Implementing robust client authentication in IKEv2 is a cornerstone of securing VPNs used by enterprises, service providers, and developers. While IKEv2 already improves on earlier IPsec variants with better resilience and extensibility, ensuring that only authorized clients can establish Phase 1 (IKE SA) and Phase 2 (IPsec SA) connections requires careful enforcement of client identity. This article provides practical, actionable steps and configuration examples to enforce client identity in IKEv2 deployments, covering certificate-based authentication, EAP-based methods, RADIUS integration, and server-side validation techniques.

Why enforce client identity in IKEv2?

Enforcing client identity prevents unauthorized access and mitigates risks such as stolen credentials, rogue devices, and credential replay. In IKEv2, authentication occurs during the IKE_AUTH exchange; however, the protocol allows several auth methods (pre-shared keys, certificates, and EAP). Each method has different security properties and operational trade-offs:

  • Certificates provide strong, non-reusable credentials that support mutual authentication and granular validation (SubjectAltName, EKU, CRL/OCSP).
  • EAP (e.g., EAP-MSCHAPv2, EAP-TLS) is useful for integrating with directory services and RADIUS but introduces complexity around identity mapping and per-user auth.
  • Pre-Shared Keys are simple but scale poorly and cannot uniquely identify clients beyond per-group secrets.

Design principles

Before diving into configs, adopt these guiding principles:

  • Prefer certificate-based client authentication (EAP-TLS or mutual certs) for server-to-client identity assurance and non-repudiation.
  • Enforce strict identity matching on the server side. Do not accept ambiguous or wildcard identities unless explicitly required.
  • Use revocation checking (CRL/OCSP) to revoke compromised client certs promptly.
  • Harden cryptographic proposals with modern algorithms (AES-GCM, CHACHA20-POLY1305, ECDH P-256/Curve25519, SHA-2/384).
  • Log and audit authentication attempts and integrate with SIEM or monitoring.

Certificate-based authentication: practical steps

Certificate-based authentication is the most secure and flexible option. Below are the steps to implement it effectively.

1. CA and certificate issuance

Run an internal CA or use a PKI provider. Certificates for clients should include:

  • SubjectAltName (SAN) with a unique identifier such as user@domain or device serial number (use rfc822Name or otherName as appropriate).
  • Key Usage and Extended Key Usage (EKU) set appropriately (e.g., Digital Signature, Key Encipherment; EKU: TLS Web Client Authentication or specifically IPsec end entity OIDs if required).
  • Short lifetimes for client certs to limit exposure from compromise.

2. Server-side validation: identity matching

Servers must explicitly check the client identity contained in the IDi payload against an allowlist. Accepting the client’s claim blindly (e.g., using wildcard or not checking SAN) creates risk. Enforce checks by:

  • Mapping certificate SAN (or DN) to an account in your AAA system or local policy.
  • Using implementation-specific settings like rightid/leftid in strongSwan, or peer identity matching in vendor devices.
  • Rejecting connections when the ID type is not what you expect (e.g., require rfc822Name rather than CN).

3. Example strongSwan (swanctl.conf) configuration

strongSwan offers precise controls over identities. Below is a minimal server block enforcing client cert identity.

[conns.myvpn] is illustrative; in WordPress editor paste as normal text:

connections {
myvpn = {
local_addrs = 198.51.100.10
local {
cert = serverCert.pem
id = vpn.example.com
}
remote {
auth = pubkey
certs = clientCerts.pem
id = %any
}
children {
net = {
local_ts = 0.0.0.0/0
remote_ts = 10.0.0.0/24
}
}
}
}

To enforce specific client identities, replace remote.id = %any with an explicit remote.id = user@example.com or use an include list, and combine with certificate checks on SAN.

4. CRL and OCSP checking

Enable CRL or OCSP validation. strongSwan, for example, supports OCSP stapling and CRL checks. Configure your server to fetch and cache CRLs and to perform OCSP queries during the IKE_AUTH validation phase. This ensures revoked certificates cannot authenticate even if otherwise valid.

EAP-based authentication and RADIUS integration

EAP methods let you authenticate users against a centralized directory (Active Directory, FreeIPA) via RADIUS. EAP-TLS uses client certificates; EAP-MSCHAPv2 uses username/password and is commonly used on Windows clients. Key considerations:

1. Use EAP-TLS when possible

EAP-TLS combines certificate-based client auth with RADIUS centralization and provides the same non-repudiation benefits. Configure the RADIUS server to map the client certificate SAN to a username and to enforce certificate validation and revocation checking.

2. EAP-MSCHAPv2: mitigate risks

EAP-MSCHAPv2 is weaker and susceptible to offline brute-force attacks if intercepted. If you must use it:

  • Combine it with machine certificates for device enrollment and perform dual-factor checks.
  • Use NTLMv2 enforcement and ensure strong passwords or passphrases.
  • Monitor failed auth rates and lock or quarantine suspicious clients.

3. strongSwan + FreeRADIUS example snippet

In strongSwan, enable EAP and configure the server to proxy to FreeRADIUS. Example (swanctl.conf):

plugins {
include strongswan.d/charon/*.conf
}
secrets {
eap {
eap_method = mschapv2
}
}

On FreeRADIUS, ensure the authorize and authenticate sections map cert SANs/usernames and respond with appropriate tunnel attributes (VLAN, IP pool).

Server configuration best practices

Beyond the core identity checks, the server should be hardened:

  • Restrict cryptographic proposals: prefer AES-GCM or ChaCha20-Poly1305 for ESP, and ECDH groups like Curve25519 or P-256 for IKE.
  • Disable weak transforms: avoid legacy 3DES, MD5, or SHA-1 where possible.
  • Enforce encapsulation policies: NAT traversal (NAT-T) is necessary in complex networks; ensure UDP encapsulation is allowed and logged.
  • Protect against identity spoofing: explicitly require ID payload types (e.g., ID_FQDN, ID_RFC822_ADDR) and match to SAN entries.
  • Enable anti-replay and replay window checks: default IKEv2 implementations provide these, but verify your parameters.

Client-side considerations

Client devices must be configured to present the correct identity and certificate. Common issues arise when the OS inserts its own default ID (e.g., using the machine FQDN). Steps to avoid mismatches:

  • Configure explicit identity on the client (e.g., “username@domain” or certificate CN/SAN) to match server policy.
  • Install client certificate with private key and verify Key Usage/EKU; some OSes will not use certs missing the appropriate EKU.
  • Validate server certificate and CA trust chain on the client; do not skip server cert validation during testing in production.

Operational responses: revocation and incident response

Enforcing identity is only part of the lifecycle. Prepare operational workflows:

  • Certificate revocation: automate CRL publication or OCSP responder updates and ensure servers refresh status periodically.
  • Compromise response: when a device is lost, revoke its cert immediately and, if possible, quarantine related sessions on the VPN concentrator.
  • Logging and alerting: log IKE failures with identity details and integrate with SIEM to detect brute-force or replay attempts.
  • Periodic audits: audit which client identities are active, their certificates, and last-auth timestamps; remove stale accounts.

Troubleshooting common problems

Below are typical issues and how to address them:

Mismatched identity leading to “no matching connection” errors

Check the client’s ID payload (IDi) and server policy. On strongSwan, enable charon logging to capture the exact ID presented. Update server rightid or mapping table to include the expected ID type.

Revoked certificate still authenticates

Confirm CRL/OCSP is enabled and reachable. Verify the server’s OCSP responder URL or CRL distribution point is correct and that the server can fetch updates.

EAP failures with RADIUS

Enable verbose logging on the RADIUS server and check for username mapping mismatches. Ensure the server is returning the proper attributes (Framed-IP-Address, Tunnel-Type) for child SA creation.

Advanced options: policy-driven identity enforcement

For larger deployments, adopt policy-driven authorization:

  • Map certificate SAN values to LDAP attributes to authorize network access scopes (e.g., VLANs, internal subnets).
  • Use RADIUS reply attributes to enforce per-user resource constraints like split-tunnel policies or assigned IP pools.
  • Integrate with endpoint posture checks that verify device health before allowing full network access.

Ultimately, enforcing client identity in IKEv2 is about eliminating ambiguity and increasing assurance that each VPN session corresponds to an authorized, traceable entity. Adopt certificate-based authentication whenever possible, enable strict identity matching and revocation checks, and integrate with RADIUS/LDAP for scalable user management. Combine these controls with hardened crypto policies, robust logging, and operational procedures for revocation and auditing to maintain a resilient VPN environment.

For implementation guides, configuration templates and managed solution options, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/ for more resources and in-depth tutorials.