otm-bridge

OpenWrt package that turns a Linux monitor interface into an opentrafficmap.org C-ITS receiver.

It captures raw 802.11(p) frames (with radiotap headers) on a configurable monitor interface and publishes each frame as-is to OTM's public MQTT ingest, matching the wire format of the ESP32-C5 reference firmware.

Wire format (mimics the ESP32-C5 firmware)

Broker: mqtts://cits1.opentrafficmap.org:8883 — TLS, server-cert only, no client auth, no registration required.

Topics (with default node-id = phy0 MAC, lowercase 12 hex chars, no separators):

Topic	Payload	When
its/<node>/status	online / offline (LWT, retained)	on connect / disconnect
its/<node>/info	{"emac":"aa:bb:..","ver":"otm-bridge-<v>","hwv":"FRITZ!Box 3390"}	on connect
its/<node>/packet	raw 802.11 frame (radiotap-prefixed bytes)	per captured frame, QoS 0

What's in the box

The package ships:

• /usr/bin/otm-bridge — the C daemon (libpcap + libmosquitto-ssl)
• /etc/config/otm-bridge — UCI config (interface, broker, node-id, capture options)
• /etc/init.d/otm-bridge — procd init with two instances:
  • sniffer: rotating tcpdump to /tmp/mon0-YYYYMMDD-HHMMSS.pcapN (default 5 × 1 MB ring, hourly rotation) — capture starts immediately after mon0 is up so no frames are lost during NTP sync
  • bridge: the MQTT publisher, gated by NTP sync (/var/state/dnsmasqsec marker + 15 s slack so the first TLS handshake doesn't trip on an in-transit clock-step)
• /etc/uci-defaults/99-otm-setup — runs once on first boot to configure:
  • lan1 as WAN (DHCP + IPv6 RA)
  • SSH allowed from the WAN zone
  • hostname opentrafficmap

The init script also flips the WLAN LED trigger from phy0tpt (throughput) to phy0rx (per-frame), so the LED visibly blinks on every received frame.

Using this as an OpenWrt feed

echo "src-git otm-bridge https://github.com/MPW1412/openwrt-otm-bridge.git" \
    >> openwrt/feeds.conf.default
cd openwrt
./scripts/feeds update otm-bridge
./scripts/feeds install -a -p otm-bridge
# enable in menuconfig: Utilities → otm-bridge

For the full image-build orchestrator that pre-applies the necessary mac80211 / ath9k / wireless-regdb V2X patches needed for 5,9 GHz operation, see avm-fritz3390-802.11p-otm.

UCI reference (/etc/config/otm-bridge)

config otm-bridge 'main'
    option enabled         '1'
    option iface           'mon0'                                  # monitor interface
    option phy             'phy0'                                  # physical radio
    option freq            '5890'                                  # ITS-G5 CCH
    option bw              '10MHz'
    option broker          'mqtts://cits1.opentrafficmap.org'
    option node            ''                                      # empty -> phy0 MAC
    option verbose         '1'
    option capture         '1'
    option capture_dir     '/tmp'
    option capture_size    '1'                                     # MB per file
    option capture_files   '5'                                     # ring length

Hardware requirements

Anything with an ath9k-driven radio that can be put into monitor mode on 5,9 GHz, and a kernel that knows about the DSRC channels. The companion repo avm-fritz3390-802.11p-otm ships the four mac80211/ath9k/wireless-regdb patches needed to make this happen on stock OpenWrt 25.12; the patches are non-invasive and should apply against other ath9k targets (PC Engines APU, Mikrotik R11e-2HnD, generic mPCIe Atheros cards) with minor adjustments.

Limitations

• Receive-only by default. The OCB mode for transmitting V2X frames is available in the underlying ath9k driver but is not wired up here — monitor mode is enough to feed OTM and avoids any regulatory questions about transmitting on 5,9 GHz.
• No CAM/DENM decoding on the device. Frames are forwarded raw; OTM's backend does the ASN.1 UPER parsing.

License

WTFPL (Do What The Fuck You Want To Public License). See LICENSE.

The package depends at runtime on libpcap (BSD), libmosquitto (EPL/EDL), mbedTLS (Apache-2.0) and OpenWrt's package framework (GPL-2.0). Those projects keep their own licenses.

Acknowledgements

• opentrafficmap.org and the ESP32-C5 receiver firmware for the wire format / broker.
• Francesco Raviglione for the original V2X patches we forward-ported.
• The HPI Potsdam 11p-on-linux project for documenting the Linux 802.11p story.