Re-implementation of the "iLnk"/"iLnkP2P"/"PPPP" protocol used on some cheap (<$5) IP cameras (sometimes branded as 'X5' or 'A9').
Per pictures of the X5, A9 the main chip is TXW817 (chinese, eng, google translate)
To execute the HTTP server, run make run; you can access the JPEG stream at http://localhost:1234/ and a file audio.pcm will be created.
The roundtrip delay when using MJPEG is ~350ms.
There's a basic UI which can display multiple cameras:
The server will send a broadcast packet every few seconds to discover all the cameras available; this means that it must run in the same broadcast domain (VLAN) as your cameras.
Clicking on the image will take you to a page that has audio streaming enabled.
Connect to the device's access point and run SSID=<your ssid> PSK=<your password> make pair.
The protocol is weirdly complex, though very little communication is necessary to use the device
The base structure of a packet is:
The payload is command-dependent; most commands have only a literal payload, but the Drw (0xf1d0) command has a framing scheme:
By using the second byte in the payload as a discriminant, we can split the payload into two types of subcommands:
Control packets:
The payload on control packets is "encrypted" when the length is > 5.
Data packets:
Data packets further discriminate based on the first 4 bytes into: Audio Data (0x55aa15a8), Video data.
To establish a session, a few control packets are sent.
---
title: Establish session
---
sequenceDiagram
autonumber
App->>+Cam: [C] LanSearch
Cam->>-App: [C] PunchPkt (SerialNo)
App->>+Cam: [C] P2PRdy
Cam->>-App: [C] P2PRdy
App->>+Cam: [C] ConnectUser
Cam->>-App: [C] ConnectUserAck (Ticket)
loop Every 400-500ms
Cam-->>+App: [C] P2PAlive
App-->>-Cam: [C] P2PAliveAck
end
To start a stream, a single control packet is sent.
The received stream is broken up into 1028 byte payloads, along with a sequence number.
Stitching the payloads together yields JPEG frames for video, and 8KHz A-law PCM for audio.
---
title: Stream audio/video
---
sequenceDiagram
App->>Cam: [C] StreamStart (with Ticket)
loop
Cam-->>+App: [D] Audio/Video Payload
App-->>-Cam: [C] DrwAck
end
The A9 cameras have a TX/RX test points - connecting with UART at 921600 8N1 gives read only access to some debug logs.
- Wifi Strength
- A9 reports '100%' strength
- X5 reports different strength values
I bricked two cameras by patching out part of the WiFi setup - unclear yet which commands.
After bricking itself, it reports very broken configuration via serial:
network interface: ƀ (Default)
MTU: 51050
MAC: 06 18 40 06 3e 51 b4 e2 c6 80 06 3f 77 30 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 74 00 00 00 01 00 00 00 9c ea 01 20 00 00 00 00 00 00 00 00 00 28 60 00 00 00 00 00 00 00 00 00 06 4e 00 20 2a 00 2a 00 80 00 00 00 00 00 ff ff ff ff ff ff 3e 51 b4 e2 c6 80 08 06 00 01 08 00 06 04 00 01 3e 51 b4 e2 c6 80 01 01 01 01 00 00 00 00 00 00 01 01 01 01 00 28 74 00 00 00 00 00 00 00 00 00 58 4e 00 20 48 00 48 00 80 00 00 00 00 00 ff ff ff ff ff ff 3e 51 b4 e2 c6 80 08 06 45 00 00 48 00 51 00 00 ff 11 c8 be 01 01 01 01 23 9c cc f7 7d 6c
FLAGS: DOWN LINK_DOWN IGMP
ip address: 1.1.1.1
gw address: 1.1.1.1
net mask : 1.1.1.1
network i
nterface: ^@^@
MTU: 0
MAC:
FLAGS: DOWN LINK_DOWN
ip address: 127.0.0.1
gw address: 127.0.0.1
net mask : 255.0.0.0
When connecting the camera to a network, it tries to send a HELLO (?) to 4 IP addresses:
139.155.68.77 - Shenzhen Tencent Computer Systems Company Limited
119.45.114.92 - Shenzhen Tencent Computer Systems Company Limited
162.62.63.154 - Tencent Building, Kejizhongyi Avenue
3.132.215.40 - ec2-3-132-215-40.us-east-2.compute.amazonaws.com
With the payload
0000 f1 10 00 28 42 41 54 43 00 00 00 00 00 09 4d 2c ...(BATC......M,
0010 48 56 44 43 53 00 00 00 08 00 02 01 00 00 6c 7d HVDCS.........l}
0020 65 28 a8 c0 00 00 00 00 00 00 00 00 e(..........
which is DevLogin
These addresses are decoded (script at scripts/dec_svr.py) from the string SWPNPDPFLVAOLNSXPHSQPIEOPAIDENLXHXEHIFLKPGLRHUARSTLQEEEPSUIHPDLSPEAOICLOSQEMLPPALNIBIAERHZLKHXEJHYHUEIEHELEEEKEG.
Every 8-10s
There are some other strings in the APK ending in -$$ which decode to other ips/hostnames.
These little cameras have quite some packet loss - I tried to deal with it by splicing around it on the JPEG payloads, but it's probably wrong, I expected artifacts like this:
but most of the time got:
which moves the rest of the image, causing more visual noise.
For now, images on which there was packet loss get skipped. The algorithm to "fix" packet loss can be enabled as an option.
The interesting implementation is in libvdp.so, part of the apk bundle.
Protocol reversing was done with a combination of static analysis of the shared object with Ghidra and dynamic analysis with Frida.
The headers reversed with Ghidra are at types/all.h. They are almost not used by this minimal implementation though.
The hooks used with frida are at frida-hooks.js, but it's mostly a playground - some useful functions got deleted once I understood the protocol.
There's also a partial Wireshark dissector at dissector.lua. You can install it with make install-wireshark-dissector.
adb shell pm list packages | grep ysx
adb shell pm path com.ysxlite.cam
adb shell pm path com.ysxlite.cam | while read -r line ; do adb pull $(echo $line | cut -d: -f2-) ; done
adb install-multiple *apk
adb shell 'su -c nohup /data/local/tmp/frida-server-16.1.11-android-arm64 &'