Port 671 is officially assigned to a service called VACDSM-APP. Both TCP and UDP. Registered with IANA, documented in Cisco's protocol libraries, recognized by firewalls and network monitoring tools.
And yet: almost nobody knows what it actually does.
What We Know
The official record is sparse:
- Port: 671 (TCP and UDP)
- Service name: vacdsm-app
- Range: Well-known ports (0-1023, assigned by IANA)
- Traffic type: Bulk-data, other business relevance1
- Encryption: None
- IPv6 support: Yes
That's it. The protocol exists in every network reference database. But what VACDSM stands for, what the application does, who uses it—those details are absent from public documentation.
The Mystery
There's a related protocol on port 670 called VACDSM-SWS. The existence of two adjacent ports suggests a system: one for the application, one for something else (maybe a web service, given the "SWS" suffix). But we're speculating now.
The protocol appears in Cisco NBAR2 (Network-Based Application Recognition) protocol libraries and Clavister firewall signatures.2 Network equipment knows how to identify VACDSM-APP traffic. But search for information about what the traffic actually contains, and you find almost nothing.
This isn't unusual. It's the norm.
The Reality of Port Assignments
The well-known port range (0-1023) contains 1,024 possible port numbers. Some carry the Internet's most critical traffic: HTTP on 80, HTTPS on 443, SSH on 22, DNS on 53. Others carry protocols from a different era: Telnet on 23, Finger on 79, Gopher on 70.
And many carry protocols like VACDSM-APP—officially assigned, technically documented, and practically invisible. Services that may have been vital to some organization in 1995, or experimental protocols that never gained adoption, or proprietary systems whose vendors have long since disappeared.
The port space is full of ghosts.
What This Tells Us
Port 671 reveals something honest about how the Internet actually works: assignment doesn't mean usage. IANA can register a service. Cisco can document it. Your firewall can recognize it. And you may never see a single packet on that port in your entire career.
The well-known port range is finite. Every assignment is permanent. Once IANA allocates a port number, it stays allocated. This means the registry is part museum—preserving the archaeology of protocols past—and part active directory.
How to Check Port 671
If you're curious whether anything is actually using port 671 on your system:
On Linux or macOS:
On Windows:
Most likely, you'll find nothing. Port 671 sits there, officially assigned, waiting for traffic that may never come.
Why Unassigned and Obscure Ports Matter
The existence of ports like 671 is part of what makes the port system work. The well-known range provides a stable namespace where every number has an official meaning—even if that meaning is "a protocol someone needed in 1992."
This permanence creates reliability. When you see traffic on port 443, you know it's HTTPS. When you see traffic on port 671, you know it's supposed to be VACDSM-APP—even if you don't know what that is. The registry provides ground truth.
And occasionally, these ghost ports come back to life. An old system gets reactivated. A proprietary protocol gets documented. Someone finds the original RFC buried in an archive. The port was there, waiting.
The Honest List
Here's what we can say with certainty about port 671:
- It's assigned to VACDSM-APP
- It's a well-known port
- It supports TCP and UDP
- It's recognized by network equipment
- Almost nobody uses it
- The details of what it does are lost or were never public
That's more honest than pretending we know the full story. Port 671 exists. It's assigned. And it's mostly a mystery.
This is what most of the port space actually looks like.
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