What Port 3723 Is
Port 3723 sits in the registered port range (1024-49151). These are ports that organizations can formally claim with IANA for a specific service. The claim requires a request, a name, and a protocol. It does not require anyone to actually use the port.
IANA's official record lists port 3723 TCP as sychrond — the Sychron Service Daemon, an enterprise virtualization service registered in 2003. If you've never heard of it, you're not alone. In practice, the port became famous for something else entirely.
The Battle.net Story
Blizzard Entertainment used port 3723 (TCP and UDP) for Battle.net, their online gaming service, across a generation of iconic titles:
- StarCraft (1998) and StarCraft: Brood War
- Diablo II (2000)
- Warcraft III: Reign of Chaos (2002) and The Frozen Throne
Battle.net launched on December 31, 1996, with Diablo — the first online gaming service built directly into the games themselves rather than bolted on externally. 1 Port 3723 handled game lobbies, matchmaking, player authentication, and real-time multiplayer traffic. It supported both TCP (for reliable session management) and UDP (for low-latency gameplay data).
For much of the early 2000s, if you were playing a Blizzard game online, packets were flowing through 3723.
Security Note
Port 3723 also appeared in security databases as associated with Mantis, an older remote access trojan. This is worth knowing if you see unexpected traffic on 3723 in an environment that doesn't run Blizzard games or Sychron software. 2
Unexpected open ports are worth investigating regardless of what any registry says they should be.
What's Actually on This Port
The only way to know what's listening on port 3723 on any given machine is to check directly.
On macOS or Linux:
On Windows:
On Linux with ss:
If nothing is listening, the port is closed. If something is, those commands will tell you the process name and PID.
Why Unassigned Ports Matter
The registered port range contains thousands of entries. Many are assigned to services that never gained traction, were superseded, or exist only in enterprise niches. Meanwhile, software authors — gaming companies, peer-to-peer applications, internal tools — pick ports for reasons that have nothing to do with IANA registration, and those choices stick.
Port 3723 is a clean example: millions of people had firewall rules and router port-forwarding entries for it, written entirely because Blizzard chose it, not because IANA did. The official record and the practical reality diverged, and the practical reality won.
That's normal. The port numbering system is a coordination mechanism, not a mandate.
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