networkd: permit DNS "DefaultRoute" configuration in .network files

man/systemd.network.xml

@@ -547,6 +547,17 @@
             name servers limited to a specific link.</para>
           </listitem>
         </varlistentry>
+        <varlistentry>
+          <term><varname>DNSDefaultRoute=</varname></term>
+          <listitem>
+            <para>Takes a boolean argument. If true, this link's configured DNS servers are used for resolving domain
+            names that do not match any link's configured <varname>Domains=</varname> setting. If false, this link's
+            configured DNS servers are never used for such domains, and are exclusively used for resolving names that
+            match at least one of the domains configured on this link. If not specified defaults to an automatic mode:
+            queries not matching any link's configured domains will be routed to this link if it has no routing-only
+            domains configured.</para>
+          </listitem>
+        </varlistentry>
         <varlistentry>
           <term><varname>NTP=</varname></term>
           <listitem>

src/network/networkd-link.c

@@ -3929,6 +3929,8 @@ int link_save(Link *link) {
                         resolve_support_to_string(link->network->llmnr));
                 fprintf(f, "MDNS=%s\n",
                         resolve_support_to_string(link->network->mdns));
+                if (link->network->dns_default_route >= 0)
+                        fprintf(f, "DNS_DEFAULT_ROUTE=%s\n", yes_no(link->network->dns_default_route));
 
                 if (link->network->dns_over_tls_mode != _DNS_OVER_TLS_MODE_INVALID)
                         fprintf(f, "DNS_OVER_TLS=%s\n",

src/network/networkd-network-gperf.gperf

@@ -58,6 +58,7 @@ Network.Address,                        config_parse_address,
 Network.Gateway,                        config_parse_gateway,                           0,                             0
 Network.Domains,                        config_parse_domains,                           0,                             0
 Network.DNS,                            config_parse_dns,                               0,                             0
+Network.DNSDefaultRoute,                config_parse_tristate,                          0,                             offsetof(Network, dns_default_route)
 Network.LLMNR,                          config_parse_resolve_support,                   0,                             offsetof(Network, llmnr)
 Network.MulticastDNS,                   config_parse_resolve_support,                   0,                             offsetof(Network, mdns)
 Network.DNSOverTLS,                     config_parse_dns_over_tls_mode,                 0,                             offsetof(Network, dns_over_tls_mode)

src/network/networkd-network.c

@@ -168,6 +168,7 @@ int network_load_one(Manager *manager, const char *filename) {
 
                 .lldp_mode = LLDP_MODE_ROUTERS_ONLY,
 
+                .dns_default_route = -1,
                 .llmnr = RESOLVE_SUPPORT_YES,
                 .mdns = RESOLVE_SUPPORT_NO,
                 .dnssec_mode = _DNSSEC_MODE_INVALID,
@@ -657,7 +658,6 @@ int config_parse_domains(
                          * routing domain, unconditionally. */
                         is_route = true;
                         domain = "."; /* make sure we don't allow empty strings, thus write the root domain as "." */
-
                 } else {
                         r = dns_name_normalize(domain, 0, &normalized);
                         if (r < 0) {

src/network/networkd-network.h

@@ -260,17 +260,20 @@ struct Network {
         Hashmap *prefixes_by_section;
         Hashmap *rules_by_section;
 
+        /* All kinds of DNS configuration */
         struct in_addr_data *dns;
         unsigned n_dns;
-
+        char **search_domains, **route_domains;
-        char **search_domains, **route_domains, **ntp, **bind_carrier;
+        int dns_default_route;
-
         ResolveSupport llmnr;
         ResolveSupport mdns;
         DnssecMode dnssec_mode;
         DnsOverTlsMode dns_over_tls_mode;
         Set *dnssec_negative_trust_anchors;
 
+        char **ntp;
+        char **bind_carrier;
+
         LIST_FIELDS(Network, networks);
 };
 

test/fuzz/fuzz-network-parser/directives.network

@@ -159,6 +159,7 @@ InvertRule=
 RouterPreference=
 DNSLifetimeSec=
 DNS=
+DNSDefaultRoute=
 RouterLifetimeSec=
 Domains=
 EmitDNS=