Introduction

Managing L2TP/IPsec VPN access for hundreds or thousands of users is a recurring challenge for ISPs, enterprises, and hosting providers. Manual creation of ppp/chap-secrets or one-off configuration files quickly becomes untenable. This article lays out a practical, hands-free approach to L2TP user provisioning at scale, combining RADIUS-backed authentication, script-driven account lifecycle management, automated IP address allocation, and orchestration tooling for consistent, auditable deployments.

Why automation matters for L2TP VPN services

At scale, the problems with manual VPN user management are:

  • Human error in credentials and configuration leading to security incidents.
  • Poor auditability: hard to trace who provisioned or de-provisioned accounts and when.
  • Operational overhead when onboarding or offboarding many users rapidly.
  • Inability to integrate with existing identity sources (SSO/HR systems).

Automated provisioning addresses these by integrating VPN authentication with centralized identity stores, using APIs and orchestration to apply policies consistently, and enabling temporary or scheduled accounts without manual intervention.

Core architecture components

A reliable hands-free L2TP deployment typically includes the following components:

  • IPsec/L2TP Daemons — strongSwan or Libreswan for IPsec and xl2tpd for the L2TP layer.
  • Authentication Backend — FreeRADIUS with an SQL backend (MySQL/Postgres) or LDAP for storing user credentials and attributes.
  • Provisioning Orchestrator — Scripts, a REST API service, or an orchestration tool like Ansible/HashiCorp Nomad to create, modify, and delete user records and firewall rules.
  • IP Address Management (IPAM) — an IP pool system to allocate client IPs dynamically and avoid overlap; can be implemented with database tables and scripts or an external IPAM solution.
  • Logging, Monitoring, and Auditing — syslog, RADIUS accounting, and centralized logging (ELK/Prometheus) to track connections and aids troubleshooting.

Why FreeRADIUS + SQL is the recommended pattern

FreeRADIUS is battle-tested and supports various authentication types needed by L2TP/IPsec: PAP/CHAP for PPP, EAP for next-gen authentication, and accounting packets for session tracking. Storing users and attributes in SQL allows programmatic CRUD operations and easy integration with existing user directories. Using SQL also enables efficient queries to manage IP pools, concurrent-session limits, and expiration times.

Detailed provisioning workflow

Below is an end-to-end workflow for hands-free user provisioning:

  • User creation request arrives via API/portal/SSO connector.
  • Provisioning service creates a row in radius.radcheck (or equivalent), optionally in radreply for attributes like Framed-IP-Address.
  • Provisioner allocates a private client IP from the IPAM pool and writes that mapping to a sessions table and/or radreply attribute (Framed-IP-Address).
  • FreeRADIUS picks up the account (SQL config set to read from radcheck/radreply) and authenticates the user during login. Accounting records are written to radacct.
  • Orchestration pushes firewall rules / NAT / route policies to the VPN gateway(s) to ensure traffic for the client IP is handled correctly.
  • Provisioner triggers TTL-based logic for temporary accounts or schedules a job for deprovisioning.

Example SQL schema elements

Important tables include:

  • radcheck — stores username and password or hashed secret.
  • radreply — stores session attributes such as Framed-IP-Address, MS-CHAP-Reply etc.
  • radacct — stores accounting records for session start, stop, bytes, and duration.
  • ip_pool — custom table tracking available addresses, lease time, and owner username.

By writing Framed-IP-Address in radreply, you ensure deterministic IP assignment for the user. Alternatively, use DHCP-like lease allocation and dynamically write temporary Framed-IP-Address values when the session starts.

Automating IP allocation and collision avoidance

IP allocation must be atomic and resilient against race conditions. Implement atomic allocation using database transactions: select an available IP row FOR UPDATE, mark it as allocated to the user, and commit. This prevents two parallel provisioners from allocating the same IP.

When users are short-lived, implement TTL columns and a periodic sweeper that releases expired leases. The sweeper can also reconcile radacct active sessions against ip_pool to detect stale allocations.

Scaling across multiple gateways

In multi-gateway setups, the provisioner should:

  • Store a mapping of which gateway a user is allowed to connect to (optional).
  • Replicate IP pools across gateways or partition pools per gateway to avoid cross-gateway conflicts.
  • Use a central RADIUS cluster so any gateway can authenticate any user; RADIUS proxies are supported for geographic distribution.

Hands-free credential lifecycle

Full automation must also handle password rotation, expiration, and recovery without manual steps. Options include:

  • On-demand password generation accessible via secure API tokens. The API returns a one-time password or a generated pre-shared key (PSK) for IPsec, stored hashed in radcheck.
  • Time-bound credentials using an expiration attribute in radcheck and scheduled jobs to disable expired accounts.
  • Integration with SSO (SAML/OAuth) for identity assertion, combined with short-lived VPN credentials issued by the provisioning service.

IPsec PSK vs. User-level authentication

A common deployment uses a shared PSK for the IPsec phase and user credentials for PPP. For higher security, use certificate-based IPsec (strongSwan) to eliminate shared PSKs. Certificates can be issued and revoked programmatically using a PKI (e.g., easy-rsa, step-ca) and integrated into the provisioning lifecycle.

Orchestration and configuration management

Use tools like Ansible, Terraform, or custom REST APIs to manage the VPN server fleet. Typical automation tasks include:

  • Deploying updated strongSwan/ipsec.conf and ipsec.secrets based on templates and environment variables.
  • Reloading xl2tpd or strongSwan without interrupting existing sessions when configuration updates are required.
  • Distributing firewall/NAT rules in a consistent, idempotent way — for example using nftables/iptables state synchronization or centralized services that push rules via API to edge nodes.

Pro tip: avoid writing per-user entries to ipsec.secrets or ppp/chap-secrets for large user bases. Let FreeRADIUS handle authentication and keep the server config generic.

Monitoring, accounting, and troubleshooting

Collect and centralize logs and metrics from:

  • FreeRADIUS accounting (radacct) — session duration, bytes in/out.
  • strongSwan/ipsec logs — tunnel negotiation failures, SA lifetimes.
  • System metrics — CPU, memory, conntrack table sizes, network throughput.

Useful monitoring alerts include high connection churn, persistent authentication failures (possible brute force), and exhausted IP pools. Use dashboards to correlate RADIUS accounting and gateway metrics so you can quickly identify overloaded nodes or misconfigured clients.

Troubleshooting checklist

  • Verify IPsec SA: ensure phase 1 and phase 2 negotiates correctly (strongSwan logs).
  • Confirm FreeRADIUS authentication: check SQL queries and whether radreply attributes are sent.
  • Check IP allocation: verify ip_pool entries and ensure no duplicate Framed-IP-Address assignments.
  • Inspect firewall/NAT rules to ensure traffic from client IPs is permitted and SNAT/DNAT is correct.

Security considerations

Automation does not remove responsibility for secure defaults and defense-in-depth:

  • Store passwords and secrets hashed using strong algorithms (e.g., salted SHA-2) where supported. FreeRADIUS supports hashed secrets and MS-CHAP variants; when using PAP, secure transport and short-lived credentials are essential.
  • Protect the RADIUS backend: restrict database access, use least-privilege DB users, and enable TLS for any remote connections.
  • Rotate PSKs or certificates regularly and provide an automated revocation path for compromised credentials.
  • Rate-limit authentication attempts and monitor for brute-force patterns. Implement fail2ban-like rules for repeated failures.

Operational patterns for large deployments

For organizations operating many gateways or tens of thousands of users, adopt these patterns:

  • Stateless gateway design: Keep gateways as stateless as possible; use centralized RADIUS and IPAM so gateways can be replaced without complex migration.
  • Blue/green rollouts: Roll out new configuration templates to a subset of gateways and monitor metrics before full rollout.
  • Multi-tenant separation: Enforce per-tenant IP pools and rate limits. Use authorization attributes (e.g., Filter-Id) from radreply to apply tenant-level firewall policies.
  • Backup and reconciliation: Regularly reconcile ip_pool, radacct, and gateway state to detect orphaned leases or configuration drift.

Conclusion

Automating L2TP user provisioning at scale is achievable by combining FreeRADIUS with an SQL backend, a robust IPAM scheme, and a provisioning service that integrates with orchestration tools. Focus on atomic IP allocation, auditable lifecycle events, and operational telemetry to maintain reliability and security. Where possible, migrate away from shared secrets toward certificate-based IPsec and short-lived user credentials to reduce blast radius.

For implementation-ready examples, templates for FreeRADIUS SQL schemas, and orchestration playbooks tailored to common hosting environments, visit Dedicated-IP-VPN at https://dedicated-ip-vpn.com/.