txthinking/brook · error

data too long

Error message

data too long

What it means

NewSimpleStreamServer reads a 2-byte length at b[32:34] and rejects values greater than 2048 with 'data too long', since the remainder cannot fit in the fixed 2048-byte pool buffer. This guards against buffer overflow from malformed or malicious length fields.

Source

Thrown at simplestreamserver.go:59

		if err := client.SetDeadline(time.Now().Add(time.Duration(timeout) * time.Second)); err != nil {
			return nil, err
		}
	}
	s := &SimpleStreamServer{Client: client, Timeout: timeout, src: src}
	b := x.BP2048.Get().([]byte)
	if _, err := io.ReadFull(s.Client, b[:32+2]); err != nil {
		x.BP2048.Put(b)
		return nil, err
	}
	if bytes.Compare(password, b[:32]) != 0 {
		x.BP2048.Put(b)
		WaitReadErr(s.Client)
		return nil, errors.New("Password is wrong")
	}
	l := int(binary.BigEndian.Uint16(b[32:34]))
	if l > 2048 {
		x.BP2048.Put(b)
		return nil, errors.New("data too long")
	}
	if _, err := io.ReadFull(s.Client, b[:l]); err != nil {
		x.BP2048.Put(b)
		return nil, err
	}
	i := int64(binary.BigEndian.Uint32(b[:4]))
	if time.Now().Unix()-i > 60 {
		x.BP2048.Put(b)
		WaitReadErr(s.Client)
		return nil, errors.New("Expired request")
	}
	if i%2 == 0 {
		s.network = "tcp"
		s.RB = b
		s.WB = x.BP2048.Get().([]byte)
	}
	if i%2 == 1 {
		s.network = "udp"

View on GitHub (pinned to 5cd13ef3b1)

Solutions

1. Ensure the client writes the exact handshake layout: 32-byte password hash, then BigEndian uint16 length of the address at offset 32
2. Check both ends run the same protocol version and same code path (tcp vs http wrapping)
3. Log the offending l value and source IP to identify misbehaving or malicious clients
4. Cap the address size on the client well below 2048 before sending

Example fix

// before (client, wrong layout)
binary.BigEndian.PutUint16(b[34:36], uint16(len(dst)))
// after
binary.BigEndian.PutUint16(b[32:34], uint16(len(dst)))

if len(dst) > 2048 {
    return errors.New("destination exceeds protocol handshake limit of 2048 bytes")
}

s, err := NewSimpleStreamServer(password, conn, ...)
if err != nil && err.Error() == "data too long" {
    log.Printf("malformed handshake (oversized length) from %s", conn.RemoteAddr())
    conn.Close()
}

When it happens

Trigger: The client writes a length prefix larger than 2048 (or a >16-bit-integer/garbage value lands at b[32:34]) in the handshake — a protocol violation, version mismatch, or a hostile client crafting a huge length.

Common situations: Client protocol version writing a different handshake layout (length at wrong offset); corrupted stream causing misaligned reads; attacker sending oversized length to probe the server; client sends raw address without the expected length framing.

Understand the failure class

Background: payload too large / request exceeds maximum size: why libraries cap bytes and how to fix oversize payloads — this error's family across 50 libraries.

Related errors

• dst too long
• invalid type
• Invalid prefer
• Invalid dial with IP
• no question

AI-assisted analysis of txthinking/brook@5cd13ef3b1 (2026-09-06). Data as JSON: /api/errors/badace258226a862. Report an issue: GitHub.