networkd: route - support 'onlink' routes (#5734)

This work based on Tom's original patch
teg@1312172

By setting GatewayOnlink=yes, the kernel will assume that the gateway is onlink
even if there is no route to it.

Resolves issue #1283.

src/network/networkd-network-gperf.gperf

@@ -86,6 +86,7 @@ Route.Metric,                           config_parse_route_priority,
 Route.Scope,                            config_parse_route_scope,                       0,                             0
 Route.PreferredSource,                  config_parse_preferred_src,                     0,                             0
 Route.Table,                            config_parse_route_table,                       0,                             0
+Route.GatewayOnlink,                    config_parse_gateway_onlink,                    0,                             0
 DHCP.ClientIdentifier,                  config_parse_dhcp_client_identifier,            0,                             offsetof(Network, dhcp_client_identifier)
 DHCP.UseDNS,                            config_parse_bool,                              0,                             offsetof(Network, dhcp_use_dns)
 DHCP.UseNTP,                            config_parse_bool,                              0,                             offsetof(Network, dhcp_use_ntp)

src/network/networkd-route.c

@@ -939,3 +939,44 @@ int config_parse_route_table(const char *unit,
 
         return 0;
 }
+
+int config_parse_gateway_onlink(const char *unit,
+                                const char *filename,
+                                unsigned line,
+                                const char *section,
+                                unsigned section_line,
+                                const char *lvalue,
+                                int ltype,
+                                const char *rvalue,
+                                void *data,
+                                void *userdata) {
+        Network *network = userdata;
+        _cleanup_route_free_ Route *n = NULL;
+        int r;
+
+        assert(filename);
+        assert(section);
+        assert(lvalue);
+        assert(rvalue);
+        assert(data);
+
+        r = route_new_static(network, filename, section_line, &n);
+        if (r < 0)
+                return r;
+
+        r = parse_boolean(rvalue);
+        if (r < 0) {
+                log_syntax(unit, LOG_ERR, filename, line, r,
+                           "Could not parse gateway onlink \"%s\", ignoring assignment: %m", rvalue);
+                return 0;
+        }
+
+        if (r)
+                n->flags |= RTNH_F_ONLINK;
+        else
+                n->flags &= ~RTNH_F_ONLINK;
+
+        n = NULL;
+
+        return 0;
+}

src/network/networkd-route.h

@@ -75,3 +75,4 @@ int config_parse_destination(const char *unit, const char *filename, unsigned li
 int config_parse_route_priority(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
 int config_parse_route_scope(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
 int config_parse_route_table(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
+int config_parse_gateway_onlink(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);