network: dhcp: create explicit host route to gateway

Some DHCP servers gives you a netmask of 255.255.255.255 so the gateway is not
routable. Other DHCP client implementations look through the existing routes to
figure out if they should add an explicit host route. See below for a link.

However, it makes sense to just create the route explicitly whether it is
needed or not since it is explicit, makes the dhcp route entries independent of
other entries and saves us from knowing the state of the kernel tables.

After patch route table on a machine with a network (common case):

default via 10.0.2.2 dev ens3
10.0.2.0/24 dev ens3  proto kernel  scope link  src 10.0.2.15
10.0.2.2 dev ens3  scope link

After patch route table on a machine without a network (this case):

default via 10.240.0.1 dev ens4v1
10.240.0.1 dev ens4v1  scope link

The code from dhcpcd that works around this issue is on line 637.
https://android.googlesource.com/platform/external/dhcpcd/+/master/configure.c

diff --git a/src/network/networkd-link.c b/src/network/networkd-link.c
index a7ba466..14cc871 100644 (file)
--- a/src/network/networkd-link.c
+++ b/src/network/networkd-link.c
@@ -228,6 +228,7 @@ static int link_enter_set_routes(Link *link) {
 
         if (link->dhcp_lease) {
                 _cleanup_route_free_ Route *route = NULL;
+                _cleanup_route_free_ Route *route_gw = NULL;
                 struct in_addr gateway;
 
                 r = sd_dhcp_lease_get_router(link->dhcp_lease, &gateway);
@@ -244,6 +245,30 @@ static int link_enter_set_routes(Link *link) {
                         return r;
                 }
 
+                r = route_new_dynamic(&route_gw);
+                if (r < 0) {
+                        log_error_link(link, "Could not allocate route: %s",
+                                       strerror(-r));
+                        return r;
+                }
+
+                /* The dhcp netmask may mask out the gateway. Add an explicit
+                 * route for the gw host so that we can route no matter the
+                 * netmask or existing kernel route tables. */
+                route_gw->family = AF_INET;
+                route_gw->dst_addr.in = gateway;
+                route_gw->dst_prefixlen = 32;
+                route_gw->scope = RT_SCOPE_LINK;
+
+                r = route_configure(route_gw, link, &route_handler);
+                if (r < 0) {
+                        log_warning_link(link,
+                                         "could not set host route: %s", strerror(-r));
+                        return r;
+                }
+
+                link->route_messages ++;
+
                 route->family = AF_INET;
                 route->in_addr.in = gateway;