resolved: implement minimal EDNS0 support

This is a minimal implementation of RFC6891. Only default values
are used, so in reality this will be a noop.

EDNS0 support is dependent on the current server's feature level,
so appending the OPT pseudo RR is done when the packet is emitted,
rather than when it is assembled. To handle different feature
levels on retransmission, we strip off the OPT RR again after
sending the packet.

Similarly, to how we fall back to TCP if UDP fails, we fall back
to plain UDP if EDNS0 fails (but if EDNS0 ever succeeded we never
fall back again, and after a timeout we will retry EDNS0).

src/resolve/resolved-dns-packet.c

@@ -267,7 +267,7 @@ static int dns_packet_extend(DnsPacket *p, size_t add, void **ret, size_t *start
         return 0;
 }
 
-static void dns_packet_truncate(DnsPacket *p, size_t sz) {
+void dns_packet_truncate(DnsPacket *p, size_t sz) {
         Iterator i;
         char *s;
         void *n;

src/resolve/resolved-dns-packet.h

@@ -162,6 +162,8 @@ 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);
 
+void dns_packet_truncate(DnsPacket *p, size_t sz);
+
 int dns_packet_read(DnsPacket *p, size_t sz, const void **ret, size_t *start);
 int dns_packet_read_blob(DnsPacket *p, void *d, size_t sz, size_t *start);
 int dns_packet_read_uint8(DnsPacket *p, uint8_t *ret, size_t *start);

src/resolve/resolved-dns-scope.c

@@ -158,12 +158,13 @@ void dns_scope_packet_lost(DnsScope *s, usec_t usec) {
                 s->resend_timeout = MIN(s->resend_timeout * 2, MULTICAST_RESEND_TIMEOUT_MAX_USEC);
 }
 
-int dns_scope_emit(DnsScope *s, int fd, DnsPacket *p) {
+int dns_scope_emit(DnsScope *s, int fd, DnsServer *server, DnsPacket *p) {
         union in_addr_union addr;
         int ifindex = 0, r;
         int family;
         uint16_t port;
         uint32_t mtu;
+        size_t saved_size = 0;
 
         assert(s);
         assert(p);
@@ -178,9 +179,19 @@ int dns_scope_emit(DnsScope *s, int fd, DnsPacket *p) {
 
         switch (s->protocol) {
         case DNS_PROTOCOL_DNS:
+                assert(server);
+
                 if (DNS_PACKET_QDCOUNT(p) > 1)
                         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);
+                        if (r < 0)
+                                return r;
+
+                        DNS_PACKET_HEADER(p)->arcount = htobe16(be16toh(DNS_PACKET_HEADER(p)->arcount) + 1);
+                }
+
                 if (p->size > DNS_PACKET_UNICAST_SIZE_MAX)
                         return -EMSGSIZE;
 
@@ -191,6 +202,12 @@ int dns_scope_emit(DnsScope *s, int fd, DnsPacket *p) {
                 if (r < 0)
                         return r;
 
+                if (saved_size > 0) {
+                        dns_packet_truncate(p, saved_size);
+
+                        DNS_PACKET_HEADER(p)->arcount = htobe16(be16toh(DNS_PACKET_HEADER(p)->arcount) - 1);
+                }
+
                 break;
 
         case DNS_PROTOCOL_LLMNR:
@@ -739,7 +756,7 @@ static int on_conflict_dispatch(sd_event_source *es, usec_t usec, void *userdata
                         return 0;
                 }
 
-                r = dns_scope_emit(scope, -1, p);
+                r = dns_scope_emit(scope, -1, NULL, p);
                 if (r < 0)
                         log_debug_errno(r, "Failed to send conflict packet: %m");
         }

src/resolve/resolved-dns-scope.h

@@ -80,7 +80,7 @@ DnsScope* dns_scope_free(DnsScope *s);
 void dns_scope_packet_received(DnsScope *s, usec_t rtt);
 void dns_scope_packet_lost(DnsScope *s, usec_t usec);
 
-int dns_scope_emit(DnsScope *s, int fd, DnsPacket *p);
+int dns_scope_emit(DnsScope *s, int fd, DnsServer *server, DnsPacket *p);
 int dns_scope_tcp_socket(DnsScope *s, int family, const union in_addr_union *address, uint16_t port, DnsServer **server);
 int dns_scope_udp_dns_socket(DnsScope *s, DnsServer **server);
 

src/resolve/resolved-dns-server.c

@@ -478,5 +478,6 @@ void manager_next_dns_server(Manager *m) {
 static const char* const dns_server_feature_level_table[_DNS_SERVER_FEATURE_LEVEL_MAX] = {
         [DNS_SERVER_FEATURE_LEVEL_TCP] = "TCP",
         [DNS_SERVER_FEATURE_LEVEL_UDP] = "UDP",
+        [DNS_SERVER_FEATURE_LEVEL_EDNS0] = "UDP+EDNS0",
 };
 DEFINE_STRING_TABLE_LOOKUP(dns_server_feature_level, DnsServerFeatureLevel);

src/resolve/resolved-dns-server.h

@@ -34,6 +34,7 @@ typedef enum DnsServerType {
 typedef enum DnsServerFeatureLevel {
         DNS_SERVER_FEATURE_LEVEL_TCP,
         DNS_SERVER_FEATURE_LEVEL_UDP,
+        DNS_SERVER_FEATURE_LEVEL_EDNS0,
         _DNS_SERVER_FEATURE_LEVEL_MAX,
         _DNS_SERVER_FEATURE_LEVEL_INVALID = -1
 } DnsServerFeatureLevel;

src/resolve/resolved-dns-transaction.c

@@ -545,7 +545,7 @@ static int dns_transaction_emit(DnsTransaction *t) {
                 t->server = dns_server_ref(server);
         }
 
-        r = dns_scope_emit(t->scope, t->dns_udp_fd, t->sent);
+        r = dns_scope_emit(t->scope, t->dns_udp_fd, t->server, t->sent);
         if (r < 0)
                 return r;