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:

One Ubuntu 18.04 server configured by following the Ubuntu 18.04 initial server setup guide, including a sudo non-root user and a firewall.

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:

ip route | grep default

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

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:

-----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:

Press WINDOWS+R to bring up the Run dialog, and enter mmc.exe to launch the Windows Management Console.

From the File menu, navigate to Add or Remove Snap-in, select Certificates from the list of available snap-ins, and click Add.

We want the VPN to work with any user, so select Computer Account and click Next.

We’re configuring things on the local computer, so select Local Computer, then click Finish.

Under the Console Root node, expand the Certificates (Local Computer) entry, expand Trusted Root Certification Authorities, and then select the Certificates entry:

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

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

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

Click Finish to import the certificate.

Then configure the VPN with these steps:

Launch Control Panel, then navigate to the Network and Sharing Center.
Click on Set up a new connection or network, then select Connect to a workplace.
Select Use my Internet connection (VPN).
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:

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.”
Enter your password, then click on Modify Keychain
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:

Go to System Preferences and choose Network.
Click on the small “plus” button on the lower-left of the list of networks.
In the popup that appears, Set Interface to VPN, set the VPN Type to IKEv2, and give the connection a name.
In the Server and Remote ID field, enter the server’s domain name or IP address. Leave the Local ID blank.
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

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

Update your local package cache: sudo apt update
Install charon-cmd and related software sudo apt install charon-cmd libcharon-extra-plugins
Move to the directory where you copied the CA certificate: cd <^>/path/to/ca-cert.pem
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
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:

Send yourself an email with the root certificate attached.
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.
Go to Settings, General, VPN and tap Add VPN Configuration. This will bring up the VPN connection configuration screen.
Tap on Type and select IKEv2.
In the Description field, enter a short name for the VPN connection. This could be anything you like.
In the Server and Remote ID field, enter the server’s domain name or IP address. The Local ID field can be left blank.
Enter your username and password in the Authentication section, then tap Done.
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:

Send yourself an email with the CA certificate attached. Save the CA certificate to your downloads folder.
Download the StrongSwan VPN client from the Play Store.
Open the app. Tap the “more” icon in the upper-right corner (the three dots icon) and select CA certificates.
Tap the “more” icon in the upper-right corner again. Select Import certificate.
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:

In the app, tap ADD VPN PROFILE at the top.
Fill out the Server with your VPN server’s domain name or public IP address.
Make sure IKEv2 EAP (Username/Password) is selected as the VPN Type.
Fill out the Username and Password with the credentials you defined on the server.
Deselect Select automatically in the CA certificate section and click Select CA certificate.
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).
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.