resolved: make sure a non-existing PTR record never gets mangled into NODATA

Previously, if a PTR query is seen for a non-existing record, we'd
generate an empty response (but not NXDOMAIN or so). Fix that. If we
have no data about an IP address, then let's say so, so that the
original error is returned, instead of anything synthesized.

Fixes: #6543

1 files changed, 26 insertions, 9 deletions

diff --git a/src/resolve/resolved-dns-synthesize.c b/src/resolve/resolved-dns-synthesize.c
index 38e99cd9c7..ca8f5da804 100644
--- a/src/resolve/resolved-dns-synthesize.c
+++ b/src/resolve/resolved-dns-synthesize.c
@@ -186,6 +186,7 @@ static int answer_add_addresses_ptr(
                 unsigned n_addresses,
                 int af, const union in_addr_union *match) {
 
+        bool added = false;
         unsigned j;
         int r;
 
@@ -215,9 +216,11 @@ static int answer_add_addresses_ptr(
                 r = dns_answer_add(*answer, rr, addresses[j].ifindex, DNS_ANSWER_AUTHENTICATED);
                 if (r < 0)
                         return r;
+
+                added = true;
         }
 
-        return 0;
+        return added;
 }
 
 static int synthesize_system_hostname_rr(Manager *m, const DnsResourceKey *key, int ifindex, DnsAnswer **answer) {
@@ -263,6 +266,7 @@ static int synthesize_system_hostname_rr(Manager *m, const DnsResourceKey *key,
 
 static int synthesize_system_hostname_ptr(Manager *m, int af, const union in_addr_union *address, int ifindex, DnsAnswer **answer) {
         _cleanup_free_ struct local_address *addresses = NULL;
+        bool added = false;
         int n, r;
 
         assert(m);
@@ -290,18 +294,26 @@ static int synthesize_system_hostname_ptr(Manager *m, int af, const union in_add
                 if (r < 0)
                         return r;
 
-                return 0;
+                return 1;
         }
 
         n = local_addresses(m->rtnl, ifindex, af, &addresses);
-        if (n < 0)
+        if (n <= 0)
                 return n;
 
         r = answer_add_addresses_ptr(answer, m->llmnr_hostname, addresses, n, af, address);
         if (r < 0)
                 return r;
+        if (r > 0)
+                added = true;
+
+        r = answer_add_addresses_ptr(answer, m->mdns_hostname, addresses, n, af, address);
+        if (r < 0)
+                return r;
+        if (r > 0)
+                added = true;
 
-        return answer_add_addresses_ptr(answer, m->mdns_hostname, addresses, n, af, address);
+        return added;
 }
 
 static int synthesize_gateway_rr(Manager *m, const DnsResourceKey *key, int ifindex, DnsAnswer **answer) {
@@ -335,7 +347,7 @@ static int synthesize_gateway_ptr(Manager *m, int af, const union in_addr_union
         assert(answer);
 
         n = local_gateways(m->rtnl, ifindex, af, &addresses);
-        if (n < 0)
+        if (n <= 0)
                 return n;
 
         return answer_add_addresses_ptr(answer, "_gateway", addresses, n, af, address);
@@ -396,14 +408,19 @@ int dns_synthesize_answer(
                                 return log_error_errno(r, "Failed to synthesize localhost PTR RRs: %m");
 
                 } else if (dns_name_address(name, &af, &address) > 0) {
+                        int v, w;
 
-                        r = synthesize_system_hostname_ptr(m, af, &address, ifindex, &answer);
-                        if (r < 0)
+                        v = synthesize_system_hostname_ptr(m, af, &address, ifindex, &answer);
+                        if (v < 0)
                                 return log_error_errno(r, "Failed to synthesize system hostname PTR RR: %m");
 
-                        r = synthesize_gateway_ptr(m, af, &address, ifindex, &answer);
-                        if (r < 0)
+                        w = synthesize_gateway_ptr(m, af, &address, ifindex, &answer);
+                        if (w < 0)
                                 return log_error_errno(r, "Failed to synthesize gateway hostname PTR RR: %m");
+
+                        if (v == 0 && w == 0) /* This IP address is neither a local one nor a gateway */
+                                continue;
+
                 } else
                         continue;