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Install a strongSwan IPSec VPN Server on CentOS 7

Author: Asineth Martin

Last Updated: Tue, Aug 24, 2021
CentOS Security

Introduction

strongSwan is a cross-platform IPSec-based VPN solution that implements the IKEv1 and IKEv2 protocols for key exchange, IPv4 and IPv6 support, and authentication with X.509 certificates. This guide explains how to install strongSwan on CentOS 7. When following this guide, you should replace the domain name vpn.example.com with your server's fully-qualified domain name.

Prerequisites

1. Configure Networking

  1. Ports 80/TCP, 500/UDP, and 4500/UDP must be open in the firewall for strongSwan. Add these ports as shown:

    $ sudo firewall-cmd --permanent --add-port=80/tcp
    $ sudo firewall-cmd --permanent --add-port=500/udp
    $ sudo firewall-cmd --permanent --add-port=4500/udp
    
  2. Allow NAT packet forwarding, also known as IP masquerade.

    $ sudo firewall-cmd --permanent --add-masquerade
    
  3. Reload the firewall to apply the changes.

    $ sudo firewall-cmd --reload
    
  4. Enable IP packet forwarding in the kernel options.

    $ echo "net.ipv4.ip_forward=1" | sudo tee /etc/sysctl.conf
    $ sudo sysctl -p
    

2. Add the EPEL Repository and Snap

  1. Install the Extra Packages for Enterprise Linux (EPEL) repository, which contains packages required for strongSwan and Let's Encrypt.

    $ sudo yum install -y epel-release
    
  2. Install Snap, which is required for Let's Encrypt.

    $ sudo yum install snapd
    
  3. Enable the systemd unit that manages the main snap communication socket.

    $ sudo systemctl enable --now snapd.socket
    
  4. Create a symbolic link between /var/lib/snapd/snap and /snap to enable classic Snap support.

    $ sudo ln -s /var/lib/snapd/snap /snap
    
  5. Reboot the server to activate Snap.

    $ sudo reboot
    
  6. Reconnect to the server via SSH as your non-root user account.

  7. Update Snap.

    $ sudo snap install core
    $ sudo snap refresh core
    

3. Install a Let's Encrypt Certificate

  1. Install the Let's Encrypt Certbot utility. By using a Let's Encrypt certificate instead of a self-signed one, the clients do not need to import certificates on their devices manually.

    $ sudo snap install --classic certbot
    
  2. Link Certbot to /usr/bin.

    $ sudo ln -s /snap/bin/certbot /usr/bin/certbot
    
  3. Request a new certificate.

    Certbot requires a fully qualified domain name that points to your server's IP address, as described before in the Prerequisites section. Replace user@example.com with your email address and vpn.example.com with your domain name.

    $ sudo certbot certonly --standalone --agree-tos --no-eff-email -m user@example.com -d vpn.example.com
    
  4. Certbot installs automatic renewal scripts for your system. You can verify the automatic renewal with this command:

    $ sudo certbot renew --dry-run
    

4. Install strongSwan

  1. Install the strongSwan package.

    $ sudo yum install -y strongswan
    
  2. Link the TLS certificate and key from Certbot for use with strongSwan. Make sure to replace vpn.example.com with your domain name.

    $ sudo ln -s /etc/letsencrypt/live/vpn.example.com/fullchain.pem /etc/strongswan/ipsec.d/certs
    $ sudo ln -s /etc/letsencrypt/live/vpn.example.com/privkey.pem /etc/strongswan/ipsec.d/private
    $ sudo ln -s /etc/letsencrypt/live/vpn.example.com/chain.pem /etc/strongswan/ipsec.d/cacerts
    
  3. Move the default strongSwan configuration files to a backup location.

    $ sudo mv /etc/strongswan/ipsec.conf{,.old}
    $ sudo mv /etc/strongswan/ipsec.secrets{,.old}
    
  4. Create a new strongSwan configuration file.

    $ sudo nano /etc/strongswan/ipsec.conf
    
  5. Paste the following lines. Replace vpn.example.com with your server's domain name.

    config setup
            strictcrlpolicy=yes
            uniqueids=never
    
    conn ikev2
            auto=add
            keyexchange=ikev2
            forceencaps=yes
            ike=chacha20poly1305-sha512-curve25519-prfsha512,aes256gcm16-sha384-prfsha384-ecp384,aes256-sha1-modp1024,aes128-sha1-modp1024,3des-sha1-modp1024!
            esp=chacha20poly1305-sha512,aes256gcm16-ecp384,aes256-sha256,aes256-sha1,3des-sha1!
            dpdaction=clear
            dpddelay=300s
            rekey=no
            left=%any
            leftid=@vpn.example.com
            leftcert=fullchain.pem
            leftsendcert=always
            leftsubnet=0.0.0.0/0
            right=%any
            rightid=%any
            rightauth=eap-mschapv2
            rightdns=8.8.8.8,8.8.4.4
            rightsourceip=10.8.0.0/16
            rightsendcert=never
            eap_identity=%identity
    
  6. Save and exit the file.

  7. Create the secrets file for strongSwan.

    $ sudo nano /etc/strongswan/ipsec.secrets
    
  8. Insert the following lines. Replace user1, user2, user3, and password with the desired credentials to access the VPN.

    : RSA "privkey.pem"
    user1 : EAP "password"
    user2 : EAP "password"
    user3 : EAP "password"
    

    You can create as many user/password lines as needed. Delete a line to revoke access for a user account.

  9. Save and exit the file.

  10. Start strongSwan and enable the service to start at boot.

    $ sudo systemctl enable --now strongswan
    

This completes the server configuration.

Client Connections

To connect to your new strongSwan server, choose the instructions for your client operating system.

Windows

  1. Open Settings.
  2. Select Network & internet.
  3. Select VPN.
  4. Select Add a VPN configuration.
  5. Configure the following settings:

    • VPN provider: Windows (built-in)
    • Connection name: (any string)
    • Server name or address: (fully-qualified domain name)
    • VPN type: IKEv2
    • Type of sign-in info: User name and password
    • User name: (username)
    • Password: (password)
  6. Click Save.

  7. Click your VPN configuration.
  8. Click Connect.

macOS

  1. Open System Preferences.
  2. Select Network.
  3. Add a new service with the + icon.
  4. Configure the following settings:

    • Interface: VPN
    • VPN Type: IKEv2
    • Service Name (any string)
  5. Click Create.

  6. Configure the following settings:

    • Server Address: (fully-qualified domain name)
    • Remote ID: (fully-qualified domain name)
    • Local ID: (leave blank)
  7. Click Authentication Settings.

  8. Enter your username and password.
  9. Click OK.
  10. Click Connect.

iOS

  1. Open Settings.
  2. Select VPN.
  3. Select Add VPN Configuration.
  4. Configure the following settings:

    • Type: IKEv2
    • Description: (can be any string)
    • Server: (fully-qualified domain name)
    • Remote ID: (fully-qualified domain name)
    • Local ID: (leave blank)
    • User Authentication: Username
    • Username: (username)
    • Password: (password)
    • Proxy: Off
  5. Select Done.

  6. Tap on the switch to connect or disconnect.

Android

  1. Open the Play Store.
  2. Search and install "strongSwan VPN Client".
  3. Launch the app.
  4. Select Add VPN Profile.
  5. Configure the following settings:

    • Server: (fully-qualified domain name)
    • VPN Type: IKEv2
    • Username: (username)
    • Password: (password)
    • CA certificate: (Select automatically)
    • Profile name: (any string)
    • Server identity: (fully-qualified domain name)
  6. Select Save.

  7. Select Connect.

More Information

For more information, please see the strongSwan documentation.

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