Raspberry Pi cannot reconnect to paired Bluetooth device

Question

I am using Raspberry Pi 3 with BlueZ v5.5 to communicate with a Bluetooth scale.

Problem: I can pair the scale and communicate with it (read/write characteristics) immediately after pairing. But if I disconnect from the scale, I cannot reconnect back.

What I do on the Raspberry Pi's side:

sudo bluetoothctl
(by default I have agent KeyboardDisplay registered)
scan on
(I see the device's ADV)
scan off
trust MAC ADDRESS
pair MAC ADDRESS
(I enter 6-digit pin code shown on the scale's screen)
(connection established)
trust MAC ADDRESS
disconnect MAC ADDRESS
connect MAC ADDRESS
(connection failed)

Here's the output from btmon:

< HCI Command: LE Create Connection (0x08|0x000d) plen 25                                                                                                        
        Scan interval: 60.000 msec (0x0060)
        Scan window: 60.000 msec (0x0060)
        Filter policy: White list is not used (0x00)
        Peer address type: Public (0x00)
        Peer address: MAC ADDRESS
        Own address type: Public (0x00)
        Min connection interval: 10.00 msec (0x0008)
        Max connection interval: 20.00 msec (0x0010)
        Connection latency: 0 (0x0000)
        Supervision timeout: 6000 msec (0x0258)
        Min connection length: 0.000 msec (0x0000)
        Max connection length: 0.000 msec (0x0000)
> HCI Event: Command Status (0x0f) plen 4                                                                                                                        
      LE Create Connection (0x08|0x000d) ncmd 1
        Status: Success (0x00)
> HCI Event: LE Meta Event (0x3e) plen 19                                                                                                                        
      LE Connection Complete (0x01)
        Status: Success (0x00)
        Handle: 64
        Role: Master (0x00)
        Peer address type: Public (0x00)
        Peer address: MAC ADDRESS
        Connection interval: 18.75 msec (0x000f)
        Connection latency: 0 (0x0000)
        Supervision timeout: 6000 msec (0x0258)
        Master clock accuracy: 0x00
@ MGMT Event: Device Connected (0x000b) plen 37                                                                                                             
        LE Address: MAC ADDRESS
        Flags: 0x00000000
        Data length: 24
        Flags: 0x06
          LE General Discoverable Mode
          BR/EDR Not Supported
        16-bit Service UUIDs (partial): 1 entry
          Weight Scale (0x181d)
        Appearance: Weight Scale (0x0c80)
        Unknown EIR field 0x1b: 00610601d3ca5d
        Service Data (UUID 0x181d): 
@ MGMT Event: Device Connected (0x000b) plen 37                                                                                                             
        LE Address: MAC ADDRESS
        Flags: 0x00000000
        Data length: 24
        Flags: 0x06
          LE General Discoverable Mode
          BR/EDR Not Supported
        16-bit Service UUIDs (partial): 1 entry
          Weight Scale (0x181d)
        Appearance: Weight Scale (0x0c80)
        Unknown EIR field 0x1b: 00610601d3ca5d
        Service Data (UUID 0x181d): 
< HCI Command: LE Read Remote Used Features (0x08|0x0016) plen 2                                                                                                 
        Handle: 64
> HCI Event: Command Status (0x0f) plen 4                                                                                                                        
      LE Read Remote Used Features (0x08|0x0016) ncmd 1
        Status: Success (0x00)
> HCI Event: LE Meta Event (0x3e) plen 12                                                                                                                        
      LE Read Remote Used Features (0x04)
        Status: Success (0x00)
        Handle: 64
        Features: 0x01 0x00 0x00 0x00 0x00 0x00 0x00 0x00
          LE Encryption
< HCI Command: LE Start Encryption (0x08|0x0019) plen 28                                                                                                         
        Handle: 64
        Random number: 0x3a0760acb556aa15
        Encrypted diversifier: 0xa2f4
        Long term key: 1639a13299a91d0000519e1041643419
> HCI Event: Command Status (0x0f) plen 4                                                                                                                        
      LE Start Encryption (0x08|0x0019) ncmd 1
        Status: Success (0x00)
> HCI Event: Encryption Change (0x08) plen 4                                                                                                                     
        Status: PIN or Key Missing (0x06)
        Handle: 64
        Encryption: Disabled (0x00)
< HCI Command: Disconnect (0x01|0x0006) plen 3                                                                                                                   
        Handle: 64
        Reason: Authentication Failure (0x05)
> HCI Event: Command Status (0x0f) plen 4                                                                                                                        
      Disconnect (0x01|0x0006) ncmd 1
        Status: Success (0x00)
> HCI Event: Disconnect Complete (0x05) plen 4                                                                                                                   
        Status: Success (0x00)
        Handle: 64
        Reason: Connection Terminated By Local Host (0x16)
@ MGMT Event: Device Disconnected (0x000c) plen 8                                                                                                           
        LE Address: MAC ADDRESS
        Reason: Connection terminated due to authentication failure (0x04)

I have looked through tons of pages and threads but could not solve this issue. Any suggestion would be appreciated. Thanks a lot!

Reading the Weight Scale Profile 1.0 specification. It seems to say that a client can connect to the scales without having bonded first. Have you tried to remove the scales from bluetoothctl; then discover and connect without pairing first? It also says that the scales will disconnect if there is not weight data to send. Could that be happening. Also, when you connect through pairing, what characteristics are discovered? What is the value of 0x2A9E (Weight Scale Feature) and 0x2A9F (User Control Point)? — ukBaz
From the btmon log you can see that your pin code is being rejected.... — Martijn van Welie
@ukBaz, I have tried to connect without pairing. The connection is being established and then broken after 1-2 seconds, most likely to the same reason - failing to established encryption. — Roman
@MartijnvanWelie, this log is from the reconnection procedure when I do not enter any pin code. — Roman
Hi there, I have the exact same issue, did you manage to solve this? — Boiethios

Roman Roman · Accepted Answer · 2021-08-12T06:43:09

So, the solution was to look closer at SMP pairing packets.

My SMP Pairing Request (from the master) contained the following parameters:

Initiator key distribution: EncKey Sign LinkKey (0x0d)
Responder key distribution: EncKey IdKey Sign LinkKey (0x0f)

The SMP Pairing Response (from the slave) had it differently:

Initiator key distribution: IdKey (0x02)
Responder key distribution: EncKey IdKey (0x03)

So the slave required the IdKey from the master and was serious about it. Instead the BlueZ stack simply ignored the demand and did not provide the IdKey.

The quick fix was to send some hard-coded id key to the slave.

Raspberry Pi cannot reconnect to paired Bluetooth device

1 Answers