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Step-by-Step Guide: How to Set Up IKEv2 VPN

Read this in other languages: English, 简体中文.

Important: This guide is for advanced users only. Other users please use IPsec/L2TP or IPsec/XAuth mode.

Introduction

Modern operating systems (such as Windows 7 and newer) support the IKEv2 standard. Internet Key Exchange (IKE or IKEv2) is the protocol used to set up a Security Association (SA) in the IPsec protocol suite. Compared to IKE version 1, IKEv2 contains improvements such as Standard Mobility support through MOBIKE, and improved reliability.

Libreswan can authenticate IKEv2 clients on the basis of X.509 Machine Certificates using RSA signatures. This method does not require an IPsec PSK, username or password. It can be used with:

  • Windows 7, 8.x and 10
  • OS X (macOS)
  • Android 4.x and newer (using the strongSwan VPN client)
  • iOS (iPhone/iPad)

Set up IKEv2 on the VPN server

Important: As a prerequisite to using this guide, and before continuing, you must make sure that you have successfully set up your own VPN server, and (optional but recommended) upgraded Libreswan to the latest version.

The following example shows how to configure IKEv2 with Libreswan. Commands below must be run as root.

  1. Find the VPN server's public IP, save it to a variable and check.

    PUBLIC_IP=$(dig @resolver1.opendns.com -t A -4 myip.opendns.com +short)
[ -z "$PUBLIC_IP" ] && PUBLIC_IP=$(wget -t 3 -T 15 -qO- http://ipv4.icanhazip.com)
printf '%s\n' "$PUBLIC_IP"

    Check to make sure the output matches the server's public IP. This variable is required in the steps below.

    Note: Alternatively, you may specify the server's DNS name here. e.g. PUBLIC_IP=myvpn.example.com.

  2. Add a new IKEv2 connection to /etc/ipsec.conf:

    cat >> /etc/ipsec.conf <<EOF

conn ikev2-cp
  left=%defaultroute
  leftcert=$PUBLIC_IP
  leftid=@$PUBLIC_IP
  leftsendcert=always
  leftsubnet=0.0.0.0/0
  leftrsasigkey=%cert
  right=%any
  rightid=%fromcert
  rightaddresspool=192.168.43.10-192.168.43.250
  rightca=%same
  rightrsasigkey=%cert
  narrowing=yes
  dpddelay=30
  dpdtimeout=120
  dpdaction=clear
  auto=add
  ikev2=insist
  rekey=no
  pfs=no
  ike-frag=yes
  ike=aes256-sha2,aes128-sha2,aes256-sha1,aes128-sha1,aes256-sha2;modp1024,aes128-sha1;modp1024
  phase2alg=aes_gcm-null,aes128-sha1,aes256-sha1,aes128-sha2,aes256-sha2
EOF

    We need to add a few more lines to that file. First check your Libreswan version, then run one of the following commands:

    ipsec --version

    For Libreswan 3.23 and newer:

    cat >> /etc/ipsec.conf <<EOF
  modecfgdns="8.8.8.8 8.8.4.4"
  encapsulation=yes
  mobike=no
EOF

    Note: If your server runs Debian or CentOS/RHEL and you wish to enable MOBIKE support, replace mobike=no with mobike=yes in the command above. DO NOT enable this option on Ubuntu systems.

    For Libreswan 3.19-3.22:

    cat >> /etc/ipsec.conf <<EOF
  modecfgdns1=8.8.8.8
  modecfgdns2=8.8.4.4
  encapsulation=yes
EOF

    For Libreswan 3.18 and older:

    cat >> /etc/ipsec.conf <<EOF
  modecfgdns1=8.8.8.8
  modecfgdns2=8.8.4.4
  forceencaps=yes
EOF

  3. Generate Certificate Authority (CA) and VPN server certificates:

    Note: Specify the certificate validity period (in months) with "-v". e.g. "-v 36".

    certutil -z <(head -c 1024 /dev/urandom) \
  -S -x -n "IKEv2 VPN CA" \
  -s "O=IKEv2 VPN,CN=IKEv2 VPN CA" \
  -k rsa -g 4096 -v 36 \
  -d sql:/etc/ipsec.d -t "CT,," -2

    Generating key.  This may take a few moments...

Is this a CA certificate [y/N]?
y
Enter the path length constraint, enter to skip [<0 for unlimited path]: >
Is this a critical extension [y/N]?
N

    Note: If you specified the server's DNS name (instead of its IP address) in step 1 above, you must replace --extSAN "ip:$PUBLIC_IP,dns:$PUBLIC_IP" in the command below with --extSAN "dns:$PUBLIC_IP".

    certutil -z <(head -c 1024 /dev/urandom) \
  -S -c "IKEv2 VPN CA" -n "$PUBLIC_IP" \
  -s "O=IKEv2 VPN,CN=$PUBLIC_IP" \
  -k rsa -g 4096 -v 36 \
  -d sql:/etc/ipsec.d -t ",," \
  --keyUsage digitalSignature,keyEncipherment \
  --extKeyUsage serverAuth \
  --extSAN "ip:$PUBLIC_IP,dns:$PUBLIC_IP"

    Generating key.  This may take a few moments...

  4. Generate client certificate(s), then export the .p12 file that contains the client certificate, private key, and CA certificate:

    certutil -z <(head -c 1024 /dev/urandom) \
  -S -c "IKEv2 VPN CA" -n "vpnclient" \
  -s "O=IKEv2 VPN,CN=vpnclient" \
  -k rsa -g 4096 -v 36 \
  -d sql:/etc/ipsec.d -t ",," \
  --keyUsage digitalSignature,keyEncipherment \
  --extKeyUsage serverAuth,clientAuth -8 "vpnclient"

    Generating key.  This may take a few moments...

    pk12util -o vpnclient.p12 -n "vpnclient" -d sql:/etc/ipsec.d

    Enter password for PKCS12 file:
Re-enter password:
pk12util: PKCS12 EXPORT SUCCESSFUL

    Enter a secure password to protect the exported .p12 file (when importing into an iOS or macOS device, this password cannot be empty). You may repeat this step to generate certificates for additional VPN clients, but make sure to replace every vpnclient with vpnclient2, etc.

    Note: To connect multiple VPN clients simultaneously, you must generate a unique certificate for each.

  5. (For macOS and iOS clients) Export the CA certificate as vpnca.cer:

    certutil -L -d sql:/etc/ipsec.d -n "IKEv2 VPN CA" -a -o vpnca.cer

  6. The database should now contain:

    certutil -L -d sql:/etc/ipsec.d

    Certificate Nickname                               Trust Attributes
                                                   SSL,S/MIME,JAR/XPI

IKEv2 VPN CA                                       CTu,u,u
($PUBLIC_IP)                                       u,u,u
vpnclient                                          u,u,u

    Note: To display a certificate, use certutil -L -d sql:/etc/ipsec.d -n "Nickname". To delete a certificate, replace -L with -D. For other certutil usage, read this page.

  7. (Important) Restart IPsec service:

    service ipsec restart

The IKEv2 setup on the VPN server is now complete. Follow instructions below to configure your VPN clients.

Configure IKEv2 VPN clients

Note: If you specified the server's DNS name (instead of its IP address) in step 1 above, you must enter the DNS name in the Server and Remote ID fields.

Windows 7, 8.x and 10

  1. Securely transfer vpnclient.p12 to your computer, then import it into the "Computer account" certificate store. Make sure that the client cert is placed in "Personal -> Certificates", and the CA cert is placed in "Trusted Root Certification Authorities -> Certificates".

    Detailed instructions:
    https://wiki.strongswan.org/projects/strongswan/wiki/Win7Certs

  2. On the Windows computer, add a new IKEv2 VPN connection:
    https://wiki.strongswan.org/projects/strongswan/wiki/Win7Config

  3. Start the new VPN connection, and enjoy your IKEv2 VPN!
    https://wiki.strongswan.org/projects/strongswan/wiki/Win7Connect

  4. (Optional) Enable stronger ciphers by adding the registry key NegotiateDH2048_AES256 and reboot. Read more here.

OS X (macOS)

First, securely transfer both vpnca.cer and vpnclient.p12 to your Mac, then double-click to import them one by one into the login keychain in Keychain Access. Next, double-click on the imported IKEv2 VPN CA certificate, expand Trust and select Always Trust from the IP Security (IPsec) drop-down menu. When finished, check to make sure both vpnclient and IKEv2 VPN CA are listed under the Certificates category of login keychain.

  1. Open System Preferences and go to the Network section.
  2. Click the + button in the lower-left corner of the window.
  3. Select VPN from the Interface drop-down menu.
  4. Select IKEv2 from the VPN Type drop-down menu.
  5. Enter anything you like for the Service Name.
  6. Click Create.
  7. Enter Your VPN Server IP (or DNS name) for the Server Address.
  8. Enter Your VPN Server IP (or DNS name) for the Remote ID.
  9. Leave the Local ID field blank.
  10. Click the Authentication Settings... button.
  11. Select None from the Authentication Settings drop-down menu.
  12. Select the Certificate radio button, then select the vpnclient certificate.
  13. Click OK.
  14. Check the Show VPN status in menu bar checkbox.
  15. Click Apply to save the VPN connection information.
  16. Click Connect.

Android 4.x and newer

  1. Securely transfer vpnclient.p12 to your Android device.
  2. Install strongSwan VPN Client from Google Play.
  3. Launch the VPN client and tap Add VPN Profile.
  4. Enter Your VPN Server IP (or DNS name) in the Server field.
  5. Select IKEv2 Certificate from the VPN Type drop-down menu.
  6. Tap Select user certificate, then tap Install certificate.
  7. Choose the .p12 file you copied from the VPN server, and follow the prompts.
  8. Save the new VPN connection, then tap to connect.

iOS (iPhone/iPad)

First, securely transfer both vpnca.cer and vpnclient.p12 to your iOS device, then import them one by one as iOS profiles. To transfer the files, you may use AirDrop. Alternatively, host the files on a secure website of yours, then download and import them in Mobile Safari. When finished, check to make sure both vpnclient and IKEv2 VPN CA are listed under Settings -> General -> Profiles.

  1. Go to Settings -> General -> VPN.
  2. Tap Add VPN Configuration....
  3. Tap Type. Select IKEv2 and go back.
  4. Tap Description and enter anything you like.
  5. Tap Server and enter Your VPN Server IP (or DNS name).
  6. Tap Remote ID and enter Your VPN Server IP (or DNS name).
  7. Leave the Local ID field blank.
  8. Tap User Authentication. Select None and go back.
  9. Make sure the Use Certificate switch is ON.
  10. Tap Certificate. Select vpnclient and go back.
  11. Tap Done.
  12. Slide the VPN switch ON.

Once successfully connected, you can verify that your traffic is being routed properly by looking up your IP address on Google. It should say "Your public IP address is Your VPN Server IP".

Known issues

  1. The built-in VPN client in Windows may not support IKEv2 fragmentation. On some networks, this can cause the connection to fail or have other issues. You may instead try the IPsec/L2TP or IPsec/XAuth mode.
  2. If using the strongSwan Android VPN client, you must upgrade Libreswan on your server to version 3.26 or above.
  3. If your VPN client can connect but cannot open any website, try editing /etc/ipsec.conf on the VPN server. Find the line phase2alg= under section conn ikev2-cp and delete aes_gcm-null,. Save the file and run service ipsec restart.
  4. Ubuntu 18.04 and CentOS users may encounter the error "The password you entered is incorrect" when trying to import the generated .p12 file into Windows. This is due to a bug in NSS. Read more here.
  5. Connecting multiple IKEv2 clients simultaneously from behind the same NAT (e.g. home router) is not supported at this time. For this use case, please instead use IPsec/XAuth mode.

References