OpenVPN : Configure VPN Server2025/10/08

Install OpenVPN to Configure Virtual Private Network.

This example is based on the environment like follows.
By settings of OpenVPN Server/Client, [tun] interface will be configured automatically and when connecting with VPN from Client to Server, Client can access to the the local network of the Server.

On this example, it needs to set IP Masquerading on Router, too.

  +----------------------+
  | [  OpenVPN Server  ] |172.16.100.1
  |    dlp.srv.world     +--------+
  |                      |tun     |
  +-----------+----------+        |
        enp1s0|10.0.0.30          |
              |                   |
              |  Local Network    |
      10.0.0.1|                   |
       +------+-----+             |
-------|   Router   |-------------|-----
       +------+-----+             |
              |                   |
              |  Internet         |
--------------+-------------------|-----
              |                   |
              |  Local Network    |
  192.168.0.30|                   |
  +-----------+----------+        |
  |                      |tun     |
  |      VPN Client      +--------+
  |                      |172.16.100.x
  +----------------------+

[1]	Install OpenVPN.

root@dlp:~#

apt -y install openvpn easy-rsa iptables

[2]	Create CA and Certificates.

root@dlp:~#

cd /usr/share/easy-rsa

# initialize

root@dlp:/usr/share/easy-rsa#

./easyrsa init-pki


Notice
------
'init-pki' complete; you may now create a CA or requests.

Your newly created PKI dir is:
* /usr/share/easy-rsa/pki

Using Easy-RSA configuration:
* undefined

# create CA

root@dlp:/usr/share/easy-rsa#

./easyrsa build-ca


# set any pass-phrase
Enter New CA Key Passphrase:
Re-Enter New CA Key Passphrase:

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.
-----
# set any name
Common Name (eg: your user, host, or server name) [Easy-RSA CA]:Server-CA

Notice
------
CA creation complete. Your new CA certificate is at:
* /usr/share/easy-rsa/pki/ca.crt

Create an OpenVPN TLS-AUTH|TLS-CRYPT-V1 key now: See 'help gen-tls'

Build-ca completed successfully.

# create server certificates
# any name is OK for [server1] name
# (it is set for file name of certs or commonName)

root@dlp:/usr/share/easy-rsa#

./easyrsa build-server-full server1 nopass


Notice
------
Private-Key and Public-Certificate-Request files created.
Your files are:
* req: /usr/share/easy-rsa/pki/reqs/server1.req
* key: /usr/share/easy-rsa/pki/private/server1.key

You are about to sign the following certificate:

  Requested CN:     'server1'
  Requested type:   'server'
  Valid for:        '825' days


subject=
    commonName                = server1

Type the word 'yes' to continue, or any other input to abort.
  Confirm request details: yes

Using configuration from /usr/share/easy-rsa/pki/837d6ab0/temp.6.1
# answer with pass-phrase set on CA
Enter pass phrase for /usr/share/easy-rsa/pki/private/ca.key:
Check that the request matches the signature
Signature ok
The Subject's Distinguished Name is as follows
commonName            :ASN.1 12:'server1'
Certificate is to be certified until Jan 11 00:28:20 2028 GMT (825 days)

Write out database with 1 new entries
Database updated

Notice
------
Inline file created:
* /usr/share/easy-rsa/pki/inline/private/server1.inline


Notice
------
Certificate created at:
* /usr/share/easy-rsa/pki/issued/server1.crt

# create client certificates
# any name is OK for [client1] name
# (it is set for file name of certs or commonName)

root@dlp:/usr/share/easy-rsa#

./easyrsa build-client-full client1 nopass


Notice
------
Private-Key and Public-Certificate-Request files created.
Your files are:
* req: /usr/share/easy-rsa/pki/reqs/client1.req
* key: /usr/share/easy-rsa/pki/private/client1.key

You are about to sign the following certificate:

  Requested CN:     'client1'
  Requested type:   'client'
  Valid for:        '825' days


subject=
    commonName                = client1

Type the word 'yes' to continue, or any other input to abort.
  Confirm request details: yes

Using configuration from /usr/share/easy-rsa/pki/d3663b07/temp.6.1
# answer with pass-phrase set on CA
Enter pass phrase for /usr/share/easy-rsa/pki/private/ca.key:
Check that the request matches the signature
Signature ok
The Subject's Distinguished Name is as follows
commonName            :ASN.1 12:'client1'
Certificate is to be certified until Jan 11 00:30:53 2028 GMT (825 days)

Write out database with 1 new entries
Database updated

Notice
------
Inline file created:
* /usr/share/easy-rsa/pki/inline/private/client1.inline


Notice
------
Certificate created at:
* /usr/share/easy-rsa/pki/issued/client1.crt

# generate Diffie Hellman ( DH ) parameter

root@dlp:/usr/share/easy-rsa#

./easyrsa gen-dh

Generating DH parameters, 2048 bit long safe prime

DH parameters appear to be ok.

Notice
------

DH parameters of size 2048 created at:
* /usr/share/easy-rsa/pki/dh.pem

# create TLS-Auth key

root@dlp:/usr/share/easy-rsa#

openvpn --genkey secret ./pki/ta.key

# copy generated certs

root@dlp:/usr/share/easy-rsa#

cp -pR /usr/share/easy-rsa/pki/{issued,private,ca.crt,dh.pem,ta.key} /etc/openvpn/server/

[3]	Configure OpenVPN. It based on the environment Firewalld is running because of using routing rules.

# copy sample configuration

root@dlp:~#

cp /usr/share/doc/openvpn/examples/sample-config-files/server.conf /etc/openvpn/server/

root@dlp:~#

vi /etc/openvpn/server/server.conf

# line 32 : change if need (listening port of OpenVPN)

port 1194

# line 35 : change if need (use udp on this example)

;proto tcp
proto udp

# line 53 : change if need (use tun on this example)

;dev tap
dev tun

# line 86 : specify certificates

ca.crt

cert

issued/server1.crt

key

private/server1.key

# line 93 : specify DH file

dh.pem

# line 115 : specify network to be used on VPN
# any network are OK except your local network

server

172.16.100.0 255.255.255.0

# line 156 : uncomment and change to your local network

push "route

10.0.0.0 255.255.255.0

# line 245 : keepalive settings

keepalive 10 120

# line 258 : uncomment and specify TLS-Auth key

tls-auth

ta.key 0

# line 276 : enable persist options

persist-key
persist-tun

# line 301 : specify log level (0 - 9, 9 means debug level)

verb 3

root@dlp:~#

vi /etc/openvpn/server/add-bridge.sh

# create new

#!/bin/bash

# network interface which can connect to local network
IF=enp1s0
# interface VPN tunnel uses
# for the case of this example like specifying [tun] on the config, generally this param is [tun0]
VPNIF=tun0

echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -A FORWARD -i ${VPNIF} -j ACCEPT
iptables -t nat -A POSTROUTING -o ${IF} -j MASQUERADE

root@dlp:~#

vi /etc/openvpn/server/remove-bridge.sh

# create new

#!/bin/bash

# network interface which can connect to local network
IF=enp1s0
# interface VPN tunnel uses
# for the case of this example like specifying [tun] on the config, generally this param is [tun0]
VPNIF=tun0

echo 0 > /proc/sys/net/ipv4/ip_forward
iptables -D FORWARD -i  ${VPNIF} -j ACCEPT
iptables -t nat -D POSTROUTING -o ${IF} -j MASQUERADE

root@dlp:~#

chmod 700 /etc/openvpn/server/{add-bridge.sh,remove-bridge.sh}

root@dlp:~#

systemctl edit openvpn-server@server

# add following lines
[Service]
ExecStartPost=/etc/openvpn/server/add-bridge.sh
ExecStopPost=/etc/openvpn/server/remove-bridge.sh

root@dlp:~#

systemctl enable --now openvpn-server@server

[4]	Transfer certs follows you generated to Client Host you'd like to connect with VPN. It's OK all for VPN Server settings. * /etc/openvpn/server/ca.crt * /etc/openvpn/server/ta.key * /etc/openvpn/server/issued/client1.crt * /etc/openvpn/server/private/client1.key

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