Port 657 is where IBM Power Systems talk to their controllers. Every partition starting up, every resource being reallocated, every health metric being collected—it all flows through this port using IBM's Resource Monitoring and Control (RMC) protocol.
What Runs Here
IBM RMC (Resource Monitoring and Control) — a specialized protocol that connects IBM Power Systems logical partitions (LPARs) to their Hardware Management Console (HMC) or Integrated Virtualization Manager. It's part of IBM's Reliable Scalable Cluster Technology (RSCT), the clustering infrastructure that's been part of AIX and IBM enterprise systems for decades.12
RMC operates on both TCP and UDP port 657. The protocol handles:
- Starting and stopping logical partitions
- Dynamic LPAR operations (adding CPU, memory on the fly)
- Collecting system health metrics
- Synchronizing status between hardware and virtual components
- Managing cluster resources across multiple nodes
How It Works
When an IBM Power Systems LPAR boots, one of the first things that happens is RMC daemon startup. These daemons—controlled by the System Resource Controller (SRC)—immediately reach out to the HMC on port 657 to establish bidirectional communication.3
The HMC sends management commands down. The LPAR sends monitoring data up. Both directions matter. If port 657 is blocked, you can't perform dynamic partition operations. The "No RMC Connection" error becomes your new reality.4
This isn't web traffic. This isn't user-facing. This is infrastructure talking to infrastructure, in a language designed for systems that measure uptime in years.
The Enterprise Context
Port 657 exists in a specific world: IBM Power Systems running AIX or Linux, managed through Hardware Management Consoles, often in enterprise data centers where "dynamic partition management" isn't a feature—it's a requirement.
If you're not running IBM Power Systems, you'll never see this port in use. But if you are, port 657 must be open—both TCP and UDP—between every LPAR and the HMC public interface. Block it and you've broken systems management.5
Security Considerations
Because port 657 carries commands that can start, stop, and reconfigure partitions, access should be strictly controlled:
- Restrict by source IP — Only the HMC and authorized management systems should reach port 657
- Firewall both directions — RMC communication is bidirectional; both sides need access
- Monitor for unexpected traffic — Port 657 traffic from unknown sources suggests reconnaissance or compromise
- Keep RSCT updated — IBM releases security updates for the RSCT components; apply them6
This port should never be exposed to the public Internet. It's an internal management channel, meant to be protected by network segmentation.
Checking What's Listening
On AIX or Linux systems with RSCT installed:
On other systems, port 657 should be closed unless you're deliberately running IBM infrastructure.
Related Ports
IBM's systems management uses several ports in concert:
- Port 22 (SSH) — Secure shell access to LPARs and HMC
- Port 5989 (HTTPS) — HMC web interface for some management operations
- Port 2049 (NFS) — Sometimes used for HMC data collection
The Bigger Picture
Port 657 is a reminder that beneath the cloud abstractions and container orchestration, there's still a world of bare metal systems management. IBM Power Systems running mission-critical databases and applications, managed through protocols designed when "high availability" meant something you built yourself, not something you bought from a cloud provider.
This port won't show up in your home network scan. But in the enterprise data centers running the world's banking systems, insurance claims, and ERP installations, port 657 is carrying the traffic that keeps partitions running and resources balanced.
Every port tells a story. Port 657's story is about systems that don't go down.
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