[Fix] Regenerate DNS transaction ID before copying packet to TCP buffer

vstakhov · vstakhov · commit c774de1a529f · 2025-11-10T09:38:35.000Z
When switching from UDP to TCP (e.g., on truncated response), the code
was copying the packet to the TCP output buffer before regenerating the
transaction ID to avoid collisions. This resulted in both UDP and TCP
packets having the same transaction ID.

Fix by moving the ID regeneration and IO channel switch logic before the
memcpy to the TCP output buffer, ensuring the TCP packet contains the
updated transaction ID.
diff --git a/contrib/librdns/resolver.c b/contrib/librdns/resolver.c
@@ -484,19 +484,6 @@ rdns_reschedule_req_over_tcp(struct rdns_request *req, struct rdns_server *serv)
 			return false;
 		}
 
-		oc->write_buf = ((unsigned char *) oc) + sizeof(*oc);
-		memcpy(oc->write_buf, req->packet, req->pos);
-		oc->next_write_size = htons(req->pos);
-
-		DL_APPEND(ioc->tcp->output_chain, oc);
-
-		if (ioc->tcp->async_write == NULL) {
-			ioc->tcp->async_write = resolver->async->add_write(
-				resolver->async->data,
-				ioc->sock, ioc);
-		}
-
-		req->state = RDNS_REQUEST_TCP;
 		/* Switch IO channel from UDP to TCP */
 		rdns_request_remove_from_hash(req);
 		req->io = ioc;
@@ -518,6 +505,20 @@ rdns_reschedule_req_over_tcp(struct rdns_request *req, struct rdns_server *serv)
 			}
 		}
 
+		oc->write_buf = ((unsigned char *) oc) + sizeof(*oc);
+		memcpy(oc->write_buf, req->packet, req->pos);
+		oc->next_write_size = htons(req->pos);
+
+		DL_APPEND(ioc->tcp->output_chain, oc);
+
+		if (ioc->tcp->async_write == NULL) {
+			ioc->tcp->async_write = resolver->async->add_write(
+				resolver->async->data,
+				ioc->sock, ioc);
+		}
+
+		req->state = RDNS_REQUEST_TCP;
+
 		req->async_event = resolver->async->add_timer(resolver->async->data,
 													  req->timeout, req);
 
