IKEv2 is widely adopted for modern VPN deployments due to its robustness, mobility support and simplified state machine compared with IKEv1. However, secure and reliable IKEv2 deployments require careful configuration and ongoing operational controls. This checklist provides practical, technically detailed steps for administrators, developers and site operators to harden IKEv2/IPsec VPNs on both server and client sides.

1. Choose strong cryptographic primitives and enforce them

Start by selecting algorithms that balance security and performance. As of current best practices, prefer authenticated encryption and strong DH groups:

  • IKE SA crypto: Use AES-GCM (AES-128-GCM or AES-256-GCM) for combined confidentiality and integrity. If AES-GCM isn’t available, use AES-CBC with HMAC-SHA2-256 or higher.
  • ESP (child SA): Prefer AES-GCM for ESP as well. AES-GCM avoids separate integrity algorithms and is faster on hardware with AES-NI.
  • DH groups: Enforce strong Diffie-Hellman groups such as ECP groups (19, 20, 21 for 256/384/521-bit curves) or MODP groups 14 (2048-bit) and above (e.g., 18/19/20). Avoid legacy groups like 2 (1024-bit).
  • Key lengths: Use 128-bit or 256-bit keys depending on required security level; prefer AES-256 for high-sensitivity environments.

Tip: Configure strict proposal order and disable weak fallback algorithms to prevent negotiation downgrades. On strongSwan, set esp and ike proposals explicitly.

2. Prefer certificate-based authentication over PSKs

Pre-shared keys (PSKs) are convenient but have major limitations for scalability and security (single shared secret, poor rotation). Instead:

  • Use X.509 certificates for both server and clients. Certificates enable per-client revocation and stronger identity binding.
  • Implement a dedicated internal CA or integrate with enterprise PKI. Keep CA private keys offline and use intermediate issuing CAs for day-to-day certificate issuance.
  • Deploy certificate revocation mechanisms: OCSP stapling on servers where supported, and regularly maintained CRLs on clients that require them.
  • Consider hardware-backed key protection: store private keys in an HSM or use TPM-backed keys on endpoints to prevent key extraction.

3. Configure lifetimes, rekeying and PFS

IKE and child SA lifetimes and rekeying behavior directly influence both security and availability.

  • Set conservative IKE SA lifetimes (e.g., 8–24 hours) and shorter child SA lifetimes (e.g., 1–2 hours) depending on traffic patterns.
  • Enable Perfect Forward Secrecy (PFS) for rekeying by requiring a new DH exchange for child SAs. This reduces exposure if a long-term key is compromised.
  • Tune rekey intervals and retransmission retries to balance resilience across lossy networks. Avoid excessively frequent rekeys that increase overhead.

4. Harden network and NAT traversal settings

Most deployments will encounter NAT. IKEv2 has strong NAT traversal support (NAT‑T) and MOBIKE for mobility, but proper configuration matters:

  • Open and monitor UDP ports 500 (IKE) and 4500 (NAT-T). If a firewall or DDoS protection is in front of the VPN server, allow and rate-limit those ports.
  • Enable NAT traversal and ESP encapsulation where needed. IKEv2 sends ESP over UDP/4500 when NAT is detected.
  • Enable MOBIKE if you support roaming clients (mobile IP changes). MOBIKE lets IKEv2 rebind SAs to new IPs without a full rekey.
  • Use NAT keepalive or UDP encapsulation to prevent NAT bindings from expiring on networks with aggressive timeouts.

5. Configure strong firewall and packet filtering

Tight firewall rules reduce attack surface and enforce correct traffic flow.

  • Only allow IKE (UDP 500) and NAT-T (UDP 4500) to VPN gateways from known client IP ranges where feasible. Rate-limit connection attempts to mitigate brute-force and amplification attacks.
  • Restrict ESP (protocol 50) to the VPN endpoints if ESP is used without NAT-T.
  • Implement stateful inspection and allow only expected traffic selectors. Deny traffic by default and explicitly allow necessary subnets and service ports.
  • Protect administrative access (SSH, web UI) with IP whitelisting, MFA and non-default ports. Disable GUI/admin exposure on public interfaces.

6. Manage clients and split tunneling carefully

Client configuration impacts privacy and attack surface.

  • Decide whether to enforce full-tunnel or split-tunnel policies. Full-tunnel (all traffic through VPN) protects clients on hostile networks; split-tunnel reduces bandwidth but can leak sensitive traffic if misconfigured.
  • Use traffic selectors (TSi/TSr) or VTI policies to tightly bind allowed source/destination pairs. Avoid overly permissive selectors like 0.0.0.0/0 unless intended.
  • Push security policies and DNS over the tunnel to prevent DNS leaks. Configure DNS servers reachable only via the VPN when privacy is required.

7. Implement monitoring, logging and alerting

Visibility is essential for detecting attacks, performance issues and failed rekeys.

  • Enable verbose but structured logging on your IKE daemon (strongSwan/libreswan/Windows RRAS). Capture IKE exchanges, authentication events and child SA status.
  • Forward logs to a central SIEM or log aggregator. Correlate IKE event patterns (failed handshakes, repeated rekey attempts) with other network events.
  • Monitor resource metrics: CPU (crypto ops), memory, and connection counts. Enforce limits per-user and global connection caps to guard against resource exhaustion.
  • Set alerts for suspicious patterns: mass authentication failures, repeated NAT-T fallbacks, unusual client rebinds (MOBIKE), and certificate revocation events.

8. Secure key lifecycle and rotation

Cryptography is only as good as how keys are managed.

  • Rotate server and client certificates on a predictable schedule. Use automation for issuance and renewal (ACME is less common for VPN certs but internal tooling can help).
  • Immediately revoke any compromised credentials and ensure clients poll OCSP or check CRLs within acceptable latency.
  • Minimize key material exposure: use short-lived credentials for ephemeral clients and prefer client certs over stored PSKs.

9. Protect against DoS and resource exhaustion

IKEv2 endpoints are potential targets for DoS due to expensive crypto operations.

  • Enable cookie protection (IKEv2 has a cookie mechanism during initial handshake) to mitigate spoofed initial packets.
  • Rate-limit IKE initiation per source IP and globally. Employ SYN/UDP flood protection techniques on the perimeter.
  • Offload crypto to hardware where possible (AES-NI, HSM) to reduce CPU pressure and improve throughput.

10. Test interoperability and client behavior extensively

Different clients (Windows, macOS, iOS, Android, strongSwan clients) implement variants of IKEv2 with specific quirks.

  • Test key scenarios: initial connect, rekey, MOBIKE rebinds, NAT traversal, roaming between networks, and failures.
  • Validate fragmentation and MTU behavior. Large packets can be dropped due to MTU/DF settings; consider enabling MSS clamping or IPSec fragmentation handling.
  • Confirm behavior with split tunneling, DNS push, and client routing tables across OS versions.

11. Operational practices and compliance

Operational discipline translates to long-term security.

  • Document configuration baselines and maintain version-controlled configuration templates.
  • Perform periodic security reviews and cipher-suite audits. Remove deprecated algorithms proactively.
  • Use change control for cryptographic changes and include rollback plans in case clients cannot handle new proposals.
  • Ensure compliance with relevant regulations (PCI, HIPAA, GDPR) for data in transit and data retention in logs.

Quick practical config hints

  • On strongSwan, set ike=aes256gcm16-sha384-modp2048 and esp=aes256gcm16 in your conn profiles.
  • On Windows Server RRAS, enable IKEv2 and configure ECDH (P-256/P-384) with certificate Authentication.
  • When using NAT-T, ensure your firewall allows UDP 4500 and that anti-spoofing doesn’t drop encapsulated ESP packets.

By implementing these measures—strong cryptography, certificate-based auth, careful lifetime and NAT handling, tight firewalling, comprehensive logging, and solid operational processes—you will significantly reduce the attack surface and increase the resilience of your IKEv2 VPN deployment. Hardening is iterative: schedule periodic reviews and adapt to new cryptographic guidance and client behavior changes.

For more resources and deployment guides, visit Dedicated-IP-VPN. Dedicated-IP-VPN provides further technical articles and configuration walkthroughs to help you design and secure production VPNs.