Introduction

IKEv2 is a modern, resilient VPN protocol that delivers fast reconnection, robust security, and native support on most operating systems. For organizations and developers who require stronger trust controls than third-party certificate providers, deploying an IKEv2 VPN with a custom Certificate Authority (CA) is a common and secure approach. This guide walks you through the full life cycle: creating a custom CA, issuing server and client certificates, configuring a popular VPN server (strongSwan on Linux), and provisioning common clients. It assumes familiarity with the Linux command line, OpenSSL, and basic networking.

Why use a custom CA for IKEv2?

Using a custom CA provides several concrete advantages for businesses and developers:

  • Full trust control — You determine certificate issuance, validity periods, revocation, and key usage policies.
  • Improved security posture — Minimizes external dependency on third-party CAs and allows shorter, safer lifetimes and algorithm control (e.g., ECDSA vs RSA).
  • Operational flexibility — Automate client provisioning, revoke access quickly with CRLs or OCSP, and integrate with internal PKI workflows.

Prerequisites and security considerations

Before beginning, ensure the following:

  • A Linux server (Ubuntu/Debian/CentOS) with root or sudo access to run strongSwan and OpenSSL.
  • A static public IP or DNS name for the VPN endpoint.
  • Open ports: UDP 500 (IKE) and UDP 4500 (NAT-T). Also ensure IP forwarding and firewall NAT rules permit client traffic.
  • Strong cryptographic defaults: use modern curves (e.g., secp521r1 or prime256v1) or RSA 3072+, and set sensible lifetimes (e.g., CA 10 years, server 2-3 years, client 1 year by default).

Security tip: Keep the CA private key offline if possible. Perform signing on an isolated machine or HSM. Maintain a CRL and/or OCSP responder to revoke compromised client certs promptly.

Step 1 — Create a minimal custom CA with OpenSSL

We will create a self-signed root CA and use it to sign server and client certificates. Adjust CNs, subjects, and extensions to match your policy.

1) Generate CA private key (ECDSA recommended):

openssl ecparam -name prime256v1 -genkey -noout -out ca.key

2) Create a self-signed CA certificate (validity 10 years):

openssl req -x509 -new -nodes -key ca.key -sha256 -days 3650 -subj “/CN=MyCorp VPN Root CA/O=Example Corp/C=US” -out ca.crt

3) Prepare a config template for extensions (server/client) to set key usage and extended key usage appropriately. Example extension file can declare keyUsage and extendedKeyUsage for serverAuth and clientAuth. Keep these files backed up with your CA metadata.

Step 2 — Issue server certificate

Generate a server key and CSR, sign it with the CA, and include SubjectAltName (SAN) for your public IP or DNS.

1) Server key:

openssl genpkey -algorithm RSA -out server.key -pkeyopt rsa_keygen_bits:3072

2) CSR (replace vpn.example.com with your hostname):

openssl req -new -key server.key -subj “/CN=vpn.example.com/O=Example Corp” -out server.csr

3) Sign the server CSR with the CA (include SANs). Example using a temporary config file to add SANs:

openssl x509 -req -in server.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out server.crt -days 825 -sha256 -extfile <(printf "subjectAltName=DNS:vpn.example.com,IP:203.0.113.5")

Note: On systems without process substitution, create a file for extfile instead.

Step 3 — Issue client certificates

Client certificates are crucial for certificate-based IKEv2 authentication. Each client should have its own key pair and certificate to enable per-user revocation.

1) Client key (prefer ECDSA for smaller keys):

openssl ecparam -name prime256v1 -genkey -noout -out client1.key

2) Client CSR:

openssl req -new -key client1.key -subj “/CN=client1@example.com/O=Example Corp” -out client1.csr

3) Sign with the CA (clientAuth EKU):

openssl x509 -req -in client1.csr -CA ca.crt -CAkey ca.key -CAcreateserial -out client1.crt -days 365 -sha256 -extfile <(printf "extendedKeyUsage = clientAuth")

4) Package key and cert for client installation (e.g., PKCS#12 for Windows/macOS):

openssl pkcs12 -export -inkey client1.key -in client1.crt -certfile ca.crt -out client1.p12 -name “client1@example.com”

Step 4 — Install and configure strongSwan on the server

strongSwan is a mature IPsec/IKEv2 implementation. Below are the essentials for Ubuntu/Debian; adapt package names for other distros.

1) Install:

sudo apt update && sudo apt install strongswan strongswan-pki libcharon-standard-plugins

2) Place certificates and keys under /etc/ipsec.d/:

  • /etc/ipsec.d/private/server.key
  • /etc/ipsec.d/certs/server.crt
  • /etc/ipsec.d/cacerts/ca.crt

3) Update /etc/ipsec.conf with an IKEv2 connection block. Example minimal config (conceptual):

conn ikev2-vpn

left=%any

leftcert=server.crt

leftid=”vpn.example.com”

leftsubnet=0.0.0.0/0

right=%any

rightauth=pubkey

rightsourceip=10.10.10.0/24

ike=aes256gcm16-prfsha384-ecp521!

esp=aes256gcm16-ecp521!

keyexchange=ikev2

auto=add

Note: Use modern proposals (nonce and AEAD if possible) and force strong algorithms. The exclamation (!) forces exact match.

4) Enable IP forwarding and NAT masquerading (for clients to reach internet):

echo 1 | sudo tee /proc/sys/net/ipv4/ip_forward

iptables -t nat -A POSTROUTING -s 10.10.10.0/24 -o eth0 -j MASQUERADE

Persist ip_forward via /etc/sysctl.conf and use your distro’s firewall tooling to persist iptables rules (or use nftables).

5) Restart strongSwan:

sudo systemctl restart strongswan

Step 5 — Configure CRL and revocation handling

When using a custom CA you must maintain certificate revocation to remove compromised clients. Generate a CRL and serve it where the VPN server can fetch it or load it locally:

1) Create CRL:

openssl ca -config /etc/ssl/openssl.cnf -revoke client1.crt -keyfile ca.key -cert ca.crt

openssl ca -gencrl -out crl.pem -keyfile ca.key -cert ca.crt

2) Copy crl.pem to /etc/ipsec.d/crls/ and ensure strongSwan is configured to check CRLs for client validation.

Alternatively, implement an OCSP responder for real-time revocation checks if your environment requires faster response and lower operational windows.

Step 6 — Client setup examples

macOS and iOS (Built-in IKEv2)

Use a PKCS#12 bundle (.p12) that contains the client key and certificate, plus the root CA installed as trusted. On iOS, you can distribute the .p12 via MDM or secure download; install the profile and trust the CA. Create a new VPN configuration: type IKEv2, server = vpn.example.com, remote ID = vpn.example.com, local ID = client certificate CN. Set authentication to Certificate.

Windows 10/11

Import the .p12 into the Windows certificate store (Current User – Personal). Import the CA certificate into Trusted Root Certification Authorities. Create a new VPN connection (type = IKEv2) in Settings > Network & Internet > VPN. Under Security properties, choose “Use machine certificates” or user certificate depending on your config. For domain-joined endpoints, you can deploy certificates via GPO.

Linux (strongSwan or NetworkManager)

Use strongSwan’s swanctl or NetworkManager’s key files. For swanctl, install client cert/key into /etc/ipsec.d and configure swanctl.conf with local and remote auth using public key. For NetworkManager, import the .p12 and select IKEv2 with certificate authentication.

Operational tips and monitoring

  • Enable logging in strongSwan but keep log levels appropriate for production to avoid disk exhaustion. Use syslog or journald centralization.
  • Monitor certificate expirations. Automate reminders for renewals at least 30 days before expiry.
  • Use short client certificate lifetimes if you plan frequent rotation; keep CA lifetime longer but protect its key carefully.
  • Regularly review IKE/ESP proposals and keep cryptographic libraries up to date to mitigate newly discovered vulnerabilities.

Troubleshooting common issues

If clients fail to connect, check these items first:

  • Server logs: sudo journalctl -u strongswan or check /var/log/syslog for IKE errors.
  • Certificate chain issues: ensure the server presents the full chain (server cert + CA) if required by the client.
  • NAT traversal: ensure UDP 4500 is open and that strongSwan is configured to handle NAT-T.
  • IP forwarding and firewall rules: confirm traffic is forwarded and NATed correctly; verify routing table entries for client subnets.
  • CRL/OCSP: ensure revoked certificates are reflected if a client was revoked; misconfigured CRLs can block valid clients.

Conclusion

Deploying IKEv2 with a custom CA gives you granular trust control and the ability to manage client identities securely. By combining careful CA practices (offline key storage where possible, short client lifetimes, CRL/OCSP for revocation) with strong server-side cryptographic choices and correct firewall/NAT configuration, you can provide a robust VPN environment for employees, customers, or devices. Follow change control and automation practices for certificate issuance and revocation to scale securely.

For more in-depth tutorials, tool-specific examples, and managed dedicated-IP solutions, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/