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      How to Set Up an IKEv2 VPN Server with StrongSwan on Ubuntu 18.04


      Introduction

      A virtual private network, or VPN, allows you to securely encrypt traffic as it travels through untrusted networks, such as those at the coffee shop, a conference, or an airport.

      IKEv2, or Internet Key Exchange v2, is a protocol that allows for direct IPSec tunneling between the server and client. In IKEv2 VPN implementations, IPSec provides encryption for the network traffic. IKEv2 is natively supported on some platforms (OS X 10.11+, iOS 9.1+, and Windows 10) with no additional applications necessary, and it handles client hiccups quite smoothly.

      In this tutorial, you’ll set up an IKEv2 VPN server using StrongSwan on an Ubuntu 18.04 server and connect to it from Windows, macOS, Ubuntu, iOS, and Android clients.

      Prerequisites

      To complete this tutorial, you will need:

      Step 1 — Installing StrongSwan

      First, we’ll install StrongSwan, an open-source IPSec daemon which we’ll configure as our VPN server. We’ll also install the public key infrastructure component so that we can create a certificate authority to provide credentials for our infrastructure.

      Update the local package cache and install the software by typing:

      • sudo apt update
      • sudo apt install strongswan strongswan-pki

      Now that everything’s installed, let’s move on to creating our certificates.

      Step 2 — Creating a Certificate Authority

      An IKEv2 server requires a certificate to identify itself to clients. To help us create the certificate required, the strongswan-pki package comes with a utility to generate a certificate authority and server certificates. To begin, let’s create a few directories to store all the assets we’ll be working on. The directory structure matches some of the directories in /etc/ipsec.d, where we will eventually move all of the items we create. We’ll lock down the permissions so that our private files can’t be seen by other users:

      • mkdir -p ~/pki/{cacerts,certs,private}
      • chmod 700 ~/pki

      Now that we have a directory structure to store everything, we can generate a root key. This will be a 4096-bit RSA key that will be used to sign our root certificate authority.

      Execute these commands to generate the key:

      • ipsec pki --gen --type rsa --size 4096 --outform pem > ~/pki/private/ca-key.pem

      Now that we have a key, we can move on to creating our root certificate authority, using the key to sign the root certificate:

      • ipsec pki --self --ca --lifetime 3650 --in ~/pki/private/ca-key.pem
      • --type rsa --dn "CN=VPN root CA" --outform pem > ~/pki/cacerts/ca-cert.pem

      You can change the distinguished name (DN) values to something else to if you would like. The common name here is just the indicator, so it doesn’t have to match anything in your infrastructure.

      Now that we’ve got our root certificate authority up and running, we can create a certificate that the VPN server will use.

      Step 3 — Generating a Certificate for the VPN Server

      We’ll now create a certificate and key for the VPN server. This certificate will allow the client to verify the server’s authenticity using the CA certificate we just generated.

      First, create a private key for the VPN server with the following command:

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

      Now, create and sign the VPN server certificate with the certificate authority’s key you created in the previous step. Execute the following command, but change the Common Name (CN) and the Subject Alternate Name (SAN) field to your VPN server’s DNS name or IP address:

      • ipsec pki --pub --in ~/pki/private/server-key.pem --type rsa
      • | ipsec pki --issue --lifetime 1825
      • --cacert ~/pki/cacerts/ca-cert.pem
      • --cakey ~/pki/private/ca-key.pem
      • --dn "CN=server_domain_or_IP" --san "server_domain_or_IP"
      • --flag serverAuth --flag ikeIntermediate --outform pem
      • > ~/pki/certs/server-cert.pem

      Now that we’ve generated all of the TLS/SSL files StrongSwan needs, we can move the files into place in the /etc/ipsec.d directory by typing:

      • sudo cp -r ~/pki/* /etc/ipsec.d/

      In this step, we’ve created a certificate pair that would be used to secure communications between the client and the server. We’ve also signed the certificates with the CA key, so the client will be able to verify the authenticity of the VPN server using the CA certificate. Now that have all of the certificates ready, we’ll move on to configuring the software.

      Step 4 — Configuring StrongSwan

      StrongSwan has a default configuration file with some examples, but we will have to do most of the configuration ourselves. Let’s back up the file for reference before starting from scratch:

      • sudo mv /etc/ipsec.conf{,.original}

      Create and open a new blank configuration file by typing:

      • sudo nano /etc/ipsec.conf

      First, we’ll tell StrongSwan to log daemon statuses for debugging and allow duplicate connections. Add these lines to the file:

      /etc/ipsec.conf

      config setup
          charondebug="ike 1, knl 1, cfg 0"
          uniqueids=no
      

      Then, we’ll create a configuration section for our VPN. We’ll also tell StrongSwan to create IKEv2 VPN Tunnels and to automatically load this configuration section when it starts up. Append the following lines to the file:

      /etc/ipsec.conf

      . . .
      conn ikev2-vpn
          auto=add
          compress=no
          type=tunnel
          keyexchange=ikev2
          fragmentation=yes
          forceencaps=yes
      

      We’ll also configure dead-peer detection to clear any “dangling” connections in case the client unexpectedly disconnects. Add these lines:

      /etc/ipsec.conf

      . . .
      conn ikev2-vpn
          . . .
          dpdaction=clear
          dpddelay=300s
          rekey=no
      

      Then, we’ll configure the server (left) side IPSec parameters. Add this to the file:

      /etc/ipsec.conf

      . . .
      conn ikev2-vpn
          . . .
          left=%any
          leftid=@server_domain_or_IP
          leftcert=server-cert.pem
          leftsendcert=always
          leftsubnet=0.0.0.0/0
      

      Note: When configuring the server ID (leftid), only include the @ character if your VPN server will be identified by a domain name:

          leftid=@vpn.example.com
      

      If the server will be identified by its IP address, just put the IP address in:

          leftid=203.0.113.7
      

      Next, we can configure the client (right) side IPSec parameters, like the private IP address ranges and DNS servers to use:

      /etc/ipsec.conf

      . . .
      conn ikev2-vpn
          . . .
          right=%any
          rightid=%any
          rightauth=eap-mschapv2
          rightsourceip=10.10.10.0/24
          rightdns=8.8.8.8,8.8.4.4
          rightsendcert=never
      

      Finally, we’ll tell StrongSwan to ask the client for user credentials when they connect:

      /etc/ipsec.conf

      . . .
      conn ikev2-vpn
          . . .
          eap_identity=%identity
      

      The configuration file should look like this:

      /etc/ipsec.conf

      config setup
          charondebug="ike 1, knl 1, cfg 0"
          uniqueids=no
      
      conn ikev2-vpn
          auto=add
          compress=no
          type=tunnel
          keyexchange=ikev2
          fragmentation=yes
          forceencaps=yes
          dpdaction=clear
          dpddelay=300s
          rekey=no
          left=%any
          leftid=@server_domain_or_IP
          leftcert=server-cert.pem
          leftsendcert=always
          leftsubnet=0.0.0.0/0
          right=%any
          rightid=%any
          rightauth=eap-mschapv2
          rightsourceip=10.10.10.0/24
          rightdns=8.8.8.8,8.8.4.4
          rightsendcert=never
          eap_identity=%identity
      

      Save and close the file once you’ve verified that you’ve configured things as shown.

      Now that we’ve configured the VPN parameters, let’s move on to creating an account so our users can connect to the server.

      Step 5 — Configuring VPN Authentication

      Our VPN server is now configured to accept client connections, but we don’t have any credentials configured yet. We’ll need to configure a couple things in a special configuration file called ipsec.secrets:

      • We need to tell StrongSwan where to find the private key for our server certificate, so the server will be able to authenticate to clients.
      • We also need to set up a list of users that will be allowed to connect to the VPN.

      Let’s open the secrets file for editing:

      • sudo nano /etc/ipsec.secrets

      First, we’ll tell StrongSwan where to find our private key:

      /etc/ipsec.secrets

      : RSA "server-key.pem"
      

      Then, we’ll define the user credentials. You can make up any username or password combination that you like:

      /etc/ipsec.secrets

      your_username : EAP "your_password"
      

      Save and close the file. Now that we’ve finished working with the VPN parameters, we’ll restart the VPN service so that our configuration is applied:

      • sudo systemctl restart strongswan

      Now that the VPN server has been fully configured with both server options and user credentials, it’s time to move on to configuring the most important part: the firewall.

      Step 6 — Configuring the Firewall & Kernel IP Forwarding

      With the StrongSwan configuration complete, we need to configure the firewall to forward and allow VPN traffic through.

      If you followed the prerequisite tutorial, you should have a very basic UFW firewall enabled. If you don’t yet have UFW configured, you can create a baseline configuration and enable it by typing:

      • sudo ufw allow OpenSSH
      • sudo ufw enable

      Now, add a rule to allow UDP traffic to the standard IPSec ports, 500 and 4500:

      • sudo ufw allow 500,4500/udp

      Next, we will open up one of UFW’s configuration files to add a few low-level policies for routing and forwarding IPSec packets. Before we do, we need to find which network interface on our server is used for internet access. We can find that by querying for the interface associated with the default route:

      Your public interface should follow the word "dev". For example, this result shows the interface named eth0, which is highlighted below:

      Output

      default via 203.0.113.7 dev eth0 proto static

      When you have your public network interface, open the /etc/ufw/before.rules file in your text editor:

      • sudo nano /etc/ufw/before.rules

      Near the top of the file (before the *filter line), add the following configuration block:

      /etc/ufw/before.rules

      *nat
      -A POSTROUTING -s 10.10.10.0/24 -o eth0 -m policy --pol ipsec --dir out -j ACCEPT
      -A POSTROUTING -s 10.10.10.0/24 -o eth0 -j MASQUERADE
      COMMIT
      
      *mangle
      -A FORWARD --match policy --pol ipsec --dir in -s 10.10.10.0/24 -o eth0 -p tcp -m tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1361:1536 -j TCPMSS --set-mss 1360
      COMMIT
      
      *filter
      :ufw-before-input - [0:0]
      :ufw-before-output - [0:0]
      :ufw-before-forward - [0:0]
      :ufw-not-local - [0:0]
      . . .
      

      Change each instance of eth0 in the above configuration to match the interface name you found with ip route. The *nat lines create rules so that the firewall can correctly route and manipulate traffic between the VPN clients and the internet. The *mangle line adjusts the maximum packet segment size to prevent potential issues with certain VPN clients.

      Next, after the *filter and chain definition lines, add one more block of configuration:

      /etc/ufw/before.rules

      . . .
      *filter
      :ufw-before-input - [0:0]
      :ufw-before-output - [0:0]
      :ufw-before-forward - [0:0]
      :ufw-not-local - [0:0]
      
      -A ufw-before-forward --match policy --pol ipsec --dir in --proto esp -s 10.10.10.0/24 -j ACCEPT
      -A ufw-before-forward --match policy --pol ipsec --dir out --proto esp -d 10.10.10.0/24 -j ACCEPT
      

      These lines tell the firewall to forward ESP (Encapsulating Security Payload) traffic so the VPN clients will be able to connect. ESP provides additional security for our VPN packets as they're traversing untrusted networks.

      When you're finished, save and close the file.

      Before we restart the firewall, we'll change some network kernel parameters to allow routing from one interface to another. Open UFW's kernel parameters configuration file:

      • sudo nano /etc/ufw/sysctl.conf

      We'll need to configure a few things here:

      • First, we'll enable IPv4 packet forwarding.
      • We'll disable Path MTU discovery to prevent packet fragmentation problems.
      • We also won't accept ICMP redirects nor send ICMP redirects to prevent man-in-the-middle attacks.

      The changes you need to make to the file are highlighted in the following code:

      /etc/ufw/sysctl.conf

      
      . . .
      
      # Enable forwarding
      # Uncomment the following line
      net/ipv4/ip_forward=1
      
      . . .
      
      # Do not accept ICMP redirects (prevent MITM attacks)
      # Ensure the following line is set
      net/ipv4/conf/all/accept_redirects=0
      
      # Do not send ICMP redirects (we are not a router)
      # Add the following lines
      net/ipv4/conf/all/send_redirects=0
      net/ipv4/ip_no_pmtu_disc=1
      

      Save the file when you are finished. UFW will apply these changes the next time it starts.

      Now, we can enable all of our changes by disabling and re-enabling the firewall:

      • sudo ufw disable
      • sudo ufw enable

      You'll be prompted to confirm the process. Type Y to enable UFW again with the new settings.

      Step 7 – Testing the VPN Connection on Windows, iOS, and macOS

      Now that you have everything set up, it's time to try it out. First, you'll need to copy the CA certificate you created and install it on your client device(s) that will connect to the VPN. The easiest way to do this is to log into your server and output the contents of the certificate file:

      • cat /etc/ipsec.d/cacerts/ca-cert.pem

      You'll see output similar to this:

      Output

      -----BEGIN CERTIFICATE----- MIIFQjCCAyqgAwIBAgIIFkQGvkH4ej0wDQYJKoZIhvcNAQEMBQAwPzELMAkGA1UE . . . EwbVLOXcNduWK2TPbk/+82GRMtjftran6hKbpKGghBVDPVFGFT6Z0OfubpkQ9RsQ BayqOb/Q -----END CERTIFICATE-----

      Copy this output to your computer, including the -----BEGIN CERTIFICATE----- and -----END CERTIFICATE----- lines, and save it to a file with a recognizable name, such as ca-cert.pem. Ensure the file you create has the .pem extension.

      Alternatively, use SFTP to transfer the file to your computer.

      Once you have the ca-cert.pem file downloaded to your computer, you can set up the connection to the VPN.

      Connecting from Windows

      First, import the root certificate by following these steps:

      1. Press WINDOWS+R to bring up the Run dialog, and enter mmc.exe to launch the Windows Management Console.
      2. From the File menu, navigate to Add or Remove Snap-in, select Certificates from the list of available snap-ins, and click Add.
      3. We want the VPN to work with any user, so select Computer Account and click Next.
      4. We're configuring things on the local computer, so select Local Computer, then click Finish.
      5. Under the Console Root node, expand the Certificates (Local Computer) entry, expand Trusted Root Certification Authorities, and then select the Certificates entry:
        Certificates view

      6. From the Action menu, select All Tasks and click Import to display the Certificate Import Wizard. Click Next to move past the introduction.

      7. On the File to Import screen, press the Browse button and select the certificate file that you've saved. Then click Next.

      8. Ensure that the Certificate Store is set to Trusted Root Certification Authorities, and click Next.

      9. Click Finish to import the certificate.

      Then configure the VPN with these steps:

      1. Launch Control Panel, then navigate to the Network and Sharing Center.
      2. Click on Set up a new connection or network, then select Connect to a workplace.
      3. Select Use my Internet connection (VPN).
      4. Enter the VPN server details. Enter the server's domain name or IP address in the Internet address field, then fill in Destination name with something that describes your VPN connection. Then click Done.

      Your new VPN connection will be visible under the list of networks. Select the VPN and click Connect. You'll be prompted for your username and password. Type them in, click OK, and you'll be connected.

      Connecting from macOS

      Follow these steps to import the certificate:

      1. Double-click the certificate file. Keychain Access will pop up with a dialog that says "Keychain Access is trying to modify the system keychain. Enter your password to allow this."
      2. Enter your password, then click on Modify Keychain
      3. Double-click the newly imported VPN certificate. This brings up a small properties window where you can specify the trust levels. Set IP Security (IPSec) to Always Trust and you'll be prompted for your password again. This setting saves automatically after entering the password.

      Now that the certificate is important and trusted, configure the VPN connection with these steps:

      1. Go to System Preferences and choose Network.
      2. Click on the small "plus" button on the lower-left of the list of networks.
      3. In the popup that appears, Set Interface to VPN, set the VPN Type to IKEv2, and give the connection a name.
      4. In the Server and Remote ID field, enter the server's domain name or IP address. Leave the Local ID blank.
      5. Click on Authentication Settings, select Username, and enter your username and password you configured for your VPN user. Then click OK.

      Finally, click on Connect to connect to the VPN. You should now be connected to the VPN.

      Connecting from Ubuntu

      To connect from an Ubuntu machine, you can set up and manage StrongSwan as a service or use a one-off command every time you wish to connect. Instructions are provided for both.

      Managing StrongSwan as a Service

      1. Update your local package cache: sudo apt update
      2. Install StrongSwan and the related software sudo apt install strongswan libcharon-extra-plugins
      3. Copy the CA certificate to the /etc/ipsec.d/cacerts directory: sudo cp /tmp/ca-cert.pem /etc/ipsec.d/cacerts
      4. Disable StrongSwan so that the VPN doesn't start automatically: sudo systemctl disable --now strongswan
      5. Configure your VPN username and password in the /etc/ipsec.secrets file: your_username : EAP "your_password"
      6. Edit the /etc/ipsec.conf file to define your configuration.

      /etc/ipsec.conf

      config setup
      
      conn ikev2-rw
          right=server_domain_or_IP
          # This should match the `leftid` value on your server's configuration
          rightid=server_domain_or_IP
          rightsubnet=0.0.0.0/0
          rightauth=pubkey
          leftsourceip=%config
          leftid=username
          leftauth=eap-mschapv2
          eap_identity=%identity
          auto=start
      

      To connect to the VPN, type:

      • sudo systemctl start strongswan

      To disconnect again, type:

      • sudo systemctl stop strongswan

      Using a Simple Client for One-Off Connections

      1. Update your local package cache: sudo apt update
      2. Install charon-cmd and related software sudo apt install charon-cmd libcharon-extra-plugins
      3. Move to the directory where you copied the CA certificate: cd <^>/path/to/ca-cert.pem
      4. Connect to the VPN server with charon-cmd using the server's CA certificate, the VPN server's IP address, and the username you configured: sudo charon-cmd --cert ca-cert.pem --host vpn_domain_or_IP --identity your_username
      5. When prompted, provide the VPN user's password.

      You should now be connected to the VPN. To disconnect, press CTRL+C and wait for the connection to close.

      Connecting from iOS

      To configure the VPN connection on an iOS device, follow these steps:

      1. Send yourself an email with the root certificate attached.
      2. Open the email on your iOS device and tap on the attached certificate file, then tap Install and enter your passcode. Once it installs, tap Done.
      3. Go to Settings, General, VPN and tap Add VPN Configuration. This will bring up the VPN connection configuration screen.
      4. Tap on Type and select IKEv2.
      5. In the Description field, enter a short name for the VPN connection. This could be anything you like.
      6. In the Server and Remote ID field, enter the server's domain name or IP address. The Local ID field can be left blank.
      7. Enter your username and password in the Authentication section, then tap Done.
      8. Select the VPN connection that you just created, tap the switch on the top of the page, and you'll be connected.

      Connecting from Android

      Follow these steps to import the certificate:

      1. Send yourself an email with the CA certificate attached. Save the CA certificate to your downloads folder.
      2. Download the StrongSwan VPN client from the Play Store.
      3. Open the app. Tap the "more" icon in the upper-right corner (the three dots icon) and select CA certificates.
      4. Tap the "more" icon in the upper-right corner again. Select Import certificate.
      5. Browse to the CA certificate file in your downloads folder and select it to import it into the app.

      Now that the certificate is imported into the StrongSwan app, you can configure the VPN connection with these steps:

      1. In the app, tap ADD VPN PROFILE at the top.
      2. Fill out the Server with your VPN server's domain name or public IP address.
      3. Make sure IKEv2 EAP (Username/Password) is selected as the VPN Type.
      4. Fill out the Username and Password with the credentials you defined on the server.
      5. Deselect Select automatically in the CA certificate section and click Select CA certificate.
      6. Tap the IMPORTED tab at the top of the screen and choose the CA you imported (it will be named "VPN root CA" if you didn't change the "DN" earlier).
      7. If you'd like, fill out Profile name (optional) with a more descriptive name.

      When you wish to connect to the VPN, click on profile you just created in the StrongSwan application.

      Troubleshooting Connections

      If you are unable to import the certificate, ensure the file has the .pem extension, and not .pem.txt.

      If you're unable to connect to the VPN, check the server name or IP address you used. The server's domain name or IP address must match what you've configured as the common name (CN) while creating the certificate. If they don't match, the VPN connection won't work. If you set up a certificate with the CN of vpn.example.com, you must use vpn.example.com when you enter the VPN server details. Double-check the command you used to generate the certificate, and the values you used when creating your VPN connection.

      Finally, double-check the VPN configuration to ensure the leftid value is configured with the @ symbol if you're using a domain name:

          leftid=@vpn.example.com
      

      And if you're using an IP address, ensure that the @ symbol is omitted.

      Conclusion

      In this tutorial, you've built a VPN server that uses the IKEv2 protocol. Now you can be assured that your online activities will remain secure wherever you go!

      To add or remove users, just take a look at Step 5 again. Each line is for one user, so adding or removing users is as simple as editing the file.

      From here, you might want to look into setting up a log file analyzer, because StrongSwan dumps its logs into syslog. The tutorial  How To Install and Use Logwatch Log Analyzer and Reporter on a VPS has more information on setting that up.

      You might also be interested in this guide from the EFF about online privacy.



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