Port 4443 carries a quiet kind of importance. Officially registered with IANA for Pharos, a print management system, it has evolved into something far more significant: the Internet's most trusted alternate HTTPS port.
When port 443 is occupied, when firewalls block the expected paths, when systems need a second encrypted channel, they turn to 4443. The number itself is a signal: four-four-four-three. It's 443 with a prefix, a deliberate echo that says "I'm doing what 443 does, just over here."
What Port 4443 Does
Port 4443 serves as an alternate port for TLS-encrypted HTTP traffic. It follows the same protocols as port 443, the standard HTTPS port, but operates on a non-privileged port number that doesn't require root access to bind on Unix systems.
The traffic that flows through port 4443 is indistinguishable from regular HTTPS: TLS handshakes, encrypted HTTP requests, certificate validation. The only difference is the door it enters through.
The Official Registration: Pharos
IANA officially registered port 4443 to Pharos Systems International, a company founded in Auckland, New Zealand in 1992 by Paul Reddy and Geoff Shaw.1 Pharos builds enterprise print management software, the kind that tracks who prints what, allocates costs, and enforces print policies in universities and corporations.
For over 30 years, Pharos has helped organizations manage their printing infrastructure.2 Their software uses TCP/IP for client-server communications, with port 4443 designated for secure administrative connections.3
But the port's official purpose tells only part of its story.
How Port 4443 Actually Gets Used
In practice, port 4443 has become the Internet's go-to alternate HTTPS port. Here's where you'll find it:
Kubernetes Metrics Server: Starting with metrics-server version 0.5.2, Kubernetes changed the default secure port from 443 to 4443.4 This means every Kubernetes cluster running metrics collection, which enables horizontal pod autoscaling, communicates through port 4443.
Shibboleth Identity Provider: In Docker deployments of Shibboleth, port 4443 serves as the standard TLS port for browser-based authentication traffic.5 When you authenticate to a university system or enterprise application using SAML single sign-on, that traffic often flows through 4443.
VMware vRealize Suite: VMware's lifecycle manager uses port 4443 for product upgrades and suite management.6
OpenVPN Servers: VPN administrators frequently configure OpenVPN to listen on TCP 4443 to traverse restrictive firewalls, making the VPN traffic appear as alternate HTTPS.7
Apache Tomcat: When running multiple HTTPS services on a single server, Tomcat commonly uses 4443 or 8443 as alternative secure ports.8
Oracle Demantra: Oracle's demand management web servers use port 4443 for HTTPS connections by default instead of the standard 443.9
Why Use a Non-Standard Port?
Several situations drive traffic to port 4443:
Port 443 is already in use. On a server running multiple services, only one can bind to port 443. Everything else needs an alternative.
Firewall traversal. Some networks block unusual ports but allow traffic that looks like HTTPS. Port 4443 often passes through where other high ports would be blocked.
Non-root binding. On Unix systems, binding to ports below 1024 requires root privileges. Port 4443 can be bound by regular user processes.
Service isolation. Running administrative interfaces on a different port from public services provides a layer of separation.
The Kubernetes Admission Webhook Problem
One of the more interesting port 4443 stories comes from Kubernetes. When Google Kubernetes Engine (GKE) runs, the master control plane operates on a separate network from the worker nodes. By default, GKE creates firewall rules that only allow traffic from the master to nodes on ports 443 and 10250.10
This creates a problem for admission webhooks, the validation and mutation services that intercept Kubernetes API requests. If your webhook runs on port 4443 (a common configuration), the master literally cannot reach it. The firewall silently drops the packets.
The solution? Add a firewall rule for port 4443. But the debugging journey to discover this, watching webhooks mysteriously time out with no obvious cause, has frustrated countless Kubernetes operators.
Security Considerations
Port 4443 itself carries no inherent security risk. The traffic is TLS-encrypted, subject to the same certificate validation and cryptographic protections as port 443.
However, some historical context: port 4443 has been flagged in trojan databases, not because malware specifically targets this port, but because some trojans have used it for command and control traffic in the past.11 This is true of many ports. The presence of an open port 4443 isn't suspicious on its own, but unexpected services on any port warrant investigation.
The real security consideration is visibility. Traffic on port 443 is expected and monitored. Traffic on port 4443 might slip past security tools configured only to inspect standard ports. This makes it attractive for both legitimate alternate services and potentially for traffic trying to avoid detection.
Related Ports
| Port | Service | Relationship |
|---|---|---|
| 443 | HTTPS | The standard secure web port that 4443 shadows |
| 8443 | HTTPS-alt | Another common alternate HTTPS port, one step further from standard |
| 4444 | KRB524 | Sequential neighbor, also historically used by malware |
| 8080 | HTTP-alt | The HTTP equivalent of an alternate port |
Checking What's on Port 4443
To see if anything is listening on port 4443:
To test connectivity to a service on port 4443:
Summary
Port 4443 is officially Pharos's port, but it has become everyone's backup plan for encrypted traffic. It carries Kubernetes metrics, Shibboleth authentication, VMware management, and countless other services that needed a secure channel when the standard one wasn't available.
There's something deeply practical about it. No committee decided port 4443 should be the alternate HTTPS port. It just happened, organically, because engineers needed somewhere to put their encrypted traffic and 4443 was memorable: four before 443. The Internet's conventions are often like this, emerging from practical necessity rather than grand design.
Every time a Kubernetes cluster scales a deployment based on CPU metrics, those metrics flowed through port 4443. Every time someone logs into their university portal through Shibboleth, the authentication often traverses this port. It's invisible infrastructure, the understudy that performs every night while the audience assumes they're watching the star.
Frequently Asked Questions
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