XTLS/Xray-core · error

write bytes: %w

Error message

write bytes: %w

What it means

After the length prefix, Bytes.writeTo writes the payload slice and wraps any Write failure with this message. The payload itself is opaque; failure is always the underlying writer (connection closed/reset, deadline exceeded, or a custom writer erroring).

Source

Thrown at transport/internet/finalmask/xmc/protocol.go:362

}

func (v *RestBytes) writeTo(w io.Writer) error {
	if _, err := w.Write(*v); err != nil {
		return fmt.Errorf("write remaining bytes: %w", err)
	}
	return nil
}

func (v *Bytes) writeTo(w io.Writer) error {
	length := Varint(len(*v))
	err := length.writeTo(w)
	if err != nil {
		return fmt.Errorf("write bytes length: %w", err)
	}

	_, err = w.Write(*v)
	if err != nil {
		return fmt.Errorf("write bytes: %w", err)
	}

	return nil
}

func readByte(r io.Reader) (byte, error) {
	var buf [1]byte
	_, err := io.ReadFull(r, buf[:])
	if err != nil {
		return 0, fmt.Errorf("read byte: %w", err)
	}

	return buf[0], nil
}

func writePacket(w io.Writer, packetID int, fields ...field) error {
	_, err := writePacketWithLength(w, packetID, fields...)
	return err

View on GitHub (pinned to 7d214f8b09)

Solutions

1. On first write error, mark the connection dead and stop the write loop — subsequent writes will only fail again.
2. Use write deadlines proportional to payload size.
3. If errors appear immediately at connect, verify the endpoint and protocol version.
4. Capture which packet ID was being written when the failure occurred for faster diagnosis.

if err := bytesField.writeTo(w); err != nil {
    markConnDead() // first write failure is terminal
    return fmt.Errorf("connection failed during payload write: %w", err)
}

When it happens

Trigger: Writing the body of a byte-array field to a conn that has been closed by the peer or by local error handling; write deadline expiring mid-payload on slow links.

Common situations: Half-closed TCP after server restart; MTU/throughput issues making large writes exceed deadlines; concurrent close from a monitor goroutine.

Related errors

• write boolean: %w
• write UUID: %w
• read bytes: %w
• read remaining bytes: %w
• write remaining bytes: %w

AI-assisted analysis of XTLS/Xray-core@7d214f8b09 (2026-08-15). Data as JSON: /api/errors/9184270145596f03. Report an issue: GitHub.