StrongSwan is an open source IPsec-based VPN Solution. It supports both the IKEv1 and IKEv2 key exchange protocols in conjunction with the native NETKEY IPsec stack of the Linux kernel. This tutorial will show you how to use strongSwan to set up an IPSec VPN server on CentOS 7.

Install strongSwan

The strongSwan packages are available in the Extra Packages for Enterprise Linux (EPEL) repository. We should enable EPEL first, then install strongSwan.

yum install http://ftp.nluug.nl/pub/os/Linux/distr/fedora-epel/7/x86_64/Packages/e/epel-release-7-11.noarch.rpm
yum install strongswan openssl

Generate certificates

Both the VPN client and server need a certificate to identify and authenticate themselves. I have prepared two shell scripts to generate and sign the certificates. First, we download these two scripts into the folder /etc/strongswan/ipsec.d.

cd /etc/strongswan/ipsec.d
wget https://raw.githubusercontent.com/michael-loo/strongswan_config/for_vultr/server_key.sh
chmod a+x server_key.sh
wget https://raw.githubusercontent.com/michael-loo/strongswan_config/for_vultr/client_key.sh
chmod a+x client_key.sh

In these two .sh files, I have set the organization name as VULTR-VPS-CENTOS. If you want to change it, open the .sh files and replace O=VULTR-VPS-CENTOS with O=YOUR_ORGANIZATION_NAME.

Next, use server_key.sh with the IP address of your server to generate the certificate authority (CA) key and certificate for server. Replace SERVER_IP with the IP address of your Vultr VPS.

./server_key.sh SERVER_IP

Generate the client key, certificate, and P12 file. Here, I will create the certificate and P12 file for the VPN user "john".

./client_key.sh john john@gmail.com

Replace "john" and his email with yours before running the script.

After the certificates for client and server are generated, copy /etc/strongswan/ipsec.d/john.p12 and /etc/strongswan/ipsec.d/cacerts/strongswanCert.pem to your local computer.

Configure strongSwan

Open the strongSwan IPSec configuration file.

vi /etc/strongswan/ipsec.conf

Replace its content with the following text.

config setup
    uniqueids=never
    charondebug="cfg 2, dmn 2, ike 2, net 0"

conn %default
    left=%defaultroute
    leftsubnet=0.0.0.0/0
    leftcert=vpnHostCert.pem
    right=%any
    rightsourceip=172.16.1.100/16

conn CiscoIPSec
    keyexchange=ikev1
    fragmentation=yes
    rightauth=pubkey
    rightauth2=xauth
    leftsendcert=always
    rekey=no
    auto=add

conn XauthPsk
    keyexchange=ikev1
    leftauth=psk
    rightauth=psk
    rightauth2=xauth
    auto=add

conn IpsecIKEv2
    keyexchange=ikev2
    leftauth=pubkey
    rightauth=pubkey
    leftsendcert=always
    auto=add

conn IpsecIKEv2-EAP
    keyexchange=ikev2
    ike=aes256-sha1-modp1024!
    rekey=no
    leftauth=pubkey
    leftsendcert=always
    rightauth=eap-mschapv2
    eap_identity=%any
    auto=add

Edit the strongSwan configuration file, strongswan.conf.

vi /etc/strongswan/strongswan.conf

Delete everything and replace it with the following.

charon {
    load_modular = yes
    duplicheck.enable = no
    compress = yes
    plugins {
            include strongswan.d/charon/*.conf
    }
    dns1 = 8.8.8.8
    dns2 = 8.8.4.4
    nbns1 = 8.8.8.8
    nbns2 = 8.8.4.4
}

include strongswan.d/*.conf

Edit the IPsec secret file to add a user and password.

vi /etc/strongswan/ipsec.secrets

Add a user account "john" into it.

: RSA vpnHostKey.pem
: PSK "PSK_KEY"
john %any : EAP "John's Password"
john %any : XAUTH "John's Password"

Please note that both sides of the colon ':' need a white-space.

Allow IPv4 forwarding

Edit /etc/sysctl.conf to allow forwarding in the Linux kernel.

vi /etc/sysctl.conf

Add the following line into the file.

net.ipv4.ip_forward=1

Save the file, then apply the change.

sysctl -p

Configure the firewall

Open the firewall for your VPN on the server.

firewall-cmd --permanent --add-service="ipsec"
firewall-cmd --permanent --add-port=4500/udp
firewall-cmd --permanent --add-masquerade
firewall-cmd --reload

Start VPN

systemctl start strongswan
systemctl enable strongswan

StrongSwan is now is running on your server. Install the strongswanCert.pem and .p12 certificate files into your client. You will now be able to join your private network.