route("192.0.2.2/32", Some("198.51.100.9"), Some(3)),
        );
        assert_eq!(
            resolve(&topology, &v4, Some(&v4_hop("192.0.2.2")), peer),
            Ok(target("192.0.2.2", 2))
        );
    }

    #[test]
    fn the_kernel_is_asked_only_for_what_changes() {
        let mut plan = Plan::default();
        let p = prefix("203.0.113.0/24");
        let (a, b) = (target("192.0.2.2", 2), target("192.0.2.3", 2));

        assert_eq!(
            plan.best(p, Ok(a)),
            Some(Op::Install {
                prefix: p,
                target: a
            })
        );
        // The best route changes to one with the same next hop: nothing
        // for the kernel.
        assert_eq!(plan.best(p, Ok(a)), None);
        assert_eq!(
            plan.best(p, Ok(b)),
            Some(Op::Install {
                prefix: p,
                target: b
            })
        );
        assert_eq!(plan.installed(), 1);
        // A best route that cannot be installed takes the old one out.
        assert_eq!(
            plan.best(p, Err(Unresolved::NoRoute)),
            Some(Op::Remove { prefix: p })
        );
        assert_eq!((plan.installed(), plan.unresolved()), (0, 1));
        assert_eq!(
            plan.unresolved_by_reason().collect::<Vec<_>>(),
            [(Unresolved::NoRoute, 1)]
        );
        assert_eq!(plan.withdrawn(p), None);
        assert_eq!(plan.unresolved_by_reason().count(), 0);
        assert_eq!(plan.unresolved(), 0);

        plan.best(p, Ok(a));
        assert_eq!(plan.withdrawn(p), Some(Op::Remove { prefix: p }));
        assert_eq!(plan.withdrawn(p), None);
    }

    #[test]
    fn what_a_whole_table_does_not_confirm_is_removed() {
        let mut plan = Plan::default();
        let (kept, changed, stale, odd) = (
            prefix("203.0.113.0/24"),
            prefix("198.18.0.0/15"),
            prefix("192.88.99.0/24"),
            prefix("2001:db8:5::/48"),
        );
        let a = target("192.0.2.2", 2);
        // Left in the kernel by an earlier run.
        plan.adopt(kept, Some(a), false);
        plan.adopt(changed, Some(a), false);
        plan.adopt(stale, Some(a), false);
        plan.adopt(odd, None, false);
        assert_eq!(plan.installed(), 4);

        plan.reset();
        // The same as the kernel has: nothing to do.
        assert_eq!(plan.best(kept, Ok(a)), None);
        let b = target("192.0.2.3", 2);
        assert_eq!(
            plan.best(changed, Ok(b)),
            Some(Op::Install {
                prefix: changed,
                target: b
            })
        );
        let mut removed = plan.synced();
        removed.sort_by_key(|op| *op.prefix());
        let mut expected = vec![Op::Remove { prefix: stale }, Op::Remove { prefix: odd }];
        expected.sort_by_key(|op| *op.prefix());
        assert_eq!(removed, expected);
        assert_eq!(plan.installed(), 2);
        // A second completion finds nothing more.
        assert_eq!(plan.synced(), []);
    }

    /// When the daemon starts its table is empty, and stays so until its
    /// peers have sent theirs. What an earlier run left in the kernel is
    /// kept meanwhile.
    #[test]
    fn routes_of_an_earlier_run_are_kept_until_released() {
        let mut plan = Plan::default();
        let (still_right, moved, gone) = (
            prefix("203.0.113.0/24"),
            prefix("198.18.0.0/15"),
            prefix("192.88.99.0/24"),
        );
        let (a, b) = (target("192.0.2.2", 2), target("192.0.2.3", 2));
        for p in [still_right, moved, gone] {
            plan.adopt(p, Some(a), true);
        }
        // The first whole table is the empty one: nothing goes.
        plan.reset();
        assert_eq!(plan.synced(), []);
        assert_eq!((plan.installed(), plan.kept()), (3, 3));

        // The peers come back.
        assert_eq!(plan.best(still_right, Ok(a)), None);
        assert_eq!(
            plan.best(moved, Ok(b)),
            Some(Op::Install {
                prefix: moved,
                target: b
            })
        );
        assert_eq!(plan.kept(), 1);
        // Time is up: what nobody announced again goes, and only that.
        assert_eq!(plan.release(), [Op::Remove { prefix: gone }]);
        assert_eq!((plan.installed(), plan.kept()), (2, 0));
        assert_eq!(plan.release(), []);

        // From then on a whole table decides as always.
        plan.reset();
        plan.best(moved, Ok(b));
        assert_eq!(
            plan.synced(),
            [Op::Remove {
                prefix: still_right
            }]
        );

        // A route that is withdrawn while kept goes at once.
        let mut plan = Plan::default();
        plan.adopt(gone, Some(a), true);
        plan.best(gone, Ok(a));
        assert_eq!(plan.withdrawn(gone), Some(Op::Remove { prefix: gone }));
    }

    #[test]
    fn a_route_the_kernel_refuses_is_not_counted_as_there() {
        let mut plan = Plan::default();
        let p = prefix("203.0.113.0/24");
        let (a, b) = (target("192.0.2.2", 2), target("192.0.2.3", 2));
        let install = plan.best(p, Ok(a)).unwrap();
        // What it refused to put there may have left an older one of ours.
        assert_eq!(
            plan.refused_by_kernel(&install),
            Some(Op::Remove { prefix: p })
        );
        assert_eq!((plan.installed(), plan.refused()), (0, 1));
        // Refusing to remove what is not there is as wanted.
        assert_eq!(plan.refused_by_kernel(&Op::Remove { prefix: p }), None);
        // The next best route is tried afresh.
        assert!(plan.best(p, Ok(b)).is_some());
        assert_eq!((plan.installed(), plan.refused()), (1, 0));
        // News of a refusal of what has since been replaced is ignored.
        assert_eq!(plan.refused_by_kernel(&install), None);
        assert_eq!(plan.installed(), 1);

        let mut cleared = plan.clear();
        assert_eq!(cleared.pop(), Some(Op::Remove { prefix: p }));
        assert_eq!(plan.installed(), 0);
    }

    /// What happens to a table of best routes.
    #[derive(Debug, Clone)]
    enum Step {
        Best(u8, Option<u8>),
        Withdrawn(u8),
        /// A reader that starts over: the whole table again.
        Resync,
    }

    fn any_step() -> impl Strategy<Value = Step> {
        prop_oneof![
            4 => (0u8..12, proptest::option::weighted(0.8, 0u8..3)).prop_map(|(p, t)| Step::Best(p, t)),
            2 => (0u8..12).prop_map(Step::Withdrawn),
            1 => Just(Step::Resync),
        ]
    }

    proptest! {
        /// Whatever happens to the best routes, and whatever an earlier
        /// run left behind, a kernel that does as it is asked ends up with
        /// exactly the best routes that resolve.
        #[test]
        fn the_kernel_ends_up_with_the_best_routes_that_resolve(
            left_behind in proptest::collection::vec((0u8..16, 0u8..3), 0..6),
            steps in proptest::collection::vec(any_step(), 0..60),
        ) {
            let p = |n: u8| IpPrefix::new(IpVersion::Ipv4, [10, n, 0, 0], 16).unwrap();
            let t = |n: u8| target(&format!("192.0.2.{}", n + 2), 2);
            let mut kernel: HashMap<IpPrefix, Target> = HashMap::default();
            let mut plan = Plan::default();
            for (prefix, gateway) in left_behind {
                kernel.insert(p(prefix), t(gateway));
                plan.adopt(p(prefix), Some(t(gateway)), false);
            }
            // The best routes: the prefix's next hop, if it resolves.
            let mut table: HashMap<u8, Option<u8>> = HashMap::default();
            let apply = |kernel: &mut HashMap<IpPrefix, Target>, op: Option<Op>| match op {
                Some(Op::Install { prefix, target }) => { kernel.insert(prefix, target); }
                Some(Op::Remove { prefix }) => { kernel.remove(&prefix); }
                None => {}
            };
            let resolved = |gateway: Option<u8>| gateway.map(t).ok_or(Unresolved::NoRoute);
            // A new reader is first given the whole table.
            let steps = std::iter::once(Step::Resync).chain(steps);
            for step in steps {
                match step {
                    Step::Best(prefix, gateway) => {
                        table.insert(prefix, gateway);
                        apply(&mut kernel, plan.best(p(prefix), resolved(gateway)));
                    }
                    Step::Withdrawn(prefix) => {
                        table.remove(&prefix);
                        apply(&mut kernel, plan.withdrawn(p(prefix)));
                    }
                    Step::Resync => {
                        plan.reset();
                        for (prefix, gateway) in &table {
                            apply(&mut kernel, plan.best(p(*prefix), resolved(*gateway)));
                        }
                        for op in plan.synced() {
                            apply(&mut kernel, Some(op));
                        }
                    }
                }
                let wanted: HashMap<IpPrefix, Target> = table
                    .iter()
                    .filter_map(|(prefix, gateway)| Some((p(*prefix), t((*gateway)?))))
                    .collect();
                prop_assert_eq!(&kernel, &wanted);
                prop_assert_eq!(plan.installed(), wanted.len());
                prop_assert_eq!(plan.unresolved(), table.len() - wanted.len());
                prop_assert_eq!(
                    plan.unresolved_by_reason().map(|(_, count)| count).sum::<usize>(),
                    plan.unresolved()
                );
            }
        }
    }
}