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Available since the Bluetooth 4.0 specification, BLE uses only 40 channels, covering the range of 2400 to 2483.5 MHz. In contrast, traditional Bluetooth uses 79 channels in that same range.
BLE devices communicate is by sending **advertising packets** (**beacons**), these packets broadcast the BLE device’s existence to other nearby devices. These beacons sometimes **send data**, too.
The listening device, also called a central device, can respond to an advertising packet with a **SCAN request** sent specifically to the advertising device. The **response** to that scan uses the same structure as the **advertising** packet with additional information that couldn’t fit on the initial advertising request, such as the full device name.
The preamble byte synchronizes the frequency, whereas the four-byte access address is a **connection identifier**, which is used in scenarios where multiple devices are trying to establish connections on the same channels. Next, the Protocol Data Unit (**PDU**) contains the **advertising data**. There are several types of PDU; the most commonly used are ADV\_NONCONN\_IND and ADV\_IND. Devices use the **ADV\_NONCONN\_IND** PDU type if they **don’t accept connections**, transmitting data only in the advertising packet. Devices use **ADV\_IND** if they **allow connections** and **stop sending advertising** packets once a **connection** has been **established**.
The **Generic Attribute Profile** (GATT) defines how the **device should format and transfer data**. When you’re analyzing a BLE device’s attack surface, you’ll often concentrate your attention on the GATT (or GATTs), because it’s how **device functionality gets triggered** and how data gets stored, grouped, and modified. The GATT lists a device’s characteristics, descriptors, and services in a table as either 16- or 32-bits values. A **characteristic** is a **data** value **sent** between the central device and peripheral. These characteristics can have **descriptors** that **provide additional information about them**. **Characteristics** are often **grouped** in **services** if they’re related to performing a particular action.
**GATTool** allows to **establish** a **connection** with another device, listing that device’s **characteristics**, and reading and writing its attributes.\
GATTTool can launch an interactive shell with the `-I` option:
```bash
gatttool -i hci0 -I
[ ][LE]> connect 24:62:AB:B1:A8:3E Attempting to connect to A4:CF:12:6C:B3:76 Connection successful
[A4:CF:12:6C:B3:76][LE]> characteristics
handle: 0x0002, char properties: 0x20, char value handle:
- Do you work in a **cybersecurity company**? Do you want to see your **company advertised in HackTricks**? or do you want to have access to the **latest version of the PEASS or download HackTricks in PDF**? Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
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- **Share your hacking tricks by submitting PRs to the [hacktricks repo](https://github.com/carlospolop/hacktricks) and [hacktricks-cloud repo](https://github.com/carlospolop/hacktricks-cloud)**.
