resolved: respond to local resolver requests on 127.0.0.53:53

In order to improve compatibility with local clients that speak DNS directly
(and do not use NSS or our bus API) listen locally on 127.0.0.53:53 and process
any queries made that way.

Note that resolved does not implement a full DNS server on this port, but
simply enough to allow normal, local clients to resolve RRs through resolved.
Specifically it does not implement queries without the RD bit set (these are
requests where recursive lookups are explicitly disabled), and neither queries
with DNSSEC DO set in combination with DNSSEC CD (i.e. DNSSEC lookups with
validation turned off). It also refuses zone transfers and obsolete RR types.
All lookups done this way will be rejected with a clean error code, so that the
client side can repeat the query with a reduced feature set.

The code will set the DNSSEC AD flag however, depending on whether the data
resolved has been validated (or comes from a local, trusted source).

Lookups made via this mechanisms are propagated to LLMNR and mDNS as necessary,
but this is only partially useful as DNS packets cannot carry IP scope data
(i.e. the ifindex), and hence link-local addresses returned cannot be used
properly (and given that LLMNR/mDNS are mostly about link-local communication
this is quite a limitation). Also, given that DNS tends to use IDNA for
non-ASCII names, while LLMNR/mDNS uses UTF-8 lookups cannot be mapped 1:1.

In general this should improve compatibility with clients bypassing NSS but
it is highly recommended for clients to instead use NSS or our native bus API.

This patch also beefs up the DnsStream logic, as it reuses the code for local
TCP listening. DnsStream now provides proper reference counting for its
objects.

In order to avoid feedback loops resolved will no silently ignore 127.0.0.53
specified as DNS server when reading configuration.

resolved listens on 127.0.0.53:53 instead of 127.0.0.1:53 in order to leave
the latter free for local, external DNS servers or forwarders.

This also changes the "etc.conf" tmpfiles snippet to create a symlink from
/etc/resolv.conf to /usr/lib/systemd/resolv.conf by default, thus making this
stub the default mode of operation if /etc is not populated.

1 files changed, 24 insertions, 13 deletions

diff --git a/src/resolve/resolved-dns-scope.c b/src/resolve/resolved-dns-scope.c
index 66e763cb7d..ed0c6aa105 100644
--- a/src/resolve/resolved-dns-scope.c
+++ b/src/resolve/resolved-dns-scope.c
@@ -232,7 +232,7 @@ static int dns_scope_emit_one(DnsScope *s, int fd, DnsPacket *p) {
                 if (fd < 0)
                         return fd;
 
-                r = manager_send(s->manager, fd, ifindex, family, &addr, LLMNR_PORT, p);
+                r = manager_send(s->manager, fd, ifindex, family, &addr, LLMNR_PORT, NULL, p);
                 if (r < 0)
                         return r;
 
@@ -257,7 +257,7 @@ static int dns_scope_emit_one(DnsScope *s, int fd, DnsPacket *p) {
                 if (fd < 0)
                         return fd;
 
-                r = manager_send(s->manager, fd, ifindex, family, &addr, MDNS_PORT, p);
+                r = manager_send(s->manager, fd, ifindex, family, &addr, MDNS_PORT, NULL, p);
                 if (r < 0)
                         return r;
 
@@ -668,11 +668,11 @@ static void dns_scope_verify_conflicts(DnsScope *s, DnsPacket *p) {
 }
 
 void dns_scope_process_query(DnsScope *s, DnsStream *stream, DnsPacket *p) {
-        _cleanup_(dns_packet_unrefp) DnsPacket *reply = NULL;
         _cleanup_(dns_answer_unrefp) DnsAnswer *answer = NULL, *soa = NULL;
+        _cleanup_(dns_packet_unrefp) DnsPacket *reply = NULL;
         DnsResourceKey *key = NULL;
         bool tentative = false;
-        int r, fd;
+        int r;
 
         assert(s);
         assert(p);
@@ -694,7 +694,7 @@ void dns_scope_process_query(DnsScope *s, DnsStream *stream, DnsPacket *p) {
 
         r = dns_packet_extract(p);
         if (r < 0) {
-                log_debug_errno(r, "Failed to extract resources from incoming packet: %m");
+                log_debug_errno(r, "Failed to extract resource records from incoming packet: %m");
                 return;
         }
 
@@ -724,9 +724,21 @@ void dns_scope_process_query(DnsScope *s, DnsStream *stream, DnsPacket *p) {
                 return;
         }
 
-        if (stream)
+        if (stream) {
                 r = dns_stream_write_packet(stream, reply);
-        else {
+                if (r < 0) {
+                        log_debug_errno(r, "Failed to enqueue reply packet: %m");
+                        return;
+                }
+
+                /* Let's take an extra reference on this stream, so that it stays around after returning. The reference
+                 * will be dangling until the stream is disconnected, and the default completion handler of the stream
+                 * will then unref the stream and destroy it */
+                if (DNS_STREAM_QUEUED(stream))
+                        dns_stream_ref(stream);
+        } else {
+                int fd;
+
                 if (!ratelimit_test(&s->ratelimit))
                         return;
 
@@ -748,12 +760,11 @@ void dns_scope_process_query(DnsScope *s, DnsStream *stream, DnsPacket *p) {
                  * verified uniqueness for all records. Also see RFC
                  * 4795, Section 2.7 */
 
-                r = manager_send(s->manager, fd, p->ifindex, p->family, &p->sender, p->sender_port, reply);
-        }
-
-        if (r < 0) {
-                log_debug_errno(r, "Failed to send reply packet: %m");
-                return;
+                r = manager_send(s->manager, fd, p->ifindex, p->family, &p->sender, p->sender_port, NULL, reply);
+                if (r < 0) {
+                        log_debug_errno(r, "Failed to send reply packet: %m");
+                        return;
+                }
         }
 }