CentOS Stream 8
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OpenVPN : Configure VPN Server
2021/09/17
 
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  ] |192.168.100.1
  |    dlp.srv.world     +--------+
  |                      |tun     |
  +-----------+----------+        |
          eth0|10.0.0.29          |
              |                   |
              |  Local Network    |
      10.0.0.1|                   |
       +------+-----+             |
-------|   Router   |-------------|-----
       +------+-----+             |
              |                   |
              |  Internet         |
--------------+-------------------|-----
              |                   |
              |  Local Network    |
 192.168.0.100|                   |
  +-----------+----------+        |
  |                      |tun     |
  |      VPN Client      +--------+
  |                      |192.168.100.x
  +----------------------+

[1] Install OpenVPN.
# install from EPEL

[root@dlp ~]#
dnf --enablerepo=epel -y install openvpn easy-rsa net-tools
[2] Create CA and Certificates.
[root@dlp ~]#
cd /usr/share/easy-rsa/3
# initialize

[root@dlp 3]#
./easyrsa init-pki

init-pki complete; you may now create a CA or requests.
Your newly created PKI dir is: /usr/share/easy-rsa/3/pki

# create CA

[root@dlp 3]#
./easyrsa build-ca


Using SSL: openssl OpenSSL 1.1.1k  FIPS 25 Mar 2021

# set any pass-phrase
Enter New CA Key Passphrase:
Re-Enter New CA Key Passphrase:
Generating RSA private key, 2048 bit long modulus (2 primes)
.......+++++
...........................................+++++
e is 65537 (0x010001)
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

CA creation complete and you may now import and sign cert requests.
Your new CA certificate file for publishing is at:
/usr/share/easy-rsa/3/pki/ca.crt

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

[root@dlp 3]#
./easyrsa build-server-full server1 nopass


Using SSL: openssl OpenSSL 1.1.1k  FIPS 25 Mar 2021
Generating a RSA private key
..................................+++++
...................................+++++
writing new private key to '/usr/share/easy-rsa/3/pki/private/server1.key.Mxv13uLARL'
-----
Using configuration from /usr/share/easy-rsa/3/pki/safessl-easyrsa.cnf
# answer with pass-phrase set on CA
Enter pass phrase for /usr/share/easy-rsa/3/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 Dec 20 07:17:08 2023 GMT (825 days)

Write out database with 1 new entries
Data Base Updated

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

[root@dlp 3]#
./easyrsa build-client-full client1 nopass


Using SSL: openssl OpenSSL 1.1.1k  FIPS 25 Mar 2021
Generating a RSA private key
.............+++++
................+++++
writing new private key to '/usr/share/easy-rsa/3/pki/private/client1.key.JZkbIpNr3Y'
-----
Using configuration from /usr/share/easy-rsa/3/pki/safessl-easyrsa.cnf
# answer with pass-phrase set on CA
Enter pass phrase for /usr/share/easy-rsa/3/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 Dec 20 07:17:32 2023 GMT (825 days)

Write out database with 1 new entries
Data Base Updated

# generate Diffie Hellman ( DH ) parameter

[root@dlp 3]#
./easyrsa gen-dh


Using SSL: openssl OpenSSL 1.1.1k  FIPS 25 Mar 2021
Generating DH parameters, 2048 bit long safe prime, generator 2
This is going to take a long time

............++*++*++*++*

DH parameters of size 2048 created at /usr/share/easy-rsa/3/pki/dh.pem

# create TLS-Auth key

[root@dlp 3]#
openvpn --genkey --secret ./pki/ta.key
# copy generated certs

[root@dlp 3]#
cp -pR /usr/share/easy-rsa/3/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 ruoting rules.
# copy sample configuration

[root@dlp ~]#
cp /usr/share/doc/openvpn/sample/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 78 : specify certificates

ca
ca.crt

cert
issued/server1.crt

key
private/server1.key
# line 85 : specify DH file

dh
dh.pem
# line 101 : specify network to be used on VPN
# any network are OK except your local network

server
192.168.100.0 255.255.255.0
# line 142 : uncomment and change to your local network

push "route
10.0.0.0 255.255.255.0
"
# line 231 : keepalive settings

keepalive 10 120
# line 244 : specify TLS-Auth key

tls-auth
ta.key 0
# line 263 : uncomment (enable compress)

comp-lzo
# line 281 : enable persist options

persist-key
persist-tun
# line 287 : change log path

status
/var/log/openvpn-status.log
# line 296 : change log path

log         /var/log/openvpn.log
log-append  /var/log/openvpn.log

# line 306 : specify log level (0 - 9, 9 means debug lebel)

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
# listening port of OpenVPN
PORT=1194

firewall-cmd --zone=public --add-masquerade
firewall-cmd --direct --add-rule ipv4 filter FORWARD 0 -i ${VPNIF} -o ${IF} -j ACCEPT
firewall-cmd --direct --add-rule ipv4 nat POSTROUTING 0 -o ${IF} -j MASQUERADE
firewall-cmd --add-port=${PORT}/udp

[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
# listening port of OpenVPN
PORT=1194

firewall-cmd --zone=public --remove-masquerade
firewall-cmd --direct --remove-rule ipv4 filter FORWARD 0 -i ${VPNIF} -o ${IF} -j ACCEPT
firewall-cmd --direct --remove-rule ipv4 nat POSTROUTING 0 -o ${IF} -j MASQUERADE
firewall-cmd --remove-port=${PORT}/udp

[root@dlp ~]#
chmod 700 /etc/openvpn/server/{add-bridge.sh,remove-bridge.sh}

[root@dlp ~]#
systemctl edit openvpn-server@server
# create new

[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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