fix: handle default routes in brutalctl

[morecola]

fix: handle default routes in brutalctl

Summary

Fix brutalctl handling of 0.0.0.0/0 and ::/0 rules.

The kernel rule can be created successfully for a default prefix while the route that selects brutal fails to install, leaving brutalctl list with ROUTE=no, MEMBERS=0, and no traffic using the rule.

Root cause

For IPv4, route_lookup() strips the prefix and runs ip -4 route get 0.0.0.0. On systems where this resolves to the local route, brutalctl treats the destination as local and never installs a route-level congctl brutal setting.

For default routes, reconstructing a route from only via and dev is also insufficient. Attributes such as onlink and metric can be required, especially on VPS IPv6 configurations.

Default routes also need different delete semantics: deleting a /0 Brutal rule must not delete the system default route itself.

Changes

• Detect IPv4 and IPv6 default prefixes explicitly.
• Read the existing default route and update it in place while preserving the attributes printed by ip route.
• Add or remove only the route-level congctl brutal setting for default routes.
• Make brutalctl list detect brutal directly on the default route.
• Keep the existing protocol-233 route behavior unchanged for non-default prefixes.
• Make del and flush preserve the underlying default route.

Reproduction before the fix

$ brutalctl add 0.0.0.0/0 1000 gain=20
brutalctl: rule added, but no route installed: destination is a local address

$ brutalctl list
DESTINATION  RATE(Mbps)  GAIN  LOCK  ROUTE  MEMBERS  SENT(MB)
0.0.0.0/0       1000.00    20   yes     no        0       0.0

The IPv6 default rule similarly failed on a route requiring onlink.

Verification

Tested against upstream master commit 377d2a0e9324ef585ff90ea91779baf276cf6a50 on:

Debian 12 (bookworm)
Linux 6.1.0-52-cloud-amd64
KVM

Default route shapes used for the test:

IPv4: default via <gateway> dev ens3 onlink
IPv6: default via <gateway> dev ens3 metric 1024 onlink pref medium

Build check:

cc -std=c11 -O2 -Wall -Wextra -Werror tools/brutalctl.c

Regression checks passed for both IPv4 and IPv6:

1. add 0.0.0.0/0 / add ::/0 results in ROUTE=yes.
2. The default route retains its gateway, device, metric, onlink, and other attributes.
3. del removes congctl brutal but leaves the default route intact.
4. Re-adding the rule restores congctl lock brutal.
5. IPv4 and IPv6 connectivity remain working after add/delete cycles.
6. Live TCP sockets selected brutal, and rule membership / sent-byte counters increased during traffic.