Port 500: ISAKMP/IKE
Port 500 is used by IKE, the Internet Key Exchange protocol that negotiates and manages the security keys for an IPsec VPN tunnel. NAT traversal for the same negotiation falls back to port 4500.
| Port number | 500 |
|---|---|
| Service | Internet Key Exchange (IPsec) |
| Protocol | UDP |
| Category | Routing |
| Default encryption | No |
How do I check whether port 500 is open?
Port 500 is UDP, which changes the answer: there is no handshake to complete, so nothing outside the host can prove the port is open. Asking the host itself is the reliable route.
| Question | Command | What to know |
|---|---|---|
| Is it open on a remote host? | sudo nmap -sU -p 500 example.com | UDP has no handshake, so there is nothing to complete. nmap infers the state from whether an ICMP port-unreachable comes back, needs root, and is slow and easy to misread. A silent port may be open, filtered, or simply not replying to a probe it does not recognise. |
| What is listening locally? | ss -ulnp | grep ':500 ' | Shows the UDP socket bound to port 500. This is the reliable way to answer the question, because you are asking the host itself rather than guessing from outside. |
Replace example.com with the host you are testing. A blocked port and a port with nothing listening on it look identical from the outside, so if a service should be running, check locally before blaming the firewall.
Frequently asked questions
What is port 500 used for?
Port 500 is used by IKE, the Internet Key Exchange protocol that negotiates and manages the security keys for an IPsec VPN tunnel. NAT traversal for the same negotiation falls back to port 4500.
Is port 500 TCP or UDP?
Port 500 (ISAKMP/IKE) uses UDP.
Is port 500 secure?
Port 500 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 500 on my firewall?
Only if you specifically need ISAKMP/IKE. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.