just extend the orignal utls.ApplicationSettingsExtension

type ApplicationSettingsExtension struct {
	*utls.ApplicationSettingsExtension
}

func (e *ApplicationSettingsExtension) Len() int {
	bLen := 2 + 2 + 2 // Type + Length + ALPS Extension length
	for _, s := range e.SupportedProtocols {
		bLen += 1 + len(s) // Supported ALPN Length + actual length of protocol
	}
	return bLen
}

func (e *ApplicationSettingsExtension) Read(b []byte) (int, error) {
	if len(b) < e.Len() {
		return 0, io.ErrShortBuffer
	}

	// Read Type.
	b[0] = byte(17613 >> 8)   // hex: 44 dec: 68
	b[1] = byte(17613 & 0xff) // hex: 69 dec: 105

	lengths := b[2:] // get the remaining buffer without Type
	b = b[6:]        // set the buffer to the buffer without Type, Length and ALPS Extension Length (so only the Supported ALPN list remains)

	stringsLength := 0
	for _, s := range e.SupportedProtocols {
		l := len(s)            // Supported ALPN Length
		b[0] = byte(l)         // Supported ALPN Length in bytes hex: 02 dec: 2
		copy(b[1:], s)         // copy the Supported ALPN as bytes to the buffer
		b = b[1+l:]            // set the buffer to the buffer without the Supported ALPN Length and Supported ALPN (so we can continue to the next protocol in this loop)
		stringsLength += 1 + l // Supported ALPN Length (the field itself) + Supported ALPN Length (the value)
	}

	lengths[2] = byte(stringsLength >> 8) // ALPS Extension Length hex: 00 dec: 0
	lengths[3] = byte(stringsLength)      // ALPS Extension Length hex: 03 dec: 3
	stringsLength += 2                    // plus ALPS Extension Length field length
	lengths[0] = byte(stringsLength >> 8) // Length hex:00 dec: 0
	lengths[1] = byte(stringsLength)      // Length hex: 05 dec: 5

	return e.Len(), io.EOF
}

func (e *ApplicationSettingsExtension) UnmarshalJSON(b []byte) error {
	var applicationSettingsSupport struct {
		SupportedProtocols []string `json:"supported_protocols_new"`
	}

	if err := json.Unmarshal(b, &applicationSettingsSupport); err != nil {
		return err
	}

	e.SupportedProtocols = applicationSettingsSupport.SupportedProtocols
	return nil
}

// Write implementation copied from ALPNExtension.Write
func (e *ApplicationSettingsExtension) Write(b []byte) (int, error) {
	fullLen := len(b)
	extData := cryptobyte.String(b)
	// https://datatracker.ietf.org/doc/html/draft-vvv-tls-alps-01
	var protoList cryptobyte.String
	if !extData.ReadUint16LengthPrefixed(&protoList) || protoList.Empty() {
		return 0, errors.New("unable to read ALPN extension data")
	}
	alpnProtocols := []string{}
	for !protoList.Empty() {
		var proto cryptobyte.String
		if !protoList.ReadUint8LengthPrefixed(&proto) || proto.Empty() {
			return 0, errors.New("unable to read ALPN extension data")
		}
		alpnProtocols = append(alpnProtocols, string(proto))

	}
	e.SupportedProtocols = alpnProtocols
	return fullLen, nil
}

How to implement the "application_settings (17613)," extended #330

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