infra: override default route when running in a netns

[rjarry]

When grout runs inside a k8s pod, the default route installed by k8s has priority over grout's own default route (which uses metric UINT32_MAX). This causes TCP connections to fail over grout.

When the environment variable GROUT_OVERRIDE_DEFAULT_ROUTE is set to a truthy value (1, true, on, yes), install the default route with metric 0 and NLM_F_REPLACE to take over the existing one. Set the variable in the default systemd environment file (which runs with PrivateNetwork=true), in the container quadlet and in the Containerfile.

Link: #625

Overview

Adds an explicit override mechanism for the default route driven by a new configuration flag (gr_config.override_default_route) and the GROUT_OVERRIDE_DEFAULT_ROUTE environment variable. When enabled, grout will install default routes into the main routing table using NLM_F_REPLACE and without the high-priority metric so the installed default takes precedence over existing defaults (useful for container/netns deployments).

Changes

• main/config.h

  • Adds bool override_default_route to struct gr_config.

• main/main.c

  • Adds parse_bool_env() helper to parse truthy environment values ("1", "true", "on", "yes", case-insensitive).
  • Reads GROUT_OVERRIDE_DEFAULT_ROUTE into gr_config.override_default_route at startup.
  • Emits a NOTICE log when GROUT_OVERRIDE_DEFAULT_ROUTE is set.

• modules/infra/control/netlink.c

  • When adding routes for RT_TABLE_MAIN, set netlink message flags to include NLM_F_REPLACE (nlmsg_flags |= NLM_F_CREATE | NLM_F_REPLACE) if gr_config.override_default_route is true; otherwise preserve previous behavior using NLM_F_EXCL.
  • Only set RTA_PRIORITY = UINT32_MAX when override_default_route is false (omit the high-priority metric when override is enabled), allowing a route with metric 0 to win.

• Documentation and defaults

  • docs/grout.8.scdoc: Documents GROUT_OVERRIDE_DEFAULT_ROUTE in ENVIRONMENT with accepted truthy values and the effect (override existing default route with metric 0).
  • Containerfile: Sets ENV GROUT_OVERRIDE_DEFAULT_ROUTE=true in the final image.
  • main/grout.default and smoke/_init.sh: Set GROUT_OVERRIDE_DEFAULT_ROUTE=true in defaults/test harness so runtime behavior overrides existing default routes in those environments.