Port 500 IKE · UDP

What runs on port 500

IKE (Internet Key Exchange) for IPsec VPNs: the negotiation that sets up the tunnel. When either side is behind NAT, the exchange moves to UDP 4500 (NAT-T) and the ESP packets are encapsulated in UDP as well. Site-to-site VPNs and the built-in VPN clients in Windows, macOS, iOS and Android all use it.

Software that uses it: strongSwan · Libreswan · Cisco ASA/Firepower · Fortinet · Windows/macOS built-in IKEv2 clients

Should port 500 be open?

Designed to face the internet. This port exists to be reachable from anywhere, so the usual question is not whether to open it but how the service behind it is configured and patched.

• Open UDP 500 and 4500; without 4500 the tunnel works in the office and fails from every hotel.
• ESP is IP protocol 50, not a port; a firewall that only knows ports will need NAT-T.

Check port 500

# on the host: is anything listening, and which process?
ss -lunp 'sport = :500'
sudo lsof -i :500

# from another machine: is it reachable?
nc -zuv host 500
sudo nmap -sU -p 500 host

A port that answers from the internet when it should not is the finding; nmap from a machine outside your network (or a VPS) is the honest test, because scans from inside the LAN pass the firewall you are trying to verify.

Allow or block port 500

# ufw (Ubuntu/Debian)
sudo ufw allow from 10.0.0.0/8 to any port 500 proto udp
sudo ufw deny 500/udp

# nftables
nft add rule inet filter input ip saddr 10.0.0.0/8 udp dport 500 accept
nft add rule inet filter input udp dport 500 drop

# iptables (legacy)
iptables -A INPUT -p udp --dport 500 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p udp --dport 500 -j DROP

# firewalld
sudo firewall-cmd --permanent --add-rich-rule='rule family=ipv4 source address=10.0.0.0/8 port port=500 protocol=udp accept'

Replace 10.0.0.0/8 with the networks that need access. Allowing by source address is the rule that matters; "allow 500" with no source is just "open".

Related ports

• 4500 IPsec NAT-T — IKE/ESP when behind NAT
• 1194 OpenVPN
• 51820 WireGuard
• 1723 PPTP — Obsolete VPN

Part of the port number reference. Ports are 16-bit numbers; see also the subnet calculator and the packet decoder, which shows ports inside TCP and UDP headers.

Questions