XTLS/Xray-core · error

read bytes: %w

Error message

read bytes: %w

What it means

Bytes.readFrom first reads a Varint length prefix; this message wraps a failure of that Varint read (itself usually 'read varint: %w'). It means the connection failed or ended while the length prefix of a byte-array field was being read — the payload was never reached.

Source

Thrown at transport/internet/finalmask/xmc/protocol.go:316

	}
	return nil
}

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

type Bytes []byte

func (v *Bytes) readFrom(r io.Reader) error {
	var length Varint
	err := length.readFrom(r)
	if err != nil {
		return fmt.Errorf("read bytes: %w", err)
	}

	if length < 0 || length >= 1024 {
		return fmt.Errorf("read bytes: invalid size: %d", length)
	}

	buf := make([]byte, length)

	_, err = io.ReadFull(r, buf)
	if err != nil {
		return fmt.Errorf("read bytes: %w", err)
	}

	*v = append([]byte(*v), buf...)

	return nil
}

View on GitHub (pinned to 7d214f8b09)

Solutions

1. Treat io.EOF / io.ErrUnexpectedEOF on this path as 'peer ended the stream', and RST as abrupt close — log and reconnect.
2. Confirm both endpoints run compatible padding presets; desync makes later reads fail here.
3. Set read deadlines so dead peers fail fast instead of hanging before this error.
4. Reproduce with verbose logging of packet IDs to find which field read failed.

if err := bytesField.readFrom(r); err != nil {
    if errors.Is(err, io.EOF) || errors.Is(err, io.ErrUnexpectedEOF) {
        return fmt.Errorf("peer closed during length prefix: %w", err)
    }
    return err // classify timeouts/resets separately
}

When it happens

Trigger: Peer closes or resets the connection between a packet header and a Bytes field; a read deadline expires mid-varint; the stream is desynchronized so the reader is at EOF where a varint should start.

Common situations: Server drops the client during login (bad profile, rate limit); half-open connections after network changes; framing desync caused by mismatched padding schedules making the reader consume the wrong bytes.

Related errors

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

AI-assisted analysis of XTLS/Xray-core@7d214f8b09 (2026-08-15). Data as JSON: /api/errors/de43385ef792e753. Report an issue: GitHub.