twislaandClaude Opus 5.5
3339d9ee59
The first two MeshCore packets heard were these: a search for repeaters and a repeater answering. Both are in clear; they are the test's bytes. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_0162FokPdvY2KsS4NBfwyWPk
4.4 KiB
+++ title = "LoRa Scanner" description = "Listen to the radio: every packet it hears, a survey of signal strength from 863 to 870 MHz, and captures for Wireshark. It only listens." weight = 2 [extra] tag = "LoRa Scanner" screens = ["sniffer.png", "sweep.png"] +++
The Scanner uses the Cap LoRa-1262. It never transmits: it listens and shows.
Sniffer
The first view lists what the radio hears, newest first: the time, the RSSI (signal strength, in dBm) and the SNR (signal over noise, in dB). For a Meshtastic packet it also shows the sender and the receiver (the last four hex digits of their numbers) and the number of hops.
For a MeshCore packet it shows what the packet is, who it concerns and how many repeaters are in its path:
| Row | Is |
|---|---|
adv Brussels-R1 2h |
A node saying who it is, by its name, heard through 2 repeaters |
txt a3>7f |
A private message from the node whose key starts with a3 to the one starting with 7f (req, rsp and path look the same) |
chan #11 1h |
A message on a channel; #11 is the channel's byte, and 11 is the public channel's |
who's there? |
Someone nearby searching for nodes (repeaters, usually) |
here 18a6 |
A node answering that search; 18a6 starts its key |
ack, anon, trace, ctl |
An acknowledgement, a login, a trace, other control packets |
| Key | Does |
|---|---|
| Enter | The details of a packet: what is never encrypted (below), and a hex dump |
| p | Picks what to listen to: one of the 7 Meshtastic presets allowed in EU868 (LongFast by default), or MeshCore |
| c | Starts or stops a capture |
| Tab | Switches to the Sweep |
The Scanner shows the header and the bytes; it does not decrypt the message itself, which Meshtastic and MeshCore both encrypt.
One network at a time. The radio listens on one frequency with one setting, and the two networks share neither: on a Meshtastic preset no MeshCore packet is heard, and the other way round.
MeshCore
The MeshCore preset is the setting its networks use in Europe, "EU/UK (Narrow)": 869.618 MHz, 62.5 kHz wide, SF8, coding rate 4/8. A network set up differently needs lora custom in the Shell.
Enter on a MeshCore packet shows:
- what it is and how it travels: flood (every repeater passes it on, and adds itself to the path) or direct (it carries the route to follow);
- the path, one repeater a hop, each named by the first byte of its key;
- for an advert, which is signed and never encrypted: the node's name, its kind (chat node, repeater, room server, sensor), the start of its key, and its position if it gives one;
- for a search and its answers, also in clear: what kind of node is wanted; and in an answer the node's kind, the start of its key, and how well it heard the search;
- for a message: the first byte of the sender's and the receiver's keys, or the channel's byte, and how many bytes are encrypted.
A byte is all a packet says about who it is from: two nodes can share it. The name comes only with a node's advert, which a node sends now and then, so a quiet network can take a while to show its names.
Capture
A capture records packets into a pcap file with LoRaTap headers in /captures/lora/ on the SD card, to open in Wireshark. It keeps recording with the App closed (the Status Bar shows CAP); the radio sleeps when the App is not open and no capture is running. Captures are never deleted unless you ask, in the Storage App, which can also show a pcap's packets on the device.
Sweep
Tab switches to the Sweep: the signal strength across 863 to 870 MHz in 100 kHz steps, drawn as bars with a mark at each peak, and a waterfall under them. The frequency the Sniffer listens on is marked. The Sniffer is paused during a Sweep and picks up where it was. The Status Bar shows SW.
The GNSS receiver raises the noise
The GNSS receiver on the same Cap makes the radio's noise floor about 8 dB worse while it runs. Settings → Pause GNSS for LoRa (off by default) puts the receiver on standby while the radio listens, except while a Track is being recorded.
The keys, as the device lists them
What Fn + h shows on these screens. These tables are generated from the firmware's own lists, so they are always the current ones.
{{ keys(scopes=["lora", "lora-packet", "lora-presets", "lora-sweep"]) }}