Port 555: dsf — The forgotten assignment that became a backdoor

Port 555 sits in the well-known ports range (0-1023), officially assigned to a service called "dsf" since at least 1992.¹ The problem: nobody seems to know what dsf actually was.

The ghost protocol

The Internet Assigned Numbers Authority (IANA) lists port 555 for both TCP and UDP under the service name "dsf."² It appears in RFC 1340 from July 1992 and RFC 1700 from October 1994.³ The assignment is real. The protocol behind it seems to have vanished.

This happens more often than you'd think. Someone builds a protocol, gets a port assignment, the protocol never catches on or gets replaced, and the port number remains—a fossil in the registry.

What moved in

When official services abandon ports, unofficial ones move in. Port 555 became known for trojans and backdoors:⁴

• PhaseZero — A backdoor trojan that listened on port 555
• Stealth Spy — Another backdoor using the same port
• Ini-Killer — A trojan variant associated with 555

These weren't sophisticated attacks. They were opportunistic—malware scanning for open ports and finding one that nobody was watching because nobody remembered what it was supposed to be for.

Why this matters

Port 555 tells you something about how the Internet works at scale. The well-known ports range (0-1023) was supposed to be the organized, official registry. Reserved for important protocols. Assigned through formal review processes.

But time moves on. Protocols die. Organizations disappear. The registry remains.

Now we have ports with official assignments to services that don't exist, sitting open on systems that don't know they should be closed, being exploited by attackers who know exactly what they're looking for.

Checking port 555

To see if anything is listening on port 555:

Linux/Mac:

sudo lsof -i :555
netstat -an | grep 555

Windows:

netstat -ano | findstr :555

If something is listening and you don't know what it is, that's worth investigating. Legitimate software rarely uses port 555. If you find it open, either you're running something very old, or something is running that you didn't install.

The unassigned ports problem

Port 555 isn't technically unassigned—it has the "dsf" assignment. But in practice, it's functionally abandoned. The service is gone. The port remains.

This is why security professionals treat "officially assigned but practically unused" ports the same as unassigned ports—both can become hiding places for malicious software. The official registry tells you what ports should be used for. Network monitoring tells you what they actually are used for.

Sometimes those are very different things.