resolved: rework OPT RR generation logic

This moves management of the OPT RR out of the scope management and into
the server and packet management. There are now explicit calls for
appending and truncating the OPT RR from a packet
(dns_packet_append_opt() and dns_packet_truncate_opt()) as well as a
call to do the right thing depending on a DnsServer's feature level
(dns_server_adjust_opt()).

This also unifies the code to pick a server between the TCP and UDP code
paths, and makes sure the feature level used for the transaction is
selected at the time the server is picked, and not changed until the
next time we pick a server. The server selction code is now unified in
dns_transaction_pick_server().

This all fixes problems when changing between UDP and TCP communication
for the same server, and makes sure the UDP and TCP codepaths are more
alike. It also makes sure we never keep the UDP port open when switchung
to TCP, so that we don't have to handle incoming datagrams on the latter
we don't expect.

As the new code picks the DNS server at the time we make a connection,
we don't need to invalidate the DNS server anymore when changing to the
next one, thus dns_transaction_next_dns_server() has been removed.

1 files changed, 28 insertions, 0 deletions

diff --git a/src/resolve/resolved-dns-server.c b/src/resolve/resolved-dns-server.c
index b0db5bbb16..e6e4feeedd 100644
--- a/src/resolve/resolved-dns-server.c
+++ b/src/resolve/resolved-dns-server.c
@@ -342,6 +342,34 @@ DnsServerFeatureLevel dns_server_possible_features(DnsServer *s) {
         return s->possible_features;
 }
 
+int dns_server_adjust_opt(DnsServer *server, DnsPacket *packet, DnsServerFeatureLevel level) {
+        size_t packet_size;
+        bool edns_do;
+        int r;
+
+        assert(server);
+        assert(packet);
+        assert(packet->protocol == DNS_PROTOCOL_DNS);
+
+        /* Fix the OPT field in the packet to match our current feature level. */
+
+        r = dns_packet_truncate_opt(packet);
+        if (r < 0)
+                return r;
+
+        if (level < DNS_SERVER_FEATURE_LEVEL_EDNS0)
+                return 0;
+
+        edns_do = level >= DNS_SERVER_FEATURE_LEVEL_DO;
+
+        if (level >= DNS_SERVER_FEATURE_LEVEL_LARGE)
+                packet_size = DNS_PACKET_UNICAST_SIZE_LARGE_MAX;
+        else
+                packet_size = server->received_udp_packet_max;
+
+        return dns_packet_append_opt(packet, packet_size, edns_do, NULL);
+}
+
 static void dns_server_hash_func(const void *p, struct siphash *state) {
         const DnsServer *s = p;