Port 733 is a ghost. It once carried NetInfo traffic—Apple's RPC-based directory service that managed user accounts, network configuration, and system resources across networked Macintosh computers. But NetInfo has been dead since 2007, when Mac OS X Leopard replaced it with Open Directory.1
If you find port 733 listening on a modern system, you're either looking at a very old Mac that hasn't been updated in nearly two decades, or something unusual is happening.
What NetInfo Was
NetInfo was a distributed network configuration database that came from NeXTSTEP—the operating system NeXT Computer built in the late 1980s.2 When Apple acquired NeXT in 1997 and built Mac OS X on top of NeXTSTEP's foundation, NetInfo came along for the ride.
NetInfo stored administrative data in a hierarchical database. User accounts. Group memberships. Email configurations. NFS mounts. Printer definitions. Network settings. All the information a Unix system needs to know about itself and the network it belongs to.3
The service used RPC (Remote Procedure Call) to communicate across the network, and port 733 was its door.
The NeXT Connection
NetInfo wasn't created for Mac OS X—it was created for NeXTSTEP version 0.9, released in the late 1980s. It replaced both traditional Unix configuration files (like /etc/passwd and /etc/hosts) and Sun Microsystems' Network Information Service (NIS, originally called Yellow Pages).2
NeXT's vision was elegant: instead of maintaining separate configuration files on every machine, administrators could manage a hierarchical NetInfo database that propagated across the network. Change a user's password once, and it updates everywhere.
When Mac OS X launched in 2001, NetInfo became the default directory service for all local accounts and groups, continuing through Mac OS X Tiger (10.4) in 2005.
Why It Disappeared
NetInfo had limitations. It wasn't compatible with industry-standard directory protocols like LDAP (Lightweight Directory Access Protocol). As organizations needed to integrate Mac systems with Windows Active Directory and other enterprise directory services, NetInfo became a barrier rather than a solution.
Apple began transitioning to Open Directory in Mac OS X Server 10.2 (2002), which supported LDAPv3 and could integrate with existing directory infrastructure.1 By Mac OS X Leopard (10.5) in 2007, NetInfo was completely removed from the operating system.
Port 733 became obsolete the moment Leopard shipped.
What Runs on Port 733 Now
Almost nothing.
The IANA registry still lists port 733 as assigned to Apple for "Mac OS X RPC-based services."4 But that assignment is historical—NetInfo is gone, and no modern macOS service uses this port.
If you find port 733 listening on a system today, you're probably looking at:
- A very old Mac running OS X 10.4 Tiger or earlier
- Legacy server software that was never updated
- Malware masquerading as a legitimate service (rare, but possible)
To check what's actually listening on port 733:
macOS/Linux:
Windows:
If something's there, investigate it. It shouldn't be.
Why Unassigned Ports Matter
Port 733 isn't technically unassigned—it's assigned but obsolete. The distinction matters.
Truly unassigned ports in the well-known range (0-1023) are rare. IANA controls this range tightly, assigning ports only to services that genuinely need them. When a service dies—like NetInfo—the port number remains in the registry as a historical marker.
These ghost ports serve a purpose: they prevent new services from claiming numbers that old systems might still be using. Some organization somewhere might have a Mac running Tiger in a closet, still serving NetInfo to a network that forgot it existed. Port 733 stays reserved so nothing collides with that ghost.
Eventually, IANA might reclaim obsolete assignments and reassign them to new services. But that process takes decades. Port numbers are cheap—there are 65,535 of them—so there's no rush.
The Well-Known Range
Port 733 sits in the well-known port range (0-1023), which requires root/administrator privileges to bind to on Unix-like systems. This restriction exists for security: if a service runs on a well-known port, you can trust (in theory) that it was started by a system administrator, not a random user process.
In practice, this protection is less meaningful today than it was in the 1980s when the port system was designed. But the convention persists.
Security Considerations
Port 733 should not be accessible from the Internet. If you find it open during a network scan, that's a red flag—either you've discovered an ancient Mac that's somehow still connected to the public Internet, or something is pretending to be NetInfo.
Since NetInfo is obsolete, there's no legitimate reason for port 733 to accept connections from outside your local network. If it does, investigate immediately.
The Legacy
NetInfo is gone, but its influence lingers. The hierarchical database design influenced how Apple thought about directory services. Open Directory, which replaced NetInfo, kept the idea of a unified system configuration database—it just used industry-standard protocols to make it work with the rest of the world.
Port 733 is a monument to a dead protocol. A numbered door that once opened to NeXT's vision of distributed system administration, now mostly silent.
If you're curious about computing history, NetInfo is worth studying. It was an elegant solution to a real problem, built by people who thought deeply about how systems should be configured. It just couldn't survive in a world that standardized on LDAP.
The port remains. The protocol does not.
Frequently Asked Questions
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