Port 751 is a well-known port (in the 0-1023 range reserved by IANA) with an unusual characteristic: it has two official service assignments that never quite resolved into a single winner. This is what happens when protocol evolution moves faster than port registry cleanup.
What Runs on Port 751
Port 751 has two registered services:
kerberos_master — A legacy administrative port from Kerberos 4, used by tools like kpasswd and kadmin for password changes and administrative operations. This predates the modern Kerberos standard, which uses port 88 for KDC (Key Distribution Center) operations and port 749 for admin server communication.12
pump — The Packet Usage Measurement Protocol, an experimental protocol designed for network usage measurement and accounting. Documented in RFC 1340, but never widely implemented beyond research environments.3
Both services are assigned to the same port number for TCP and UDP. In practice, neither sees significant use on modern networks.
The Kerberos Story
Kerberos has a messy port history. Early versions used unofficial ports, including port 750 and port 751 for various functions. When Kerberos V5 was standardized, port 88 became the official KDC port relatively early in development. The plan was for KDCs to listen on both the old ports and port 88, then migrate all clients to port 88.12
Port 751 (kerberos_master) is a remnant of this transition period. It was used for intra-realm administrative operations—password changes via kpasswd and administrative commands via kadmin—but modern Kerberos deployments use port 749 for admin server operations instead. Port 751 never became formally reserved and has faded into obscurity.2
If you're running a modern Kerberos environment, you're not using port 751. You're using port 88 for authentication and port 749 for admin operations.
The PUMP Protocol
PUMP (Packet Usage Measurement Protocol) was designed to provide standardized network traffic measurement and resource utilization accounting across IP networks. It operates on both TCP and UDP port 751.3
The protocol never escaped experimental status. You won't find PUMP running in production environments—it's a protocol that existed in RFCs and research papers but never gained the adoption needed to become infrastructure.
What This Port Demonstrates
Port 751 is an example of how IANA port assignments reflect the history of Internet protocol development. Sometimes multiple services get assigned to the same port because different communities are working in parallel. Sometimes protocols get assigned ports and then fail to achieve adoption. Sometimes standards evolve and leave behind ports that were useful in version N but obsolete in version N+1.
The well-known port range (0-1023) contains hundreds of these historical artifacts—ports assigned to services that made sense in 1985 or 1995 but never became critical infrastructure.
Security Considerations
Port 751 is rarely open in modern networks. If you discover something listening on port 751:
Check if it's actually kerberos_master — Unlikely unless you're running a very old Kerberos implementation. Modern Kerberos uses port 88 and port 749.
Check if it's PUMP — Even more unlikely. PUMP never achieved widespread deployment.
Check if it's something else entirely — More likely. Malware or unauthorized services sometimes bind to officially-assigned but rarely-used ports because administrators don't expect traffic there. Investigate unfamiliar services listening on port 751.
Like all well-known ports, port 751 requires elevated privileges to bind on Unix-like systems, which provides some protection against casual misuse.
How to Check What's Listening on Port 751
On Linux or macOS:
On Windows:
You'll almost certainly find nothing. Port 751 is a quiet corner of the port space where protocols were assigned but never thrived.
Why Unassigned and Rarely-Used Ports Matter
The port numbering system has 65,535 ports. Only a fraction see regular use. Ports like 751—officially assigned but practically dormant—serve as a reminder that the Internet's infrastructure evolved through experimentation, standardization, abandoned projects, and constant iteration.
These ports also provide options. If you're developing a new protocol or need a port for a custom service, you could theoretically request an assignment or use a rarely-occupied well-known port for internal purposes. But modern practice favors using the registered port range (1024-49151) or dynamic ports (49152-65535) for new services, leaving the well-known range as a historical record of what the Internet tried to become.
Port 751 isn't carrying the weight of the Internet. But it's part of the complete map of how we got here.
Frequently Asked Questions About Port 751
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