Port 4500: IPsec NAT-T

What is port 4500 used for?

VPNs where a NAT device sits in the path.

Is it safe to expose port 4500 to the internet?

Needed on a VPN gateway.

If port 4500 seems closed or unreachable

1. Check that the IPsec NAT-T service is running and listening on port 4500, not only on 127.0.0.1.
2. Check the firewall on the host itself (Windows Defender Firewall, iptables, nftables, ufw) for a rule that allows UDP 4500.
3. If the host is behind a router, confirm the forwarding rule points to the correct internal address and that your internet provider does not use carrier-grade NAT.
4. Test from outside your own network. A port can be open locally and blocked at the edge, or the other way round.

Using port 4500 safely

Port 4500 can be exposed when the host exists to provide this service, but add controls: restrict source addresses where you can, require strong authentication and encryption, and monitor the logs for repeated failures.

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