Read this in other languages: English, 简体中文.
Note: You may also connect using IKEv2 (recommended) or IPsec/XAuth mode.
After setting up your own VPN server, follow these steps to configure your devices. IPsec/L2TP is natively supported by Android, iOS, OS X, and Windows. There is no additional software to install. Setup should only take a few minutes. In case you are unable to connect, first check to make sure the VPN credentials were entered correctly.
ℹ️ You may also connect using IKEv2 mode (recommended).
- Right-click on the wireless/network icon in your system tray.
- Select Network and Internet settings, then on the page that opens, click VPN.
- Click the Add VPN button.
- Select Windows (built-in) in the VPN provider drop-down menu.
- Enter anything you like in the Connection name field.
- Enter
Your VPN Server IP
in the Server name or address field. - Select L2TP/IPsec with pre-shared key in the VPN type drop-down menu.
- Enter
Your VPN IPsec PSK
in the Pre-shared key field. - Enter
Your VPN Username
in the User name field. - Enter
Your VPN Password
in the Password field. - Check the Remember my sign-in info checkbox.
- Click Save to save the VPN connection details.
Note: This one-time registry change is required if the VPN server and/or client is behind NAT (e.g. home router).
To connect to the VPN: Click the Connect button, or click on the wireless/network icon in your system tray, click VPN, then select the new VPN entry and click Connect. If prompted, enter Your VPN Username
and Password
, then click OK. 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
".
If you get an error when trying to connect, see Troubleshooting.
- Right-click on the wireless/network icon in your system tray.
- Select Open Network & Internet settings, then on the page that opens, click Network and Sharing Center.
- Click Set up a new connection or network.
- Select Connect to a workplace and click Next.
- Click Use my Internet connection (VPN).
- Enter
Your VPN Server IP
in the Internet address field. - Enter anything you like in the Destination name field, and then click Create.
- Return to Network and Sharing Center. On the left, click Change adapter settings.
- Right-click on the new VPN entry and choose Properties.
- Click the Security tab. Select "Layer 2 Tunneling Protocol with IPsec (L2TP/IPSec)" for the Type of VPN.
- Click Allow these protocols. Check the "Challenge Handshake Authentication Protocol (CHAP)" and "Microsoft CHAP Version 2 (MS-CHAP v2)" checkboxes.
- Click the Advanced settings button.
- Select Use preshared key for authentication and enter
Your VPN IPsec PSK
for the Key. - Click OK to close the Advanced settings.
- Click OK to save the VPN connection details.
Note: This one-time registry change is required if the VPN server and/or client is behind NAT (e.g. home router).
To connect to the VPN: Click on the wireless/network icon in your system tray, select the new VPN entry, and click Connect. If prompted, enter Your VPN Username
and Password
, then click OK. 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
".
If you get an error when trying to connect, see Troubleshooting.
Alternatively, instead of following the steps above, you may create the VPN connection using these Windows PowerShell commands. Replace Your VPN Server IP
and Your VPN IPsec PSK
with your own values, enclosed in single quotes:
# Disable persistent command history
Set-PSReadlineOption –HistorySaveStyle SaveNothing
# Create VPN connection
Add-VpnConnection -Name 'My IPsec VPN' -ServerAddress 'Your VPN Server IP' -L2tpPsk 'Your VPN IPsec PSK' -TunnelType L2tp -EncryptionLevel Required -AuthenticationMethod Chap,MSChapv2 -Force -RememberCredential -PassThru
# Ignore the data encryption warning (data is encrypted in the IPsec tunnel)
- Click on the Start Menu and go to the Control Panel.
- Go to the Network and Internet section.
- Click Network and Sharing Center.
- Click Set up a new connection or network.
- Select Connect to a workplace and click Next.
- Click Use my Internet connection (VPN).
- Enter
Your VPN Server IP
in the Internet address field. - Enter anything you like in the Destination name field.
- Check the Don't connect now; just set it up so I can connect later checkbox.
- Click Next.
- Enter
Your VPN Username
in the User name field. - Enter
Your VPN Password
in the Password field. - Check the Remember this password checkbox.
- Click Create, and then Close.
- Return to Network and Sharing Center. On the left, click Change adapter settings.
- Right-click on the new VPN entry and choose Properties.
- Click the Options tab and uncheck Include Windows logon domain.
- Click the Security tab. Select "Layer 2 Tunneling Protocol with IPsec (L2TP/IPSec)" for the Type of VPN.
- Click Allow these protocols. Check the "Challenge Handshake Authentication Protocol (CHAP)" and "Microsoft CHAP Version 2 (MS-CHAP v2)" checkboxes.
- Click the Advanced settings button.
- Select Use preshared key for authentication and enter
Your VPN IPsec PSK
for the Key. - Click OK to close the Advanced settings.
- Click OK to save the VPN connection details.
Note: This one-time registry change is required if the VPN server and/or client is behind NAT (e.g. home router).
To connect to the VPN: Click on the wireless/network icon in your system tray, select the new VPN entry, and click Connect. If prompted, enter Your VPN Username
and Password
, then click OK. 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
".
If you get an error when trying to connect, see Troubleshooting.
ℹ️ You may also connect using IKEv2 (recommended) or IPsec/XAuth mode.
- Open System Preferences and go to the Network section.
- Click the + button in the lower-left corner of the window.
- Select VPN from the Interface drop-down menu.
- Select L2TP over IPSec from the VPN Type drop-down menu.
- Enter anything you like for the Service Name.
- Click Create.
- Enter
Your VPN Server IP
for the Server Address. - Enter
Your VPN Username
for the Account Name. - Click the Authentication Settings button.
- In the User Authentication section, select the Password radio button and enter
Your VPN Password
. - In the Machine Authentication section, select the Shared Secret radio button and enter
Your VPN IPsec PSK
. - Click OK.
- Check the Show VPN status in menu bar checkbox.
- (Important) Click the Advanced button and make sure the Send all traffic over VPN connection checkbox is checked.
- (Important) Click the TCP/IP tab, and make sure Link-local only is selected in the Configure IPv6 section.
- Click OK to close the Advanced settings, and then click Apply to save the VPN connection information.
To connect to the VPN: Use the menu bar icon, or go to the Network section of System Preferences, select the VPN and choose Connect. 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
".
If you get an error when trying to connect, see Troubleshooting.
ℹ️ You may also connect using IKEv2 (recommended) or IPsec/XAuth mode. Android 12 only supports IKEv2 mode.
- Launch the Settings application.
- Tap "Network & internet". Or, if using Android 7 or earlier, tap More... in the Wireless & networks section.
- Tap VPN.
- Tap Add VPN Profile or the + icon at top-right of screen.
- Enter anything you like in the Name field.
- Select L2TP/IPSec PSK in the Type drop-down menu.
- Enter
Your VPN Server IP
in the Server address field. - Leave the L2TP secret field blank.
- Leave the IPSec identifier field blank.
- Enter
Your VPN IPsec PSK
in the IPSec pre-shared key field. - Tap Save.
- Tap the new VPN connection.
- Enter
Your VPN Username
in the Username field. - Enter
Your VPN Password
in the Password field. - Check the Save account information checkbox.
- Tap Connect.
Once connected, you will see a VPN icon in the notification bar. 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
".
If you get an error when trying to connect, see Troubleshooting.
ℹ️ You may also connect using IKEv2 (recommended) or IPsec/XAuth mode.
- Go to Settings -> General -> VPN.
- Tap Add VPN Configuration....
- Tap Type. Select L2TP and go back.
- Tap Description and enter anything you like.
- Tap Server and enter
Your VPN Server IP
. - Tap Account and enter
Your VPN Username
. - Tap Password and enter
Your VPN Password
. - Tap Secret and enter
Your VPN IPsec PSK
. - Make sure the Send All Traffic switch is ON.
- Tap Done.
- Slide the VPN switch ON.
Once connected, you will see a VPN icon in the status bar. 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
".
If you get an error when trying to connect, see Troubleshooting.
- If you haven't already, sign in to your Chromebook.
- Click the status area, where your account picture appears.
- Click Settings.
- In the Internet connection section, click Add connection.
- Click Add OpenVPN / L2TP.
- Enter
Your VPN Server IP
for the Server hostname. - Enter anything you like for the Service name.
- Make sure Provider type is L2TP/IPSec + pre-shared key.
- Enter
Your VPN IPsec PSK
for the Pre-shared key. - Enter
Your VPN Username
for the Username. - Enter
Your VPN Password
for the Password. - Click Connect.
Once connected, you will see a VPN icon overlay on the network status icon. 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
".
If you get an error when trying to connect, see Troubleshooting.
ℹ️ You may also connect using IKEv2 mode (recommended).
Ubuntu 18.04 (and newer) users can install the network-manager-l2tp-gnome package using apt
, then configure the IPsec/L2TP VPN client using the GUI.
- Go to Settings -> Network -> VPN. Click the + button.
- Select Layer 2 Tunneling Protocol (L2TP).
- Enter anything you like in the Name field.
- Enter
Your VPN Server IP
for the Gateway. - Enter
Your VPN Username
for the User name. - Right-click the ? in the Password field, select Store the password only for this user.
- Enter
Your VPN Password
for the Password. - Leave the NT Domain field blank.
- Click the IPsec Settings... button.
- Check the Enable IPsec tunnel to L2TP host checkbox.
- Leave the Gateway ID field blank.
- Enter
Your VPN IPsec PSK
for the Pre-shared key. - Expand the Advanced section.
- Enter
aes128-sha1-modp2048
for the Phase1 Algorithms. - Enter
aes128-sha1
for the Phase2 Algorithms. - Click OK, then click Add to save the VPN connection information.
- Turn the VPN switch ON.
If you get an error when trying to connect, try this fix.
Once 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
".
Fedora 28 (and newer) and CentOS 8/7 users can connect using IPsec/XAuth mode.
First check here to see if the network-manager-l2tp
and network-manager-l2tp-gnome
packages are available for your Linux distribution. If yes, install them (select strongSwan) and follow the instructions above. Alternatively, you may configure Linux VPN clients using the command line.
Read this in other languages: English, 简体中文.
See also: Check logs and VPN status, IKEv2 troubleshooting and Advanced usage.
- Windows error 809
- Windows error 789 or 691
- Windows error 628 or 766
- Windows 10 connecting
- Windows 10 upgrades
- Windows DNS leaks and IPv6
- Android MTU/MSS issues
- Android 6 and 7
- macOS send traffic over VPN
- iOS 13+ and macOS 10.15/11+
- iOS/Android sleep mode
- Debian 11/10 kernel
- Other errors
- Check logs and VPN status
Error 809: The network connection between your computer and the VPN server could not be established because the remote server is not responding. This could be because one of the network devices (e.g, firewalls, NAT, routers, etc) between your computer and the remote server is not configured to allow VPN connections. Please contact your Administrator or your service provider to determine which device may be causing the problem.
Note: The registry change below is only required if you use IPsec/L2TP mode to connect to the VPN. It is NOT required for the IKEv2 and IPsec/XAuth modes.
To fix this error, a one-time registry change is required because the VPN server and/or client is behind NAT (e.g. home router). Download and import the .reg
file below, or run the following from an elevated command prompt. You must reboot your PC when finished.
-
For Windows Vista, 7, 8, 10 and 11 (download .reg file)
REG ADD HKLM\SYSTEM\CurrentControlSet\Services\PolicyAgent /v AssumeUDPEncapsulationContextOnSendRule /t REG_DWORD /d 0x2 /f
-
For Windows XP ONLY (download .reg file)
REG ADD HKLM\SYSTEM\CurrentControlSet\Services\IPSec /v AssumeUDPEncapsulationContextOnSendRule /t REG_DWORD /d 0x2 /f
Although uncommon, some Windows systems disable IPsec encryption, causing the connection to fail. To re-enable it, run the following command and reboot your PC.
-
For Windows XP, Vista, 7, 8, 10 and 11 (download .reg file)
REG ADD HKLM\SYSTEM\CurrentControlSet\Services\RasMan\Parameters /v ProhibitIpSec /t REG_DWORD /d 0x0 /f
Error 789: The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer.
Error 691: The remote connection was denied because the user name and password combination you provided is not recognized, or the selected authentication protocol is not permitted on the remote access server.
For error 789, click here for troubleshooting information. For error 691, you may try removing and recreating the VPN connection, by following the instructions in this document. Make sure that the VPN credentials are entered correctly.
Error 628: The connection was terminated by the remote computer before it could be completed.
Error 766: A certificate could not be found. Connections that use the L2TP protocol over IPSec require the installation of a machine certificate, also known as a computer certificate.
To fix these errors, please follow these steps:
- Right-click on the wireless/network icon in your system tray.
- Select Open Network and Sharing Center. Or, if using Windows 10 version 1709 or newer, select Open Network & Internet settings, then on the page that opens, click Network and Sharing Center.
- On the left, click Change adapter settings. Right-click on the new VPN and choose Properties.
- Click the Security tab. Select "Layer 2 Tunneling Protocol with IPsec (L2TP/IPSec)" for Type of VPN.
- Click Allow these protocols. Check the "Challenge Handshake Authentication Protocol (CHAP)" and "Microsoft CHAP Version 2 (MS-CHAP v2)" checkboxes.
- Click the Advanced settings button.
- Select Use preshared key for authentication and enter
Your VPN IPsec PSK
for the Key. - Click OK to close the Advanced settings.
- Click OK to save the VPN connection details.
For reference, see this screenshot of the VPN connection properties dialog.
If using Windows 10 and the VPN is stuck on "connecting" for more than a few minutes, try these steps:
- Right-click on the wireless/network icon in your system tray.
- Select Open Network & Internet settings, then on the page that opens, click VPN on the left.
- Select the new VPN entry, then click Connect. If prompted, enter
Your VPN Username
andPassword
, then click OK.
After upgrading Windows 10 version (e.g. from 1709 to 1803), you may need to re-apply the fix above for Windows Error 809 and reboot.
Windows 8, 10 and 11 use "smart multi-homed name resolution" by default, which may cause "DNS leaks" when using the native IPsec VPN client if your DNS servers on the Internet adapter are from the local network segment. To fix, you may either disable smart multi-homed name resolution, or configure your Internet adapter to use DNS servers outside your local network (e.g. 8.8.8.8 and 8.8.4.4). When finished, clear the DNS cache and reboot your PC.
In addition, if your computer has IPv6 enabled, all IPv6 traffic (including DNS queries) will bypass the VPN. Learn how to disable IPv6 in Windows. If you need a VPN with IPv6 support, you could instead try OpenVPN.
Some Android devices have MTU/MSS issues, that they are able to connect to the VPN using IPsec/XAuth ("Cisco IPsec") mode, but cannot open websites. If you encounter this problem, try running the following commands on the VPN server. If successful, you may add these commands to /etc/rc.local
to persist after reboot.
iptables -t mangle -A FORWARD -m policy --pol ipsec --dir in \
-p tcp -m tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1361:1536 \
-j TCPMSS --set-mss 1360
iptables -t mangle -A FORWARD -m policy --pol ipsec --dir out \
-p tcp -m tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1361:1536 \
-j TCPMSS --set-mss 1360
echo 1 > /proc/sys/net/ipv4/ip_no_pmtu_disc
Docker users: Instead of running the commands above, you may apply this fix by adding VPN_ANDROID_MTU_FIX=yes
to your env file, then re-create the Docker container.
If your Android 6.x or 7.x device cannot connect, try these steps:
- Tap the "Settings" icon next to your VPN profile. Select "Show advanced options" and scroll down to the bottom. If the option "Backward compatible mode" exists (see screenshot), enable it and reconnect the VPN. If not, try the next step.
- Edit
/etc/ipsec.conf
on the VPN server. Find the linesha2-truncbug
and toggle its value. i.e. Replacesha2-truncbug=no
withsha2-truncbug=yes
, or replacesha2-truncbug=yes
withsha2-truncbug=no
. Save the file and runservice ipsec restart
. Then reconnect the VPN.
Docker users: You may set sha2-truncbug=yes
(default is no
) in /etc/ipsec.conf
by adding VPN_SHA2_TRUNCBUG=yes
to your env file, then re-create the Docker container.
OS X (macOS) users: If you can successfully connect using IPsec/L2TP mode, but your public IP does not show Your VPN Server IP
, read the OS X section above and complete these steps. Save VPN configuration and re-connect.
- Click the Advanced button and make sure the Send all traffic over VPN connection checkbox is checked.
- Click the TCP/IP tab, and make sure Link-local only is selected in the Configure IPv6 section.
After trying the steps above, if your computer is still not sending traffic over the VPN, check the service order. From the main network preferences screen, select "set service order" in the cog drop down under the list of connections. Drag the VPN connection to the top.
If your device running iOS 13+, macOS 10.15 (Catalina), macOS 11 (Big Sur) or above cannot connect, try these steps: Edit /etc/ipsec.conf
on the VPN server. Find sha2-truncbug=yes
and replace it with sha2-truncbug=no
. Save the file and run service ipsec restart
. Then reconnect the VPN.
In addition, users running macOS Big Sur 11.0 should update to version 11.1 or newer, to fix some issues with VPN connections. To check your macOS version and update, refer to this article.
To save battery, iOS devices (iPhone/iPad) will automatically disconnect Wi-Fi shortly after the screen turns off (sleep mode). As a result, the IPsec VPN disconnects. This behavior is by design and cannot be configured.
If you need the VPN to auto-reconnect when the device wakes up, you may connect using IKEv2 mode (recommended) and enable the "VPN On Demand" feature. Alternatively, you may try OpenVPN instead, which has support for options such as "Reconnect on Wakeup" and "Seamless Tunnel".
Android devices will also disconnect Wi-Fi shortly after entering sleep mode, unless the option "Keep Wi-Fi on during sleep" is enabled. This option is no longer available in Android 8 (Oreo) and newer. Alternatively, you may try enabling the "Always-on VPN" option to stay connected. Learn more here.
Debian 11 or 10 users: Run uname -r
to check your server's Linux kernel version. If it contains the word "cloud", and /dev/ppp
is missing, then the kernel lacks ppp
support and cannot use IPsec/L2TP mode. The VPN setup scripts try to detect this and show a warning. In this case, you may instead use IKEv2 or IPsec/XAuth mode to connect to the VPN.
To fix the issue with IPsec/L2TP mode, you may switch to the standard Linux kernel by installing e.g. the linux-image-amd64
package. Then update the default kernel in GRUB and reboot your server.
If you encounter other errors, refer to the links below:
- http://www.tp-link.com/en/faq-1029.html
- https://documentation.meraki.com/MX-Z/Client_VPN/Troubleshooting_Client_VPN#Common_Connection_Issues
- https://stackoverflow.com/questions/25245854/windows-8-1-gets-error-720-on-connect-vpn
Commands below must be run as root
(or using sudo
).
First, restart services on the VPN server:
service ipsec restart
service xl2tpd restart
Docker users: Run docker restart ipsec-vpn-server
.
Then reboot your VPN client device, and retry the connection. If still unable to connect, try removing and recreating the VPN connection. Make sure that the VPN credentials are entered correctly.
Check the Libreswan (IPsec) and xl2tpd logs for errors:
# Ubuntu & Debian
grep pluto /var/log/auth.log
grep xl2tpd /var/log/syslog
# CentOS/RHEL, Rocky Linux, AlmaLinux, Oracle Linux & Amazon Linux 2
grep pluto /var/log/secure
grep xl2tpd /var/log/messages
# Alpine Linux
grep pluto /var/log/messages
grep xl2tpd /var/log/messages
Check the status of the IPsec VPN server:
ipsec status
Show currently established VPN connections:
ipsec trafficstatus
After setting up your own VPN server, follow these steps to configure Linux VPN clients using the command line. Alternatively, you may connect using IKEv2 mode (recommended), or configure using the GUI. Instructions below are based on the work of Peter Sanford. Commands must be run as root
on your VPN client.
To set up the VPN client, first install the following packages:
# Ubuntu and Debian
apt-get update
apt-get install strongswan xl2tpd net-tools
# Fedora
yum install strongswan xl2tpd net-tools
# CentOS
yum install epel-release
yum --enablerepo=epel install strongswan xl2tpd net-tools
Create VPN variables (replace with actual values):
VPN_SERVER_IP='your_vpn_server_ip'
VPN_IPSEC_PSK='your_ipsec_pre_shared_key'
VPN_USER='your_vpn_username'
VPN_PASSWORD='your_vpn_password'
Configure strongSwan:
cat > /etc/ipsec.conf <<EOF
# ipsec.conf - strongSwan IPsec configuration file
conn myvpn
auto=add
keyexchange=ikev1
authby=secret
type=transport
left=%defaultroute
leftprotoport=17/1701
rightprotoport=17/1701
right=$VPN_SERVER_IP
ike=aes128-sha1-modp2048
esp=aes128-sha1
EOF
cat > /etc/ipsec.secrets <<EOF
: PSK "$VPN_IPSEC_PSK"
EOF
chmod 600 /etc/ipsec.secrets
# For CentOS and Fedora ONLY
mv /etc/strongswan/ipsec.conf /etc/strongswan/ipsec.conf.old 2>/dev/null
mv /etc/strongswan/ipsec.secrets /etc/strongswan/ipsec.secrets.old 2>/dev/null
ln -s /etc/ipsec.conf /etc/strongswan/ipsec.conf
ln -s /etc/ipsec.secrets /etc/strongswan/ipsec.secrets
Configure xl2tpd:
cat > /etc/xl2tpd/xl2tpd.conf <<EOF
[lac myvpn]
lns = $VPN_SERVER_IP
ppp debug = yes
pppoptfile = /etc/ppp/options.l2tpd.client
length bit = yes
EOF
cat > /etc/ppp/options.l2tpd.client <<EOF
ipcp-accept-local
ipcp-accept-remote
refuse-eap
require-chap
noccp
noauth
mtu 1280
mru 1280
noipdefault
defaultroute
usepeerdns
connect-delay 5000
name "$VPN_USER"
password "$VPN_PASSWORD"
EOF
chmod 600 /etc/ppp/options.l2tpd.client
The VPN client setup is now complete. Follow the steps below to connect.
Note: You must repeat all steps below every time you try to connect to the VPN.
Create xl2tpd control file:
mkdir -p /var/run/xl2tpd
touch /var/run/xl2tpd/l2tp-control
Restart services:
service strongswan restart
# For Ubuntu 20.04, if strongswan service not found
ipsec restart
service xl2tpd restart
Start the IPsec connection:
# Ubuntu and Debian
ipsec up myvpn
# CentOS and Fedora
strongswan up myvpn
Start the L2TP connection:
echo "c myvpn" > /var/run/xl2tpd/l2tp-control
Run ifconfig
and check the output. You should now see a new interface ppp0
.
Check your existing default route:
ip route
Find this line in the output: default via X.X.X.X ...
. Write down this gateway IP for use in the two commands below.
Exclude your VPN server's public IP from the new default route (replace with actual value):
route add YOUR_VPN_SERVER_PUBLIC_IP gw X.X.X.X
If your VPN client is a remote server, you must also exclude your Local PC's public IP from the new default route, to prevent your SSH session from being disconnected (replace with actual value):
route add YOUR_LOCAL_PC_PUBLIC_IP gw X.X.X.X
Add a new default route to start routing traffic via the VPN server:
route add default dev ppp0
The VPN connection is now complete. Verify that your traffic is being routed properly:
wget -qO- http://ipv4.icanhazip.com; echo
The above command should return Your VPN Server IP
.
To stop routing traffic via the VPN server:
route del default dev ppp0
To disconnect:
# Ubuntu and Debian
echo "d myvpn" > /var/run/xl2tpd/l2tp-control
ipsec down myvpn
# CentOS and Fedora
echo "d myvpn" > /var/run/xl2tpd/l2tp-control
strongswan down myvpn
This document was adapted from the Streisand project, maintained by Joshua Lund and contributors.
Note: This license applies to this document only.
Copyright (C) 2016-2022 Lin Song
Based on the work of Joshua Lund (Copyright 2014-2016)
This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.