Set up OpenConnect VPN Server (ocserv) on Ubuntu 16.04/18.04 with Let’s Encrypt

This tutorial is going to show you how to install OpenConnect VPN server on Ubuntu 16.04/18.04. OpenConnect VPN server, aka ocserv, is an open-source implementation of Cisco AnyConnnect VPN protocol, which is popular among businesses and universities. AnyConnect is a SSL-based VPN protocol that allows individual users to connect to a remote network.

Features of OpenConnect VPN server:

  • Lightweight and fast. In my test, I can watch YouTube in 4k with OpenConnect VPN. YouTube is blocked in my country.
  • Compatible with Cisco AnyConnect client
  • Supports password authentication and certificate authentication
  • Easy to set up

I particularly like that 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 Debian-based Linux distro on my computer and want to quickly unblock websites or hide my IP address, I install OpenConnect client and connect to the server with just two lines of commands:

sudo apt install openconnect

sudo openconnect -b vpn.mydomain.com

The gnutls-bin software package 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.

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 memory high performance KVM VPS for just $2.5 per month, which is perfect for your private VPN server. Once you have a VPS, install Ubuntu 16.04 or Ubuntu 18.04 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.

Update: 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.

Installing OpenConnect VPN Server on Ubuntu 16.04/18.04

Log into your Ubuntu 16.04/18.04 server. Then use apt to install the ocserv package,which is included in Ubuntu repository since 16.04.

sudo apt install ocserv

Once installed, the OpenConnect VPN server is automatically started. You can check its status with:

systemctl status ocserv

Sample output:

● ocserv.service - OpenConnect SSL VPN server
   Loaded: loaded (/lib/systemd/system/ocserv.service; enabled; vendor preset: enabled
   Active: active (running) since Thu 2017-11-30 05:45:07 UTC; 11s ago
     Docs: man:ocserv(8)
 Main PID: 19235 (ocserv-main)
   CGroup: /system.slice/ocserv.service
           ├─19235 ocserv-main                                                  
           └─19242 ocserv-secm 

If it’s not running, then you can start it with:

sudo systemctl start ocserv

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

Installing Let’s Encrypt Client (Certbot) on Ubuntu 16.04/18.04 Server

Run the following commands to install the latest version of certbot from the official PPA. software-properties-common is required if you want to install packages from PPA. It may be missing on your Ubuntu server.

sudo apt install software-properties-common

sudo add-apt-repository ppa:certbot/certbot

sudo apt update

sudo apt install certbot

To check version number, run

certbot --version

Sample output:

certbot 0.31.0

Obtaining a TLS Certificate from Let’s Encrypt

Standalone Plugin

If there’s no web server running on your Ubuntu 16.04/18.04 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. Run the following command. Don’t forget to set A record for your domain name.

sudo certbot certonly --standalone --preferred-challenges http --agree-tos --email your-email-address -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. By default the standalone plugin will perform tls-sni challenge, which uses port 443. Since port 443 is already used by OpenConnect VPN server, we need to change the default behavior.
  • --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 the from the following screenshot, I successfully obtained the certificate.

Using webroot Plugin

If your Ubuntu 16.04/18.04 server has a web server listening on port 80 and 443, and you want OpenConnect VPN server to use a different port, 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, then

sudo nano /etc/apache2/sites-available/vpn.example.com.conf

And paste the following lines into the file.

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

        DocumentRoot /var/www/vpn.example.com
</VirtualHost>

Save and close the file. Then create the web root directory.

sudo mkdir /var/www/vpn.example.com

Set www-data (Apache user) as the owner of the web root.

sudo chown www-data:www-data /var/www/vpn.example.com -R

Enable this virtual host.

sudo a2ensite vpn.example.com

Reload Apache for the changes to take effect.

sudo systemctl reload apache2

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 your-email-address -d vpn.example.com -w /var/www/vpn.example.com

Nginx

If you are using Nginx, then

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 /var/www/vpn.example.com/;

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

Save and close the file. Then create the web root directory.

sudo mkdir -p /var/www/vpn.example.com

Set www-data (Nginx user) as the owner of the web root.

sudo chown www-data:www-data /var/www/vpn.example.com -R

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 your-email-address -d vpn.example.com -w /var/www/vpn.example.com

Editing 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 Ubuntu system accounts to login from VPN clients. This behavior can be disabled by commenting out the following line.

auth = "pam[gid-min=1000]"

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. 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, 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/ssl/certs/ssl-cert-snakeoil.pem
server-key = /etc/ssl/private/ssl-cert-snakeoil.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

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

max-clients = 16

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 = 2

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

try-mtu-discovery = false

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 (10.10.10.0/24) to avoid IP address collision, so change the value of ipv4-network to

ipv4-network = 10.10.10.0

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

Note: It’s a good practice to run your own DNS resolver on the same server, especially if you are a VPN provider. 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 # symbol at the beginning of the following four 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

Save and close the file  Then restart the VPN server for the changes to take effect.

sudo systemctl restart ocserv

Fixing DTLS Handshake Failure

On Ubuntu 16.04 and Ubuntu 18.04, ocserv daemon ocserv.socket does not respect “listen-host” value from configuration file, which will cause the following error when clients connect to VPN server.

DTLS handshake failed: Resource temporarily unavailable, try again.

To fix this error, we need to edit the ocserv.service file. We first copy the original file in /lib/systemd/system/ directory to /etc/systemd/system/ directory, then edit it, because we don’t want new version of ocserv package to override our modifications. (To learn more about systemd unit files, run man systemd.unit.)

sudo cp /lib/systemd/system/ocserv.service /etc/systemd/system/ocserv.service
sudo nano /etc/systemd/system/ocserv.service

Comment out the following two lines.

Requires=ocserv.socket

Also=ocserv.socket

Save and close the file. Then reload systemd

sudo systemctl daemon-reload

Stop ocserv.socket and disable it.

sudo systemctl stop ocserv.socket

sudo systemctl disable ocserv.socket

Restart ocserv service.

sudo systemctl restart ocserv.service

The ocserv systemd service won’t output any message if it fails to restart, so we need to check the status to make sure it’s actually running.

systemctl status ocserv

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.

Enable IP Forwarding

In order for the VPN server to route packets between VPN client and the outside world, 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

Configure Firewall for IP Masquerading

We need to set up IP masquerading in the server firewall, so that the server becomes a virtual router for VPN clients. I will use UFW, which is a front end to the iptables firewall. Install UFW on Ubuntu with:

sudo apt install ufw

First, you need to allow SSH traffic.

sudo ufw allow 22/tcp

Next, find the name of your server’s main network interface.

ip addr

As you can see, it’s named ens3 on my Ubuntu server.

openconnect ubuntu command line

To configure IP masquerading, we have to add iptables command in a UFW configuration file.

sudo nano /etc/ufw/before.rules

By default, there are some rules for the filter table. Add the following lines at the end of this file. Replace ens3 with your own network interface name.

# NAT table rules
*nat
:POSTROUTING ACCEPT [0:0]
-A POSTROUTING -o ens3 -j MASQUERADE

# End each table with the 'COMMIT' line or these rules won't be processed
COMMIT

In Nano text editor, you can go to the end of the file by pressing Ctrl+W, then pressing Ctrl+V.

The above lines will append (-A) a rule to the end of of POSTROUTING chain of nat table. It will link your virtual private network with the Internet. And also hide your 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.

By default, UFW forbids packet forwarding. We can allow forwarding for our private network. Find the ufw-before-forward chain in this file and add the following 3 lines, which will accept packet forwarding if the source IP or destination IP is in the 10.10.10.0/24 range.

# allow forwarding for trusted network
-A ufw-before-forward -s 10.10.10.0/24 -j ACCEPT
-A ufw-before-forward -d 10.10.10.0/24 -j ACCEPT

ufw allow packet fowarding

Save and close the file. Then enable UFW.

sudo ufw enable

If you have enabled UFW before, then you can use systemctl to restart UFW.

sudo systemctl restart ufw

Now if you list the rules in the POSTROUTING chain of the NAT table by using the following command:

sudo iptables -t nat -L POSTROUTING

You can see the Masquerade rule.

ocserv iptables masquerading

Open Port 443 in Firewall

Run the following command to open TCP and UDP port 443. If you configured a different port for ocserv, then change 443 to your configured port.

sudo ufw allow 443/tcp
sudo ufw allow 443/udp

Now OpenConnect VPN server is ready to accept client connections.

If you specified 10.10.10.1 as the DNS server for VPN clients, then you must allow VPN clients to connect to port 53 with the following UFW rule.

sudo ufw insert 1 allow in from 10.10.10.0/24

You also need to edit the BIND DNS server’s configuration to allow VPN clients to send recursive DNS queries like below.

allow-recursion { 127.0.0.1; 10.10.10.0/24; };

How to Install and Use OpenConnect VPN client on Ubuntu 16.04/18.04 Desktop

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

sudo apt install openconnect

You can Connect to VPN from the command line like below. -b flag will make it run in the background after connection is established.

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 tun0 as 192.168.1.139, using SSL
Established DTLS connection (using GnuTLS). Ciphersuite (DTLS1.2)-(RSA)-(AES-256-GCM).

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 want to use Network Manager to manage VPN connection, then you also need to install these packages.

sudo apt install network-manager-openconnect network-manager-openconnect-gnome

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. I once had a typo in my iptables command, which caused my computer not being able to browse the Internet.

Auto-Connect on System Startup

To let 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 systemd-resolved.service
  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'
  ExecStop=/bin/bash -c '/sbin/resolvconf -d tun0 && /usr/bin/pkill -SIGINT openconnect && /sbin/ip route flush 12.34.56.78'
  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 systemd-resolved.service and Wants=network-online.target make this service run after network is up. We want the openconnect.service start after the systemd-resolved.service because that will ensure the DNS server address set by OpenConnect won’t be overridden by systemd-resolved.service.
  • 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 comand 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 ExecStop directive is for stopping VPN connection. First, we use resolvconf command to revert DNS server settings. tun0 is the default name for the TUN device, which you can see with ip link command. Then we use pkill command to stop openconenct process. Lastly, we restore the Linux kernel routing table with the ip route flush command. Replace 12.34.56.78 with the IP address of your VPN server.

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. 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.

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.

  • How to Easily boost Ubuntu Network Performance by enabling TCP BBR

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 encounter any problem, then check OpenConnect VPN server log.

sudo journalctl -xe -u ocserv.service

I found that if I change port 443 to a different port, the great firewall of China will block this VPN connection. Sometimes the speed drops to around 1Mbps, probably due to the fact that the great firewall of China is interrupting VPN connections because if I use Shadowsocks proxy on the same server, I can watch YouTube at 50Mbps.

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

Please read the following article:

  • Run OpenConnect VPN Server & Apache/Nginx on the Same Box with 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, just add -VERS-TLS1.0 and -VERS-TLS1.1 in the line.

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

Save and close the file. Then restart ocserv.

sudo systemctl restart ocserv

Now ocserv will only accept TLS 1.2. 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

I tried to enable TLS 1.3, but it is not supported yet in the ocserv package on Ubuntu.

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 trick to allow my another VPS (virtual private server) to connect to this VPN server without disrupting normal traffic, so the tun device (vpns0) of my VPN server is always turned on, which means my VPN server will always have the private IP address 10.10.10.1.

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

sudo systemctl restart ocserv

Wrapping Up

That’s it! I hope this tutorial helped you install and configure OpenConnect VPN on Ubuntu 16.04 and Ubuntu 18.04. As always, if you found this post useful, then subscribe to our free newsletter to get more tips and tricks 🙂

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