Quote commit message:
all: allow configuring default-routes as manual, static routes Up until now, a default-route (with prefix length zero) could not be configured directly. The user could only set ipv4.gateway, ipv4.never-default, ipv4.route-metric and ipv4.route-table to influence the setting of the default-route (respectively for IPv6). That is a problematic limitation. For one, whether a route has prefix length zero or non-zero does not make a fundamental difference. Also, it makes it impossible to configure all the routing attributes that one can configure otherwise for static routes. For example, the default-route could not be configured as "onlink", could not have a special MTU, nor could it be placed in a dedicated routing table. Fix that by lifting the restriction. Note that "ipv4.never-default" does not apply to /0 manual routes. Likewise, the previous manners of configuring default-routes ("ipv4.gateway") don't conflict with manual default-routes. Server-side this all the pieces are already in place to accept a default-route as static routes. This was done by earlier commits like 5c299454b49b ('core: rework tracking of gateway/default-route in ip-config'). A long time ago, NMIPRoute would assert that the prefix length is positive. That was relaxed by commit a2e93f2de4ac ('libnm: allow zero prefix length for NMIPRoute'), already before 1.0.0. Using libnm from before 1.0.0 would result in assertion failures. Note that the default-route-metric-penalty based on connectivity checking applies to all /0 routes, even these static routes. Be they added due to DHCP, "ipv4.gateway", "ipv4.routes" or "wireguard.peer-routes". I wonder whether doing that unconditionally is desirable, and maybe there should be a way to opt-out/opt-in for the entire profile or even per-routes.
Also, it makes it possible to configure default-routes for WireGuard in a separate routing-table, which is relevant to manually configure the policy-routing-based solution akin to
Please consider especially what about the default-route-metric-penalty.