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).

1 files changed, 5 insertions, 3 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;