Deploying an L2TP VPN backed by a proper Certificate Authority (CA) enhances authentication and secures the IKE/IPsec tunnel used in L2TP/IPsec. This guide walks you through setting up a private CA, issuing server and client certificates, integrating them with IPsec (strongSwan example), configuring xl2tpd/PPP, and implementing certificate revocation and best practices. The instructions assume a Linux server (Debian/Ubuntu/CentOS) and root or sudo access.
Why use an internal CA for L2TP/IPsec?
Using X.509 certificates provides several advantages over pre-shared keys (PSKs):
- Stronger authentication and non-repudiation via public key cryptography.
- Per-client certificates allow granular revocation and key management.
- Scalable and auditable distribution for enterprise environments.
- Compatibility with IKEv2 and modern IPsec stacks; can be integrated with existing PKI.
High-level workflow
- Install required packages (OpenSSL, strongSwan, xl2tpd, ppp).
- Create your CA (private key + self-signed certificate).
- Generate server key and CSR, sign with CA to get server cert.
- Generate client keys and client certificates signed by CA.
- Configure strongSwan to use certificates for IKE, enable proper IKE/ESP algorithms.
- Configure xl2tpd and PPP to accept authenticated clients.
- Harden, test, and implement CRL/OCSP for revocation.
1. Installing packages
On Debian/Ubuntu: apt update && apt install -y strongswan xl2tpd ppp openssl. On CentOS/RHEL: yum install -y strongswan xl2tpd ppp openssl. Confirm versions and kernel support for IPsec modules (esp, ah, xfrm).
2. Building a simple private CA with OpenSSL
Create a directory for the CA and initialize files:
mkdir -p /etc/ipsec/certs /etc/ipsec/private /root/ca && cd /root/ca
Create an openssl.cnf or use system default; a minimal command to create a CA key and self-signed cert:
openssl genrsa -out private/ca.key.pem 4096
openssl req -x509 -new -nodes -key private/ca.key.pem -sha256 -days 3650 -out certs/ca.cert.pem -subj “/C=US/ST=State/L=City/O=YourOrg/OU=VPN/CN=YourVPN-CA”
Keep private/ca.key.pem highly protected. Consider hardware security module (HSM) or at-rest encryption for production.
3. Issuing the server certificate
Generate the server key and CSR. The Common Name (CN) should match the VPN server public name or IP. Include Subject Alternative Names (SAN) for multiple hostnames/IPs using a config file.
openssl genrsa -out private/server.key.pem 4096
Create a CSR:
openssl req -new -key private/server.key.pem -out csr/server.csr.pem -subj “/C=US/ST=State/L=City/O=YourOrg/OU=VPN/CN=vpn.example.com”
Sign the CSR with CA:
openssl x509 -req -in csr/server.csr.pem -CA certs/ca.cert.pem -CAkey private/ca.key.pem -CAcreateserial -out certs/server.cert.pem -days 1825 -sha256
Verify:
openssl verify -CAfile certs/ca.cert.pem certs/server.cert.pem
4. Issuing client certificates
Create per-device keys and CSRs. Example for client1:
openssl genrsa -out private/client1.key.pem 2048
openssl req -new -key private/client1.key.pem -out csr/client1.csr.pem -subj “/C=US/ST=State/L=City/O=YourOrg/OU=VPN/CN=client1@example.com”
Sign:
openssl x509 -req -in csr/client1.csr.pem -CA certs/ca.cert.pem -CAkey private/ca.key.pem -CAcreateserial -out certs/client1.cert.pem -days 825 -sha256
For client distribution, provide the client1.key.pem and client1.cert.pem and the CA cert (ca.cert.pem). Use secure channels for transporting keys (encrypted ZIP + passphrase, secure file transfer, or device provisioning systems).
5. Enabling revocation: CRL
Maintain a Certificate Revocation List (CRL) so servers can reject revoked client certificates. Create CRL initial file:
openssl ca -config openssl.cnf -gencrl -keyfile private/ca.key.pem -cert certs/ca.cert.pem -out certs/ca.crl.pem
When revoking:
openssl ca -config openssl.cnf -revoke certs/client1.cert.pem -keyfile private/ca.key.pem -cert certs/ca.cert.pem
Then regenerate CRL:
openssl ca -config openssl.cnf -gencrl -keyfile private/ca.key.pem -cert certs/ca.cert.pem -out certs/ca.crl.pem
Place ca.crl.pem where strongSwan can access it and configure frequent updates.
6. strongSwan configuration (IKEv1/IPsec for L2TP)
For L2TP/IPsec you typically use IKEv1 with certificates (or IKEv2 with EAP). Example minimal /etc/ipsec.conf:
config setup
uniqueids=no
conn L2TP-PSK-NAT
left=%any
leftcert=server.cert.pem
leftsendcert=always
leftsubnet=0.0.0.0/0
right=%any
rightprotoport=17/1701
auto=add
keyexchange=ikev1
ike=aes256-sha2_256-modp2048
esp=aes256-sha2_256
Place your certificates and CA in /etc/ipsec.d/certs and keys in /etc/ipsec.d/private. Update /etc/ipsec.secrets to reference private keys only if needed; with certificate auth, no PSK required for server identity. Restart strongSwan: systemctl restart strongswan.
7. xl2tpd and PPP configuration
Configure /etc/xl2tpd/xl2tpd.conf with a LAC/LNS section. Example /etc/ppp/options.xl2tpd:
- require-mschap-v2
- ms-dns (your DNS)
- name l2tpd
- refuse-eap
Use /etc/ppp/chap-secrets or better integrate RADIUS for user management. L2TP handles PPP authentication after IPsec is established; certificate auth is used for IPsec phase1.
8. Firewall and NAT
Open UDP 500 and 4500 for IKE and NAT-T, and allow UDP 1701 for L2TP if not NATed. Example iptables/NFT rules:
- Allow UDP 500/4500 and UDP 1701 (or accept established related).
- Enable IP forwarding: sysctl -w net.ipv4.ip_forward=1.
- Add NAT MASQUERADE if clients need internet access: iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE.
9. Client configuration
On clients, install the CA certificate as a trusted root for the VPN profile. Configure the VPN to use certificate-based authentication and supply the client certificate and private key. For Windows built-in L2TP/IPsec, certificate support is available; macOS and Linux (NetworkManager) support certificate-based L2TP/IPsec profiles. Ensure private keys are stored securely (PKCS#12 import with passphrase recommended).
Creating PKCS#12 for client distribution
Bundle key and certificate into a secure PKCS#12 (.p12) file:
openssl pkcs12 -export -inkey private/client1.key.pem -in certs/client1.cert.pem -name “client1” -certfile certs/ca.cert.pem -out client1.p12
Protect this file with a strong passphrase and distribute via secure channels.
10. Testing and troubleshooting
- Check strongSwan logs: journalctl -u strongswan -f or tail -f /var/log/syslog.
- Verify certificate chains: openssl verify -CAfile certs/ca.cert.pem certs/client1.cert.pem.
- Confirm IKE/ESP SA via strongSwan’s ipsec status output: ipsec statusall.
- If phase1 fails, confirm correct CN/SAN and that certificates are readable by the IPsec process.
11. Security hardening and best practices
- Use strong cryptography: AES-256, SHA-2 families, and at least 2048-bit RSA or better (EC keys preferred where supported).
- Limit CA access: store CA private key offline and sign CSRs from a hardened signing host.
- Short certificate lifetimes for clients and frequent CRL updates reduce exposure on key compromise.
- Use OCSP or CRL distribution points in certs so servers can check revocation.
- Monitor logs and implement alerting for repeated failed auth attempts.
- Consider automated provisioning systems (SCEP/EST) for large fleets.
12. Maintenance: rotating keys and certificate renewal
Plan certificate rotation: renew server cert before expiry and deploy to all nodes with minimal downtime. For clients, implement an automated renewal workflow if possible. Maintain an audit log of issued and revoked certificates.
Implementing a CA and certificate-based authentication for L2TP/IPsec yields a robust and manageable VPN solution suited for businesses and development environments. The approach above covers the core technical steps—from CA creation, issuing certs, integrating with strongSwan and xl2tpd, to operational practices like CRLs and key rotation. For enterprise-scale deployments, integrate with existing PKI, use hardware-backed keys, and automate certificate lifecycle management.
For additional setup examples, client configuration tips, and managed dedicated IP solutions, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.