shadow1ng/fscan · error

domain_length_invalid

Error message

domain_length_invalid

What it means

Guard in the domain branch of the SOCKS5 address parser: either the declared domain length is zero, or io.ReadFull failed while reading the domainLen+2 payload (domain bytes plus 2-byte port). Both cases mean the domain section is empty or truncated and the target address cannot be reconstructed.

Source

Thrown at plugins/local/socks5proxy.go:228

	var targetHost string
	var targetPort int

	switch addrType {
	case 0x01: // IPv4
		addr := make([]byte, 6)
		if _, err := io.ReadFull(clientConn, addr); err != nil {
			return nil, 0, fmt.Errorf("%s", i18n.GetText("ipv4_address_invalid"))
		}
		targetHost = fmt.Sprintf("%d.%d.%d.%d", addr[0], addr[1], addr[2], addr[3])
		targetPort = int(addr[4])<<8 + int(addr[5])
	case 0x03: // 域名
		lenBuf := make([]byte, 1)
		if _, err := io.ReadFull(clientConn, lenBuf); err != nil {
			return nil, 0, fmt.Errorf("%s", i18n.GetText("domain_format_invalid"))
		}
		domainLen := int(lenBuf[0])
		if domainLen == 0 {
			return nil, 0, fmt.Errorf("%s", i18n.GetText("domain_length_invalid"))
		}
		addr := make([]byte, domainLen+2)
		if _, err := io.ReadFull(clientConn, addr); err != nil {
			return nil, 0, fmt.Errorf("%s", i18n.GetText("domain_length_invalid"))
		}
		targetHost = string(addr[:domainLen])
		targetPort = int(addr[domainLen])<<8 + int(addr[domainLen+1])
	case 0x04: // IPv6
		addr := make([]byte, 18)
		if _, err := io.ReadFull(clientConn, addr); err != nil {
			return nil, 0, fmt.Errorf("%s", i18n.GetText("ipv6_address_invalid"))
		}
		// IPv6地址解析（简化实现）
		targetHost = net.IP(addr[:16]).String()
		targetPort = int(addr[16])<<8 + int(addr[17])
	default:
		// 发送不支持的地址类型响应
		response := []byte{0x05, 0x08, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}

View on GitHub (pinned to 95cc12e753)

Solutions

1. Reject zero-length domain prefixes early on the client side
2. Ensure the full domain+port payload is written in one request
3. Distinguish the length==0 case from the read-error case with separate messages for easier diagnosis

When it happens

Trigger: Thrown at plugins/local/socks5proxy.go:228 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of shadow1ng/fscan@95cc12e753 (2026-09-06). Data as JSON: /api/errors/a34becb9921d6018. Report an issue: GitHub.