Port 1104 is registered with IANA for XRL (XORP Resource Locator), the inter-process communication protocol used by XORP—the eXtensible Open Router Platform.12
What XRL Does
XRL is how different parts of a XORP router talk to each other. Instead of building routers as monolithic software where everything is tightly coupled, XORP breaks routing into independent modules—one for BGP, one for OSPF, one for managing interfaces. XRL is the language they use to coordinate.3
An XRL describes a procedure call. It specifies:
- Which protocol family to use for transport (TCP, UDP, Unix sockets)
- Which interface and version of the target being called
- Which method to invoke
- What arguments to pass
The syntax looks like a URL, because that's exactly the pattern it follows—a resource locator for procedure calls inside a router.4
The XORP Story
XORP was founded by Mark Handley at the International Computer Science Institute (ICSI) in Berkeley in 2000. The project emerged from frustration with commercial router software—closed, inflexible, impossible to extend for research.5
The first public paper appeared at the HotNets workshop in 2002. The vision: build an open-source router platform that researchers could actually modify, where new routing protocols could be developed without reverse-engineering proprietary systems.6
XRL was designed as the core communication mechanism. Instead of hard-coding where each module lives or how to reach it, XRLs are resolved dynamically through a central Finder process. A module can move, restart, or change transport protocols, and the rest of the system adapts.7
The first production release shipped in July 2004.
Why This Matters
Most people never think about how routers work internally. They're appliances—you configure them, they route packets, that's it. But XORP represented a different philosophy: routers should be platforms, not products. Open, modular, extensible.
XRL made that possible. By creating a standard way for routing modules to communicate, XORP allowed researchers to replace one piece—say, the BGP implementation—without rewriting the entire system. You could experiment with new protocols, test optimizations, study routing behavior in ways that were impossible with closed systems.
Port 1104 carries the messages that make this work. When the BGP module needs to tell the RIB (Routing Information Base) about a new route, it sends an XRL. When the interface manager detects a link going down, it sends an XRL to notify every module that cares.
Security Considerations
XRL itself doesn't define security mechanisms—it's an IPC protocol designed for communication between trusted modules within the same router. The assumption is that if someone can inject XRL messages into your router, you have bigger problems.
In production deployments, XORP processes typically communicate over Unix domain sockets (for performance) or localhost TCP connections. External access to port 1104 would be unusual and worth investigating.
Checking What's Listening
On Linux or macOS:
On Windows:
If you see something listening on port 1104 and you're not running XORP, investigate. It could be legitimate software using the registered port, or something that shouldn't be there.
Related Ports
XORP doesn't have a fixed set of related ports—XRL can use any available port depending on configuration. The Finder process (which resolves XRL targets) typically runs on a configured port, but this varies by deployment.
The Registered Port Range
Port 1104 sits in the registered port range (1024–49151). These ports are registered with IANA for specific services, but they're not as universally standardized as well-known ports (0–1023). You'll find ports registered for protocols that serve specific communities—like XRL for XORP router deployments—rather than universal Internet services.
The registration prevents conflicts. If two systems both tried to use port 1104 for different protocols, you'd have collisions in mixed deployments. IANA registration is the Internet saying "this port means XRL, at least officially."
Frequently Asked Questions
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