Policy scripts: the operator's own rules for routes, in Rhai

A new crate, bgp-policy, runs functions of a script on routes: on their
way in, after the general checks of a peer's import and before the
checks that can change (RPKI, next hop reachability); and on their way
out to a peer. A function is given the route and the peer, may refuse
the route, and may change its communities, large communities,
LOCAL_PREF and MED, and on the way out prepend our AS:

    fn from_peer(route, peer) {
        if route.has_community("65535:666") { return false; }
        route.add_community(LEARNED_FROM[peer.role]);
        route.local_pref = PREFERENCE[peer.role];
        true
    }

Configuration: policy { path | source, import_function, export_function }
and, for a peer, import { function } and export { function }. The script
is compiled at startup; `bgp --check` reads it and says which function
is whose. A function that fails on a route has refused it. Scripts have
no files, network or clock, and are stopped after 200,000 steps.

Route selection now goes by a LOCAL_PREF on a route from an external
peer: the decoder never lets a peer's own through, so one that is there
was set by policy.

What it costs was measured before it was built on (benches/policy.rs):
a call that tests a few communities and changes two takes 1 to 3
microseconds, some two hundred times what Rust would. So a function is
asked once for the routes of an UPDATE, which share their attributes,
unless it asks for the prefix. One peer sending 1.23 M routes over a
session: 0.37 s without a function, 0.80 s with one that marks and
ranks every route, 3.4 s if it also looks at the prefix.

Export applies to what is announced, which today is what the daemon
originates; learned routes are not passed on yet.

Tested by unit tests of the crate, sessions end to end, a fuzz target on
the example script (testdata/policy.rhai), and a lab phase in which a
script changes which of FRR's and BIRD's route is chosen and what each
of them is told.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

testdata/policy.rhai

@@ -0,0 +1,61 @@
+// An example policy script; see "Policy scripts" in bin/bgp/README.md for
+// everything a function can read and change.
+//
+// A function is given a route and the peer it comes from (or goes to). It
+// may change the route, and returns whether the route is taken (or
+// announced). Which function is for which peer is said in the
+// configuration: import { function: "from_peer" }.
+//
+// Constants defined here are for every function below.
+
+// Where a route was learned: what the rules for announcing it go by.
+const LEARNED_FROM = #{
+    customer: "65000:1000",
+    peer: "65000:2000",
+    provider: "65000:3000",
+    route_server: "65000:2000",
+};
+
+// A customer's route is preferred to a peer's, and that to a provider's:
+// the first earns money, the last costs it.
+const PREFERENCE = #{ customer: 300, peer: 200, route_server: 200, provider: 100 };
+
+const BLACKHOLE = "65535:666";
+const NO_EXPORT = "65535:65281";
+
+// Routes on their way in.
+fn from_peer(route, peer) {
+    // Not ours to act on, from just anyone.
+    if route.has_community(BLACKHOLE) { return false; }
+
+    // Marks in our own range mean something here: nobody else's word for
+    // them is taken.
+    route.remove_community("65000:*");
+    if peer.role in LEARNED_FROM {
+        route.add_community(LEARNED_FROM[peer.role]);
+        route.local_pref = PREFERENCE[peer.role];
+    }
+
+    // A peer can ask to be a last resort: 65000:80 on a route of its own.
+    if route.has_community(peer.asn % 65536, 80) {
+        route.local_pref = 80;
+    }
+    true
+}
+
+// Routes on their way out.
+fn to_peer(route, peer) {
+    if route.has_community(NO_EXPORT) { return false; }
+
+    // Only customers are told what we learned from peers and providers.
+    let from_above = route.has_community("65000:2000") || route.has_community("65000:3000");
+    if from_above && peer.role != "customer" { return false; }
+
+    // Our marks are our own business.
+    route.remove_community("65000:*");
+
+    // Traffic from this one is wanted by another way: the path looks
+    // longer to it.
+    if peer.name == "backup-transit" { route.prepend(2); }
+    true
+}