Port / UDP
4500IPsec NAT-T
IPsec NAT traversal encapsulates ESP inside UDP so tunnels survive NAT devices that would otherwise drop them.
If it is internet-facing: Port 4500 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 4500 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.
Registered with IANA on request, but bindable by any unprivileged user. Registration records intent — it does not reserve the port, so collisions between products are common.
Grouped with the other network services ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
1 note- A VPN endpoint needs both 500 and 4500 open; opening only 500 produces tunnels that establish and then carry no traffic.
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 4500/udpiptables
sudo iptables -A INPUT -p udp --dport 4500 -j ACCEPTCheck what is really there
Listening locally
sudo ss -ulpn 'sport = :4500'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:4500macOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sUV -Pn -p 4500 <host>A version probe confirms whether IPsec NAT-T is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vzu <host> 4500The quick yes/no when you only need to know whether a path exists through the firewall.