resolved: filter out our own stub resolvers when parsing servers

We get "upstream" dns server config from ~three places: /etc/resolv.conf,
config files, and runtime config via dbus. With this commit, we'll filter out
our own stub listeners if they are configured in either of the first two
sources. For /etc/resolv.conf this is done quitely, and for our own config
files, a LOG_INFO message is emitted, since this is a small inconsistency in
the config.

Setting loops like this over dbus is still allowed. The reason is that in the
past we didn't treat this as an error, and if we were to start responding with
an error, we could break a scenario that worked previously. E.g. NM sends us a
list of servers, and one happens to be the our own. We would just not use that
stub server before, but it'd still be shown in the dbus properties and such.
We would have to return error for the whole message, also rejecting the other
valid servers. I think it's easier to just keep that part unchanged.

Test case:
$ ls -l /etc/resolv.conf
-rw-r--r-- 1 root root 57 Dec 15 10:26 /etc/resolv.conf
$ cat /etc/resolv.conf
nameserver 192.168.150.1
options edns0 trust-ad
search .
$ cat /etc/systemd/resolved.conf.d/stub.conf
[Resolve]
DNSStubListenerExtra=192.168.150.1

$ resolvectl
...
Global
    resolv.conf mode: foreign
         DNS Servers: 192.168.150.1
Fallback DNS Servers: ...
(with the patch):
Global
    resolv.conf mode: foreign
Fallback DNS Servers: ...

5 files changed, 34 insertions, 23 deletions

diff --git a/src/resolve/resolved-conf.c b/src/resolve/resolved-conf.c
index 930313b844..7873c363b3 100644
--- a/src/resolve/resolved-conf.c
+++ b/src/resolve/resolved-conf.c
@@ -35,15 +35,16 @@ static int manager_add_dns_server_by_string(Manager *m, DnsServerType type, cons
         if (r < 0)
                 return r;
 
-        /* Silently filter out 0.0.0.0, 127.0.0.53, 127.0.0.54 (our own stub DNS listener) */
-        if (!dns_server_address_valid(family, &address))
-                return 0;
-
-        /* By default, the port number is determined with the transaction feature level.
+        /* By default, the port number is determined by the transaction feature level.
          * See dns_transaction_port() and dns_server_port(). */
         if (IN_SET(port, 53, 853))
                 port = 0;
 
+        /* Refuse 0.0.0.0, 127.0.0.53, 127.0.0.54 and the rest of our own stub DNS listeners. */
+        if (!dns_server_address_valid(family, &address) ||
+            manager_server_address_is_stub(m, family, &address, port ?: 53))
+                return -ELOOP;
+
         /* Filter out duplicates */
         s = dns_server_find(manager_get_first_dns_server(m, type), family, &address, port, ifindex, server_name);
         if (s) {
@@ -56,7 +57,7 @@ static int manager_add_dns_server_by_string(Manager *m, DnsServerType type, cons
         return dns_server_new(m, NULL, type, NULL, family, &address, port, ifindex, server_name);
 }
 
-int manager_parse_dns_server_string_and_warn(Manager *m, DnsServerType type, const char *string) {
+int manager_parse_dns_server_string_and_warn(Manager *m, DnsServerType type, const char *string, bool ignore_self_quietly) {
         int r;
 
         assert(m);
@@ -70,7 +71,10 @@ int manager_parse_dns_server_string_and_warn(Manager *m, DnsServerType type, con
                         return r;
 
                 r = manager_add_dns_server_by_string(m, type, word);
-                if (r < 0)
+                if (r == -ELOOP)
+                        log_full(ignore_self_quietly ? LOG_DEBUG : LOG_INFO,
+                                 "DNS server string '%s' points to our own listener, ignoring.", word);
+                else if (r < 0)
                         log_warning_errno(r, "Failed to add DNS server address '%s', ignoring: %m", word);
         }
 }
@@ -151,7 +155,7 @@ int config_parse_dns_servers(
                 dns_server_unlink_all(manager_get_first_dns_server(m, ltype));
         else {
                 /* Otherwise, add to the list */
-                r = manager_parse_dns_server_string_and_warn(m, ltype, rvalue);
+                r = manager_parse_dns_server_string_and_warn(m, ltype, rvalue, false);
                 if (r < 0) {
                         log_syntax(unit, LOG_WARNING, filename, line, r,
                                    "Failed to parse DNS server string '%s', ignoring.", rvalue);
@@ -159,8 +163,7 @@ int config_parse_dns_servers(
                 }
         }
 
-        /* If we have a manual setting, then we stop reading
-         * /etc/resolv.conf */
+        /* If we have a manual setting, then we stop reading /etc/resolv.conf */
         if (ltype == DNS_SERVER_SYSTEM)
                 m->read_resolv_conf = false;
         if (ltype == DNS_SERVER_FALLBACK)
@@ -202,8 +205,7 @@ int config_parse_search_domains(
                 }
         }
 
-        /* If we have a manual setting, then we stop reading
-         * /etc/resolv.conf */
+        /* If we have a manual setting, then we stop reading /etc/resolv.conf */
         m->read_resolv_conf = false;
 
         return 0;
@@ -485,7 +487,7 @@ int manager_parse_config_file(Manager *m) {
                 return r;
 
         if (m->need_builtin_fallbacks) {
-                r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_FALLBACK, DNS_SERVERS);
+                r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_FALLBACK, DNS_SERVERS, false);
                 if (r < 0)
                         return r;
         }
diff --git a/src/resolve/resolved-conf.h b/src/resolve/resolved-conf.h
index 07ce2591a9..4639cefbd8 100644
--- a/src/resolve/resolved-conf.h
+++ b/src/resolve/resolved-conf.h
@@ -8,7 +8,7 @@
 int manager_parse_config_file(Manager *m);
 
 int manager_parse_search_domains_and_warn(Manager *m, const char *string);
-int manager_parse_dns_server_string_and_warn(Manager *m, DnsServerType type, const char *string);
+int manager_parse_dns_server_string_and_warn(Manager *m, DnsServerType type, const char *string, bool ignore_self_quietly);
 
 const struct ConfigPerfItem* resolved_gperf_lookup(const char *key, GPERF_LEN_TYPE length);
 const struct ConfigPerfItem* resolved_dnssd_gperf_lookup(const char *key, GPERF_LEN_TYPE length);
diff --git a/src/resolve/resolved-manager.c b/src/resolve/resolved-manager.c
index 6b32ee4cf0..223ef36691 100644
--- a/src/resolve/resolved-manager.c
+++ b/src/resolve/resolved-manager.c
@@ -1620,30 +1620,37 @@ bool manager_next_dnssd_names(Manager *m) {
         return tried;
 }
 
-bool manager_server_is_stub(Manager *m, DnsServer *s) {
+bool manager_server_address_is_stub(Manager *m, int family, const union in_addr_union *address, uint16_t port) {
         DnsStubListenerExtra *l;
 
         assert(m);
-        assert(s);
+        assert(address);
 
         /* Safety check: we generally already skip the main stub when parsing configuration. But let's be
          * extra careful, and check here again */
-        if (s->family == AF_INET &&
-            s->address.in.s_addr == htobe32(INADDR_DNS_STUB) &&
-            dns_server_port(s) == 53)
+        if (family == AF_INET &&
+            address->in.s_addr == htobe32(INADDR_DNS_STUB) &&
+            port == 53)
                 return true;
 
         /* Main reason to call this is to check server data against the extra listeners, and filter things
          * out. */
         ORDERED_SET_FOREACH(l, m->dns_extra_stub_listeners)
-                if (s->family == l->family &&
-                    in_addr_equal(s->family, &s->address, &l->address) &&
-                    dns_server_port(s) == dns_stub_listener_extra_port(l))
+                if (family == l->family &&
+                    in_addr_equal(family, address, &l->address) &&
+                    port == dns_stub_listener_extra_port(l))
                         return true;
 
         return false;
 }
 
+bool manager_server_is_stub(Manager *m, DnsServer *s) {
+        assert(m);
+        assert(s);
+
+        return manager_server_address_is_stub(m, s->family, &s->address, dns_server_port(s));
+}
+
 int socket_disable_pmtud(int fd, int af) {
         int r;
 
diff --git a/src/resolve/resolved-manager.h b/src/resolve/resolved-manager.h
index 35e0068a83..e969738978 100644
--- a/src/resolve/resolved-manager.h
+++ b/src/resolve/resolved-manager.h
@@ -207,6 +207,7 @@ void manager_cleanup_saved_user(Manager *m);
 
 bool manager_next_dnssd_names(Manager *m);
 
+bool manager_server_address_is_stub(Manager *m, int family, const union in_addr_union *address, uint16_t port);
 bool manager_server_is_stub(Manager *m, DnsServer *s);
 
 int socket_disable_pmtud(int fd, int af);
diff --git a/src/resolve/resolved-resolv-conf.c b/src/resolve/resolved-resolv-conf.c
index 100894d6b2..e9785ab964 100644
--- a/src/resolve/resolved-resolv-conf.c
+++ b/src/resolve/resolved-resolv-conf.c
@@ -143,7 +143,8 @@ int manager_read_resolv_conf(Manager *m) {
 
                 a = first_word(l, "nameserver");
                 if (a) {
-                        r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_SYSTEM, a);
+                        r = manager_parse_dns_server_string_and_warn(m, DNS_SERVER_SYSTEM, a,
+                                                                     true /* don't warn about loops to our own stub listeners */);
                         if (r < 0)
                                 log_warning_errno(r, "Failed to parse DNS server address '%s', ignoring.", a);