NEWS

+Knot Resolver 4.x.y (2019-0m-dd)
+================================
+
+Bugfixes
+--------
+- TCP to upstream: don't send wrong message length (unlikely, !816)
+
+
 Knot Resolver 4.0.0 (2019-04-18)
 ================================
 

daemon/bindings/net.rst

@@ -283,14 +283,16 @@ Following commands are useful in special situations and can be usef with and wit
 
 .. function:: net.bufsize([udp_bufsize])
 
-   Get/set maximum EDNS payload available. Default is 4096.
-   You cannot set less than 512 (512 is DNS packet size without EDNS, 1220 is minimum size for DNSSEC) or more than 65535 octets.
+   Get/set maximum EDNS payload size advertised in DNS packets. Default is 4096 bytes and the default will be lowered to value around 1220 bytes in future, once `DNS Flag Day 2020 <https://dnsflagday.net/>`_ becomes effective.
+
+   Minimal value allowed by standard :rfc:`6891` is 512 bytes, which is equal to DNS packet size without Extension Mechanisms for DNS. Value 1220 bytes is minimum size required in DNSSEC standard :rfc:`4035`.
 
    Example output:
 
    .. code-block:: lua
 
-	> net.bufsize 4096
+	> net.bufsize(4096)
+	nil
 	> net.bufsize()
 	4096
 

daemon/worker.c

@@ -626,13 +626,28 @@ static int qr_task_send(struct qr_task *task, struct session *session,
 		ret = uv_udp_send(send_req, (uv_udp_t *)handle, &buf, 1, addr, &on_send);
 	} else if (handle->type == UV_TCP) {
 		uv_write_t *write_req = (uv_write_t *)ioreq;
-		uint16_t pkt_size = htons(pkt->size);
-		uv_buf_t buf[2] = {
-			{ (char *)&pkt_size, sizeof(pkt_size) },
-			{ (char *)pkt->wire, pkt->size }
+		/* We need to write message length in native byte order,
+		 * but we don't have a convenient place to store those bytes.
+		 * The problem is that all memory referenced from buf[] MUST retain
+		 * its contents at least until on_write() is called, and I currently
+		 * can't see any convenient place outside the `pkt` structure.
+		 * So we use directly the *individual* bytes in pkt->size.
+		 * The call to htonl() and the condition will probably be inlinable. */
+		int lsbi, slsbi; /* (second) least significant byte index */
+		if (htonl(1) == 1) { /* big endian */
+			lsbi  = sizeof(pkt->size) - 1;
+			slsbi = sizeof(pkt->size) - 2;
+		} else {
+			lsbi  = 0;
+			slsbi = 1;
+		}
+		uv_buf_t buf[3] = {
+			{ (char *)&pkt->size + slsbi, 1 },
+			{ (char *)&pkt->size + lsbi,  1 },
+			{ (char *)pkt->wire, pkt->size },
 		};
 		write_req->data = task;
-		ret = uv_write(write_req, (uv_stream_t *)handle, buf, 2, &on_write);
+		ret = uv_write(write_req, (uv_stream_t *)handle, buf, 3, &on_write);
 	} else {
 		assert(false);
 	}