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Configure strongSwan VPN for P2S certificate authentication IKEv2 connections - Linux

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This article helps you connect to your Azure virtual network (VNet) using VPN Gateway point-to-site (P2S) VPN and Certificate authentication from an Ubuntu Linux client using strongSwan.

Before you begin

Before beginning, verify that you are on the correct article. The following table shows the configuration articles available for Azure VPN Gateway P2S VPN clients. Steps differ, depending on the authentication type, tunnel type, and the client OS.

Authentication Tunnel type Client OS VPN client
Certificate
IKEv2, SSTP Windows Native VPN client
IKEv2 macOS Native VPN client
IKEv2 Linux strongSwan
OpenVPN Windows Azure VPN client
OpenVPN client version 2.x
OpenVPN client version 3.x
OpenVPN macOS OpenVPN client
OpenVPN iOS OpenVPN client
OpenVPN Linux Azure VPN Client
OpenVPN client
Microsoft Entra ID
OpenVPN Windows Azure VPN client
OpenVPN macOS Azure VPN Client
OpenVPN Linux Azure VPN Client

Prerequisites

This article assumes that you've already performed the following prerequisites:

Connection requirements

To connect to Azure using the strongSwan client and certificate authentication via IKEv2 tunnel type, each connecting client requires the following items:

  • Each client must be configured to use strongSwan.
  • The client must have the correct certificates installed locally.

Workflow

The workflow for this article is:

  1. Install strongSwan.
  2. View the VPN client profile configuration files contained in the VPN client profile configuration package that you generated.
  3. Locate any necessary client certificates.
  4. Configure strongSwan.
  5. Connect to Azure.

About certificates

For certificate authentication, a client certificate must be installed on each client computer. The client certificate you want to use must be exported with the private key, and must contain all certificates in the certification path. Additionally, for some configurations, you'll also need to install root certificate information.

For more information about certificates for Linux, see the following articles:

Install strongSwan

The following configuration was used when specifying commands:

  • Computer: Ubuntu Server 18.04
  • Dependencies: strongSwan

Use the following commands to install the required strongSwan configuration:

sudo apt-get update

sudo apt-get upgrade

sudo apt install strongswan

sudo apt install strongswan-pki

sudo apt install libstrongswan-extra-plugins

sudo apt install libtss2-tcti-tabrmd0

View VPN client profile configuration files

When you generate a VPN client profile configuration package, all the necessary configuration settings for VPN clients are contained in a VPN client profile configuration zip file. The VPN client profile configuration files are specific to the P2S VPN gateway configuration for the virtual network. If there are any changes to the P2S VPN configuration after you generate the files, such as changes to the VPN protocol type or authentication type, you need to generate new VPN client profile configuration files and apply the new configuration to all of the VPN clients that you want to connect.

Locate and unzip the VPN client profile configuration package you generated and downloaded. You can find all of the information that you need for configuration in the Generic folder. Azure doesn’t provide a mobileconfig file for this configuration.

If you don't see the Generic folder, check the following items, then generate the zip file again.

  • Check the tunnel type for your configuration. It's likely that IKEv2 wasn’t selected as a tunnel type.
  • On the VPN gateway, verify that the SKU isn’t Basic. The VPN Gateway Basic SKU doesn’t support IKEv2. Then, select IKEv2 and generate the zip file again to retrieve the Generic folder.

The Generic folder contains the following files:

  • VpnSettings.xml, which contains important settings like server address and tunnel type.
  • VpnServerRoot.cer, which contains the root certificate required to validate the Azure VPN gateway during P2S connection setup.

Configure the VPN client

After viewing the VPN client profile files, continue with the steps that you want to use:

GUI steps

This section walks you through the configuration using the strongSwan GUI. The following instructions were created on Ubuntu 18.0.4. Ubuntu 16.0.10 doesn’t support strongSwan GUI. If you want to use Ubuntu 16.0.10, you’ll have to use the command line. The following examples might not match screens that you see, depending on your version of Linux and strongSwan.

  1. Open the Terminal to install strongSwan and its Network Manager by running the command in the example.

    sudo apt install network-manager-strongswan

  2. Select Settings, then select Network. Select the + button to create a new connection.

    Screenshot shows the network connections page.

  3. Select IPsec/IKEv2 (strongSwan) from the menu, and double-click.

    Screenshot shows the Add VPN page.

  4. On the Add VPN page, add a name for your VPN connection.

    Screenshot shows Choose a connection type.

  5. Open the VpnSettings.xml file from the Generic folder contained in the downloaded VPN client profile configuration files. Find the tag called VpnServer and copy the name, beginning with 'azuregateway' and ending with '.cloudapp.net'.

    Screenshot shows copy data.

  6. Paste the name in the Address field of your new VPN connection in the Gateway section. Next, select the folder icon at the end of the Certificate field, browse to the Generic folder, and select the VpnServerRoot file.

  7. In the Client section of the connection, for Authentication, select Certificate/private key. For Certificate and Private key, choose the certificate and the private key that were created earlier. In Options, select Request an inner IP address. Then, select Add.

    Screenshot shows Request an inner IP address.

  8. Turn the connection On.

    Screenshot shows copy.

CLI steps

This section walks you through the configuration using the strongSwan CLI.

  1. From the VPN client profile configuration files Generic folder, copy or move the VpnServerRoot.cer to /etc/ipsec.d/cacerts.

  2. Copy or move the files you generated to /etc/ipsec.d/certs and /etc/ipsec.d/private/ respectively. These files are the client certificate and the private key, they need to be located in their corresponding directories. Use the following commands:

    sudo cp ${USERNAME}Cert.pem /etc/ipsec.d/certs/
sudo cp ${USERNAME}Key.pem /etc/ipsec.d/private/
sudo chmod -R go-rwx /etc/ipsec.d/private /etc/ipsec.d/certs

  3. Run the following command to take note of your hostname. You’ll use this value in the next step.

    hostnamectl --static

  4. Open the VpnSettings.xml file and copy the <VpnServer> value. You’ll use this value in the next step.

  5. Adjust the values in the following example, then add the example to the /etc/ipsec.conf configuration.

    conn azure
      keyexchange=ikev2
      type=tunnel
      leftfirewall=yes
      left=%any
      # Replace ${USERNAME}Cert.pem with the key filename inside /etc/ipsec.d/certs  directory. 
      leftcert=${USERNAME}Cert.pem
      leftauth=pubkey
      leftid=%client # use the hostname of your machine with % character prepended. Example: %client
      right= #Azure VPN gateway address. Example: azuregateway-xxx-xxx.vpn.azure.com
      rightid=% #Azure VPN gateway FQDN with % character prepended. Example: %azuregateway-xxx-xxx.vpn.azure.com
      rightsubnet=0.0.0.0/0
      leftsourceip=%config
      auto=add
      esp=aes256gcm16

  6. Add the secret values to /etc/ipsec.secrets.

    The name of the PEM file must match what you have used earlier as your client key file.

    : RSA ${USERNAME}Key.pem  # Replace ${USERNAME}Key.pem with the key filename inside /etc/ipsec.d/private directory.

  7. Run the following commands:

    sudo ipsec restart
sudo ipsec up azure

Next steps

For more steps, return to the P2S Azure portal article.