resolved: automatically forget all learnt DNS server information when the network configuration changes

When the network configuration changes we should relearn everything
there is to know about the configured DNS servers, because we might talk
to the same addresses, but there might be different servers behind them.

src/resolve/resolved-dns-server.c

@@ -70,15 +70,12 @@ int dns_server_new(
 
         s->n_ref = 1;
         s->manager = m;
-        s->verified_feature_level = _DNS_SERVER_FEATURE_LEVEL_INVALID;
-        s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_BEST;
-        s->features_grace_period_usec = DNS_SERVER_FEATURE_GRACE_PERIOD_MIN_USEC;
-        s->received_udp_packet_max = DNS_PACKET_UNICAST_SIZE_MAX;
         s->type = type;
         s->family = family;
         s->address = *in_addr;
         s->ifindex = ifindex;
-        s->resend_timeout = DNS_TIMEOUT_MIN_USEC;
+
+        dns_server_reset_features(s);
 
         switch (type) {
 
@@ -828,6 +825,34 @@ void dns_server_flush_cache(DnsServer *s) {
         dns_cache_flush(&scope->cache);
 }
 
+void dns_server_reset_features(DnsServer *s) {
+        assert(s);
+
+        s->max_rtt = 0;
+        s->resend_timeout = DNS_TIMEOUT_MIN_USEC;
+
+        s->verified_feature_level = _DNS_SERVER_FEATURE_LEVEL_INVALID;
+        s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_BEST;
+
+        s->received_udp_packet_max = DNS_PACKET_UNICAST_SIZE_MAX;
+
+        s->packet_bad_opt = false;
+        s->packet_rrsig_missing = false;
+
+        s->features_grace_period_usec = DNS_SERVER_FEATURE_GRACE_PERIOD_MIN_USEC;
+
+        s->warned_downgrade = false;
+
+        dns_server_reset_counters(s);
+}
+
+void dns_server_reset_features_all(DnsServer *s) {
+        DnsServer *i;
+
+        LIST_FOREACH(servers, i, s)
+                dns_server_reset_features(i);
+}
+
 static const char* const dns_server_type_table[_DNS_SERVER_TYPE_MAX] = {
         [DNS_SERVER_SYSTEM] = "system",
         [DNS_SERVER_FALLBACK] = "fallback",

src/resolve/resolved-dns-server.h

@@ -151,3 +151,6 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(DnsServer*, dns_server_unref);
 extern const struct hash_ops dns_server_hash_ops;
 
 void dns_server_flush_cache(DnsServer *s);
+
+void dns_server_reset_features(DnsServer *s);
+void dns_server_reset_features_all(DnsServer *s);

src/resolve/resolved-link.c

@@ -111,13 +111,30 @@ Link *link_free(Link *l) {
 }
 
 void link_allocate_scopes(Link *l) {
+        bool unicast_relevant;
         int r;
 
         assert(l);
 
-        if (link_relevant(l, AF_UNSPEC, false) &&
-            l->dns_servers) {
+        /* If a link that used to be relevant is no longer, or a link that did not use to be relevant now becomes
+         * relevant, let's reinit the learnt global DNS server information, since we might talk to different servers
+         * now, even if they have the same addresses as before. */
+
+        unicast_relevant = link_relevant(l, AF_UNSPEC, false);
+        if (unicast_relevant != l->unicast_relevant) {
+                l->unicast_relevant = unicast_relevant;
+
+                dns_server_reset_features_all(l->manager->fallback_dns_servers);
+                dns_server_reset_features_all(l->manager->dns_servers);
+        }
+
+        /* And now, allocate all scopes that makes sense now if we didn't have them yet, and drop those which we don't
+         * need anymore */
+
+        if (unicast_relevant && l->dns_servers) {
                 if (!l->unicast_scope) {
+                        dns_server_reset_features_all(l->dns_servers);
+
                         r = dns_scope_new(l->manager, &l->unicast_scope, l, DNS_PROTOCOL_DNS, AF_UNSPEC);
                         if (r < 0)
                                 log_warning_errno(r, "Failed to allocate DNS scope: %m");

src/resolve/resolved-link.h

@@ -88,6 +88,8 @@ struct Link {
 
         bool loaded;
         char *state_file;
+
+        bool unicast_relevant;
 };
 
 int link_new(Manager *m, Link **ret, int ifindex);

src/resolve/resolved-manager.c

@@ -1396,6 +1396,19 @@ void manager_flush_caches(Manager *m) {
         log_info("Flushed all caches.");
 }
 
+void manager_reset_server_features(Manager *m) {
+        Iterator i;
+        Link *l;
+
+        dns_server_reset_features_all(m->dns_servers);
+        dns_server_reset_features_all(m->fallback_dns_servers);
+
+        HASHMAP_FOREACH(l, m->links, i)
+                dns_server_reset_features_all(l->dns_servers);
+
+        log_info("Resetting learnt feature levels on all servers.");
+}
+
 void manager_cleanup_saved_user(Manager *m) {
         _cleanup_closedir_ DIR *d = NULL;
         struct dirent *de;

src/resolve/resolved-manager.h

@@ -184,5 +184,6 @@ void manager_dnssec_verdict(Manager *m, DnssecVerdict verdict, const DnsResource
 bool manager_routable(Manager *m, int family);
 
 void manager_flush_caches(Manager *m);
+void manager_reset_server_features(Manager *m);
 
 void manager_cleanup_saved_user(Manager *m);

src/resolve/resolved-resolv-conf.c

@@ -26,6 +26,7 @@
 #include "fileio.h"
 #include "ordered-set.h"
 #include "resolved-conf.h"
+#include "resolved-dns-server.h"
 #include "resolved-resolv-conf.h"
 #include "string-util.h"
 #include "strv.h"
@@ -136,6 +137,11 @@ int manager_read_resolv_conf(Manager *m) {
         if (m->unicast_scope)
                 dns_cache_flush(&m->unicast_scope->cache);
 
+        /* If /etc/resolv.conf changed, make sure to forget everything we learned about the DNS servers. After all we
+         * might now talk to a very different DNS server that just happens to have the same IP address as an old one
+         * (think 192.168.1.1). */
+        dns_server_reset_features_all(m->dns_servers);
+
         return 0;
 
 clear: