Port 500 (IKE (IPsec VPN))
Key exchange for IPsec VPNs, with NAT traversal on UDP 4500.
- Transport
- UDP
- Internet exposure
- Public by design
Normally public, but keep the software patched and hardened.
Security notes
- VPN gateways are high-value targets; patch quickly when advisories are published.
- Aggressive mode IKE can leak a crackable pre-shared key hash.
When it shows up in your logs
Probing of 500 and 4500 is VPN discovery scanning.