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

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

Note: This guide is for advanced users. 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)

Using helper scripts

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. Docker users, see here.

You may use this helper script to automatically set up IKEv2 on the VPN server:

wget https://git.io/ikev2setup -O ikev2.sh && sudo bash ikev2.sh

The script must be run using bash, not sh. Follow the prompts to set up IKEv2. When finished, continue to configure IKEv2 VPN clients and check known issues. If you want to generate certificates for additional VPN clients, just run the script again.

Manually set up IKEv2 on the VPN server

The following example shows how to manually 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:

    if ! grep -qs '^include /etc/ipsec\.d/\*\.conf$' /etc/ipsec.conf; then
  echo >> /etc/ipsec.conf
  echo 'include /etc/ipsec.d/*.conf' >> /etc/ipsec.conf
fi

    cat > /etc/ipsec.d/ikev2.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
  fragmentation=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.d/ikev2.conf <<EOF
  modecfgdns="8.8.8.8 8.8.4.4"
  encapsulation=yes
  mobike=no
EOF

    Note: If your server (or Docker host) 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.d/ikev2.conf <<EOF
  modecfgdns1=8.8.8.8
  modecfgdns2=8.8.4.4
  encapsulation=yes
EOF

    For Libreswan 3.18 and older:

    cat >> /etc/ipsec.d/ikev2.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 120".

    Generate CA certificate:

    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 120 \
  -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

    Generate VPN server certificate:

    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 120 \
  -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.

    Note: You may repeat this step to generate certificates for additional VPN clients, but make sure to replace every vpnclient with vpnclient2, etc. To connect multiple VPN clients simultaneously, you must generate a unique certificate for each.

    Generate client 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 120 \
  -d sql:/etc/ipsec.d -t ",," \
  --keyUsage digitalSignature,keyEncipherment \
  --extKeyUsage serverAuth,clientAuth -8 "vpnclient"

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

    Export .p12 file:

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

    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).

  5. (For iOS clients) Export the CA certificate as ikev2vpnca.cer:

    certutil -L -d sql:/etc/ipsec.d -n "IKEv2 VPN CA" -a -o ikev2vpnca.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 revoke a client certificate, follow these steps. For other certutil usage, read here.

  7. (Important) Restart the IPsec service:

    service ipsec restart

Before continuing, you must restart the IPsec service. The IKEv2 setup on the VPN server is now complete. Follow instructions below to configure your VPN clients.

Configure IKEv2 VPN clients

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

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. If you want to generate certificates for additional VPN clients, just run the helper script again. Or you may refer to step 4 in the previous section.

Windows 7, 8.x and 10

  1. Securely transfer the generated .p12 file 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 the generated .p12 file to your Mac, then double-click to import 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. Close the dialog using the red "X" on the top-left corner. When prompted, use Touch ID or enter your password and click "Update Settings". When finished, check to make sure both the new client certificate 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. Enter Your VPN client name in the Local ID field.
    Note: This must match exactly the client name you specified during IKEv2 setup. Same as the first part of your .p12 filename.
  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 new client 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.

iOS

First, securely transfer the generated ikev2vpnca.cer and .p12 files to your iOS device, then import them one by one as iOS profiles. To transfer the files, you may use:

  1. AirDrop, or
  2. Upload to your device, tap them in the "Files" app (must first move to the "On My iPhone" folder), then follow the prompts to import, or
  3. Host the files on a secure website of yours, then download and import them in Mobile Safari.

When finished, check to make sure both the new client certificate 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. Enter Your VPN client name in the Local ID field.
    Note: This must match exactly the client name you specified during IKEv2 setup. Same as the first part of your .p12 filename.
  8. Tap User Authentication. Select None and go back.
  9. Make sure the Use Certificate switch is ON.
  10. Tap Certificate. Select the new client certificate and go back.
  11. Tap Done.
  12. Slide the VPN switch ON.

Android 10 and newer

  1. Securely transfer the generated .p12 file to your Android device.
  2. Install strongSwan VPN Client from Google Play.
  3. Launch the Settings application.
  4. Go to Security -> Advanced -> Encryption & credentials.
  5. Tap Install from storage (or SD card).
  6. Choose the .p12 file you transferred from the VPN server, and follow the prompts.
    Note: To find the .p12 file, click on the three-line menu button, then click on your device name.
  7. Launch the strongSwan VPN client and tap Add VPN Profile.
  8. Enter Your VPN Server IP (or DNS name) in the Server field.
  9. Select IKEv2 Certificate from the VPN Type drop-down menu.
  10. Tap Select user certificate, select the new client certificate and confirm.
  11. (Important) Tap Show advanced settings. Scroll down, find and enable the Use RSA/PSS signatures option.
  12. Save the new VPN connection, then tap to connect.

Android 4.x to 9.x

  1. Securely transfer the generated .p12 file to your Android device.
  2. Install strongSwan VPN Client from Google Play.
  3. Launch the strongSwan 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 transferred from the VPN server, and follow the prompts.
    Note: To find the .p12 file, click on the three-line menu button, then click on your device name.
  8. (Important) Tap Show advanced settings. Scroll down, find and enable the Use RSA/PSS signatures option.
  9. Save the new VPN connection, then tap to connect.

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".

Add a client certificate

If you want to generate certificates for additional VPN clients, just run the helper script again. Or you may refer to step 4 in this section.

Revoke a client certificate

In certain circumstances, you may need to revoke a previously generated VPN client certificate. This can be done using crlutil. See example steps below, commands must be run as root.

  1. Check the database, and identify the nickname of the client certificate you want to revoke.

    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-to-revoke                                u,u,u

    In this example, we will revoke the certificate with nickname vpnclient-to-revoke, issued by IKEv2 VPN CA.

  2. Find the serial number of this client certificate.

    certutil -L -d sql:/etc/ipsec.d -n "vpnclient-to-revoke"

    Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            00:cd:69:ff:74
... ...

    From the output, we see that the serial number is CD69FF74 in hexadecimal, which is 3446275956 in decimal. It will be used in the next steps.

  3. Create a new Certificate Revocation List (CRL). You only need to do this once for each CA.

    if ! crlutil -L -d sql:/etc/ipsec.d -n "IKEv2 VPN CA" 2>/dev/null; then
  crlutil -G -d sql:/etc/ipsec.d -n "IKEv2 VPN CA" -c /dev/null
fi

    CRL Info:
:
    Version: 2 (0x1)
    Signature Algorithm: PKCS #1 SHA-256 With RSA Encryption
    Issuer: "O=IKEv2 VPN,CN=IKEv2 VPN CA"
    This Update: Sat Jun 06 22:00:00 2020
    CRL Extensions:

  4. Add the client certificate you want to revoke to the CRL. Here we specify the certificate's serial number in decimal, and the revocation time in GeneralizedTime format (YYYYMMDDhhmmssZ) in UTC.

    crlutil -M -d sql:/etc/ipsec.d -n "IKEv2 VPN CA" <<EOF
addcert 3446275956 20200606220100Z
EOF

    CRL Info:
:
    Version: 2 (0x1)
    Signature Algorithm: PKCS #1 SHA-256 With RSA Encryption
    Issuer: "O=IKEv2 VPN,CN=IKEv2 VPN CA"
    This Update: Sat Jun 06 22:02:00 2020
    Entry 1 (0x1):
        Serial Number:
            00:cd:69:ff:74
        Revocation Date: Sat Jun 06 22:01:00 2020
    CRL Extensions:

    Note: If you want to remove a certificate from the CRL, replace addcert 3446275956 20200606220100Z above with rmcert 3446275956. For other crlutil usage, read here.

  5. Finally, let Libreswan re-read the updated CRL.

    ipsec crls

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. Ubuntu 18.04 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.
  3. If using the strongSwan Android VPN client, you must upgrade Libreswan on your server to version 3.26 or above.

References