Bidirectional Forwarding Detection control packets — the sub-second liveness check between routers.
BFD gives routing protocols a fast, protocol-independent way to notice that a neighbour has gone away, exchanging small control packets on a timer measured in milliseconds. RFC 5881 defines single-hop BFD over IPv4 and IPv6 with destination UDP port 3784; the source port comes from the 49152-65535 range, and receivers are required to check that the packet arrived with TTL/hop-limit 255. IANA also lists a TCP registration for the number, but the protocol only ever uses UDP. You will find it configured alongside BGP, OSPF, or static routes on routers, switches, and firewalls.
Do not expose
It is a routing control plane: spoofed or flooded BFD packets can tear down adjacencies. RFC 5881's TTL=255 check and BFD authentication exist precisely because this must stay between directly connected, trusted peers.
$ sudo tcpdump -ni any udp port 3784Seen 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.