When a DHCP server provides only a gateway, the networkd-dhcp4 code adds a
default route using that gateway, and sets the just-provided address as
the route's prefsrc; this was added in commit
46b0c76e2c355c0d0cc4792abb98cde07b28bc53
However, if the DHCP server has also provided classless route(s), these
are used instead of a default route using the provided gateway; the
networkd-dhcp4 code sets up the classless routes, but does not use the
just-provided dhcp address as the prefsrc of the route(s). Note that
it also doesn't set the prefsrc for static routes, though it should.
If the interface has only the dhcp-provided address, this is not usually
a problem, but if it has another address (e.g. a static address), then
traffic sent through the dhcp-provided gateway might not use the
dhcp-provided source address. If the gateway router only will route
traffic from the dhcp-provided address, then the dhcp client system's
networking through the router will not work.
Fixes: #12969
route->priority = link->network->dhcp_route_metric;
route->table = table;
route->scope = route_scope_from_address(route, &address);
+ if (IN_SET(route->scope, RT_SCOPE_LINK, RT_SCOPE_UNIVERSE))
+ route->prefsrc.in = address;
r = route_configure(route, link, dhcp4_route_handler);
if (r < 0)
route->priority = link->network->dhcp_route_metric;
route->table = table;
route->scope = route_scope_from_address(route, address);
+ if (IN_SET(route->scope, RT_SCOPE_LINK, RT_SCOPE_UNIVERSE))
+ route->prefsrc.in = *address;
if (route_present_in_routes(route, new_routes, m))
continue;