Differences

This shows you the differences between two versions of the page.

--- server:openvpn [2020/02/02 14:54] – created lucid
+++ server:openvpn [2020/02/02 14:58] (current) – removed lucid
@@ Line 1: / Line 1: @@
-======OpenVPN on Ubuntu 16.04======
-This guide is pretty much a copy paste of the guide by [[https://www.digitalocean.com/community/users/jellingwood|Justin Ellingwood]] on Digital Ocean. It includes a script to ease the generation of certificates.
-[[https://www.digitalocean.com/community/tutorials/how-to-set-up-an-openvpn-server-on-ubuntu-16-04|Digital Ocean Source]]
-=====Step 1: Install OpenVPN=====
-Install OpenVPN and Easy-RSA. How to use ChaCha20 FIXME
-<code>
-sudo apt-get update
-sudo apt-get install openvpn easy-rsa
-</code>
-=====Step 2: Set Up the CA Directory=====
-OpenVPN is an TLS/SSL VPN. This means that it utilizes certificates in order to encrypt traffic between the server and clients. In order to issue trusted certificates, we will need to set up our own simple certificate authority (CA).
-To begin, we can copy the ''easy-rsa'' template directory into our home directory with the ''make-cadir'' command:
-<code>
-make-cadir ~/openvpn-ca
-</code>
-Move into the newly created directory to being configuring the CA:
-<code>
-cd ~/openvpn-ca
-</code>
-=====Step 3: Configure the CA Variables=====
-To configure the values our CA will use, we need to edit the ''vars'' file within the directory. Open that file now in your text editor:
-<code>
-nano vars
-</code>
-Inside, you will find some variables that can be adjusted to determine how your certificates will be created. We only need to worry about a few of these.
-Towards the bottom of the file, find the settings that set field defaults for new certificates. It should look something like this:\\
-\\
-**~/openvpn-ca/vars**
-<code>
-. . .
-export KEY_COUNTRY="US"
-export KEY_PROVINCE="CA"
-export KEY_CITY="SanFrancisco"
-export KEY_ORG="Fort-Funston"
-export KEY_EMAIL="me@myhost.mydomain"
-export KEY_OU="MyOrganizationalUnit"
-. . .
-</code>
-Edit the values in red to whatever you'd prefer, but do not leave them blank:\\
-\\
-**~/openvpn-ca/vars**
-<code>
-. . .
-export KEY_COUNTRY="US"
-export KEY_PROVINCE="NY"
-export KEY_CITY="New York City"
-export KEY_ORG="DigitalOcean"
-export KEY_EMAIL="admin@example.com"
-export KEY_OU="Community"
-. . .
-</code>
-While we are here, we will also edit the ''KEY_NAME'' value just below this section, which populates the subject field. To keep this simple, we'll call it ''server'' in this guide:\\
-\\
-**~/openvpn-ca/vars**
-<code>
-export KEY_NAME="server"
-</code>
-When you are finished, save and close the file.
-=====Step 4: Build the Certificate Authority=====
-Now, we can use the variables we set and the ''easy-rsa'' utilities to build our certificate authority.
-Ensure you are in your CA directory, and then source the vars file you just edited:
-<code>
-cd ~/openvpn-ca
-source vars
-</code>
-You should see the following if it was sourced correctly:
-<code>
-Output
-NOTE: If you run ./clean-all, I will be doing a rm -rf on /home/sammy/openvpn-ca/keys
-</code>
-Make sure we're operating in a clean environment by typing:
-<code>
-./clean-all
-</code>
-Now, we can build out root CA by typing:
-<code>
-./build-ca
-</code>
-This will initiate the process of creating the root certificate authority key and certificate. Since we filled out the vars file, all of the values should be populated automatically. Just press **ENTER** through the prompts to confirm the selections:
-<code>
-Output
-Generating a 2048 bit RSA private key
-..........................................................................................+++
-...............................+++
-writing new private key to ''ca.key''
------
-You are about to be asked to enter information that will be incorporated
-into your certificate request.
-What you are about to enter is what is called a Distinguished Name or a DN.
-There are quite a few fields but you can leave some blank
-For some fields there will be a default value,
-If you enter '.', the field will be left blank.
------
-Country Name (2 letter code) [US]:
-State or Province Name (full name) [NY]:
-Locality Name (eg, city) [New York City]:
-Organization Name (eg, company) [DigitalOcean]:
-Organizational Unit Name (eg, section) [Community]:
-Common Name (eg, your name or your server's hostname) [DigitalOcean CA]:
-Name [server]:
-Email Address [admin@email.com]:
-</code>
-We now have a CA that can be used to create the rest of the files we need.
-=====Step 5: Create the Server Certificate, Key, and Encryption Files=====
-Next, we will generate our server certificate and key pair, as well as some additional files used during the encryption process.
-\\
-Start by generating the OpenVPN server certificate and key pair. We can do this by typing:
->**Note**: If you choose a name other than ''server'' here, you will have to adjust some of the instructions below. For instance, when copying the generated files to the ''/etc/openvpn directroy'', you will have to substitute the correct names. You will also have to modify the ''/etc/openvpn/server.conf'' file later to point to the correct ''.crt'' and ''.key'' files.
-<code>
-./build-key-server server
-</code>
-Once again, the prompts will have default values based on the argument we just passed in (''server'') and the contents of our ''vars'' file we sourced.
-\\
-Feel free to accept the default values by pressing **ENTER**. Do //not// enter a challenge password for this setup. Towards the end, you will have to enter **y** to two questions to sign and commit the certificate:
-<code>
-Output
-. . .
-Certificate is to be certified until May  1 17:51:16 2026 GMT (3650 days)
-Sign the certificate? [y/n]:y
-out of 1 certificate requests certified, commit? [y/n]y
-Write out database with 1 new entries
-Data Base Updated
-</code>
-Next, we'll generate a few other items. We can generate a strong Diffie-Hellman keys to use during key exchange by typing:
-<code>
-./build-dh
-</code>
-This might take a few minutes to complete.
-\\
-Afterwards, we can generate an HMAC signature to strengthen the server's TLS integrity verification capabilities:
-<code>
-openvpn --genkey --secret keys/ta.key
-</code>
-=====Step 6: Generate a Client Certificate and Key Pair=====
-Next, we can generate a client certificate and key pair. Although this can be done on the client machine and then signed by the server/CA for security purposes, for this guide we will generate the signed key on the server for the sake of simplicity.
-\\
-We will generate a single client key/certificate for this guide, but if you have more than one client, you can repeat this process as many times as you'd like. Pass in a unique value to the script for each client.
-\\
-Because you may come back to this step at a later time, we'll re-source the ''vars'' file. We will use ''client1'' as the value for our first certificate/key pair for this guide.
-\\
-To produce credentials without a password, to aid in automated connections, use the ''build-key'' command like this:
-<code>
-cd ~/openvpn-ca
-source vars
-./build-key client1
-</code>
-If instead, you wish to create a password-protected set of credentials, use the build-key-pass command:
-<code>
-cd ~/openvpn-ca
-source vars
-./build-key-pass client1
-</code>
-Again, the defaults should be populated, so you can just hit **ENTER** to continue. Leave the challenge password blank and make sure to enter **y** for the prompts that ask whether to sign and commit the certificate.
-=====Step 7: Configure the OpenVPN Service=====
-Next, we can begin configuring the OpenVPN service using the credentials and files we've generated.
-Copy the Files to the OpenVPN Directory
-To begin, we need to copy the files we need to the ''/etc/openvpn'' configuration directory.
-We can start with all of the files that we just generated. These were placed within the ''~/openvpn-ca/keys'' directory as they were created. We need to move our CA cert, our server cert and key, the HMAC signature, and the Diffie-Hellman file:
-<code>
-cd ~/openvpn-ca/keys
-sudo cp ca.crt server.crt server.key ta.key dh2048.pem /etc/openvpn
-</code>
-Next, we need to copy and unzip a sample OpenVPN configuration file into configuration directory so that we can use it as a basis for our setup:
-<code>
-gunzip -c /usr/share/doc/openvpn/examples/sample-config-files/server.conf.gz | sudo tee /etc/openvpn/server.conf
-</code>
-Adjust the OpenVPN Configuration
-Now that our files are in place, we can modify the server configuration file:
-<code>
-sudo nano /etc/openvpn/server.conf
-</code>
-Basic Configuration
-First, find the HMAC section by looking for the ''tls-auth'' directive. Remove the ";" to uncomment the ''tls-auth'' line. Below this, add the ''key-direction'' parameter set to "0":\\
-\\
-**/etc/openvpn/server.conf**
-<code>
-tls-auth ta.key 0 # This file is secret
-key-direction 0
-</code>
-Next, find the section on cryptographic ciphers by looking for the commented out 'cipher' lines. The AES-128-CBC cipher offers a good level of encryption and is well supported. Remove the ";" to uncomment the cipher AES-128-CBC line:\\
-\\
-**/etc/openvpn/server.conf**
-<code>
-cipher AES-128-CBC
-</code>
-Below this, add an auth line to select the HMAC message digest algorithm. For this, SHA256 is a good choice:\\
-\\
-**/etc/openvpn/server.conf**
-<code>
-auth SHA256
-</code>
-Finally, find the user and group settings and remove the ";" at the beginning of to uncomment those lines:\\
-\\
-**/etc/openvpn/server.conf**
-<code>
-user nobody
-group nogroup
-</code>
-====(Optional) Push DNS Changes to Redirect All Traffic Through the VPN====
-The settings above will create the VPN connection between the two machines, but will not force any connections to use the tunnel. If you wish to use the VPN to route all of your traffic, you will likely want to push the DNS settings to the client computers.
-You can do this, uncomment a few directives that will configure client machines to redirect all web traffic through the VPN. Find the ''redirect-gateway'' section and remove the semicolon ";" from the beginning of the ''redirect-gateway'' line to uncomment it:\\
-\\
-**/etc/openvpn/server.conf**
-<code>
-push "redirect-gateway def1 bypass-dhcp"
-</code>
-Just below this, find the ''dhcp-option'' section. Again, remove the ";" from in front of both of the lines to uncomment them:\\
-\\
-**/etc/openvpn/server.conf**
-<code>
-push "dhcp-option DNS 208.67.222.222"
-push "dhcp-option DNS 208.67.220.220"
-</code>
-This should assist clients in reconfiguring their DNS settings to use the VPN tunnel for as the default gateway.
-====(Optional) Adjust the Port and Protocol====
-By default, the OpenVPN server uses port 1194 and the UDP protocol to accept client connections. If you need to use a different port because of restrictive network environments that your clients might be in, you can change the ''port'' option. If you are not hosting web content your OpenVPN server, port 443 is a popular choice since this is usually allowed through firewall rules.\\
-\\
-**/etc/openvpn/server.conf**
-<code>
-# Optional!
-port 443
-</code>
-Often if the protocol will be restricted to that port as well. If so, change proto from UDP to TCP:\\
-\\
-**/etc/openvpn/server.conf**
-<code>
-# Optional!
-proto tcp
-</code>
-If you have no need to use a different port, it is best to leave these two settings as their default.
-====(Optional) Point to Non-Default Credentials====
-If you selected a different name during the ./build-key-server command earlier, modify the cert and key lines that you see to point to the appropriate .crt and .key files. If you used the default server, this should already be set correctly:\\
-\\
-**/etc/openvpn/server.conf**
-<code>
-cert server.crt
-key server.key
-</code>
-When you are finished, save and close the file.
-=====Step 8: Adjust the Server Networking Configuration=====
-Next, we need to adjust some aspects of the server's networking so that OpenVPN can correctly route traffic.
-====Allow IP Forwarding====
-First, we need to allow the server to forward traffic. This is fairly essential to the functionality we want our VPN server to provide.
-We can adjust this setting by modifying the ''/etc/sysctl.conf'' file:
-<code>
-sudo nano /etc/sysctl.conf
-</code>
-Inside, look for the line that sets ''net.ipv4.ip_forward''. Remove the "#" character from the beginning of the line to uncomment that setting:\\
-\\
-**/etc/sysctl.conf**
-<code>
-net.ipv4.ip_forward=1
-</code>
-Save and close the file when you are finished.
-To read the file and adjust the values for the current session, type:
-<code>
-sudo sysctl -p
-</code>
-====Adjust the UFW Rules to Masquerade Client Connections====
-If you followed the Ubuntu 16.04 initial server setup guide in the prerequisites, you should have the UFW firewall in place. Regardless of whether you use the firewall to block unwanted traffic (which you almost always should do), we need the firewall in this guide to manipulate some of the traffic coming into the server. We need to modify the rules file to set up masquerading, an ''iptables'' concept that provides on-the-fly dynamic NAT to correctly route client connections.
-Before we open the firewall configuration file to add masquerading, we need to find the public network interface of our machine. To do this, type:
-<code>
-ip route | grep default
-</code>
-Your public interface should follow the word "dev". For example, this result shows the interface named wlp11s0, which is highlighted below:
-<code>
-Output
-default via 203.0.113.1 dev wlp11s0  proto static  metric 600
-</code>
-When you have the interface associated with your default route, open the ''/etc/ufw/before.rules'' file to add the relevant configuration:
-<code>
-sudo nano /etc/ufw/before.rules
-</code>
-This file handles configuration that should be put into place before the conventional UFW rules are loaded. Towards the top of the file, add the highlighted lines below. This will set the default policy for the ''POSTROUTING'' chain in the ''nat'' table and masquerade any traffic coming from the VPN:
->**Note**: Remember to replace ''wlp11s0'' in the ''-A POSTROUTING'' line below with the interface you found in the above command.\\
-\\
-**/etc/ufw/before.rules**
-<code>
-#
-# rules.before
-#
-# Rules that should be run before the ufw command line added rules. Custom
-# rules should be added to one of these chains:
-#   ufw-before-input
-#   ufw-before-output
-#   ufw-before-forward
-#
-# START OPENVPN RULES
-# NAT table rules
-*nat
-:POSTROUTING ACCEPT [0:0]
-# Allow traffic from OpenVPN client to wlp11s0 (change to the interface you discovered!)
--A POSTROUTING -s 10.8.0.0/8 -o wlp11s0 -j MASQUERADE
-COMMIT
-# END OPENVPN RULES
-# Don't delete these required lines, otherwise there will be errors
-*filter
-. . .
-</code>
-Save and close the file when you are finished.
-We need to tell UFW to allow forwarded packets by default as well. To do this, we will open the ''/etc/default/ufw'' file:
-<code>
-sudo nano /etc/default/ufw
-</code>
-Inside, find the ''DEFAULT_FORWARD_POLICY'' directive. We will change the value from ''DROP'' to ''ACCEPT'':\\
-\\
-**/etc/default/ufw**
-<code>
-DEFAULT_FORWARD_POLICY="ACCEPT"
-</code>
-Save and close the file when you are finished.
-====Open the OpenVPN Port and Enable the Changes====
-Next, we'll adjust the firewall itself to allow traffic to OpenVPN.
-If you did not change the port and protocol in the ''/etc/openvpn/server.conf'' file, you will need to open up UDP traffic to port 1194. If you modified the port and/or protocol, substitute the values you selected here.
-We'll also add the SSH port in case you forgot to add it when following the prerequisite tutorial:
-<code>
-sudo ufw allow 1194/udp
-sudo ufw allow OpenSSH
-</code>
-Now, we can disable and re-enable UFW to load the changes from all of the files we've modified:
-<code>
-sudo ufw disable
-sudo ufw enable
-</code>
-Our server is now configured to correctly handle OpenVPN traffic.
-=====Step 9: Start and Enable the OpenVPN Service=====
-We're finally ready to start the OpenVPN service on our server. We can do this using systemd.
-We need to start the OpenVPN server by specifying our configuration file name as an instance variable after the systemd unit file name. Our configuration file for our server is called ''/etc/openvpn/server.conf'', so we will add @server to end of our unit file when calling it:
-<code>
-sudo systemctl start openvpn@server
-</code>
-Double-check that the service has started successfully by typing:
-<code>
-sudo systemctl status openvpn@server
-</code>
-If everything went well, your output should look something that looks like this:
-<code>
-Output
-● openvpn@server.service - OpenVPN connection to server
-   Loaded: loaded (/lib/systemd/system/openvpn@.service; disabled; vendor preset: enabled)
-   Active: active (running) since Tue 2016-05-03 15:30:05 EDT; 47s ago
-     Docs: man:openvpn(8)
-           https://community.openvpn.net/openvpn/wiki/Openvpn23ManPage
-           https://community.openvpn.net/openvpn/wiki/HOWTO
-  Process: 5852 ExecStart=/usr/sbin/openvpn --daemon ovpn-%i --status /run/openvpn/%i.status 10 --cd /etc/openvpn --script-security 2 --config /etc/openvpn/%i.conf --writepid /run/openvpn/%i.pid (code=exited, sta
- Main PID: 5856 (openvpn)
-    Tasks: 1 (limit: 512)
-   CGroup: /system.slice/system-openvpn.slice/openvpn@server.service
-           └─5856 /usr/sbin/openvpn --daemon ovpn-server --status /run/openvpn/server.status 10 --cd /etc/openvpn --script-security 2 --config /etc/openvpn/server.conf --writepid /run/openvpn/server.pid
-May 03 15:30:05 openvpn2 ovpn-server[5856]: /sbin/ip addr add dev tun0 local 10.8.0.1 peer 10.8.0.2
-May 03 15:30:05 openvpn2 ovpn-server[5856]: /sbin/ip route add 10.8.0.0/24 via 10.8.0.2
-May 03 15:30:05 openvpn2 ovpn-server[5856]: GID set to nogroup
-May 03 15:30:05 openvpn2 ovpn-server[5856]: UID set to nobody
-May 03 15:30:05 openvpn2 ovpn-server[5856]: UDPv4 link local (bound): [undef]
-May 03 15:30:05 openvpn2 ovpn-server[5856]: UDPv4 link remote: [undef]
-May 03 15:30:05 openvpn2 ovpn-server[5856]: MULTI: multi_init called, r=256 v=256
-May 03 15:30:05 openvpn2 ovpn-server[5856]: IFCONFIG POOL: base=10.8.0.4 size=62, ipv6=0
-May 03 15:30:05 openvpn2 ovpn-server[5856]: IFCONFIG POOL LIST
-May 03 15:30:05 openvpn2 ovpn-server[5856]: Initialization Sequence Completed
-</code>
-You can also check that the OpenVPN ''tun0'' interface is available by typing:
-<code>
-ip addr show tun0
-</code>
-You should see a configured interface:
-<code>
-Output
-: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN group default qlen 100
-    link/none
-    inet 10.8.0.1 peer 10.8.0.2/32 scope global tun0
-       valid_lft forever preferred_lft forever
-</code>
-If everything went well, enable the service so that it starts automatically at boot:
-<code>
-sudo systemctl enable openvpn@server
-</code>
-=====Step 10: Create Client Configuration Infrastructure=====
-Next, we need to set up a system that will allow us to create client configuration files easily.
-====Creating the Client Config Directory Structure====
-Create a directory structure within your home directory to store the files:
-<code>
-mkdir -p ~/client-configs/files
-</code>
-Since our client configuration files will have the client keys embedded, we should lock down permissions on our inner directory:
-<code>
-chmod 700 ~/client-configs/files
-</code>
-====Creating a Base Configuration====
-Next, let's copy an example client configuration into our directory to use as our base configuration:
-<code>
-cp /usr/share/doc/openvpn/examples/sample-config-files/client.conf ~/client-configs/base.conf
-</code>
-Open this new file in your text editor:
-<code>
-nano ~/client-configs/base.conf
-</code>
-Inside, we need to make a few adjustments.
-First, locate the ''remote'' directive. This points the client to our OpenVPN server address. This should be the public IP address of your OpenVPN server. If you changed the port that the OpenVPN server is listening on, change ''1194'' to the port you selected:\\
-\\
-**~/client-configs/base.conf**
-<code>
-. . .
-# The hostname/IP and port of the server.
-# You can have multiple remote entries
-# to load balance between the servers.
-remote server_IP_address 1194
-. . .
-</code>
-Be sure that the protocol matches the value you are using in the server configuration:\\
-\\
-**~/client-configs/base.conf**
-<code>
-proto udp
-</code>
-Next, uncomment the ''user'' and ''group'' directives by removing the ";":\\
-\\
-**~/client-configs/base.conf**
-<code>
-# Downgrade privileges after initialization (non-Windows only)
-user nobody
-group nogroup
-</code>
-Find the directives that set the ''ca'', ''cert'', and ''key''. Comment out these directives since we will be adding the certs and keys within the file itself:\\
-\\
-**~/client-configs/base.conf**
-<code>
-# SSL/TLS parms.
-# See the server config file for more
-# description.  It's best to use
-# a separate .crt/.key file pair
-# for each client.  A single ca
-# file can be used for all clients.
-#ca ca.crt
-#cert client.crt
-#key client.key
-</code>
-Mirror the ''cipher'' and ''auth'' settings that we set in the ''/etc/openvpn/server.conf'' file:\\
-\\
-**~/client-configs/base.conf**
-<code>
-cipher AES-128-CBC
-auth SHA256
-</code>
-Next, add the ''key-direction'' directive somewhere in the file. This **must** be set to "1" to work with the server:\\
-\\
-**~/client-configs/base.conf**
-<code>
-key-direction 1
-</code>
-Finally, add a few **commented out** lines. We want to include these with every config, but should only enable them for Linux clients that ship with a ''/etc/openvpn/update-resolv-conf'' file. This script uses the ''resolvconf'' utility to update DNS information for Linux clients.\\
-\\
-**~/client-configs/base.conf**
-<code>
-# script-security 2
-# up /etc/openvpn/update-resolv-conf
-# down /etc/openvpn/update-resolv-conf
-</code>
-If your client is running Linux and has an ''/etc/openvpn/update-resolv-conf'' file, you should uncomment these lines from the generated OpenVPN client configuration file.
-Save the file when you are finished.
-====Creating a Configuration Generation Script====
-Next, we will create a simple script to compile our base configuration with the relevant certificate, key, and encryption files. This will place the generated configuration in the ''~/client-configs/files'' directory.
-Create and open a file called ''make_config.sh'' within the ''~/client-configs'' directory:
-<code>
-nano ~/client-configs/make_config.sh
-</code>
-Inside, paste the following script:\\
-\\
-**~/client-configs/make_config.sh**
-<code>
-#!/bin/bash
-# First argument: Client identifier
-KEY_DIR=~/openvpn-ca/keys
-OUTPUT_DIR=~/client-configs/files
-BASE_CONFIG=~/client-configs/base.conf
-cat ${BASE_CONFIG} \
-    <(echo -e '<ca>') \
-    ${KEY_DIR}/ca.crt \
-    <(echo -e '</ca>\n<cert>') \
-    ${KEY_DIR}/${1}.crt \
-    <(echo -e '</cert>\n<key>') \
-    ${KEY_DIR}/${1}.key \
-    <(echo -e '</key>\n<tls-auth>') \
-    ${KEY_DIR}/ta.key \
-    <(echo -e '</tls-auth>') \
-    > ${OUTPUT_DIR}/${1}.ovpn
-</code>
-Save and close the file when you are finished.
-Mark the file as executable by typing:
-<code>
-chmod 700 ~/client-configs/make_config.sh
-</code>
-=====Step 11: Generate Client Configurations=====
-Now, we can easily generate client configuration files.
-If you followed along with the guide, you created a client certificate and key called ''client1.crt'' and ''client1.key'' respectively by running the ''./build-key client1'' command in step 6. We can generate a config for these credentials by moving into our ''~/client-configs'' directory and using the script we made:
-<code>
-cd ~/client-configs
-./make_config.sh client1
-</code>
-If everything went well, we should have a ''client1.ovpn'' file in our ''~/client-configs/files'' directory:
-<code>
-ls ~/client-configs/files
-</code>
-<code>
-Output
-client1.ovpn
-</code>
-====Transferring Configuration to Client Devices====
-We need to transfer the client configuration file to the relevant device. For instance, this could be your local computer or a mobile device.
-While the exact applications used to accomplish this transfer will depend on your choice and device's operating system, you want the application to use SFTP (SSH file transfer protocol) or SCP (Secure Copy) on the backend. This will transport your client's VPN authentication files over an encrypted connection.
-Here is an example SFTP command using our client1.ovpn example. This command can be run from your local computer (OS X or Linux). It places the ''.ovpn'' file in your home directory:
-<code>
-sftp sammy@openvpn_server_ip:client-configs/files/client1.ovpn ~/
-</code>
-=====Step 12: Install the Client Configuration=====
-Installation should be relatively straightforward on any OS. Consider consulting the documentation for your own OS if you have trouble figuring it out on your own. [[https://www.digitalocean.com/community/tutorials/how-to-set-up-an-openvpn-server-on-ubuntu-16-04#step-12-install-the-client-configuration|Here]] is a link to the documentation written on the original article
-Add guide on how to make it start at boot time on Linux FIXME
-======Revoking Client Certificates======
-Occasionally, you may need to revoke a client certificate to prevent further access to the OpenVPN server.
-To do so, enter your CA directory and re-source the vars file:
-<code>
-cd ~/openvpn-ca
-source vars
-</code>
-Next, call the ''revoke-full'' command using the client name that you wish to revoke:
-<code>
-./revoke-full client3
-</code>
-This will show some output, ending in error 23. This is normal and the process should have successfully generated the necessary revocation information, which is stored in a file called ''crl.pem'' within the keys subdirectory.
-Transfer this file to the ''/etc/openvpn'' configuration directory:
-<code>
-sudo cp ~/openvpn-ca/keys/crl.pem /etc/openvpn
-</code>
-Next, open the OpenVPN server configuration file:
-<code>
-sudo nano /etc/openvpn/server.conf
-</code>
-At the bottom of the file, add the crl-verify option, so that the OpenVPN server checks the certificate revocation list that we've created each time a connection attempt is made:\\
-\\
-**/etc/openvpn/server.conf**
-<code>
-crl-verify crl.pem
-</code>
-Save and close the file.
-Finally, restart OpenVPN to implement the certificate revocation:
-    sudo systemctl restart openvpn@server
-The client should now longer be able to successfully connect to the server using the old credential.
-To revoke additional clients, follow this process:
-  - Generate a new certificate revocation list by sourcing the vars file in the ~/openvpn-ca directory and then calling the revoke-full script on the client name.
-  - Copy the new certificate revocation list to the /etc/openvpn directory to overwrite the old list.
-  - Restart the OpenVPN service.
-This process can be used to revoke any certificates that you've previously issued for your server.