# Kernel routes

> Install the best routes learned from neighbors into the Linux routing table.

With a `[kernel]` section, the best route of every prefix learned from a neighbor with
`import = "all"` goes into the kernel's routing table (protocol `bgp`,
`ip route show proto bgp`). Linux 5.3 or newer.

```toml
[kernel]
table = 254            # main (default)
metric = 20            # FRR's for eBGP (default)
families = ["ipv4", "ipv6"]
resolve_via_default = false

[[neighbor]]
address = "10.0.0.1"
asn = 64501
import = "all"
install = true         # default with [kernel]; false keeps this neighbor's routes out
```

## Next hops

Each BGP next hop is one kernel nexthop object (ids from 3120562176 = 0xBA000000,
`ip nexthop show`), so a next hop that moves is one change, not one per route.

- A next hop is resolved through the kernel's own route to it: connected, or through that
  route's gateway (iBGP, multihop; for an ECMP route or a nexthop group, its first leg that is
  up). Routes learned by BGP and the default route do not count (`resolve_via_default = true`
  allows the latter).
- An IPv6 link-local next hop (RFC 2545) from a neighbor on a shared link goes out of the
  session's interface: used when the neighbor sends only a link-local next hop, and when its
  global one does not resolve.
- ansage watches interfaces, addresses and routes: when a next hop becomes unreachable its
  routes leave the kernel at once and the next best path takes over; what the kernel refuses
  is tried again every 30 s.
- Our own prefixes (`announce`, also after a runtime withdraw) never go in.

## Inspecting

`ansage show fib` lists the counts and every next hop (reachable through which gateway and
interface, or not); `ansage show rib ADDRESS` shows the longest matching prefix and whether it
is in the kernel. Metrics: `ansage_fib_routes`, `ansage_fib_next_hops`,
`ansage_fib_errors_total`, `ansage_fib_adopted_routes`.

## Permissions and lifecycle

> **CAP_NET_ADMIN:** ansage needs CAP_NET_ADMIN and netlink: the installer enables them in the unit when the config
> has `[kernel]`; otherwise ansage refuses to start and says what is missing.

A stop removes ansage's routes. `ansage upgrade` and a crash leave them: the next start keeps
them forwarding, replaces each as its neighbor sends it again, and removes the rest once every
installing neighbor has sent End-of-RIB (at most 6 minutes). A reload moves the routes to a
new `table` or `metric` without a gap; adding or removing `[kernel]` needs a restart.