Overview: why combine IKEv2 with Cloudflare Tunnels

IKEv2 (Internet Key Exchange v2) paired with IPsec is a widely adopted, robust VPN protocol for site-to-site and remote access deployments. It offers strong security, native client support across major OSes, and good performance. Cloudflare Tunnels (cloudflared) provides a secure way to expose internal services to the internet without opening inbound firewall ports on a public IP, and can proxy TCP services through Cloudflare’s network.

Combining an IKEv2 server (for example strongSwan) with Cloudflare Tunnels can be useful when you want to avoid exposing raw UDP ports (500/4500) on your network perimeter or when your server is behind NAT/firewalls and you prefer Cloudflare’s connectivity as the public ingress. However, because IKEv2 natively uses UDP (and ESP IP protocol 50), you must pick a deployment approach that handles UDP. This guide presents two practical methods: (A) use a UDP-to-TCP relay plus Cloudflare Tunnel, and (B) use Cloudflare Spectrum (or equivalent Enterprise UDP support) when available. Both include step-by-step setup details and troubleshooting tips.

Preparatory notes and limitations

Before you begin, consider the following constraints and tradeoffs:

  • ESP (IP protocol 50) is used for encrypted payload in IPsec. ESP is not TCP/UDP — if you tunnel only UDP 500/4500 you must still ensure ESP is forwarded or use NAT-T (NAT Traversal) so ESP is encapsulated inside UDP. Modern strongSwan/NAT-T implementations use UDP/4500 for ESP when NAT is detected.
  • Cloudflare Tunnel native UDP support is limited in standard plans. If you have Cloudflare Spectrum or an Enterprise plan with UDP support, prefer that — it’s cleaner and preserves UDP semantics. Otherwise, an application-level UDP-to-TCP relay on the VPN host is the common workaround.
  • Performance and latency — relaying UDP over TCP can introduce head-of-line blocking and degrade performance for latency-sensitive flows. Test before production deployment.

Method A — Use a UDP-to-TCP relay + cloudflared TCP tunnel (community / standard plan)

1) Install and configure strongSwan (IKEv2) on your server

Install strongSwan on your Linux server (Debian/Ubuntu example):

  • apt update && apt install -y strongswan strongswan-pki libcharon-extra-plugins
  • Generate an RSA or ECDSA CA and server certificate. Use strongSwan’s pki tools to create a CA, then sign a server certificate for the server’s hostname (commonName matches the hostname you will use in client configs).

Key configuration pointers:

  • /etc/ipsec.conf: define ikev2 connections, enable NAT-T and forceencaps if necessary.
  • /etc/ipsec.secrets: place the server key and any PSKs (prefer certificate-based authentication or EAP for user auth).
  • Enable IP forwarding: echo 1 > /proc/sys/net/ipv4/ip_forward and persist via /etc/sysctl.conf.
  • iptables: set MASQUERADE on the outbound interface and allow forwarding between ipsec0/vti interfaces and your LAN.

Example strongSwan concepts (abstracted): set up an ikev2-rw connection for remote access, use leftcert for server cert, rightauth=eap-mschapv2 and include eap users in /etc/ipsec.secrets.

2) Install a UDP-to-TCP relay on the same host

Because cloudflared can expose TCP services easily, you can run a locally bound relay that listens on TCP and converts to UDP locally for the strongSwan process. There are several tools for this:

  • udp2raw — encapsulates UDP in raw TCP/ICMP and can be tuned for reliability.
  • udprelay or ssh -w/ socat — simpler UDP->TCP forwarding but less featureful.

udp2raw example (conceptual): run udp2raw in server mode to listen on TCP port 4500 and forward to local UDP 4500:

– Start: udp2raw -s -l0.0.0.0:4500 -r 127.0.0.1:4500 –raw-mode faketcp

This creates a TCP listener that will forward traffic into the local UDP port. Do the same for port 500 if needed.

3) Run cloudflared to expose the relay TCP port via a Tunnel

Install cloudflared on the server and authenticate it to your Cloudflare account:

  • Download cloudflared binary and run cloudflared tunnel login to create a tunnel and map DNS to a hostname (e.g. vpn.example.com).
  • Configure tunnel ingress rules to expose the TCP relay port. In your tunnel config (config.yml), set:

– ingress rule for tcp: service: tcp://localhost:4500

– create a CNAME/DNS record that points vpn.example.com to the tunnel.

Start the tunnel: cloudflared tunnel run your-tunnel-name

4) Configure client IKEv2 to connect to the Cloudflare hostname

On the VPN client, configure the IKEv2 connection to use the Cloudflare hostname (vpn.example.com) as the remote gateway. Ensure the client trusts the server certificate (or use EAP username/password). Because the external transport is TCP (encapsulating UDP), the client will open UDP sockets to the remote endpoint but the cloudflared proxy will terminate TCP and forward to the local UDP relay, which then feeds strongSwan.

5) Validate and troubleshoot

  • Check strongSwan logs: journalctl -u strongswan or ipsec statusall.
  • Verify that cloudflared is forwarding connections: cloudflared logs show connection and origin data.
  • Monitor the UDP-to-TCP relay logs (udp2raw) to confirm packets are being encapsulated/decapsulated.
  • If IKE negotiation fails, inspect packet captures with tshark/tcpdump on localhost UDP ports to see if messages reach strongSwan.

Method B — Use Cloudflare Spectrum / Enterprise UDP support (recommended where available)

If your organization has access to Cloudflare Spectrum or Cloudflare for Teams features that provide UDP forwarding, you can avoid UDP->TCP encapsulation. Spectrum supports arbitrary TCP/UDP proxying at the Cloudflare edge and is an ideal match for IKEv2 VPNs.

Steps with Spectrum

  • Configure your Cloudflare account and enable Spectrum for the domain you want to use (vpn.example.com).
  • Create a Spectrum application that forwards UDP ports 500 and 4500 to your origin IP and corresponding UDP ports.
  • Ensure the origin (your VPN host) allows inbound UDP on 500/4500 from Cloudflare edge IP ranges (or use cloudflared as a connector if your host is behind NAT).
  • Configure strongSwan normally — no UDP-to-TCP relay required.

Advantages: true UDP forwarding preserves NAT-T and ESP handling, reduces latency and avoids head-of-line blocking. This is the cleanest approach if your plan supports it.

Server and client configuration tips (strongSwan specifics)

When configuring strongSwan for remote access via Cloudflare Tunnel or Spectrum, keep these technical details in mind:

  • NAT Traversal: enable nat_traversal=yes in ipsec.conf. strongSwan will use UDP/4500 for encapsulated ESP if NAT is detected.
  • Certificates: use a certificate for server identity matching the Cloudflare hostname (vpn.example.com). Export the CA certificate and import/trust it on clients.
  • EAP auth: for per-user authentication, enable eap-mschapv2 and configure /etc/ipsec.secrets with username:password entries or connect to LDAP/Radius.
  • IP forwarding and NAT: iptables -t nat -A POSTROUTING -s 10.10.0.0/24 -o eth0 -j MASQUERADE (adjust subnets and interface names).
  • ESP handling: if you expect ESP traffic (protocol 50), ensure your path supports it. When relaying only UDP 4500, NAT-T will encapsulate ESP and strongSwan will handle it.

Security considerations

When exposing a VPN server through Cloudflare:

  • Use certificate-based server authentication and strong ciphers (IKEv2 with AES-GCM and ECDHE). Avoid legacy DES/3DES and weak DH groups.
  • Harden the strongSwan config to prefer strong algorithms and use IKEv2 only for modern clients when possible.
  • Protect your cloudflared tunnel credentials: treat the tunnel token and config as sensitive secrets.
  • Monitor logs and set up alerting for repeated failed authentication attempts.

Performance testing and monitoring

After deployment, validate throughput and latency. Use iperf3 over the established VPN tunnel, test round-trip times to critical resources, and measure retransmissions to detect encapsulation inefficiencies. If you experience poor performance with Method A, consider migrating to Spectrum or hosting the VPN on a public-IP host to avoid TCP encapsulation.

When to choose which method

  • Choose Method A (UDP->TCP + cloudflared) when you do not have Enterprise Cloudflare features and cannot open ports on the public firewall, and you accept potential performance tradeoffs.
  • Choose Method B (Spectrum/Enterprise UDP) for production-grade deployments with optimal latency and native UDP handling.
  • For large enterprise or multi-site VPNs, evaluate Cloudflare Magic Transit / dedicated IP Gateway offerings, which integrate better with IPsec and routing at scale.

Final checklist before going live

  • Server cert signed and trusted by clients, or CA installed on clients.
  • Strong ciphers and IKEv2 policies configured; weak suites disabled.
  • IP forwarding and NAT rules in place for traffic breakout.
  • cloudflared tunnel created and configured with ingress rules for the relay (or Spectrum app configured for UDP).
  • Monitoring and logging configured for cloudflared and strongSwan.
  • Client configuration profiles exported (e.g., .mobileconfig for iOS/macOS or strongSwan Android/iOS setup), with clear instructions for users.

Combining IKEv2 with Cloudflare’s tunnel infrastructure gives you an option to deploy secure remote access without directly exposing UDP ports on your public perimeter. For production-grade performance and reliability, prefer solutions that preserve native UDP (Cloudflare Spectrum or similar). For smaller deployments or when constrained by network policies, a carefully implemented UDP-to-TCP relay plus cloudflared can work as a practical workaround—just be aware of its performance and reliability implications.

For additional resources and a managed dedicated IP VPN solution overview, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.