# WHATPORT(3784)

## NAME

port 3784 — BFD Control — Bidirectional Forwarding Detection control packets — the sub-second liveness check between routers.

Protocol: TCP/UDP  
IANA registry

## DESCRIPTION

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.

## EXPOSURE

**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.

## CHECK

```sh
sudo tcpdump -ni any udp port 3784
```

## SEE ALSO

- spec — [RFC 5881 — BFD for IPv4 and IPv6 (Single Hop)](https://www.rfc-editor.org/rfc/rfc5881.html) (rfc-editor.org)
- iana — [IANA port registry — 3784 (bfd-control)](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=3784) (iana.org)

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HTML: <https://whatport.net/3784/> · JSON: <https://whatport.net/3784.json> · full dataset: <https://whatport.net/ports.json>