resolved: clear the AD bit for bypass packets

When the bypass logic is invoked, such as for queries to the stub with
the DO bit set, be certain to clear the AD bit in the reply before
forwarding it if the answer is not known to be authentic.

(cherry picked from commit 13e15da)
(cherry picked from commit 3a2be65)
(cherry picked from commit 6c243bb)

Author: rpigott

src/resolve/resolved-dns-packet.h

@@ -111,6 +111,7 @@ static inline uint8_t* DNS_PACKET_DATA(const DnsPacket *p) {
 #define DNS_PACKET_AD(p) ((be16toh(DNS_PACKET_HEADER(p)->flags) >> 5) & 1)
 #define DNS_PACKET_CD(p) ((be16toh(DNS_PACKET_HEADER(p)->flags) >> 4) & 1)
 
+#define DNS_PACKET_FLAG_AD (UINT16_C(1) << 5)
 #define DNS_PACKET_FLAG_TC (UINT16_C(1) << 9)
 
 static inline uint16_t DNS_PACKET_RCODE(DnsPacket *p) {

src/resolve/resolved-dns-stub.c

@@ -685,7 +685,8 @@ static int dns_stub_send_failure(
 static int dns_stub_patch_bypass_reply_packet(
                 DnsPacket **ret,       /* Where to place the patched packet */
                 DnsPacket *original,   /* The packet to patch */
-                DnsPacket *request) {  /* The packet the patched packet shall look like a reply to */
+                DnsPacket *request,    /* The packet the patched packet shall look like a reply to */
+                bool authenticated) {
         _cleanup_(dns_packet_unrefp) DnsPacket *c = NULL;
         int r;
 
@@ -725,6 +726,10 @@ static int dns_stub_patch_bypass_reply_packet(
                 DNS_PACKET_HEADER(c)->flags = htobe16(be16toh(DNS_PACKET_HEADER(c)->flags) | DNS_PACKET_FLAG_TC);
         }
 
+        /* Ensure we don't pass along an untrusted ad flag for bypass packets */
+        if (!authenticated)
+                DNS_PACKET_HEADER(c)->flags = htobe16(be16toh(DNS_PACKET_HEADER(c)->flags) & ~DNS_PACKET_FLAG_AD);
+
         *ret = TAKE_PTR(c);
         return 0;
 }
@@ -745,7 +750,8 @@ static void dns_stub_query_complete(DnsQuery *query) {
                     q->answer_full_packet->protocol == DNS_PROTOCOL_DNS) {
                         _cleanup_(dns_packet_unrefp) DnsPacket *reply = NULL;
 
-                        r = dns_stub_patch_bypass_reply_packet(&reply, q->answer_full_packet, q->request_packet);
+                        r = dns_stub_patch_bypass_reply_packet(&reply, q->answer_full_packet, q->request_packet,
+                                        FLAGS_SET(q->answer_query_flags, SD_RESOLVED_AUTHENTICATED));
                         if (r < 0)
                                 log_debug_errno(r, "Failed to patch bypass reply packet: %m");
                         else