Install and Configure a StrongSwan Gateway VPN Server on Ubuntu 20.04

StrongSwan is an open-source tool that operates as a keying daemon and uses the Internet Key Exchange protocols (IKEv1 and IKEv2) to secure connections between two hosts. In this way, you can use StrongSwan to establish a Virtual Private Network (VPN). VPN connections from a client to the StrongSwan server are encrypted and provide a secure gateway to other resources available on the server and its network. This guide shows you how to install and configure a StrongSwan gateway VPN server on Ubuntu 20.04. You also learn how to set up and connect to a StrongSwan server from an Ubuntu, Windows, and macOS client.

Install StrongSwan on Ubuntu 20.04 Server

The steps in this section show you how to install and configure a StrongSwan gateway VPN server on Ubuntu 20.04. See the Install and Configure the StrongSwan Client section if you have already installed and configured the StrongSwan server.

Prerequisites

1. Deploy an Ubuntu 20.04 server and follow our Getting Started with Linode guide and complete the steps for setting your Linode’s hostname and timezone.

2. This guide uses sudo wherever possible. Complete the sections of our Securing Your Server guide to create a standard user account, harden SSH access, and remove unnecessary network services.

3. Update your system:

    sudo apt-get update && sudo apt-get upgrade
   

Install StrongSwan

1. SSH into your Ubuntu 20.04 server.

2. Use APT to install StrongSwan and the supporting plugins and libraries.

    sudo apt install strongswan strongswan-pki libcharon-extra-plugins libcharon-extauth-plugins libstrongswan-extra-plugins libtss2-tcti-tabrmd0 -y
   

Generate Server Keys and Certificate

1. Use the IPsec command-line utility to create your IPsec private key. In the case of this tutorial, the private key is used to create the root certificate for StrongSwan. You can also use this key to generate other certificates.

    sudo ipsec pki --gen --size 4096 --type rsa --outform pem > /etc/ipsec.d/private/ca.key.pem
   

2. Create and sign the root certificate with the configurations included below. Ensure you replace the value of the CN configuration with your own desired name for your StrongSwan VPN server.

    ipsec pki --self --in /etc/ipsec.d/private/ca.key.pem --type rsa --dn "CN=<Name of this VPN Server>" --ca --lifetime 3650 --outform pem > /etc/ipsec.d/cacerts/ca.cert.pem
   

   In the example above, the --lifetime 3650 configuration sets the certificate’s lifetime to 3650 days or approximately ten years. The lifetime of the certificate determines when it is to be regenerated and distributed to your StrongSwan server and connected clients. You can adjust this setting to your preferred value.

3. Generate the StrongSwan VPN server’s private certificate.

    ipsec pki --gen --size 4096 --type rsa --outform pem > /etc/ipsec.d/private/server.key.pem
   

4. Generate the host server certificate. There are two ways to generate the certificate, however, they cannot be mixed. The two ways are as follows:

   • Use a local resolver, like DNS, your hosts’ file, or another resolver.
   • Use a static host gateway server by providing its IPv4 address.

   Local Resolver Method The example below uses a local resolver. The IPsec utility takes the server key from step 2 and uses it as an input private certificate source, and generates a resolver-based certificate. Ensure you replace the value of CN and san with your own. The --dn “CN=<serverhost.ourdomain.tld> is a DNS or /etc/hosts call that should be changed to reflect your organization’s own hostname.

    ipsec pki --pub --in /etc/ipsec.d/private/server.key.pem --type rsa | ipsec pki --issue --lifetime 3650 --cacert /etc/ipsec.d/cacerts/ca.cert.pem --cakey /etc/ipsec.d/private/ca.key.pem --dn "CN=<serverhost.ourdomain.tld>" --san="<server.ourdomain.tld>" --flag serverAuth --flag ikeIntermediate --outform pem > /etc/ipsec.d/certs/server.cert.pem
   

   Gateway Server IPv4 Address

   The duplicate –san=”<server static IP address> configuration in the command below is correct; do not omit both configurations. Replace their values with your own gateway server’s IPv4 address.

    ipsec pki --pub --in /etc/ipsec.d/private/server.key.pem --type rsa | ipsec pki --issue --lifetime 3650 --cacert /etc/ipsec.d/cacerts/ca.cert.pem --cakey /etc/ipsec.d/private/ca.key.pem --dn "CN=<server static IP address>" –san=”<server static IP address>” --san="<server static IP address>" --flag serverAuth --flag ikeIntermediate --outform pem > /etc/ipsec.d/certs/server.cert.pem
   

At the end of this section, you should have generated the following files on your Ubuntu 20.04 server:

StrongSwan Configurations

1. The Linux kernel aids in packet forwarding between internal and external interfaces, but this is disabled by default in Ubuntu 20.04.

   Use your preferred text editor to edit your /etc/sysctl.conf file. The configurations to add enable packet forwarding for IPsec and StrongSwan on your Ubuntu system. Ensure the configurations displayed below are uncommented.

   File: /etc/sysctl.conf

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   net.ipv4.ip_forward=1 net.ipv6.conf.all.forwarding=1 net.ipv4.conf.all.accept_redirects = 0 net.ipv6.conf.all.accept_redirects = 0

2. Configure the StrongSwan file. Open your /etc/ipsec.conf file and add the configurations included in the example file below.

   Within the context of StrongSwan, the gateway host server (your Ubuntu server) is referred to as left resources. External hosts connecting to the StrongSwan VPN are referred to as right resources.

   File: /etc/ipsec.conf

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   config setup charondebug="ike 1, knl 1, cfg 0, net 1" strictcrlpolicy=no uniqueids=yes cachecrls=no conn ipsec-ikev2-vpn auto=add compress=no type=tunnel keyexchange=ikev2 fragmentation=yes forceencaps=yes dpdaction=clear dpddelay=300s rekey=no left=%any leftid=@vpn.domain.com #<Note 1> leftcert=server.cert.pem leftsendcert=always leftsubnet=0.0.0.0/0 right=%any rightid=%any rightauth=eap-mschapv2 rightsourceip=0.0.0.0/0 #<Note 2> rightdns= #<preferred external DNS server - Note 3> rightsendcert=never eap_identity=%identity 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!

   • The leftid configuration matches the tunneled network assets that are exposed to VPN clients. A route through this subnet must be reachable if a local resolver is used to access resources.

     The syntax for leftid must match the server certificate, resolver/DNS or IP address from step 4 in the Generate Server Keys and Certificate section. If the resolver/DNS method was used, place an @ before the resolved host address. Do not place an @ symbol in front of an IPv4 address.

   • The rightsourceip configuration sets the client IP addresses that are allowed to connect to the StrongSwan VPN. It is possible to limit the scope to an IP address range. This limits the number of addresses that are admitted through the tunnel created by the host server VPN gateway. If the source addresses should only be allowed from a single subnet, specify that subnet. An example would be 10.0.100.0/24. This subnet allows the 254 hosts in the 10.0.100.0 subnet. This configuration is used for internal VPN resource admittance control.

   • The rightdns value may correspond to a public server’s IPv4 address. You can also use a private DNS server address for clients to use DNS or hostname resolution. Networks using a local resolver must specify the desired resolver rightdns IPv4 address, otherwise queries made to the local tunneled resources fail.

3. Create authentication and access secrets. Access control and authentication require that StrongSwan clients provide a username and password. This information is contained in the /etc/ipsec.secrets file.

   Using a text editor, create a the /etc/ipsec.secrets file with the following contents:

   File: /etc/ipsec.secrets

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   : RSA "/etc/ipsec.d/private/server.key.pem" username : EAP "<user’s password>" another_username : EAP "<user’s password>"

   Note

   Make sure that you use unique usernames each time you add a new user to the access secrets file.

Your StrongSwan server is now ready to receive client connections. To check the status of the IPsec tunnel created by StrongSwan, use the following command:

sudo ipsec status

Install and Configure the StrongSwan Client

This section shows you how to install the StrongSwan client. The StrongSwan client is used to connect to a StrongSwan server. Ensure you have your StrongSwan server’s access credentials ready before beginning the steps corresponding to your computer’s operating system.

Connecting to StrongSwan VPN on Ubuntu

StrongSwan should be installed on Linux systems using Ubuntu 16.04. Older versions require moderate or extensive updates that may break other installed applications.

1. Update your Ubuntu system.

    sudo apt update && upgrade -y
   

2. Install the StrongSwan client and required plugins.

    sudo apt install strongswan libcharon-extra-plugins
   

3. Download or copy the StrongSwan host gateway VPN server’s certificate. The certificate is located on the VPN server in /etc/ipsec.d/cacerts/ca.cert.pem. Store the copied or downloaded certificate in the client’s /etc/ipsec.d/ directory.

4. Add the IPsec secrets file to the StrongSwan client. Using a text editor, add the /etc/ipsec.secrets file. The credentials for this user must exactly match those created on the StrongSwan VPN server.

   File: /etc/ipsec.secrets

   1 2	<username> : "<password>"

   If the username or password are changed in the StrongSwan VPN server, then the client’s secret file must be updated as well.

5. Create or modify the /etc/ipsec.conf configuration file. The file can be configured to support a host gateway VPN server configured for a resolver/DNS or to support access via an IPv4 address. Refer to the example configuration below that corresponds to your StrongSwan VPN server.

   Resolver/DNS

   File: /etc/ipsec.conf

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   config setup conn ikev2-rw right=<qualified domain name and can also be left blank> rightid=<qualified domain of this host> rightsubnet=0.0.0.0/0 rightauth=pubkey leftsourceip=%config leftid=<username as matched in /etc/ipsec.secrets> leftauth=eap-mschapv2 eap_identity=%identity auto=start

   Server IPv4 Address

   File: /etc/ipsec.conf

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   config setup conn ikev2-rw right=<IP address of the host VPN> rightid=<host as named /etc/ipsec.conf> rightsubnet=0.0.0.0/0 rightauth=pubkey leftsourceip=%config leftid=<username as matched in /etc/ipsec.secrets> leftauth=eap-mschapv2 eap_identity=%identity auto=start

6. To start the StrongSwan client VPN, use the following command:

    systemctl start strongswan-starter
   

7. Verify the StrongSwan connection from the client to server, use the following command:

    sudo ipsec status
   

If needed, the commands below show you how to start and stop StrongSwan using systemctl.

• To automatically start the VPN client after all reboots, use the following command:

    systemctl enable strongswan-starter
  

• To stop StrongSwan use the following command:

   systemctl stop strongswan-starter
  

Connecting to StrongSwan VPN on Windows 10

Importing the VPN Root Certificate on Windows 10

To connect to a StrongSwan VPN gateway server, your Windows 10 system needs a copy of the gateway VPN server’s certificate.

• Import the VPN gateway server’s certificate that is located in /etc/ipsec.d/certs/server.cert.pem. The certificate must be marked as a VPN Root Certificate.

• Use the Microsoft Management Console/MMC to configure the VPN’s IPsec information.

1. Open the Run dialog box, (Windows_key-R), or press the Windows key, and enter into the lower-left dialog box, mmc.exe. This starts the Microsoft Management Console/MMC.

2. From the File menu of the MMC, scroll to Add or Remove Snap-in. Select Certificates from the list, and click Add.

   The Snap-in asks for the account type to manage. From the list that appears, choose Computer account.

   Then, choose Local Compute unless you manage other computers that also use this certificate. Click Finish, and the process is completed.

3. The Console Root MMC displays a list of certificate types on the left side of the MMC, and in the middle, a list of certificates pertaining to the selection on the left.

   On the left of the MMC, open Trusted Root Certificate Authorities, then click the Certificates folder that appears directly under Trusted Root Certificate Authorities.

4. From the MMC Action menu, choose All Tasks, then Import. The Certificate Import Wizard appears. Choose Local Machine, then browse to the location where the server.cert.pem file was imported, and select it.

5. The Certificate Import Wizard asks where to import the certificate. The wizard recognizes the type, and places the certificate into the Trusted Root Certification Authorities certificate store. Click Finish to complete the certificate import process.

Connecting a Windows Client to the StrongSwan Gateway VPN Server

The client authentication process relies on the ipsec.secrets file located on the gateway VPN server.

1. To configure a new VPN connection on your Windows computer, launch the Control Panel from the Windows menu by pressing the Windows key. Then, select Network and Sharing Center.

2. Choose Setup a new connection or network and then, select Connect to a workplace. Next, select Choose Use my Internet Connection (VPN).

   During this step, you need some details about your gateway VPN server. You should know the server’s DNS name if that’s how it was configured in the ipsec.conf file. If, however, you used an IPv4 address when configuring the leftid value in the ipsec.conf file, provide the server’s IPv4 address. Finally, you enter a username and password that matches the VPN server’s ipsec.secrets entry.

3. Start the VPN by clicking its name from the Taskbar Networks list of choices.

4. To terminate your VPN connection, click the VPN again and you have disconnected another network.

Connecting to StrongSwan VPN on macOS

Importing the VPN Root Certificate on macOS

1. Download the ca.cert.pem file from the StrongSwan gateway VPN server host to your macOS computer using scp.

2. Click on the downloaded file to open Keychain Access. Provide your user’s administrative password, to accept the certificate. Then, click Modify Keychain.

   A dialog appears that asks you about the certificate’s trust level. Choose IP Security (IPSec) to Always Trust*, and enter the macOS user password again.

Connecting a macOS Client to the StrongSwan Gateway VPN Server

1. Open Systems Preferences from your Finder. Click on the Network icon. Add a new network by clicking on the + button. You may be prompted to enter your user password again.

2. Once the new network choice appears, set the Interface to VPN. Then, set the VPN Type to IKEv2 and provide a name for this connection. Choose the name of the StrongSwan VPN server from the list.

   You are prompted to provide the server name. Depending on how the VPN server was configured, provide its DNS name or its IPv4 address.

   Provide the username and password configured in the VPN servers ipsec.secrets for the current user.

3. To start the VPN, click on the Network icon in the top-right menu bar and choose your StrongSwan VPN server’s name from the list.

   You can also start the connection from System Preferences > Network. Then, click on your StrongSwan VPN server’s name.

4. To disconnect, click the VPN server’s name. VPN connections are persistent on macOS during sleep mode, but not after a reboot.

Troubleshooting StrongSwan

• Connection problems are frequently due to mismatched username and passwords between the host gateway VPN server (/etc/ipsec.secrets) and the VPN client settings.

• Connection issues can also be caused by your firewall settings. Ensure you check your system’s firewall settings when troubleshooting.

• Finally, check your StrongSwan VPN server’s log file (/var/log/syslog) to further investigate connection issues.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

• Introduction to StrongSwan