resolved: set the DNSSEC OK (DO) flag

This indicates that we can handle DNSSEC records (per RFC3225), even if
all we do is silently drop them. This feature requires EDNS0 support.

As we do not yet support larger UDP packets, this feature increases the
risk of getting truncated packets.

Similarly to how we fall back to plain UDP if EDNS0 fails, we will fall
back to plain EDNS0 if EDNS0+DO fails (with the same logic of remembering
success and retrying after a grace period after failure).

5 files changed, 13 insertions, 5 deletions

diff --git a/src/resolve/resolved-dns-packet.c b/src/resolve/resolved-dns-packet.c
index cb713f1f85..75ca23fd08 100644
--- a/src/resolve/resolved-dns-packet.c
+++ b/src/resolve/resolved-dns-packet.c
@@ -28,6 +28,8 @@
 #include "utf8.h"
 #include "util.h"
 
+#define EDNS0_OPT_DO (1<<15)
+
 int dns_packet_new(DnsPacket **ret, DnsProtocol protocol, size_t mtu) {
         DnsPacket *p;
         size_t a;
@@ -610,7 +612,7 @@ fail:
 }
 
 /* Append the OPT pseudo-RR described in RFC6891 */
-int dns_packet_append_opt_rr(DnsPacket *p, uint16_t max_udp_size, size_t *start) {
+int dns_packet_append_opt_rr(DnsPacket *p, uint16_t max_udp_size, bool edns0_do, size_t *start) {
         size_t saved_size;
         int r;
 
@@ -640,8 +642,8 @@ int dns_packet_append_opt_rr(DnsPacket *p, uint16_t max_udp_size, size_t *start)
         if (r < 0)
                 goto fail;
 
-        /* flags */
-        r = dns_packet_append_uint16(p, 0, NULL);
+        /* flags: DNSSEC OK (DO), see RFC3225 */
+        r = dns_packet_append_uint16(p, edns0_do ? EDNS0_OPT_DO : 0, NULL);
         if (r < 0)
                 goto fail;
 
diff --git a/src/resolve/resolved-dns-packet.h b/src/resolve/resolved-dns-packet.h
index 385a8af796..feded09db3 100644
--- a/src/resolve/resolved-dns-packet.h
+++ b/src/resolve/resolved-dns-packet.h
@@ -160,7 +160,7 @@ int dns_packet_append_label(DnsPacket *p, const char *s, size_t l, size_t *start
 int dns_packet_append_name(DnsPacket *p, const char *name, bool allow_compression, size_t *start);
 int dns_packet_append_key(DnsPacket *p, const DnsResourceKey *key, size_t *start);
 int dns_packet_append_rr(DnsPacket *p, const DnsResourceRecord *rr, size_t *start);
-int dns_packet_append_opt_rr(DnsPacket *p, uint16_t max_udp_size, size_t *start);
+int dns_packet_append_opt_rr(DnsPacket *p, uint16_t max_udp_size, bool edns0_do, size_t *start);
 
 void dns_packet_truncate(DnsPacket *p, size_t sz);
 
diff --git a/src/resolve/resolved-dns-scope.c b/src/resolve/resolved-dns-scope.c
index 42478e41e2..80070da2b9 100644
--- a/src/resolve/resolved-dns-scope.c
+++ b/src/resolve/resolved-dns-scope.c
@@ -185,7 +185,11 @@ int dns_scope_emit(DnsScope *s, int fd, DnsServer *server, DnsPacket *p) {
                         return -EOPNOTSUPP;
 
                 if (server->possible_features >= DNS_SERVER_FEATURE_LEVEL_EDNS0) {
-                        r = dns_packet_append_opt_rr(p, DNS_PACKET_UNICAST_SIZE_MAX, &saved_size);
+                        bool edns_do;
+
+                        edns_do = server->possible_features >= DNS_SERVER_FEATURE_LEVEL_DO;
+
+                        r = dns_packet_append_opt_rr(p, DNS_PACKET_UNICAST_SIZE_MAX, edns_do, &saved_size);
                         if (r < 0)
                                 return r;
 
diff --git a/src/resolve/resolved-dns-server.c b/src/resolve/resolved-dns-server.c
index f8c921e4c8..916f5dadb8 100644
--- a/src/resolve/resolved-dns-server.c
+++ b/src/resolve/resolved-dns-server.c
@@ -479,5 +479,6 @@ static const char* const dns_server_feature_level_table[_DNS_SERVER_FEATURE_LEVE
         [DNS_SERVER_FEATURE_LEVEL_TCP] = "TCP",
         [DNS_SERVER_FEATURE_LEVEL_UDP] = "UDP",
         [DNS_SERVER_FEATURE_LEVEL_EDNS0] = "UDP+EDNS0",
+        [DNS_SERVER_FEATURE_LEVEL_DO] = "UDP+EDNS0+DO",
 };
 DEFINE_STRING_TABLE_LOOKUP(dns_server_feature_level, DnsServerFeatureLevel);
diff --git a/src/resolve/resolved-dns-server.h b/src/resolve/resolved-dns-server.h
index e9b425430f..9dd4961d5f 100644
--- a/src/resolve/resolved-dns-server.h
+++ b/src/resolve/resolved-dns-server.h
@@ -35,6 +35,7 @@ typedef enum DnsServerFeatureLevel {
         DNS_SERVER_FEATURE_LEVEL_TCP,
         DNS_SERVER_FEATURE_LEVEL_UDP,
         DNS_SERVER_FEATURE_LEVEL_EDNS0,
+        DNS_SERVER_FEATURE_LEVEL_DO,
         _DNS_SERVER_FEATURE_LEVEL_MAX,
         _DNS_SERVER_FEATURE_LEVEL_INVALID = -1
 } DnsServerFeatureLevel;