core: suppress onlink flag for IPv4 routes without gateway

(cherry picked from commit e59d09b053)

src/core/nm-l3-config-data.c

@@ -2851,6 +2851,22 @@ _init_from_connection_ip(NML3ConfigData *self, int addr_family, NMConnection *co
 
         nm_utils_ip_route_attribute_to_platform(addr_family, s_route, &r.rx, -1);
 
+        if (IS_IPv4 && r.r4.gateway == 0) {
+            /* As far as kernel is concerned, an IPv4 without gateway cannot have
+             * the onlink flag set, we need to clear it. If we wouldn't clear it,
+             * then the address we would add in kernel would be entirely different than
+             * the one we create here (because the "onlink" flag is part of the
+             * identifier of a route, see nm_platform_ip4_route_cmp()).
+             *
+             * Note however that for ECMP routes we currently can only merge routes
+             * that agree in their onlink flag. So a route without gateway cannot
+             * merge with an onlink route that has a gateway. That needs fixing,
+             * by not treating the onlink flag as for the entire route, but allowing
+             * to merge ECMP routes with different onlink flag. And first, we need
+             * to track the onlink flag for the nexthop (NMPlatformIP4RtNextHop). */
+            r.r4.r_rtm_flags &= ~((unsigned) RTNH_F_ONLINK);
+        }
+
         nm_l3_config_data_add_route(self, addr_family, NULL, &r.rx);
     }
 

src/libnm-core-impl/nm-setting-ip4-config.c

@@ -1116,7 +1116,11 @@ nm_setting_ip4_config_class_init(NMSettingIP4ConfigClass *klass)
      *        <para><literal>"mtu"</literal> - an unsigned 32 bit integer.</para>
      *      </listitem>
      *      <listitem>
-     *        <para><literal>"onlink"</literal> - a boolean value.</para>
+     *        <para><literal>"onlink"</literal> - a boolean value. The onlink flag
+     *          is ignored for IPv4 routes without a gateway. That also means,
+     *          with a positive "weight" the route cannot merge with ECMP routes
+     *          which are onlink and have a gateway.
+     *        </para>
      *      </listitem>
      *      <listitem>
      *        <para><literal>"quickack"</literal> - a boolean value.</para>