Set Up OpenConnect VPN Server (ocserv) on CentOS 8/RHEL 8 with Let’s Encrypt

This tutorial is going to show you how to run your own VPN server by installing OpenConnect VPN server on CentOS 8/RHEL 8. OpenConnect VPN server, aka ocserv, is an open-source implementation of the Cisco AnyConnnect VPN protocol, which is widely-used in businesses and universities. AnyConnect is a SSL-based VPN protocol that allows individual users to connect to a remote network.

Why Set Up Your Own VPN Server?

  • Maybe you are a VPN service provider or a system administator, which behooves you to set up your own VPN server.
  • You don’t trust the no logging policy of VPN service providers, so you go the self-host route.
  • You can use VPN to implement network security policy. For example, if you run your own email server, you can require users to login only from the IP address of the VPN server by creating an IP address whitelist in the firewall. Thus, your email server is hardened to prevent hacking activities.
  • Perhaps you are just curious to know how VPN server works.

install openconnect ocserv VPN server centos 8

Features of OpenConnect VPN server

  • Lightweight and fast. In my test, I can watch YouTube 4K videos with OpenConnect VPN. YouTube is blocked in my country (China).
  • Runs on Linux and most BSD servers.
  • Compatible with Cisco AnyConnect client
  • There are OpenConnect client software for Linux, MacOS, Windows and OpenWRT. For Android and iOS, you can use the Cisco AnyConnect Client.
  • Supports password authentication and certificate authentication
  • Supports RADIUS accounting.
  • Supports virtual hosting (multiple domains)
  • Easy to set up

I particularly like the fact that compared to other VPN technologies, it is very easy and convenient for the end-user to use OpenConnect VPN. Whenever I install a Linux distro on my computer and want to quickly unblock websites or hide my IP address, I simply run the following command to connect to my OpenConnect VPN server.

sudo openconnect -b vpn.mydomain.com

The openconnect client software is available for Debian, Ubuntu, Fedora, RHEL, CentOS, Arch Linux and OpenSUSE. You can easily install it with your package manager.

sudo apt install openconnect
sudo dnf install openconnect
sudo pacman -S openconnect

Prerequisites

To follow this tutorial, you will need a VPS (Virtual Private Server) that can access blocked websites freely (Outside of your country or Internet filtering system). I recommend Vultr VPS (This is my referral link. You can get $50 free credit by creating an account at Vultr via my referral link).  They offer 512M RAM high performance KVM VPS for just $2.5 per month, which is perfect for your private VPN server. Once you have a VPS, install CentOS 8 on it and follow the instructions below.

You also need a domain name. I registered my domain name from NameCheap because the price is low and they give whois privacy protection free for life.

Note: The new Vultr $2.5/month plan includes IPv6 address only. You can select the $3.5/month plan at the New York (NJ) data center to have both IPv4 and IPv6 address.

Step 1: Install OpenConnect VPN Server (ocserv) on CentOS 8

Log into your CentOS 8 server via SSH. Then run the following commands to install the ocserv package from the EPEL repository.

sudo dnf install epel-release
sudo dnf install ocserv

Step 2: Open Ports in Firewall

Firewall on CentOS is enabled by default. It’s customary to configure ocserv to listen on port 443, so run the following commands to open TCP and UDP port 443.

sudo firewall-cmd --zone=public --permanent --add-port=443/tcp
sudo firewall-cmd --zone=public --permanent --add-port=443/udp

We also need to open TCP port 80 to obtain TLS certificate from Let’s Encrypt.

sudo firewall-cmd --zone=public --permanent --add-port=80/tcp

Reload firewall for the changes to take effect.

sudo systemctl reload firewalld

Step 3: Install Let’s Encrypt Client (Certbot) on CentOS 8 Server

The gnutls-utils package installed along with ocserv provides tools to create your own CA and server certificate, but we will obtain and install Let’s Encrypt certificate. The advantage of using Let’s Encrypt certificate is that it’s free, easier to set up and trusted by VPN client software.

Run the following command to install the Let’s Encrypt client (certbot) on CentOS 8.

sudo dnf install certbot

To check version number, run

certbot --version

Sample output:

certbot 1.0.0

Step 4: Obtain a Trusted TLS Certificate from Let’s Encrypt

I recommend using the standalone or webroot plugin to obtain TLS certificate.

Standalone Plugin

If there’s no web server running on your CentOS 8 server and you want OpenConnect VPN server to use port 443, then you can use the standalone plugin to obtain TLS certificate from Let’s Encrypt. Set DNS A record for vpn.example.com at your domain registrar’s website, then run the following command to obtain certificate.

sudo certbot certonly --standalone --preferred-challenges http --agree-tos --email you@example.com -d vpn.example.com

Explanation:

  • certonly: Obtain a certificate but don’t install it.
  • --standalone: Use the standalone plugin to obtain a certificate
  • --preferred-challenges http: Perform http-01 challenge to validate our domain, which will use port 80.
  • --agree-tos: Agree to Let’s Encrypt terms of service.
  • --email: Email address is used for account registration and recovery.
  • -d: Specify your domain name.

As you can see from the following screenshot, I successfully obtained the certificate.

ocserv centos 8 letsencrypt

Using webroot Plugin

If your CentOS 8 server has a web server listening on port 80 and 443, then it’s a good idea to use the webroot plugin to obtain a certificate because the webroot plugin works with pretty much every web server and we don’t need to install the certificate in the web server.

First, you need to create a virtual host for vpn.example.com.

Apache

If you are using Apache web server, then create virtual host under /etc/httpd/conf.d/ directory.

sudo nano /etc/httpd/conf.d/vpn.example.com.conf

And paste the following lines into the file.

<VirtualHost *:80>        
        ServerName vpn.example.com

        DocumentRoot /var/www/html/
</VirtualHost>

Save and close the file. Reload Apache for the changes to take effect.

sudo systemctl reload httpd

Once virtual host is created and enabled, run the following command to obtain Let’s Encrypt certificate using webroot plugin.

sudo certbot certonly --webroot --agree-tos --email you@example.com -d vpn.example.com -w /var/www/html/

Nginx

If you are using Nginx web server, then create virtual host under /etc/nginx/conf.d/.

sudo nano /etc/nginx/conf.d/vpn.example.com.conf

Paste the following lines into the file.

server {
      listen 80;
      server_name vpn.example.com;

      root /usr/share/nginx/html/;

      location ~ /.well-known/acme-challenge {
         allow all;
      }
}

Save and close the file. Reload Nginx for the changes to take effect.

sudo systemctl reload nginx

Once virtual host is created and enabled, run the following command to obtain Let’s Encrypt certificate using webroot plugin.

sudo certbot certonly --webroot --agree-tos --email you@example.com -d vpn.example.com -w /usr/share/nginx/html/

Step 5: Edit OpenConnect VPN Server Configuration File

Edit ocserv configuration file.

sudo nano /etc/ocserv/ocserv.conf

First, configure password authentication. By default, password authentication through PAM (Pluggable Authentication Modules) is enabled, which allows you to use CentOS system accounts to login from VPN clients. This behavior can be disabled by commenting out the following line.

auth = "pam"

If we want users to use separate VPN accounts instead of system accounts to login, we need to add the following line to enable password authentication with a password file.

auth = "plain[passwd=/etc/ocserv/ocpasswd]"

After finishing editing this config file, we will see how to use ocpasswd tool to generate the /etc/ocserv/ocpasswd file, which contains a list of usernames and encoded passwords.

Note: Ocserv supports client certificate authentication, but Let’s Encrypt does not issue client certificate. If you want to enable certificate authentication, you need to set up your own CA to issue client certificate.

Next, if you don’t want ocserv to use TCP and UDP port 443 (there’s a web server using port 443), then find the following two lines and change the port number. Otherwise leave them alone.

tcp-port = 443
udp-port = 443

Then find the following two lines. We need to change them.

server-cert = /etc/pki/ocserv/public/server.crt
server-key = /etc/pki/ocserv/private/server.key

Replace the default setting with the path of Let’s Encrypt server certificate and server key file.

server-cert = /etc/letsencrypt/live/vpn.example.com/fullchain.pem
server-key = /etc/letsencrypt/live/vpn.example.com/privkey.pem

It’s recommended to enable LZ4 compression, so uncomment the following line.

compression = true

Then, set the maximal number of clients. Default is 16. Set to zero for unlimited.

max-clients = 0

Set the number of devices a user is able to login from at the same time. Default is 2. Set to zero for unlimited.

max-same-clients = 0

Next, find the following line. Change false to true to enable MTU discovery, which can optimize VPN performance.

try-mtu-discovery = true

You can set the time that a client is allowed to stay idle before being disconnected via the following two parameters. If you prefer the client to stay connected indefinitely, then comment out these two parameters.

idle-timeout=1200
mobile-idle-timeout=2400

After that, set the default domain to vpn.example.com.

default-domain = vpn.example.com

The IPv4 network configuration is as follows by default. This will cause problems because most home routers also set the IPv4 network range to 192.168.1.0/24.

#ipv4-network = 192.168.1.0
#ipv4-netmask = 255.255.255.0

We can use another private IP address range (such as 10.10.10.0/24) to avoid IP address collision, so uncomment the above two lines and change the value of ipv4-network to 10.10.10.0.

ipv4-network = 10.10.10.0
ipv4-netmask = 255.255.255.0

Now uncomment the following line to tunnel all DNS queries via the VPN.

tunnel-all-dns = true

Change DNS resolver address. You can use Google’s public DNS server.

dns = 8.8.8.8
dns = 8.8.4.4

or Cloudflare’s public DNS server.

dns = 1.1.1.1
dns = 1.0.0.1

Note: If you are a VPN service provider, it’s a good practice to run your own DNS resolver. If there’s a DNS resolver running on the same server, then specify the DNS as

dns = 10.10.10.1

10.10.10.1 is the IP address of OpenConnect VPN server in the VPN LAN. This will speed up DNS lookups a little bit for clients because the network latency between the VPN server and the DNS resolver is eliminated.

Then comment out all the route parameters (add # character at the beginning of the following lines), which will set the server as the default gateway for the clients.

#route = 10.10.10.0/255.255.255.0
#route = 192.168.0.0/255.255.0.0
#route = fef4:db8:1000:1001::/64

#no-route = 192.168.5.0/255.255.255.0

Finally, scroll down to the end of the file (In Nano text editor, you can go to the end of the file by pressing Ctrl+W, then pressing Ctrl+V), and comment out the user-profile parameter, because the user profile XML file is not needed by OpenConnect clients or the Cisco AnyConnect client on iOS and Android.

#user-profile = profile.xml

Note: If user-profile is enabled, the Cisco AnyConnect client on iOS will produce the following error when connecting to the OpenConnect VPN server.

Failed to download the AnyConnect profile. Please try again.

Save and close the file.

Step 6: Creating VPN Accounts

Now use the ocpasswd tool to generate VPN accounts.

sudo ocpasswd -c /etc/ocserv/ocpasswd username

You will be asked to set a password for the user and the information will be saved to /etc/ocserv/ocpasswd file. To reset password, simply run the above command again.

Now we can start the ocserv serice.

sudo systemctl start ocserv

And enable auto-start at boot time.

sudo systemctl enable ocserv

You can check its status with:

systemctl status ocserv

Sample output:

 ocserv.service - OpenConnect SSL VPN server
   Loaded: loaded (/usr/lib/systemd/system/ocserv.service; enabled; vendor preset: disabled)
   Active: active (running) since Fri 2020-01-03 19:04:53 CST; 15s ago
     Docs: man:ocserv(8)
 Main PID: 19851 (ocserv-main)
    Tasks: 2 (limit: 5061)
   Memory: 3.2M
   CGroup: /system.slice/ocserv.service
           ├─19851 ocserv-main
           └─19853 ocserv-sm

Hint: If the above command doesn’t quit immediately, you can press the Q key to gain back control of the terminal.

By default OpenConnect VPN server listens on TCP and UDP port 443. If it’s being used by a web server, then the VPN server can’t be started. We will see how to make OpenConnect VPN server and web server use the same port later.

Step 7: Enable IP Forwarding in Linux Kernel

In order for the VPN server to route packets between VPN client and the Internet, we need to enable IP forwarding. Edit sysctl.conf file.

sudo nano /etc/sysctl.conf

Add the following line at the end of this file.

net.ipv4.ip_forward = 1

Save and close the file. Then apply the changes with the below command. The -p option will load sysctl settings from /etc/sysctl.conf file. This command will preserve our changes across system reboots.

sudo sysctl -p

Step 8: Configure IP Masquerading in Firewall

Run the following command to enable IP masquerading in the server firewall.

sudo firewall-cmd --zone=public --permanent --add-masquerade
sudo systemctl reload firewalld

This will hide your VPN network from the outside world. So the Internet can only see your VPN server’s IP, but can’t see your VPN client’s IP, just like your home router hides your private home network.

Now OpenConnect VPN server is ready to accept client connections.

Install and Use OpenConnect VPN client on CentOS 8/Fedora Desktop

Run the following command to install OpenConnect VPN command-line client on Fedora desktop.

sudo dnf install openconnect

On CentOS 8, you need to enable the EPEL repository in order to install the client.

sudo dnf install epel-release
sudo dnf install openconnect

Then you can connect to VPN server from the command line like below. -b flag will make it run in the background after the connection is established.

sudo openconnect -b vpn.example.com

By default, openconnect client sends request to port 443 of the server. If you configured a different port for the server, then you can add the port number.

sudo openconnect -b vpn.example.com:port-number

You will be asked to enter VPN username and password. If connection is successfully established, you will see the following message.

Got CONNECT response: HTTP/1.1 200 CONNECTED
CSTP connected. DPD 90, Keepalive 32400
Connected as 10.10.50.139, using SSL + lz4
Continuing in background; pid 2137
Established DTLS connection (using GnuTLS). Ciphersuite (DTLS1.2)-(PSK)-(AES-256-GCM).

If the connection failed, you can check the ocserv log to find out why. (Perhaps you didn’t enter the password correctly.)

sudo journaltcl -eu ocserv

To stop the connection, run:

sudo pkill openconnect

To run the client non-interactively, use the following syntax.

echo -n password | sudo openconnect -b vpn.example.com -u username --passwd-on-stdin

If you are successfully connected to the VPN server, but your public IP address doesn’t change, that’s because IP forwarding or IP masquerading is not working.

Auto-Connect on System Startup

To make OpenConnect VPN client automatically connect to the server at boot time, we can create a systemd service unit.

sudo nano /etc/systemd/system/openconnect.service

Put the following lines to the file. Replace the red text.

[Unit]
  Description=OpenConnect VPN Client
  After=network-online.target
  Wants=network-online.target

[Service]
  Type=simple
  ExecStart=/bin/bash -c '/bin/echo -n password | /usr/sbin/openconnect vpn.example.com -u username --passwd-on-stdin'
  KillSignal=SIGINT
  Restart=always
  RestartSec=2

[Install]
  WantedBy=multi-user.target

Save and close the file. Then enable this service so that it will start at boot time.

sudo systemctl enable openconnect.service

Explanation of the file content:

  • After=network-online.target and Wants=network-online.target make this service run after network is up.
  • In reality, this service can still run before network is up. We add Restart=always and RestartSec=2 to restart this service after 2 seconds if this service fails.
  • Systemd doesn’t recognise pipe redirection. So in the ExecStart directive, we wrap the command in single quotes and run it with the Bash shell.
  • Since OpenConnect VPN client will run as a systemd service, which runs in the background, there’s no need to add -b flag to the openconnect command.
  • The KillSignal directive tells Systemd to send the SIGINT signal when the systemctl stop openconnect command is issued. This will perform a clean shutdown by logging the session off, and restoring DNS server settings and the Linux kernel routing table.

To start this Systemd service immediately, run

sudo systemctl start openconnect

To stop this Systemd service, run

sudo systemctl stop openconnect

OpenConnect GUI Client for Windows and MacOS

They can be downloaded from OpenConnect GUI Github Page.

Speed

OpenConnect VPN is pretty fast. I can use it to watch 4k videos on YouTube.

ocserv letsencrypt

Auto-Renew Let’s Encrypt Certificate

Edit root user’s crontab file.

sudo crontab -e

Add the following line at the end of the file to run the Cron job daily. If the certificate is going to expire in 30 days, certbot will try to renew the certificate. It’s necessary to restart ocserv service for the VPN server to pick up new certificate and key file.

@daily certbot renew --quiet && systemctl restart ocserv

Optimization

OpenConnect by default uses TLS over UDP protocol (DTLS) to achieve faster speed, but UDP can’t provide reliable transmission. TCP is slower than UDP but can provide reliable transmission. One optimization tip I can give you is to disable DTLS, use standard TLS (over TCP), then enable TCP BBR to boost TCP speed.

You may also want to disable DTLS to bypass firewall restrictions because DTLS uses UDP port 443. Standard TLS uses TCP port 443.

To disable DTLS, comment out (add # symbol at the beginning) the following line in ocserv configuration file.

udp-port = 443

Save and close the file. Then restart ocserv service.

sudo systemctl restart ocserv.service

To enable TCP BBR, please check out the following tutorial. It’s written for Ubuntu, but also works for CentOS.

In my test, standard TLS with TCP BBR enabled is two times faster than DTLS.

Troubleshooting

Note that if you are using OpenVZ VPS, make sure you enable the TUN virtual networking device in VPS control panel. (If you use Vultr VPS, then you have KVM-based VPS, so you don’t have to worry about this.)

If you encounter any problem, then check OpenConnect VPN server log.

sudo journalctl -eu ocserv.service

I found that if I change port 443 to a different port, the great firewall of China will block this VPN connection.

Make OpenConnect VPN server and web server use port 443 at the same time

Normally a port can only be used by one process. However, we can use HAproxy (High Availability Proxy) and SNI (Server Name Indication) to make ocserv and Apache/Nginx use port 443 at the same time.

First, edit ocserv configuration file.

sudo nano /etc/ocserv/ocserv.conf

Find the following line.

#listen-host = [IP|HOSTNAME]

Change it to

listen-host = 127.0.0.1

This will make ocserv listen on 127.0.0.1 because later HAproxy will need to listen on the public IP address.

Then add the following line. This will allow ocserv to obtain the client IP address instead of HAproxy IP address.

listen-proxy-proto = true

Save and close the file. Restart ocserv for the changes to take effect.

sudo systemctl restart ocserv

Next, we also need to make the web server listen on localhost only, instead of listening on public IP address. If you use Nginx, edit the server block file.

sudo nano /etc/nginx/conf.d/www.example.com.conf

In the SSL server block, find the following directive.

listen 443 ssl;

Change it to

listen 127.0.0.2:443 ssl;

This time we make it listen on 127.0.0.2:443 because 127.0.0.1:443 is already taken by ocserv. Save and close the file. Then restart Nginx.

sudo systemctl restart nginx

Now install HAproxy.

sudo dnf install haproxy

Edit configuration file.

sudo nano /etc/haproxy/haproxy.cfg

Copy and paste the following lines to the end of the file. Replace 12.34.56.78 with the public IP address of your server. Replace vpn.example.com with the domain name used by ocserv and www.example.com with the domain name used by your web server.

frontend https
   bind 12.34.56.78:443
   mode tcp
   tcp-request inspect-delay 5s
   tcp-request content accept if { req_ssl_hello_type 1 }

   use_backend ocserv if { req_ssl_sni -i vpn.example.com }
   use_backend nginx if { req_ssl_sni -i www.example.com }
   use_backend nginx if { req_ssl_sni -i example.com }

   default_backend ocserv

backend ocserv
   mode tcp
   option ssl-hello-chk
   server ocserv 127.0.0.1:443 send-proxy-v2

backend nginx
   mode tcp
   option ssl-hello-chk
   server nginx 127.0.0.2:443 check

Save and close the file. Then restart HAproxy.

sudo systemctl restart haproxy

In the configuration above, we utilized the SNI (Server Name Indication) feature in TLS to differentiate VPN traffic and normal HTTPS traffic.

  • When vpn.example.com is in the TLS Client Hello, HAProxy redirect traffic to the ocserv backend.
  • When www.example.com is in the TLS Client Hello, HAProxy redirect traffic to the nginx backend.
  • If the client doesn’t specify the server name in TLS Client Hello, then HAproxy will use the default backend (ocserv).

You can test this setup with the openssl tool. First, run the following command multiple times.

echo | openssl s_client -connect your-server-IP:443 | grep subject

We didn’t specify server name in the above command, so HAproxy will always pass the request to the default backend (ocserv), and its certificate will be sent to the client. Next, run the following two commands.

echo | openssl s_client -servername www.example.com -connect your-server-IP:443 | grep subject

echo | openssl s_client -servername vpn.example.com -connect your-server-IP:443 | grep subject

Now we specified the server name in the commands, so HAproxy will pass request accordingly. Note that the Cisco AnyConnect App doesn’t support TLS SNI, so it’s better to set ocserv as the default backend in HAProxy configuration file.

When renewing Let’s Encrypt certificate for your website, it’s recommended that you use the http-01 challenge instead of tls-alpn-01 challenge, because HAproxy is listening on port 443 of the public IP address, so it can interfere with the renew process.

sudo certbot renew --preferred-challenges http-01

Fixing HAproxy Error

If your Nginx web server doesn’t show up in your browser and you see the following messages in haproxy log (/var/log/haproxy.log)

Server nginx/nginx is DOWN, reason: Socket error, info: "Connection reset by peer

backend nginx has no server available!

Layer6 invalid response

It maybe because your backend Nginx web server is using a TLS certificate with OCSP must staple extension. Nginx doesn’t send the OCSP staple information on the first HTTP request. To make it work, be sure to add a resolver in your Nginx virtual host configuration like below.

{
     ....
     ssl_trusted_certificate /etc/letsencrypt/live/www.example/chain.pem;
     ssl_stapling on;
     ssl_stapling_verify on;

    resolver 8.8.8.8;
    ....
}

Save and close the file. Then restart Nginx.

sudo systemctl restart nginx

Also consider removing health check for the backend server in HAproxy. So change

server nginx 127.0.0.2:443 check

To

server nginx 127.0.0.2:443

Save and close the file. Then restart HAproxy.

sudo systemctl restart haproxy

How to Disable TLS 1.0 and TLS 1.1 in ocserv

The PCI council deprecated TLS 1.0 in June 30, 2018 and main stream web browsers are going to disable TLS 1.0 and TLS 1.1 in 2020. We should do the same with VPN server. Edit the main configuration file.

sudo nano /etc/ocserv/ocserv.conf

Find the following line:

tls-priorities = "NORMAL:%SERVER_PRECEDENCE:%COMPAT:-VERS-SSL3.0"

To disable TLS 1.0 and TLS 1.1 in OpenConnect VPN server, replace it with:

tls-priorities = "NORMAL:%SERVER_PRECEDENCE:%COMPAT:-RSA:-VERS-SSL3.0:-ARCFOUR-128:-VERS-TLS1.0:-VERS-TLS1.1"

Save and close the file. Then restart ocserv.

sudo systemctl restart ocserv

Now ocserv will only accept TLS 1.3 and TLS 1.2. For further information on configuring the TLS parameter in ocserv, please see GnuTLS priority strings.

To check if TLS 1.0 is supported in your OpenConnect VPN server, run the following command.

openssl s_client -connect vpn.your-domain.com:443 -tls1

And check TLS 1.1

openssl s_client -connect vpn.your-domain.com:443 -tls1_1

If you see the following message in the output, that means the TLS version is not supported.

New, (NONE), Cipher is (NONE)
Secure Renegotiation IS NOT supported

Per User or Per Group Configuration

Ocserv allows per user and per group configurations. To enable this feature, uncomment the following two lines in /etc/ocserv/ocserv.conf file.

config-per-user = /etc/ocserv/config-per-user/
config-per-group = /etc/ocserv/config-per-group/

Save and close the file. Then create the per user and per group config directory.

sudo mkdir /etc/ocserv/config-per-user/
sudo mkdir /etc/ocserv/config-per-group/

Next, you can create a file under these two directories. For example, create the user1 file to allow custom configuration for user1.

sudo nano /etc/ocserv/config-per-user/user1

You can also create the group1 file to allow custom configuration for the group named group1.

sudo nano /etc/ocserv/config-per-group/group1

You can add something like below in the file.

route = 10.10.10.0/255.255.255.0

This means that after user1 connect to this VPN server, only traffic to the 10.10.10.0/24 network will be routed via VPN server. Traffic to other IP addresses are routed via the original gateway. I use this to allow my another VPS (virtual private server) to connect to this VPN server.

Save and close the file. Restart ocserv for the changes to take effect.

How to Set Up VPN Relay

Suppose there are two servers: server A and server B.

  • You have good connection to server A. Latency is very low and no packet is dropped.
  • You have bad connection to server B. Latency is high and there’s packet being dropped.
  • The connection between server A and server B is good.

Naturally you want to install VPN on server A. But what if you want the Internet to see your traffic coming from server B’s IP address? Well, you can install ocserv VPN on server B, then set up HAProxy on server A to proxy the traffic between your computer and server B.

On server A, install HAProxy.

sudo dnf install haproxy

Edit main configuration file.

sudo nano /etc/haproxy/haproxy.cfg

Create a front end and back end just like before. Replace 12.34.56.78 with server A’s public IP address. Replace 12.34.56.79 with server B’s public IP address.

frontend https
   bind 12.34.56.78:443
   mode tcp
   tcp-request inspect-delay 5s
   tcp-request content accept if { req_ssl_hello_type 1 }

   use_backend ocserv if { req_ssl_sni -i vpn.example.com }

   default_backend ocserv

backend ocserv
   mode tcp
   option ssl-hello-chk
   server ocserv 12.34.56.79:443 send-proxy-v2

Save and close the file. On server B, you should configure ocserv to listen on the public IP address and enable proxy protocol just like before. After that, edit the A record of vpn.example.com. It should be pointed to the IP address of server A.

Restart HAProxy and Ocserv and it should be working.

Virtual Hosting

To add a new virtual host in ocserv, first you need to use the method in step 4 to obtain a TLS certificate for the new virtual host. Then edit ocserv configuration file.

sudo nano /etc/ocserv/ocserv.conf

Go to the bottom of this file. In Nano text editor, you can press Ctrl+W, then Ctrl+V to jump to the bottom of a file. Add the following lines. Replace vpn2.example.com with the hostname of the second virtual host.

[vhost:vpn2.example.com]
#Allow password authentication and certificate authentication
enable-auth = "plain[passwd=/etc/ocserv/ocpasswd]"
auth = "certificate"

tcp-port = 443

#uncomment the two lines if ocserv runs behind HAProxy.
#listen-host = 127.0.0.1
#listen-proxy-proto = true

# SSL/TLS configuration
ca-cert = /etc/ocserv/ssl/ca-cert.pem
server-cert = /etc/letsencrypt/live/vpn2.example.com/fullchain.pem
server-key = /etc/letsencrypt/live/vpn2.example.com/privkey.pem
cert-user-oid = 0.9.2342.19200300.100.1.1

#Networking configuration. Use a different network range for this virtual host. 
device = vpns
ipv4-network = 10.10.20.0
ipv4-netmask = 255.255.255.0
route = default
dns = 8.8.8.8
tunnel-all-dns = true

compression = true
max-clients = 0
max-same-clients = 0
try-mtu-discovery = true
idle-timeout=1200
mobile-idle-timeout=2400

Save and close the file. Then restart ocserv.

sudo systemctl restart ocserv

Note that the ocserv daemon might tell you some parameters will be ignored for virtual host. However, I found that some of the ignored parameters are actually needed. For example, if you delete the device = vpns line from the virtual host, you might encounter the following error when establishing VPN connection to the virtual host.

VPN service unavailable; reason: Server configuration error

And the VPN server would produce the following error message in the log.

no networks are configured; rejecting client

Note that the AnyConnect VPN client on iOS doesn’t support TLS SNI, so iOS users will connect to the default virtual host.

Wrapping Up

That’s it! I hope this tutorial helped you install and configure OpenConnect VPN on CentOS 8/RHEL 8 server. As always, if you found this post useful, then subscribe to our free newsletter to get more tips and tricks. Take care 🙂

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20 Responses to “Set Up OpenConnect VPN Server (ocserv) on CentOS 8/RHEL 8 with Let’s Encrypt

  • Is there a way I can see who is connected to the vpn server, for how long it’s connected ?

  • I am trying to connect to the server via iOS AnyConnect.

    here is the info that I get from occtl -debug

    show sessions all
    session user vhost ip user agent created status
    uVfDbl saeed default x.x.x.x AnyConnect AppleSSLVPN_D 30m:00s authenticated

    default: disconnect user ‘saeed’ (1844) from x.x.x.x with IP 10.10.10.122 (reason: unspecified error, time: 10s)
    default: connected user ‘saeed’ (1859) from x.x.x.x with IP 10.10.10.122
    default: disconnect user ‘saeed’ (1859) from x.x.x.x with IP 10.10.10.122 (reason: unspecified error, time: 8s)
    default: connected user ‘saeed’ (1874) from x.x.x.x with IP 10.10.10.122
    default: disconnect user ‘saeed’ (1874) from x.x.x.x with IP 10.10.10.122 (reason: unspecified error, time: 4s)
    default: connected user ‘saeed’ (1891) from x.x.x.x with IP 10.10.10.122
    default: disconnect user ‘saeed’ (1891) from x.x.x.x with IP 10.10.10.122 (reason: unspecified error, time: 11s)

    Despite of the fact that my session does not drop, It constantly get disconnected on the backend.
    I have no explanation for it, do you?

    • I can also found this error in my ocserv log sudo journalctl -eu ocserv, both when using OpenConnect and Anyconnect clients. I don’t know what it means, but it doesn’t affect web browsing or any user experience.

  • Tanzil Hussain
    9 months ago

    After setup server, i am connecting with IP instead of domain coz I don’t want to use with domain and its giving a warning of the untrusted server.
    a year ago it was sorted i forget that command that was something.

    no-cert-check = true
    in /etc/ocserv/ocserv.conf file

    but that is not working now could you please guide me on which line I can put in ocserv.conf file so it will not give any warning.
    thanks a lot

  • Following this tutorial, i can setup ocserv on a centos8 in google cloud. the connection between anyconnect client and vpns1 ethernet is good. however, the forwarding between vpns1 ethernet and etho in host looks never work, despite I tried many firewalld/iptables rules as following. can someone give me some suggestions to solve this problem bothered me several days. Thanks a lot in advance!

    firewall-cmd --permanent --zone=public --add-port=443/tcp
    firewall-cmd --permanent --zone=public --add-port=443/udp
    firewall-cmd --permanent --zone=public --add-masquerade
    firewall-cmd --reload

    or

    systemctl stop firewalld
    systemctl disable firewalld
    yum -y install iptables-services
    
    iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
    iptables -I FORWARD -o eth0 -s 10.20.30.0/24 -j ACCEPT
    iptables -I INPUT -s 10.20.30.0/24 -j ACCEPT
    iptables-save > /etc/sysconfig/iptables
    systemctl restart iptables

    or

    iptables -t nat -A POSTROUTING -s 10.20.30.0/24 -o eth0 -j MASQUERADE
    iptables -A FORWARD -i vpns+ -j ACCEPT
    iptables -A FORWARD -o vpns+ -j ACCEPT
    iptables-save > /etc/sysconfig/iptables
    systemctl restart iptables
    • CentOS 8 on Google Cloud servers doesn’t use the public zone. It uses the trusted zone. You can run the following command to check the current active zone.

      sudo firewall-cmd --list-all

      Sample outout:

      trusted
        target: ACCEPT
        icmp-block-inversion: no
        interfaces: 
        sources: 
        services: 
        ports: 
        protocols: 
        masquerade: no
        forward-ports: 
        source-ports: 
        icmp-blocks: 
        rich rules: 
      

      As you can see, masquerading is disabled. To enable masquerading, run the following commands.

      sudo firewall-cmd --zone=trusted --permanent --add-masquerade
      
      sudo systemctl reload firewalld
  • How to see the connected vpn users and manage them from gui

  • asif shaikh
    7 months ago

    can you please add TLS1.2 parameter configuration ?

  • amaresh Pattanayak
    4 months ago

    can you share an article how to setup VPN for IP v6?

  • I get an error in CentOS7 when cheking via

     sudo systemctl status ocserv 

    :

     error connecting to sec-mod socket 'ocserv.sock.1056e6c7': No such file or directory 

    client connects, and clien’t IP changes. but it will not open the pages that are censored. I’m using google’d DNS in configuration. you have any idea how to solve it?

    •  error connecting to sec-mod socket 'ocserv.sock.1056e6c7': No such file or directory 

      This is normal, because the ocserv.sock file is placed under the chroot directory (/var/lib/ocserv). The ocserv daemon tries to find the socket file in the global directory hierarchy. If it can’t be found there, ocserv will find the file in the chroot directory.

      If the client’s public IP address changed to the VPN server’s public IP address, then the VPN is working correctly. If you can’t open the censored page, it means your VPN server is located in the censored area, or the page is censored by other methods, not by IP address.

  • Hi, thanks for this great guide,
    i wanted to know if in ocserv we can allow it to be used with a proxy like squid & tiny proxy
    so as all the packets/data packets goes through tcp only or http packets with tcp in wireshark log instead of tls shown in wireshark logs.
    i have a setup like mobile client connects to ocserv server but currently it only shows http-over-tls in application data.
    it would be helpfull just wanted this detail so as to make it more secure.

    Thanks in advance.

  • hi

    how can we use socks5 with ocserv, can you share any steps or config changes to be done at ocserv.conf file & client-side.

    thanks

  • 兄弟知道怎么开启ipv6支持么?

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