Deploying a secure VPN for businesses requires both robust encryption and scalable, centralized authentication. Combining IKEv2 for the VPN control plane with RADIUS for authentication gives you the best of both worlds: modern cryptographic security and centralized user management and accounting. This guide walks technical readers through the end-to-end architecture, configuration patterns, and operational considerations needed to implement IKEv2 + RADIUS in production environments.

Why IKEv2 + RADIUS?

IKEv2 (Internet Key Exchange version 2) provides a modern, resilient VPN control protocol with native support for mobility and multihoming (MOBIKE). It works efficiently with modern cipher suites and supports EAP methods for flexible authentication. RADIUS (Remote Authentication Dial-In User Service) centralizes authentication, authorization, and accounting (AAA), integrates with directories like Active Directory or LDAP, and enables multi-factor authentication (MFA) and fine-grained policy enforcement.

Combining IKEv2 and RADIUS allows you to authenticate VPN users against enterprise identity stores, apply policy decisions centrally, and maintain consistent logs and accounting for compliance.

Core components and architecture

Typical deployment components include:

  • VPN gateway(s) that support IKEv2 (e.g., strongSwan, Libreswan, Cisco/Juniper appliances, Windows RRAS).
  • RADIUS server(s) (e.g., FreeRADIUS, Microsoft NPS, Radiator) connected to a directory (Active Directory, LDAP, SQL).
  • Certificate Authority (internal PKI or public CA) to issue server and client certificates when using certificate authentication.
  • Clients that support IKEv2 — modern Windows, macOS, iOS, Android, and many Linux clients.

Authentication modes and RADIUS integration options

When using IKEv2 with RADIUS you typically choose one of the following authentication methods:

  • EAP-MSCHAPv2: Username/password negotiated via IKEv2 EAP. Works well with Active Directory and NPS for Windows Single Sign-On scenarios.
  • EAP-TLS: Certificate-based client authentication with RADIUS performing certificate validation and mapping to user accounts or groups.
  • Machine authentication + user authorization: Use client certificates for machine-level authentication and RADIUS to authorize users or apply policies.

Each method has trade-offs: EAP-TLS is strongest for non-repudiation and automated device provisioning, while EAP-MSCHAPv2 is simpler to integrate with existing AD passwords and MFA proxies.

Prerequisites

  • Root or administrative access to the VPN gateway and RADIUS server.
  • Certificates for server identity (preferably from a trusted CA) and a plan for client certificates if using EAP-TLS.
  • Network connectivity and firewall rules allowing IKE/ESP (UDP 500, UDP 4500) and any NAT traversal.
  • RADIUS shared secret configured consistently on both ends.
  • Access to user directory (AD/LDAP) or local RADIUS databases.

Example setup: strongSwan + FreeRADIUS

The following example demonstrates configuring a Linux-based VPN gateway using strongSwan and FreeRADIUS. This pattern is suitable for many Linux distributions.

1) Install components

On the VPN gateway:

apt-get update
apt-get install strongswan strongswan-plugin-eap-mschapv2 libcharon-extra-plugins

On the RADIUS server:

apt-get install freeradius freeradius-utils

2) Certificates

Generate or obtain a server certificate for the gateway. For testing, a simple CA-signed certificate suffices. Place the server certificate and private key into /etc/ipsec.d/certs and /etc/ipsec.d/private respectively. Make sure the CA certificate is available to clients.

3) Configure strongSwan

Edit /etc/ipsec.conf (simplified example for EAP-MSCHAPv2 authentication):

config setup
  charondebug="ike 2, knl 2, cfg 2"

conn %default
  keyexchange=ikev2
  ike=aes256-sha2_256-modp2048
  esp=aes256-sha2_256
  dpdaction=clear
  dpddelay=300s
  rekey=no
  left=%any
  leftid=@vpn.example.com
  leftcert=vpn.example.com.crt
  leftsendcert=always
  leftsubnet=0.0.0.0/0
  right=%any
  rightsourceip=10.10.0.0/24
  rightauth=eap-mschapv2
  rightsendcert=never
  eap_identity=%any

conn ikev2-eap
  auto=add

Edit /etc/ipsec.secrets to include the private key and (optionally) PSK for admin access:

: RSA "vpn.example.com.key"

4) Configure RADIUS plugin in strongSwan

Create /etc/strongswan.d/charon/eap-radius.conf (or modify charon config):

eap-radius {
  load = yes
  server {
    address = 192.0.2.10
    secret = your_radius_shared_secret
    auth_port = 1812
    acct_port = 1813
  }
}

Enable the plugin by ensuring charon loads the eap-radius module and restart strongSwan:

systemctl restart strongswan

5) Configure FreeRADIUS

On the RADIUS server, configure clients in /etc/freeradius/clients.conf:

client vpn-gateway {
  ipaddr = 198.51.100.5
  secret = your_radius_shared_secret
  shortname = vpn-gateway
}

Use the users file or connect to Active Directory via rlm_ldap or rlm_mschap for authentication. For EAP-MSCHAPv2 with AD, enable ntlm_auth or use the rlm_winbind module.

6) Test authentication

From a client machine, initiate an IKEv2 connection. On FreeRADIUS, run in debug mode to view authentication requests:

freeradius -X

On the gateway, monitor strongSwan logs:

journalctl -u strongswan -f

Successful logs will show an EAP request arriving at RADIUS and a subsequent Access-Accept. The VPN client should receive an IP in the rightsourceip pool and establish an IPsec SA.

Troubleshooting common issues

Some typical problems and how to resolve them:

  • RADIUS rejects EAP requests: Verify shared secret, client entry IP, and that FreeRADIUS is listening on the expected interface. Use freeradius -X to inspect attributes.
  • Certificate validation failures: Ensure the client trusts the gateway’s CA and that the server certificate’s subjectAltName matches leftid (or leftid is set correctly).
  • NAT traversal issues: Ensure UDP 4500/500 are open and NAT-T is enabled. strongSwan supports NAT-T automatically.
  • Accounting missing: Verify acct_port and that the RADIUS server accepts accounting packets; enable accounting in eap-radius config.

Windows Server (NPS) as RADIUS backend

Microsoft NPS is a common choice in Windows environments. A basic integration pattern:

  • Install Network Policy Server (NPS) role.
  • Add the VPN gateway as a RADIUS client in NPS, setting the shared secret.
  • Configure Connection Request Policies and Network Policies to allow user authentication, optionally checking group membership or vendor-specific attributes.
  • For EAP-MSCHAPv2 with AD, NPS will authenticate directly against Active Directory. For MFA, insert server-side MFA extension or configure conditional access policies.

Remember to configure appropriate RADIUS attributes (e.g., Framed-Route or Filter-Id) if you want the VPN gateway to enforce per-user routing or policy rules.

Security and operational best practices

To maintain a secure and maintainable deployment consider:

  • Use strong cryptography: Prefer AES-GCM, SHA2-based PRFs, and 2048+ DH groups (or ECDH groups) where supported.
  • Enforce certificate-based server authentication so clients can verify gateway identity and prevent man-in-the-middle attacks.
  • Deploy redundancy for RADIUS (primary/secondary) and gateways behind a load balancer to avoid single points of failure.
  • Enable logging and monitoring on both the VPN gateway and RADIUS servers for authentication events and accounting data to meet compliance needs.
  • Harden RADIUS servers: restrict management access, enable fail2ban or similar, and isolate them on a trusted management network segment.
  • Use MFA where possible—either via RADIUS integrations or by enabling EAP-TLS with certificate smartcards and a second factor.

Scaling and advanced features

As your user base grows, consider these enhancements:

  • Use RADIUS proxies to route requests to different backend realms or to failover across data centers.
  • Leverage dynamic VLAN assignment and RADIUS attributes to apply network segmentation per user or group.
  • Implement RADIUS accounting to feed SIEMs and billing systems. Store accounting records in SQL or forward to log collectors.
  • Adopt certificate lifecycle automation (e.g., SCEP, EST, or ACME variations) for client certificate provisioning when using EAP-TLS.

Conclusion

IKEv2 paired with RADIUS delivers a scalable, secure approach to VPN authentication that aligns well with enterprise identity systems and modern security practices. The combination gives administrators centralized control over policies and auditing while leveraging resilient cryptographic protocols for secure transport.

For hands-on deployments, start with a lab environment to validate certificate trust chains, RADIUS shared-secret alignment, and IKEv2 policy matches. Once operational, iterate on redundancy, logging, and MFA integration to harden your setup.

For more articles and practical guides on secure VPN deployments, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.