Registered for DXSpider, the amateur-radio DX cluster software, to link one node to another.
IANA registers 8873 tcp and udp as dxspider, "dxspider linking protocol". DXSpider (Dirk Koopman, G1TLH) is the DX cluster server that amateur radio operators run to share spots, and cluster nodes interlink over TCP. Seeing it means a ham-radio cluster node, not a general-purpose service; note it is not rsync, which is 873.
Expose with care
Cluster nodes are meant to link across the internet, but the protocol is cleartext and node-to-node trust is by configured callsign, so restrict it to the peers you actually link with.
$ nc -vz TARGET 8873Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.