Client to Server OpenVPN

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My specific purpose for these instructions is so IP phones can establish a tunnel and communicate to a SIP server without the need of worrying about NAT, STUN or port forwards. The Yealink SIP phones have inbuilt OpenVPN capabilities.

The following instructions were performed on CentOS 7.2

Server

Installing

Firstly install epel due to openvpn not been available in the default repository. Once epel has been install then... 

yum -y install openssl lzo openvpn easy-rsa

Config Files

Examples of Configuration Files 

/usr/share/doc/openvpn-2.3.12/sample/sample-config-files/server.conf

OpenVPN

vi /etc/openvpn/server.conf

local 192.168.1.1
port 1194
proto udp
dev tun

ca   /etc/openvpn/keys/ca.crt
cert /etc/openvpn/keys/server.crt
key  /etc/openvpn/keys/server.key
dh   /etc/openvpn/keys/dh2048.pem

cipher AES-256-CBC
#duplicate-cn
#tls-auth /etc/openvpn/keys/ta.key 0

server 10.121.18.0 255.255.254.0
ifconfig-pool-persist ipp.txt

push "route 10.121.18.0 255.255.254.0"

client-to-client
keepalive 10 120
comp-lzo
persist-key
persist-tun
verb 4

log access.log
status connections.log

Certificate Creation

NOTE: Make sure the KeyName is different for each certificate generated otherwise the system can't keep track of the certfiicates generated and you will get errors like: 

failed to update database
TXT_DB error number 2

The default KeyName is : EasyRSA

Preparation

mkdir /etc/openvpn/easy-rsa
cp  /usr/share/easy-rsa/2.0/* /etc/openvpn/easy-rsa/
mkdir /etc/openvpn/easy-rsa/keys
mkdir /etc/openvpn/keys
cd /etc/openvpn/easy-rsa
vi vars

change 

export EASY_RSA="`pwd`"

to 

export EASY_RSA="/etc/openvpn/easy-rsa"


Change the other fields as required. 

export KEY_COUNTRY="US"
export KEY_PROVINCE="CA"
export KEY_CITY="SanFrancisco"
export KEY_ORG="Fort-Funston"
export KEY_EMAIL="me@myhost.mydomain"
export KEY_OU="MyOrganizationalUnit"


NOTE: Do not populate the KEY_CN field if you are utilising individual client certificates. If you are generate a single certificate for all clients then you can specify a default value here. 

#export KEY_CN="my.fqdn.server.name"

Source in our new variables. 

source ./vars

Clean any previous builds. Should not be required if this is your first run at building certificates. 

./clean-all

Server Certificates

Build Certificate Authority Certificate 

./build-ca

Build Certificate and Key and name the certificate server 

./build-key-server server

Build Diffie-Hellman key exchange 

./build-dh


Copy certificate files to OpenVPN keys folder 

cd /etc/openvpn/easy-rsa/keys
cp dh2048.pem ca.crt server.crt server.key /etc/openvpn/keys/

Client Certificate

Know Issues:
1. For the tightened of security you would normally generate a client certificate for each client. In this case I will use the one certificate for all clients.

The name client in the below command is the name of the client certificate. If generating multiple certificates you will have to name them differently. 

cd /etc/openvpn/easy-rsa
source ./vars
./build-key client

2. If you are using the same client certificate name and the duplicate-cn variable then the ifconfig-pool-persist won't work as the client certificate name is used to map the IP address to keep it consistent.

tls-auth Key

openvpn --genkey --secret /etc/openvpn/keys/ta.key

Openvpn Service

Automatic

vi /etc/default/openvpn

Add or specific file without the conf. 

AUTOSTART="all"

or 

AUTOSTART="server"

Start Service 

systemctl restart openvpn

Manually

While the openvpn service is stopped. 

openvpn --config /etc/openvpn/server.conf

Client

Windows

VPN Config File

C:\Program Files\OpenVPN\config\office-vpn.ovpn

client
dev tun
proto udp
remote my-vpn.server.com 1194
resolv-retry infinite
nobind

persist-key
persist-tun

ca ca.crt
cert client.crt
key client.key
# tls-auth ta.key 1

remote-cert-tls server

cipher AES-256-CBC

comp-lzo
verb 3

Copy Certificate Files

Copy the following files from the Openvpn server to your openvpn client config folder.

Source 

/etc/openvpn/easy-rsa/keys/ca.crt
/etc/openvpn/easy-rsa/keys/client.crt
/etc/openvpn/easy-rsa/keys/client.key
/etc/openvpn/easy-rsa/keys/ta.key

Destination 

C:\Program Files\OpenVPN\config

Yealink IP Phone

NOTE: The Yealink phones don't seem to support tls-auth, which means you will nee to remove this setting from the server and client configuration files.

Copy Certificate Files

On the Linux server where the OpenVPN server is running: 

mkdir -p /tmp/yealink/keys
cd /etc/openvpn/easy-rsa/keys
cp ca.crt client.crt client.key /tmp/yealink/keys/

Create VPN client config File

vi /tmp/yealink/vpn.cnf

client
nobind
setenv SERVER_POLL_TIMEOUT 4

proto udp
remote my-vpn.server.com 1194
dev tun
dev-type tun
ns-cert-type server

persist-tun
persist-key
comp-lzo

verb 3

ca /config/openvpn/keys/ca.crt
cert /config/openvpn/keys/client.crt
key /config/openvpn/keys/client.key

cipher AES-256-CBC

tar files

cd /tmp/yealink
tar cvpf openvpn.tar *

Copy openvpn.tar to your client computer. Open a web browser and enter the IP address of the handset. Select Network, Advanced. Go down to the VPN setting, upload the openvpn.tar file and then select enable.

Once the phone reboots you should see VPN located in the top right of the screen. If you do not see VPN displayed then a VPN has not been successfully established to the server.

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