Port 694 is where high-availability clusters find their pulse. This is the official IANA-assigned port for ha-cluster (high-availability cluster) services, most notably used by the Linux-HA Heartbeat project.
What Runs on Port 694
Port 694 carries heartbeat messages between nodes in a high-availability cluster. These are periodic signals that servers send to each other asking the most essential question in failover systems: "Are you still alive?"
Both TCP and UDP can use port 694, though UDP is more common for heartbeat traffic. UDP's lightweight, connectionless nature makes it efficient for sending rapid periodic signals. TCP provides reliability when ordered delivery matters for cluster coordination.
How Heartbeat Works
In a high-availability cluster, multiple servers work together to ensure continuous service. If the primary server fails, a backup needs to take over immediately. But how does the backup know when to step in?
The heartbeat answers this. Cluster nodes send periodic messages through port 694—simple signals that say "I'm functioning normally." When a node stops receiving these signals, it knows something failed. The missing heartbeat triggers failover: the backup assumes the primary is down and takes over.
This is monitoring reduced to its essence. Not "how are you doing?" but "are you there?" The question a heartbeat can answer in milliseconds.
The Linux-HA Story
Port 694's primary use comes from the Linux-HA project, founded in 1998 by Alan Robertson at Bell Labs.12 Robertson was working on making Linux viable for mission-critical enterprise systems—the kind where downtime costs thousands of dollars per minute.
The first working code appeared on March 18, 1998. The first release followed on November 15, 1998. By 1999, the project had developed a complete two-node cluster stack with membership management, message passing, fencing, and resource management.2
Heartbeat became the core component—a GPL-licensed cluster management program that lets Linux systems provide the reliability, availability, and serviceability (RAS) that enterprise applications demand.1 Port 694 was registered with IANA as the official port for this ha-cluster service.3
The naming is perfect. In medicine, checking for a heartbeat is the first test of life. In clustering, port 694 serves the same function.
What Heartbeat Detects
When a node stops sending heartbeat signals through port 694, the cluster must determine what happened:
- Node failure — The server crashed or lost power
- Network partition — The connection between nodes failed but both are still running
- Software hang — The operating system froze but the hardware is fine
- Planned shutdown — The node is shutting down gracefully
The heartbeat itself doesn't diagnose which scenario occurred. It just reports the absence. The cluster's fencing mechanisms handle the rest—ensuring that a failed node doesn't come back online and cause split-brain scenarios where both nodes think they're primary.
Configuration and Deployment
Heartbeat clusters typically configure port 694 in the ha.cf configuration file using the udpport directive.4 The default is 694, but administrators can override this if multiple clusters share a subnet or local policy requires a different port.
Heartbeat supports both unicast and multicast communication. Unicast sends heartbeats directly between specific nodes. Multicast broadcasts to all cluster members simultaneously. Both methods use port 694 for their messaging.3
This port appears primarily in enterprise Linux deployments where high availability is non-negotiable: web servers that can't afford downtime, databases that handle financial transactions, network services that underpin critical infrastructure.
Security Considerations
Port 694 carries life-or-death messages for cluster operation. If an attacker can interfere with heartbeat traffic, they can:
- Trigger false failovers by blocking legitimate heartbeat packets
- Prevent failover by injecting fake heartbeat messages from a failed node
- Cause split-brain by selectively disrupting communication between nodes
Best practices:
- Isolate cluster traffic on a dedicated network interface separate from client-facing services
- Use authentication to verify heartbeat messages are from legitimate cluster members
- Monitor heartbeat health to detect anomalies in timing or packet patterns
- Firewall carefully to restrict port 694 access to only cluster nodes
The heartbeat network should be as reliable as possible—preferably redundant links on separate physical media.
Checking Port 694
To see what's listening on port 694:
Linux:
macOS:
Windows:
If you see port 694 in use and you're not running a high-availability cluster, investigate immediately. This port shouldn't be active on standalone systems.
Related Ports
Port 694 is part of the clustering ecosystem but focuses specifically on heartbeat messaging. Other clustering and high-availability ports include:
- Port 5405 — Corosync cluster communication (modern Linux clustering)
- Port 2224 — Pacemaker cluster resource manager
- Port 21064 — Default port for Pacemaker remote nodes
Modern high-availability stacks have evolved beyond the original Heartbeat, but port 694 remains registered and active in many production clusters running older or specialized configurations.
Why Port 694 Matters
Every system that demands uptime—financial trading platforms, hospital systems, telecommunications infrastructure—needs an answer to the question "is my backup ready?"
Port 694 provides that answer thousands of times per second in clusters around the world. The heartbeat traveling through this port is what makes "five nines" reliability (99.999% uptime) possible. That's five minutes of downtime per year. You don't achieve that without knowing instantly when something fails.
The beauty of a heartbeat is its simplicity. You don't need complex health checks or deep diagnostics. You just need to know: are you there? Port 694 asks that question, and the presence or absence of an answer determines whether services fail over or keep running.
It's the pulse that keeps critical systems alive.
Frequently Asked Questions About Port 694
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