OpenVPN

Let’s say you have an old dedicated server without AES-NI and you need 200 devices connected to it, but they only route traffic for a web server and a file server on your private network, and about 50% will be actively using the connection, and 50% will be idling, at any given time. As in the previous example this will of course vary somewhat as some users are working on other tasks and alternate this with retrieving files and data through the VPN tunnel. Let’s say you want to make sure each active users will have 10Mbps available, and let’s again assume they actually have that bandwidth on their Internet connection.

100 active users times 10Mbps is 1000Mbps or 1Gbps. Most systems nowadays have this by default, even servers that are several years old. 1000Mbps time 40MHz is about 40000MHz or 40GHz. Older servers with a dual octa-core setup with 2.5GHz will be able to get you to those requirements. With 200 connected devices in this example you would need about 2GB of memory, a fairly low amount.

PiVPN

curl -L https://install.pivpn.io | bash
nano /etc/openvpn/server.conf
: <<'END_COMMENT'
dev tun
proto udp
port 1194
ca /etc/openvpn/easy-rsa/pki/ca.crt
cert /etc/openvpn/easy-rsa/pki/issued/pi03_ca27deca-6db9-49de-98c8-f6d1fd57c9be.crt
key /etc/openvpn/easy-rsa/pki/private/pi03_ca27deca-6db9-49de-98c8-f6d1fd57c9be.key
dh /etc/openvpn/easy-rsa/pki/dh2048.pem
topology subnet
server 10.20.13.0 255.255.255.0
# Set your primary domain name server address for clients
push "dhcp-option DOMAIN dev.shahed.biz"
push "dhcp-option DNS 10.19.83.100"
push "dhcp-option DNS 10.19.83.1"
# Prevent DNS leaks on Windows
push "block-outside-dns"
# Override the Client default gateway by using 0.0.0.0/1 and
# 128.0.0.0/1 rather than 0.0.0.0/0. This has the benefit of
# overriding but not wiping out the original default gateway.
push "redirect-gateway def1"
client-to-client
client-config-dir /etc/openvpn/ccd
keepalive 15 120
remote-cert-tls client
tls-version-min 1.2
tls-auth /etc/openvpn/easy-rsa/pki/ta.key 0
cipher AES-256-CBC
auth SHA256
user openvpn
group openvpn
persist-key
persist-tun
crl-verify /etc/openvpn/crl.pem
status /var/log/openvpn-status.log 20
status-version 3
syslog
verb 3
#DuplicateCNs allow access control on a less-granular, per user basis.
#Remove # if you will manage access by user instead of device. 
#duplicate-cn
# Generated for use by PiVPN.io
END_COMMENT

pivpn add
: <<'END_COMMENT'
::: Create a client ovpn profile, optional nopass
:::
::: Usage: pivpn <-a|add> [-n|--name <arg>] [-p|--password <arg>]|[nopass] [-d|--days <number>] [-b|--bitwarden] [-i|--iOS] [-o|--ovpn] [-h|--help]
:::
::: Commands:
:::  [none]               Interactive mode
:::  nopass               Create a client without a password
:::  -n,--name            Name for the Client (default: 'pi03')
:::  -p,--password        Password for the Client (no default)
:::  -d,--days            Expire the certificate after specified number of days (default: 1080)
:::  -b,--bitwarden       Create and save a client through Bitwarden
:::  -i,--iOS             Generate a certificate that leverages iOS keychain
:::  -o,--ovpn            Regenerate a .ovpn config file for an existing client
:::  -h,--help            Show this help dialog

Enter a Name for the Client:  ios   
How many days should the certificate last?  1080
Enter the password for the client:  
Enter the password again to verify:  
spawn ./easyrsa build-client-full ios

--more-skipped--

========================================================
Done! ios.ovpn successfully created! 
ios.ovpn was copied to:
  /home/pi/ovpns
for easy transfer. Please use this profile only on one
device and create additional profiles for other devices.
========================================================
END_COMMENT

systemctl restart openvpn
tail -f /var/log/openvpn.log

nc -uv 10.20.13.1 1194
nc -uv 10.19.83.103 1194
nc -uv 10.19.83.203 1194
nc -uv vpn.shahed.biz 1194
nc -uv vpn0.dev.shahed.biz 1194

Knowledge

apt install ufw
apt install nmap
apt install telnet

ufw status
netstat -a
netstat -lpn
nmap -sT vpn0.dev.shahed.biz
nmap -sU vpn0.dev.shahed.biz

nc -uv 10.20.13.1 1194
nc -uv 10.19.83.103 1194
nc -uv 10.19.83.203 1194
nc -uv vpn.shahed.biz 1194
nc -uv vpn0.dev.shahed.biz 1194

netstat -uap|grep openvpn
tail -f /var/log/openvpn.log
nano /etc/openvpn/server.conf

telnet localhost 1194
telnet nas0.dev.shahed.biz 80
telnet nas0.dev.shahed.biz 1194

rm -f /etc/openvpn/pki/reqs/dev.shahed.biz.req
apt purge openmediavault-openvpn
rm -rf /opt/EasyRSA-v3.0.6/
rm -rf /etc/openvpn/

sysctl -w net.ipv4.tcp_window_scaling=0

References

OpenVPN Access Server System Requirements Prevent SSH freezes over an OpenVPN Client Config an OpenVPN Server on Ubuntu 18.04 Config an OpenVPN Server on Ubuntu 16.04 OpenVPN Connect Client On Windows OpenVPN not Listening on Port 1194 OpenVPN Pi Setup not Listening OpenVPN How To OpenVPN PiVPN