Port 1352 is the official port for IBM Lotus Notes and Domino servers running Notes Remote Procedure Call (NRPC). This is one of the rare ports where IANA gave a company a permanent reservation—port 1352 belongs to Domino, so non-Domino applications don't compete for it.1
What Runs on This Port
Port 1352 carries the proprietary Remote Procedure Call protocol that powers Lotus Notes and Domino collaboration. Every time a Notes client connects to a Domino server—to check email, sync a calendar, replicate a database, or query a directory—that connection happens over TCP port 1352.2
The protocol handles:
- Client-to-server communication — Notes clients accessing mail databases and directory lookups
- Server-to-server replication — Domino servers synchronizing data across distributed systems
- Administrative operations — Configuration and management tasks across the Domino infrastructure
By default, all NRPC connections use TCP port 1352. The port can be changed in Domino server configuration, but 1352 remains the standard.3
How NRPC Works
NRPC is a proprietary protocol—IBM designed it specifically for Notes and Domino, and the internals aren't publicly documented. What we know:
When a Notes client or server initiates a connection to port 1352, it's sending commands and data objects using NRPC's binary format. The protocol enables real-time collaboration features by transmitting commands across the network and returning results—hence "remote procedure call."4
The protocol is tightly integrated with Domino's architecture. It's not REST or HTTP or any standard RPC framework. It's Notes talking to Notes, in a language only Notes understands.
The History
Lotus Development Corporation released Lotus Notes in December 1989 after beta testing it for so long that people thought it was vaporware.5 The software was designed for corporate collaboration—email, shared databases, workflow—back when most office workers didn't have email yet.
IBM acquired Lotus in July 1995 for $3.5 billion, primarily to get Notes and establish a presence in client-server computing.6 The product became IBM Lotus Notes, then IBM Notes, and in 2019, HCL Technologies acquired it and rebranded it as HCL Notes. But the protocol on port 1352 remained unchanged through all the corporate transitions.
Port 1352 has been carrying Notes traffic since before HTTP existed. That's 35+ years of the same proprietary protocol on the same port number.
Security Considerations
NRPC traffic on port 1352 is not encrypted by default. Unless you've explicitly configured Domino to use encryption or you're using a VPN, everything—including authentication credentials—travels in the clear.7
This has led to security issues over the years:
Unencrypted credentials — Without encryption, passwords and session data are visible to anyone who can see the network traffic.
Denial of service vulnerabilities — IBM Lotus Domino 8.x before 8.5.2 FP4 had an authentication vulnerability that allowed remote attackers to crash the daemon with a crafted Notes RPC packet.8
Information disclosure — Earlier versions (before Domino 7.0.2 and 6.5.5 FP2) had an information disclosure issue that required setting BLOCK_LOOKUPID in the server's notes.ini file to fix.9
Recommendation: If you're running NRPC across the Internet (not just internal networks), configure Domino to encrypt all traffic on port 1352. The protocol was designed for corporate LANs in the 1980s, not the modern Internet.
Why This Port Matters
Port 1352 represents a specific moment in computing history: proprietary protocols designed for corporate collaboration before the web standardized everything.
Most modern collaboration tools use HTTP/HTTPS (ports 80/443) and standard protocols. But Lotus Notes chose its own path, built its own protocol, and got IANA to reserve a port number just for it. That port assignment still stands today.
The fact that organizations still run Domino servers—and still rely on port 1352—shows how deeply some corporate infrastructure is embedded. You don't just migrate away from a system that's been carrying your email and databases for three decades. You keep port 1352 open and the Notes clients running.
How to Check What's Listening on Port 1352
On Linux or macOS:
On Windows:
If you see something listening on port 1352 and you're not running Domino, investigate. This port is so specifically associated with Notes/Domino that anything else using it is unusual.
Related Ports
- Port 80/443 — Modern Domino servers also support HTTP/HTTPS access for web clients
- Port 1533 — IBM Lotus Sametime (another Lotus collaboration tool)
Frequently Asked Questions About Port 1352
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