XTLS/Xray-core · error

write packet ID: %w

Error message

write packet ID: %w

What it means

Returned by encodePacket when writing the packet ID varint into the in-memory frame bytes.Buffer fails. As with the frame-length write, bytes.Buffer writes are infallible for the shipped Varint type, making this a defensive error path satisfying the field.writeTo contract rather than a realistic failure.

Source

Thrown at transport/internet/finalmask/xmc/protocol.go:420

	}
	if dataBuf.Len() > maxPacketDataLength {
		return nil, fmt.Errorf("write packet: bad length: %d", dataBuf.Len())
	}

	packetIDVarint := Varint(packetID)
	bodyLength := varintSize(packetIDVarint) + dataBuf.Len()
	if bodyLength > maxPacketBodyLength {
		return nil, fmt.Errorf("write packet: bad length: %d", bodyLength)
	}

	var frame bytes.Buffer
	frame.Grow(varintSize(Varint(bodyLength)) + bodyLength)
	frameLength := Varint(bodyLength)
	if err := frameLength.writeTo(&frame); err != nil {
		return nil, fmt.Errorf("write packet length: %w", err)
	}
	if err := packetIDVarint.writeTo(&frame); err != nil {
		return nil, fmt.Errorf("write packet ID: %w", err)
	}
	frame.Write(dataBuf.Bytes())
	return frame.Bytes(), nil
}

func writeFull(w io.Writer, p []byte) error {
	for len(p) > 0 {
		n, err := w.Write(p)
		if n > 0 {
			p = p[n:]
		}
		if err != nil {
			return err
		}
		if n == 0 {
			return io.ErrShortWrite
		}
	}

View on GitHub (pinned to 7d214f8b09)

Solutions

1. Inspect the wrapped error for the real cause if using custom field types
2. Verify the local tree has not modified Varint.writeTo
3. Report upstream if reproducible with stock types

When it happens

Trigger: packetIDVarint.writeTo(&frame) returning an error at protocol.go:420 — unreachable with stock field types writing to bytes.Buffer.

Common situations: Not observed in production; only reachable via custom field implementations or a modified encoder.

Related errors

• write packet length: %w
• write packet field: %w
• write packet: bad length: %d
• no QueryStrategy available for
• Cannot start all fake dns pools

AI-assisted analysis of XTLS/Xray-core@7d214f8b09 (2026-08-15). Data as JSON: /api/errors/431c6b0463386140. Report an issue: GitHub.