IKEv2 (Internet Key Exchange version 2) is a modern, resilient protocol for establishing IPsec VPN tunnels. For organizations and developers seeking a reliable Always‑On VPN implementation, IKEv2 provides fast reconnects, built‑in NAT traversal, and robust authentication methods. This guide walks through practical configuration steps, security best practices, and operational considerations to deploy Always‑On IKEv2 across common platforms.

Why IKEv2 for Always‑On VPN?

Before diving into configuration, understand the core advantages of IKEv2 for an Always‑On deployment:

  • Fast session resumption: IKEv2 supports MOBIKE and rapid rekeying, allowing seamless reconnection during network changes (e.g., switching from cellular to Wi‑Fi).
  • Robust authentication: Multiple authentication options (certificates, EAP‑MSCHAPv2, EAP‑TLS) enable both machine and user assertions.
  • Native client support: Built into modern Windows, iOS, macOS, and many Android builds, reducing reliance on third‑party clients.
  • Scalable key management: Efficient SA lifetimes and rekeying reduce CPU and latency overhead on servers.

Design Decisions for Always‑On Deployments

Always‑On deployments require careful design decisions to balance security, performance, and user experience:

  • Tunneling mode: Choose between full tunnel (all traffic through VPN) and split tunnel (only corporate subnets routed). Full tunnel provides maximum privacy; split tunnel reduces bandwidth on the VPN gateway.
  • Authentication strategy: Prefer certificate‑based authentication for machine identity and EAP‑TLS or multi‑factor user authentication for users. Avoid cleartext credentials when possible.
  • Failover and HA: Use active/passive or active/active VPN gateways with synchronized state or short SA lifetimes to minimize disruption.
  • Monitoring and logging: Centralized logs for IKE and IPsec events help detect authentication failures, rekey problems, and DoS events.

Core IPsec / IKEv2 Parameters

Define consistent policies to ensure interoperability and security. Key recommended parameters:

  • IKE SA (phase 1): Use IKEv2, PRF: HMAC‑SHA2 (SHA256 or greater), encryption: AES‑GCM or AES‑CBC with AES‑HMAC‑SHA2, DH group: modp2048 (group 14) or stronger (group 19/20/21 or ECP groups for PFS).
  • IPsec SA (phase 2): ESP with AES‑GCM (preferred) or AES‑CBC + HMAC‑SHA2, PFS using a DH group matching policy requirements, lifetime typically 3600s (adjust to security needs).
  • NAT‑T: Enable NAT traversal (UDP encapsulation on 4500) for mobility scenarios.
  • Dead Peer Detection (DPD): Short DPD intervals (e.g., 10s checks with 3 retries) allow faster detection of dead peers in mobile environments.
  • Mobility (MOBIKE): Enable MOBIKE for client IP changes—this is essential for Always‑On on mobile devices.

Server Options: strongSwan Example

strongSwan is a popular open‑source IKEv2 implementation on Linux. Key configuration points:

  • ipsec.conf: Define connections with ike= set to desired proposals, esp= for child SAs, left/right identifiers, and leftcert for server certificate. Example parameters: ike=aes256gcm16‑ecp521‑sha512, esp=aes256gcm16.
  • ipsec.secrets: Store private keys in files referenced by leftcert. Use appropriate file permissions (600) and consider hardware security modules (HSMs) for key protection.
  • conn alwayson: Use dpdaction=clear and dpddelay=10s for quick reclaims. Set fragmentation=yes for handling large packets and enable rekeying with rekeymargin=3m and keyingtries=%forever.
  • Plugins: Use strongSwan’s EAP plugin (eap-mschapv2, eap-tls) and the stroke or VICI control interface to automate provisioning tasks.

Note: For large scale, front strongSwan with load balancers and use synchronization or persistent session storage to maintain best user experience across gateway failovers.

Client Configuration Highlights

Each client OS has nuances. Below are key notes for mainstream platforms.

Windows (Enterprise / Always On VPN)

  • Use Microsoft’s Always On VPN profile delivered via Intune or SCCM. The profile includes device tunnels (machine) and user tunnels.
  • Configure IKEv2 parameters in the XML profile (authentication via certificate or user credential). Use Certificate Revocation List (CRL) checking and specify trusted root CA IDs.
  • Use conditional access and device compliance rules to enforce MFA and endpoint posture checks before allowing traffic.

iOS / iPadOS

  • iOS supports IKEv2 natively and can be configured via MDM (e.g., Apple Configurator, Jamf, Intune). Use “On Demand” or Always‑On profiles for supervised devices.
  • Prefer certificate authentication with EAP‑TLS for users. Set “Enable NAT Traversal” and “Dead Peer Detection” options when available.

Android

  • Android OEMs vary. Use strongSwan VPN Client app for consistent IKEv2/EAP‑TLS behavior across devices; it supports Always‑On via Android’s Always‑On VPN API.
  • For enterprise-managed devices, configure via Android Enterprise (Device Owner) to enforce Always‑On and block bypass.

Linux

  • Use strongSwan or libreswan. For desktop clients, NetworkManager’s strongSwan plugin can manage IKEv2 connections and integrate with systemd for autostart at boot.
  • Scripts: Add iptables/nftables rules and ip rule/ip route entries to implement split tunneling or force all traffic through the tunnel.

PKI and Certificate Management

Certificate lifecycle is crucial for Always‑On deployments:

  • Separate CAs: Use a dedicated CA for VPN server and another for client/device certificates to limit blast radius.
  • Short lifetimes: Issue client certificates with moderate lifetimes (e.g., 1 year) and use automated renewal (SCEP, EST) for scale.
  • CRL/OCSP: Ensure clients can access CRL distribution points or OCSP responders to check revocation; configure timely revocation on device compromise.

Routing, DNS, and Split Tunneling

Carefully plan routing and DNS to avoid leakage:

  • Split DNS: Push internal DNS servers via the VPN and configure suffixes to ensure internal names resolve over the tunnel.
  • Policy vs Route Based: Choose route‑based IPsec (using virtual interfaces like xfrm) for complex routing or policy‑based for simpler networks.
  • Traffic selectors: Use precise traffic selectors in IKEv2 to control what subnets traverse the tunnel, reducing unnecessary VPN load.

Security Hardening and Best Practices

To reduce risk and maintain resilience:

  • Harden ciphers: Disable legacy ciphers like 3DES and SHA1. Prefer AES‑GCM, CHACHA20‑POLY1305 where supported.
  • Limit management access: Restrict administrative ports and APIs to specific management networks and use MFA for admin access.
  • Logging and IDS: Integrate IKE/IPsec logs with SIEM and monitor for unusual rekey rates, repeated auth failures, or DB/CPU spikes indicating abuse.
  • Patch regularly: Keep strongSwan, OS kernels, and client stacks patched—many vulnerabilities arise in crypto libraries and kernel XFRM subsystems.

Troubleshooting Checklist

Common issues and diagnostics:

  • IKE negotiation fails: Check certificates/CRL, verify IKE proposals match on both ends, ensure correct identity types (FQDN vs User FQDN vs IP) and correct cert CN/SAN.
  • DPD or MOBIKE issues: Ensure NAT‑T is enabled and check firewall for UDP 500/4500. Verify client supports MOBIKE if IP mobility is expected.
  • Routing/DNS leakage: Use packet capture (tcpdump/wireshark) and ip route/ip rule to verify traffic flow. Confirm DNS resolution path for sensitive queries.
  • Performance degradation: Profile CPU and crypto offload. Use AES‑GCM with hardware acceleration (AES‑NI) and consider EC cryptography for smaller key sizes with similar strength.

Operational Checklist Before Production Rollout

  • Define acceptable SA lifetimes and rekey policies aligned with compliance requirements.
  • Automate certificate lifecycle and client provisioning workflows (SCEP/EST, MDM profiles).
  • Pilot Always‑On on controlled user groups and measure reconnection times across mobility scenarios.
  • Prepare rollback and incident response plans for gateway compromise or certificate key exposure.

Implementing an Always‑On IKEv2 VPN requires careful coordination between cryptographic choices, CA practices, client provisioning, and network routing. By standardizing IKE/IPsec parameters, leveraging certificate‑based authentication, and enforcing monitoring and automation, organizations can achieve a resilient, low‑latency Always‑On VPN experience across diverse clients and mobile scenarios.

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