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).

1 files changed, 19 insertions, 2 deletions

diff --git a/src/resolve/resolved-dns-scope.c b/src/resolve/resolved-dns-scope.c
index a8c0ae1569..42478e41e2 100644
--- a/src/resolve/resolved-dns-scope.c
+++ b/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");
         }