Port 2746: The NAT Truce

What Range This Port Lives In

Port 2746 sits in the registered port range (1024–49151). These ports are tracked by IANA — the organization responsible for global IP address and port allocation — but unlike well-known ports (0–1023), they don't require root privileges to open and their registration isn't mandatory.

IANA lists port 2746 as unassigned: no service has officially claimed it. But "unassigned" doesn't mean "empty."

The Check Point VPN Story

The most documented use of port 2746 comes from Check Point VPN-1 SecuRemote, a remote access VPN product. The port carries something called CPUDPENCAP — Check Point UDP Encapsulation.¹

Here's the problem it solves: IPsec, the protocol that secures VPN traffic, was designed to be cryptographically sealed. Every bit of the packet is protected. NAT — Network Address Translation, the technology that lets thousands of devices share a single public IP address — works by rewriting port numbers in packet headers. These two technologies are fundamentally incompatible. IPsec says "don't touch my packets." NAT says "I have to."

When a remote access VPN client sits behind a NAT device, IPsec packets arrive at the NAT gateway with no port number to rewrite. The gateway can't track the connection. The packets die.

Check Point's fix: wrap the entire IPsec packet inside a UDP envelope. UDP has port numbers. NAT can handle UDP. Both source and destination ports in the wrapper are set to 2746. The NAT device now has something to hold onto, forwards the packet, and the VPN session survives.²

It's not elegant. It's a packet inside a packet — overhead on top of overhead. But it works, and it became standard behavior in Check Point's client software for years.

Argo Workflows

More recently, port 2746 picked up a second occupant: Argo Workflows, a Kubernetes-native workflow engine. The Argo Server — which provides both a web UI and REST/gRPC API — defaults to port 2746.³

If you're running Argo in a Kubernetes cluster, you'll typically see:

kubectl -n argo port-forward svc/argo-server 2746:2746

Then navigate to https://localhost:2746. This is convention, not standard — the port is configurable, and nothing officially reserves it for Argo.

How to Check What's on Port 2746

If you see traffic on port 2746 and want to know what's using it:

On Linux/macOS:

# See what process is listening
sudo ss -tlnp sport = :2746
# or
sudo lsof -i :2746

On Windows:

netstat -ano | findstr :2746

Check Point-specific: If you're running Check Point VPN client software and see UDP traffic on 2746, it's almost certainly the NAT traversal encapsulation working as designed.

Argo-specific: If you're running Kubernetes and see a server on 2746, check for an argo-server pod.

Why Unassigned Ports Matter

The registered port range contains 48,128 ports. IANA tracks assignments, but enforcement is voluntary — any application can listen on any unregistered port without asking permission. This creates a system where:

• Widely-used software gradually "claims" ports through convention (like Argo picking 2746)
• Vendor-specific protocols occupy ports informally (like Check Point's encapsulation)
• Conflicts happen when two applications choose the same unassigned port on the same machine

Port 2746 is a small example of how the port namespace actually works in practice: official registrations coexist with informal conventions, and "unassigned" just means IANA hasn't blessed it — not that it's unused.