Port 358 (both TCP and UDP) is officially assigned to a protocol called "Shrinkwrap" by Bill Simpson.1 But if you go looking for what Shrinkwrap actually does, you'll find almost nothing.
No RFC. No technical specification. No working implementation anyone can point to. Just an entry in the IANA registry with a name and a contact person who helped build some of the Internet's fundamental protocols.
The Ghost in the Registry
Port 358 sits in the well-known ports range (0-1023), the territory reserved for essential Internet services. Ports in this range are assigned by IANA and typically correspond to standardized protocols with detailed documentation.2
But Shrinkwrap is different. It exists in the official record but not in practice. The protocol was registered by Bill Simpson, a networking engineer who contributed significantly to Internet standards—he edited RFC 1661 (the Point-to-Point Protocol), co-authored the Photuris key management protocol, and worked on early IP next-generation efforts.3
Yet Shrinkwrap, whatever it was meant to be, never got the same treatment. No published RFC. No widespread deployment. No technical documentation in the usual places.
What Shrinkwrap Might Have Been
The name "Shrinkwrap" suggests some kind of packaging or encapsulation protocol—a way to wrap data for transmission. But that's speculation. The actual purpose is lost.
Some possibilities:
- A data compression or packaging protocol that never made it past the proposal stage
- An experimental protocol that solved a problem that was solved differently
- A placeholder reservation for work that never materialized
- A protocol used internally somewhere but never documented publicly
Whatever it was, Shrinkwrap never became part of the Internet's working infrastructure. The port was assigned, the name was recorded, and then silence.
The Well-Known Ports Range
Port 358's placement in the well-known ports range (0-1023) is significant. These ports are:
- Assigned by IANA for specific, standardized services
- Typically require privileged access to bind on Unix-like systems
- Reserved for protocols important enough to warrant official registration
The well-known ports range represents the core infrastructure of networked computing. Port 22 for SSH. Port 80 for HTTP. Port 443 for HTTPS. And port 358 for... a protocol that never was.
Why Unassigned Ports Matter
Port 358 isn't technically unassigned—it has an official assignment. But it functions like an unassigned port because nothing actually uses it. This matters because:
The registry is a historical record: Every assignment tells a story about what people thought the Internet would need. Shrinkwrap is evidence of an idea someone had, even if we don't know what it was.
Ports are finite resources: The well-known ports range only has 1,024 slots. Port 358 occupies one of them, reserved for a protocol that never materialized. This is the cost of early reservation—sometimes you hold space for something that never arrives.
The namespace is permanent: IANA doesn't reclaim old port assignments easily. Port 358 will likely remain assigned to Shrinkwrap indefinitely, a permanent marker of something that didn't happen.
Security Considerations
Some security databases note that port 358 has been used by malware in the past.4 This is common with ports that have official assignments but no active legitimate use—malware authors sometimes pick them precisely because they're registered but unused, hoping to blend in.
If you see traffic on port 358:
- It's almost certainly not the official Shrinkwrap protocol (which likely never existed in working form)
- It could be malware or unauthorized software
- It warrants investigation
Checking Port 358 on Your System
To see if anything is listening on port 358:
On Linux/Mac:
On Windows:
If something is listening, investigate what process owns it. Legitimate use of port 358 is extremely rare.
The Pattern of Forgotten Protocols
Shrinkwrap isn't alone. The IANA registry contains dozens of assigned ports with minimal or no documentation—protocols that were proposed, registered, and then abandoned. They're fossils in the registry, evidence of the Internet's evolutionary dead ends.
Bill Simpson's other protocol, Photuris (port 468), also never saw widespread deployment—it was passed over in favor of IKE for IPsec key management.3 But at least Photuris has documentation. Shrinkwrap doesn't even have that.
What This Port Teaches Us
Port 358 is a reminder that not every good idea makes it. The Internet we have is the result of countless experiments, most of which failed or were superseded. For every HTTP that succeeded, there are dozens of protocols like Shrinkwrap that never found their place.
The port remains assigned because the Internet treats its namespace with care. Once you register a port, it's yours, even if you never use it. Even if you never finish building what you planned. Even if the only trace left is a name in a database and people decades later wondering what you meant to create.
Port 358 carries nothing now. But it once carried someone's intention to build something. That's what makes it worth remembering.
Related Ports
- Port 468 (Photuris): Another Bill Simpson protocol that was documented but not widely deployed
- Port 1-1023: The well-known ports range where Shrinkwrap resides
Frequently Asked Questions
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