Introduction

Setting up a robust IKEv2 VPN server on Ubuntu is a practical approach for site operators, enterprises, and developers who need high-performance, secure remote access. IKEv2 (Internet Key Exchange version 2) paired with IPsec provides fast reconnection, native mobile support, and strong cryptography. This article walks through a complete, technically detailed deployment on Ubuntu, including certificate-based authentication, firewall configuration, system hardening, and client provisioning.

Prerequisites and system preparation

Before beginning, ensure you have:

  • A server running Ubuntu 20.04 or 22.04 LTS with a public IPv4 address and root (or sudo) access.
  • Basic familiarity with the Linux command line and network concepts (IP routing, NAT, firewall rules).
  • Ports UDP/500 and UDP/4500 open (for IKEv2 and NAT-T).

Update the system packages and install required components. You will need strongSwan (IPsec/IKEv2 implementation), libstrongswan plugins for certificate support, and utilities like iptables/iproute2.

Install and verify strongSwan

Install strongSwan and required plugins from Ubuntu repositories. The package set typically includes strongswan, strongswan-pki, and libcharon-extra-plugins. After installation, confirm the charon IKE daemon is available and systemctl can manage the service.

Key technical considerations

  • Use the latest stable strongSwan whenever possible for up-to-date cryptographic support and fixes.
  • Enable the right plugins (e.g., openssl, eap-mschapv2) depending on whether you plan to use certificate authentication, EAP, or hybrid setups.
  • Plan your IP addressing for VPN clients (a dedicated subnet avoids conflicts with local networks).

Generate a Public Key Infrastructure (PKI)

StrongSwan supports certificate-based authentication which is the recommended method for security and scalability. The PKI step creates a CA, a server certificate, and optionally client certificates. Use strongSwan’s pki tool to generate keys and certificates.

Important choices:

  • Key sizes and algorithms: Use at least 3072-bit RSA or preferably elliptic curve keys (e.g., prime256v1 or secp384r1) for better performance and smaller key sizes.
  • Validity periods: Balance operational convenience with security; typical CA validity can be 5–10 years, server/client certificates 1–3 years.
  • Certificate filenames and permissions: Keep private keys readable only by root.

Configure strongSwan (ipsec.conf and ipsec.secrets)

Main configuration occurs in /etc/ipsec.conf and /etc/ipsec.secrets. Use a modular configuration where possible. A minimum IKEv2 server config includes a connection definition using:

  • ike proposal (e.g., AES-GCM-256, CHACHA20_POLY1305 for ESP, and ECDH groups like 21/29).
  • dpd and rekey policies tuned to your environment.
  • left/right values: left usually server public IP, right=%any to accept clients from any IP.

Example configuration decisions:

  • Use IKEv2 only: set keyexchange=ikev2 to avoid legacy IKEv1.
  • Tunnel mode: choose whether to push a routed subnet or allow full tunneling; use rightsubnet=0.0.0.0/0 for full tunnel.
  • Authentication: reference the server certificate in ipsec.conf (leftcert=server-cert.pem) and manage client credentials in ipsec.secrets.

Kernel/networking and NAT configuration

Enable IP forwarding at the kernel level to allow traffic from VPN clients to traverse the server into the public network. Persist the setting via sysctl (/etc/sysctl.conf or /etc/sysctl.d/99-sysctl.conf): net.ipv4.ip_forward=1.

Set up NAT for outbound traffic from the VPN subnet if clients should access the Internet through the server. With nftables or iptables, create MASQUERADE rules targeting the server’s external interface. For example:

  • iptables -t nat -A POSTROUTING -s -o -j MASQUERADE
  • If using nftables, add an equivalent postrouting masquerade rule in the nat table.

Ensure sysctl protections like rp_filter are tuned appropriately (typically set to 0 or 1 depending on your routing design; test thoroughly to avoid dropped packets).

Firewall hardening

Carefully configure the server firewall to allow necessary IKEv2/IPsec traffic and SSH, and to drop everything else by default. Required ports:

  • UDP 500 (IKE)
  • UDP 4500 (NAT-T)
  • ESP (protocol 50) if not relying solely on NAT-T (when traversing NAT, ESP is encapsulated)

Example best practices:

  • Allow inbound IKE and NAT-T from known peers where possible (e.g., corporate office IP ranges).
  • Lock down management ports like SSH to trusted admin addresses or use key-based auth and non-standard port with rate limiting.
  • Log and monitor dropped packets to detect misconfigurations or probing attempts.

Client provisioning and profiles

For certificate-based authentication, generate a client certificate and export it in a client-friendly format (PKCS#12) containing the private key and certificate chain. For EAP with username/password, configure strongSwan to accept EAP-MSCHAPv2 and store credentials securely in ipsec.secrets or use an external RADIUS backend for centralized authentication.

Client configuration tips:

  • Provide explicit instructions for different OS clients (Windows built-in VPN client supports IKEv2, macOS/iOS have native support, Linux clients use strongSwan or NetworkManager). Include the CA certificate and client PKCS#12 bundle where applicable.
  • On mobile, enable “Connect on Demand” and trust the certificate chain; IKEv2’s MOBIKE support ensures seamless roaming.
  • For enterprise scale, automate provisioning via MDM or configuration management tools.

Testing and verification

After starting strongSwan (systemctl enable –now strongswan), validate the server side with:

  • systemctl status strongswan or journalctl -u strongswan to inspect logs.
  • sudo ipsec statusall to list active connections and charon state.
  • tcpdump -n -i udp port 500 or port 4500 and ESP protocol filters to observe IKE negotiations and NAT-T traffic.

From a client, test authentication, IP assignment, and routing. Verify DNS resolution and Internet access if full tunnel is enabled. Pay attention to split tunneling requirements and push appropriate DNS routes if clients must use internal name resolution.

Troubleshooting common issues

Typical issues and diagnostic pointers:

  • Authentication failures: Check certificate chains, subjectAltName entries, and certificate validity periods. Confirm the server certificate matches the IP or DNS name the client expects.
  • NAT traversal problems: Ensure UDP/4500 is reachable and NAT devices do not drop fragmented UDP packets. Enable path MTU discovery tuning or reduce IKE/ESP payload sizes.
  • No traffic through tunnel: Verify IP forwarding, NAT rules, and that iptables/nftables chains permit forwarding between the VPN interface and the external interface.
  • Intermittent disconnects: Tune DPD (dead peer detection), keepalive settings, and consider using MOBIKE settings for mobile clients.

Security hardening and maintenance

Operational security should be a continuous process. Key hardening measures include:

  • Rotate keys and certificates on a defined schedule and revoke compromised client certificates by maintaining a CRL or OCSP responder if your deployment requires revocation checks.
  • Limit access with firewall rules and per-client policies; use connection limits or per-user policy enforcement via strongSwan’s vici/ctrl interface or RADIUS attributes.
  • Enable logging and monitoring for authentication attempts, unusual connection patterns, or brute-force attempts. Integrate logs into your SIEM for long-term analysis.
  • Minimize attack surface by disabling unused plugins and services, applying kernel hardening, and keeping the OS and strongSwan packages patched.

Scaling and high availability

For enterprise deployments, consider redundancy and load distribution strategies:

  • Active/passive HA: Use floating IP and configuration synchronization to failover to a standby server.
  • Load balancing: Distribute client connections across multiple IKEv2 servers using DNS round-robin or dedicated load balancers that support UDP-based VPN traffic.
  • Centralized authentication: Offload user credentials and authorization to RADIUS or LDAP for consistent policy enforcement across servers.

Wrap-up and further reading

Deploying an IKEv2 VPN on Ubuntu delivers strong security and modern features suitable for site-to-site and remote-access use cases. The critical pieces are proper PKI management, secure strongSwan configuration, robust firewall/NAT rules, and ongoing maintenance. Test thoroughly in staging before rolling out to production, and automate certificate management and configuration drift control where possible.

For additional resources and examples, consult the strongSwan documentation and Ubuntu network administration guides. For assisted hosting or dedicated IP configurations, you can explore solutions tailored to managed VPN services.

Published by Dedicated-IP-VPN