Port reference
Port 4500 (UDP) – IPsec NAT-T (IKE NAT traversal)
UDP encapsulation port for IPsec NAT-Traversal, carrying IKE and ESP traffic across NAT devices.
Quick facts
- Transport
- udp
- Category
- Registered
- Risk level
- High
Frequently targeted — restrict exposure and harden it.
Default state
Open on VPN gateways and remote-access concentrators that support clients behind NAT. Typically paired with IKE on UDP 500; the security depends on IKE/IPsec configuration.
What is port 4500 used for?
Port 4500 carries IPsec NAT-Traversal (NAT-T), which lets IPsec VPNs work when one side sits behind a NAT router (almost always the case for home and mobile clients). Plain IPsec traffic can't pass through NAT cleanly, so NAT-T wraps it inside UDP on port 4500. It works alongside IKE on UDP 500 and is used by VPN clients and gateways — Windows VPN, macOS/iOS IKEv2, strongSwan, and most commercial VPN appliances.
When would you open it?
Open or forward UDP 500 and 4500 on a VPN gateway when you host an IPsec remote-access VPN that mobile or home users connect to. Client devices don't need it opened inbound — they initiate the connection outbound.
Is it safe to open?
The port itself is a normal VPN transport; risk comes from weak settings. Prefer IKEv2 with certificate or EAP authentication, enable MFA, and keep the gateway firmware patched. See the security notes below.
How to check if this port is open
ss -tulpn | grep :4500
nmap -sU -p 4500 <target>netstat -ano | findstr :4500
Test-NetConnection <host> -Port 4500 # TCP onlylsof -i :4500
nmap -sU -p 4500 <target>How to open this port on your router
To reach this service from outside your network, forward the port on your router:
- Open your router's admin page (usually http://192.168.1.1 or http://192.168.0.1) and sign in.
- Find the "Port Forwarding" section — it may be called NAT, Virtual Server, or Applications & Gaming.
- Add a rule forwarding external port 4500 to your device's local IP, internal port 4500, protocol UDP.
- Save and reboot the router if prompted, then test the port from outside your network.
Only forward ports you understand — it exposes that device to the public internet. For remote admin access, a VPN is safer than forwarding the port.
Allow this port through the firewall
sudo ufw allow 4500/udpsudo firewall-cmd --permanent --add-port=4500/udp
sudo firewall-cmd --reloadsudo iptables -A INPUT -p udp --dport 4500 -j ACCEPTNew-NetFirewallRule -DisplayName "Allow 4500" -Direction Inbound -Protocol UDP -LocalPort 4500 -Action AllowSecurity & risks
Common attacks
- IKE/VPN fingerprinting and vendor identification
- Aggressive-mode PSK capture and offline cracking
- Brute-force or password-spraying of remote-access VPN credentials
- Exploitation of vulnerable VPN gateway firmware reachable via NAT-T
Hardening
- Disable IKEv1 aggressive mode; prefer IKEv2 with strong, modern crypto
- Use certificate or EAP authentication instead of weak group PSKs
- Restrict UDP 500/4500 to expected client networks where feasible
- Enforce MFA on remote-access VPN and rate-limit authentication
- Keep VPN gateway firmware patched against known CVEs
How to block this port
sudo ufw deny 4500/udpsudo firewall-cmd --permanent --remove-port=4500/udp
sudo firewall-cmd --reloadsudo iptables -A INPUT -p udp --dport 4500 -j DROPNew-NetFirewallRule -DisplayName "Block 4500" -Direction Inbound -Protocol UDP -LocalPort 4500 -Action Blocknmap snippet
nmap -sU -p4500 --script ike-version <target>Replace <target> with the host or range you're authorized to scan.
Related ports
Frequently asked questions
- Why does IPsec need port 4500 in addition to 500?
- Standard IPsec ESP isn't a UDP/TCP protocol, so NAT devices can't track it. NAT-Traversal detects NAT and wraps IKE and ESP inside UDP on port 4500 so the tunnel survives address translation.
- Is port 4500 a vulnerability by itself?
- No. It's a legitimate VPN transport. Risk comes from weak configurations — IKEv1 aggressive mode with a guessable PSK, missing MFA, or unpatched gateway firmware — not from the port being open.