Port / UDP
500IKE (IPsec)
IKE negotiates the keys and security associations that IPsec tunnels and most site-to-site VPNs are built on.
If it is internet-facing: Port 500 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Internet-facing
Transport
Encrypted by default
Risk if public
Elevated
Exposure verdict
Port 500 is meant to answer from the internet, and it protects the session itself. What remains is authentication strength, patch level, and rate limiting.
The session is protected from the first byte, so there is no plaintext phase for an on-path attacker to strip or read.
Assigned by IANA through a review process. On Unix systems a process needs root or the CAP_NET_BIND_SERVICE capability to bind here, which is why containers so often remap these to a higher port.
Grouped with the other network services ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
2 notes- Aggressive mode with a pre-shared key exposes a hash that can be cracked offline; prefer certificates or IKEv2 with strong PSKs.
- Expect this port to be publicly reachable — it is the front door of the tunnel.
Recommended firewall rule
Allow it, and harden the service
This port is expected to answer from anywhere, so the firewall is not where its security comes from.
ufw
sudo ufw allow 500/udpiptables
sudo iptables -A INPUT -p udp --dport 500 -j ACCEPTCheck what is really there
Listening locally
sudo ss -ulpn 'sport = :500'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iUDP:500macOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sUV -Pn -p 500 <host>A version probe confirms whether IKE (IPsec) is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vzu <host> 500The quick yes/no when you only need to know whether a path exists through the firewall.