Port 3740SixXS Heartbeat Protocol

A legacy SixXS protocol used by IPv6 tunnel clients to report their current endpoint to a tunnel broker.

transport: tcp · udp
in transit: cleartext
assignment: official
risk: 1/10
lookups: 0

Technical Details

what runs on :3740

IANA registers heartbeat on both TCP and UDP port 3740. The established use is UDP: a tunnel client sends heartbeat/update datagrams to a SixXS broker so it can track the client's current IPv4 endpoint; the IPv6 tunnel data itself is carried separately, commonly as IPv6-in-IPv4 (IP protocol 41). This is a legacy, vendor-associated protocol rather than a general-purpose cluster health-check service. Do not infer a TCP application protocol from the registry's TCP assignment.

Security Information

exposure of :3740

risk score

1/ 10safe

routine exposure. this port is rarely the way in on its own — keep it patched and logged and move on.

network services averages 2.7 across 1,442 ports — this one sits 1.7 below.

in transit

cleartext

credentials and payloads are readable by anyone on path

assignment

official

registered with iana for this service — scanners fingerprint it by number

reachable over

tcp · udp

udp is connectionless — source addresses are trivially spoofed and it is a reflection favourite

security overview

The legacy heartbeat exchange is not encrypted by default, but it is not a remote-administration service or a general-purpose data channel. Since SixXS is no longer operating, there is little reason for this port to be reachable on the public Internet; identify any current listener and restrict it to required peers rather than assuming it is a routine health check.

Record

iana registry entry

port: :3740
service: SixXS Heartbeat Protocol
transport: TCPUDP
encryption: cleartext
assignment: iana registered
risk: 1/10safe
category: Network Services
usage: This is best known as the heartbeat mechanism used by SixXS IPv6 tunnel clients, including AICCU, to keep a dynamic tunnel endpoint up to date. SixXS shut down in 2017, so a listener today is more likely to be legacy software or a different service than an active public SixXS broker.
lookups: 0rank 993
added: 2026-10-01
updated: 2026-10-01
references: IANA service name registry — port 3740

Something wrong?

This record is maintained by the engineers who use port 3740. Every field on it is editable — change what's wrong, say how you know, and a reviewer publishes it, usually within a minute. Last touched 2026-10-01.

01edit any field on a page of its own
02a model checks it for accuracy and spam
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edit this record →back to the registry

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risk mix of the 8 listed

caution100%

0 of 8 encrypted

Port :3740SixXS Heartbeat Protocol

Technical Details

Security Information

Related Ports

Port 3740SixXS Heartbeat Protocol