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.

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.

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.

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.

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.

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:

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:

. . .
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:

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

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

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

    leftid=@vpn.example.com

    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:

. . .
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:

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

The configuration file should look like this:

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.

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:

: RSA "server-key.pem"

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

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.

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:

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:

*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:

. . .
*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:

. . .

# 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.

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.

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

sudo systemctl start strongswan

sudo systemctl stop strongswan

    leftid=@vpn.example.com

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.