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OpenVPN - How to Set Up a Slackware Server and a Slackware Client

1. Introduction

1.1. OpenVPN(1)

OpenVPN is an open source software application that implements virtual private network (VPN) techniques for creating secure point-to-point or site-to-site connections in routed or bridged configurations and remote access facilities. It uses a custom security protocol that utilizes SSL/TLS for key exchange. It is capable of traversing network address translators (NATs) and firewalls. It was written by James Yonan and is published under the GNU General Public License (GPL).

OpenVPN allows peers to authenticate each other using a pre-shared secret key, certificates, or username/password. When used in a multiclient-server configuration , it allows the server to release an authentication certificate for every client, using signature and Certificate authority. It uses the OpenSSL encryption library extensively, as well as the SSLv3/TLSv1 protocol, and contains many security and control features.

2. Scope and Objective

The objective of this article is to serve as a tutorial for the readers to set up a basic but functional Slackware VPN Server and Client over the Internet.

The emphasis is to provide a reliable method that can be easily followed to set-up OpenVPN on Slackware Servers and Clients. Nevertheless the process is still not free from pitfalls and require some attention and determination.

This article comprises of a selection of other similar tutorials found on the Internet particularly (2) and (3) and the documents contained in the downloaded source files. However these are reformatted to satisfy the objective.

3. Installation

Openvpn is already installed on Slackware if a default installation was followed. If this was not the case, then the package is available from the “n” directory of the Slackware DVD. Refer to other Slackware specific documents on how to go about this installation.

If you want to confirm that Openvpn is indeed installed, you can check it by listing the /var/log/packages/ directory:

# ls /var/log/packages/openvpn*

4. Requirements

Server and a Client computers would be needed. They would have to be connected to the Internet on two different Routers and different Network Routes. For the purpose of this tutorial, specific details are defined in order to enhance the readability. Of course, you will probably have a different addresses, so you will need to amend accordingly.

4.1. Server DNS

A URL is normally used to address the Server. This is not mandatory and instead you may use only the Internet IP. However it is recommended to use a URL to access the Server from the Internet, especially if it is connected to a dynamic IP, which is typical for domestic Internet connections. The author is using noip2(4) as it is free upon subscription. Noip2 is available from http://slackbuilds.org.

4.2. Server details

hostname: server1
IP: 192.168.200.195/255.255.255.0
URL:  servervpn.no-ip.org
Network Interface: eth0

4.3. Client details

hostname: client1
IP: 192.168.1.101/255.255.255.0
Network Interface: wlan0

4.3 Administrator Rights

You will need to have administrator rights to set up the Openvpn. This applies to both the Server and the Client. For simplicity, in this tutorial, it will be assumed that all actions will be performed by the root user. Naturally advanced users might be more discerning.

4.4 Possible Constraints and Possible Solutions for a WiFi equipped Client

The availability of two Routers might be challenging. Consider that interactive sessions on both the Server and Client will be needed before the VPN is set up. If the Client is equipped with a WiFi interface there might be some easy solutions that may be considered:

  1. Use a 3G smart phone's “Portable Wi-Fi Hot Spot” facility to connect the Client as the VPN Client. As 3G bandwidth is expensive you may want to minimise traffic. For example, you might want to switch off services that are not absolutely essential during the course of this exercise, such as ntpd, dropbox and tor.
  2. Connect the Client to another WiFi available in the vicinity of the Server. Some lucky people live in areas were benevolent neighbours provide them with openly accessible Internet WiFi. It is recommended to request permission before taking up this solution. In case that no such open service exists, you may find it appropriate to request a temporary password from a friendly neighbour for the private encrypted WiFi service.
  3. Nowadays, many governmental premises, such as libraries and Local Councils provide free WiFi service. Other places such as fast food outlets, pubs, cafés, etc. also provide free WiFi from their location to their valuable customers. You may access the Server via an available service such as SSH from a WiFi equipped Client. If this option is chosen for this solution, be aware that the Client may have to pass through some firewalls. Besides the VPN connection might be a breach of the terms of conditions that should be accepted before using the WiFi service.

5. Creating a Public Key Infrastructure (PKI) using the easy-rsa Scripts

The PKI may be created on any computer with a VPN installation, but it is probably more sensible to be done on both the Server and the Client as both would need it. An easy way to build the PKI is to use the easy-rsa scripts. These may be downloaded like this:

# cd
# git clone git://github.com/OpenVPN/easy-rsa

and then archive it for future purposes:

# tar cvf easy-rsa.tar easy-rsa

5.1 Create the keys and certificates for the Server

Follow these steps on the Server to create the needed keys and certificates:

# cd easy-rsa/easyrsa3

Create the PKI and the CA:

# ./easyrsa init-pki
# ./easyrsa build-ca

Enter a PEM pass phrase, reverify it and then enter a name for the server. In this article I am using the hostnames for clarity (in this case: server1), but you may choose any name.

Then generate the request:

# ./easyrsa gen-req server1

You will be prompted for another PEM pass phrase to reverify it and to confirm that the name of the entity is indeed server1. Now you may proceed to sign this request:

# ./easyrsa sign-req server server1

Confirm the request by entering “yes”, then enter original ca PEM passphrase.

Now create two additional key files:

# cd /etc/openvpn/certs/
# openssl dhparam -out dh2048.pem 2048
# cd /etc/openvpn/keys/
# /usr/sbin/openvpn --genkey --secret ta.key 

5.2 Create the keys and certificates for the Client

Follow these steps on the Client to create the needed keys and certificates:

You will need the easy-rsa scripts, so you can copy easy-rsa tarball from the Server to the Client and extract it:

# cd
# tar xvf easy-rsa.tar

Now create the PKI and generate the request:

# cd easy-rsa/easyrsa3
# ./easyrsa init-pki
# ./easyrsa gen-req client1

You will be prompted for another PEM pass phrase, to re-verify it and to confirm that the name of the entity is indeed client1. In this article I am using the hostnames for clarity (in this case: cleint1), but you may choose any name.

Copy pki/reqs/client1.req back to the Server.

5.2.1 Sign the Client's request on the Server

For the purpose of this article, it is assumed that the Client's request file (client1.req) has been transferred to the $HOME/openvpn/ directory of the Server. Now you can proceed to import and sign the client1 request:

# cd $HOME/easy-rsa/easyrsa3
# ./easyrsa import-req $HOME/openvpn/client1.req client1
# ./easyrsa sign-req client client1

When prompted enter “yes” and the server1 CA PEM pass phrase.

Copy the generated $HOME/easy-rsa/easyrsa3/pki/issued/client1.crt back to the client.

6. Setting up the Server

Copy the following files generated by the easy-rsa scripts to their respective directories in the /etc/openvpn/ directory:

# cp $HOME/easy-rsa/easyrsa3/pki/ca.crt \
> /etc/openvpn/certs/
# cp $HOME/easy-rsa/easyrsa3/pki/issued/server1.crt \
> /etc/openvpn/certs/
# cp $HOME/easy-rsa/easyrsa3/pki/private/server1.key \
> /etc/openvpn/keys/

Copy the sample server.conf from the openvpn source onto the openvpn's configuration directory. The source of openvpn may be obtained from Slackware's source DVD or your favourite Slackware mirror or from http://openvpn.net. In the following example I am downloading the source from ftp.slackware.com

# cd /tmp/
# wget -c \
> ftp://ftp.slackware.com/pub/slackware/slackware/source/n/openvpn/openvpn-*.tar.?z
# cd /usr/src/
# tar xvf /tmp/openvpn-*.tar.?z

Copy the file server.conf contained in the source to the openvpn configuration directory:

# cp openvpn-*/sample/sample-config-files/server.conf \
> /etc/openvpn/

Edit the following lines of /etc/openvpn/server.conf

From these lines:

ca ca.crt
cert server.crt
key server.key  # This file should be kept secret


dh dh1024.pem
;tls-auth ta.key 0 # This file is secret

;user nobody 
;group nobody

;log-append  openvpn.log

To:

ca /etc/openvpn/certs/ca.crt 
cert /etc/openvpn/certs/server1.crt 
key /etc/openvpn/keys/server1.key #This file should be kept secret

dh /etc/openvpn/certs/dh2048.pem

tls-auth /etc/openvpn/keys/ta.key 0 # This file is secret

user nobody 
group nobody

log-append  /var/log/openvpn.log

Finally add the following to /etc/openvpn/server.conf:

# If you want to use OpenVPN as a daemon, uncomment this line.
# Generally speaking, servers should run OpenVPN as a daemon
daemon

My full server.conf is the following:

#################################################
# Sample OpenVPN 2.0 config file for            #
# multi-client server.                          #
#                                               #
# This file is for the server side              #
# of a many-clients <-> one-server              #
# OpenVPN configuration.                        #
#                                               #
# OpenVPN also supports                         #
# single-machine <-> single-machine             #
# configurations (See the Examples page         #
# on the web site for more info).               #
#                                               #
# This config should work on Windows            #
# or Linux/BSD systems.  Remember on            #
# Windows to quote pathnames and use            #
# double backslashes, e.g.:                     #
# "C:\\Program Files\\OpenVPN\\config\\foo.key" #
#                                               #
# Comments are preceded with '#' or ';'         #
#################################################

# Which local IP address should OpenVPN
# listen on? (optional)
;local a.b.c.d

# Which TCP/UDP port should OpenVPN listen on?
# If you want to run multiple OpenVPN instances
# on the same machine, use a different port
# number for each one.  You will need to
# open up this port on your firewall.
port 1194

# TCP or UDP server?
;proto tcp
proto udp

# "dev tun" will create a routed IP tunnel,
# "dev tap" will create an ethernet tunnel.
# Use "dev tap0" if you are ethernet bridging
# and have precreated a tap0 virtual interface
# and bridged it with your ethernet interface.
# If you want to control access policies
# over the VPN, you must create firewall
# rules for the the TUN/TAP interface.
# On non-Windows systems, you can give
# an explicit unit number, such as tun0.
# On Windows, use "dev-node" for this.
# On most systems, the VPN will not function
# unless you partially or fully disable
# the firewall for the TUN/TAP interface.
;dev tap
dev tun

# Windows needs the TAP-Win32 adapter name
# from the Network Connections panel if you
# have more than one.  On XP SP2 or higher,
# you may need to selectively disable the
# Windows firewall for the TAP adapter.
# Non-Windows systems usually don't need this.
;dev-node MyTap

# SSL/TLS root certificate (ca), certificate
# (cert), and private key (key).  Each client
# and the server must have their own cert and
# key file.  The server and all clients will
# use the same ca file.
#
# See the "easy-rsa" directory for a series
# of scripts for generating RSA certificates
# and private keys.  Remember to use
# a unique Common Name for the server
# and each of the client certificates.
#
# Any X509 key management system can be used.
# OpenVPN can also use a PKCS #12 formatted key file
# (see "pkcs12" directive in man page).
ca /etc/openvpn/certs/ca.crt
cert /etc/openvpn/certs/server1.crt
key /etc/openvpn/keys/server1.key  # This file should be kept secret

# Diffie hellman parameters.
# Generate your own with:
#   openssl dhparam -out dh1024.pem 1024
# Substitute 2048 for 1024 if you are using
# 2048 bit keys. 
dh /etc/openvpn/certs/dh2048.pem

# Configure server mode and supply a VPN subnet
# for OpenVPN to draw client addresses from.
# The server will take 10.8.0.1 for itself,
# the rest will be made available to clients.
# Each client will be able to reach the server
# on 10.8.0.1. Comment this line out if you are
# ethernet bridging. See the man page for more info.
server 10.8.0.0 255.255.255.0

# Maintain a record of client <-> virtual IP address
# associations in this file.  If OpenVPN goes down or
# is restarted, reconnecting clients can be assigned
# the same virtual IP address from the pool that was
# previously assigned.
ifconfig-pool-persist ipp.txt

# Configure server mode for ethernet bridging.
# You must first use your OS's bridging capability
# to bridge the TAP interface with the ethernet
# NIC interface.  Then you must manually set the
# IP/netmask on the bridge interface, here we
# assume 10.8.0.4/255.255.255.0.  Finally we
# must set aside an IP range in this subnet
# (start=10.8.0.50 end=10.8.0.100) to allocate
# to connecting clients.  Leave this line commented
# out unless you are ethernet bridging.
;server-bridge 10.8.0.4 255.255.255.0 10.8.0.50 10.8.0.100

# Configure server mode for ethernet bridging
# using a DHCP-proxy, where clients talk
# to the OpenVPN server-side DHCP server
# to receive their IP address allocation
# and DNS server addresses.  You must first use
# your OS's bridging capability to bridge the TAP
# interface with the ethernet NIC interface.
# Note: this mode only works on clients (such as
# Windows), where the client-side TAP adapter is
# bound to a DHCP client.
;server-bridge

# Push routes to the client to allow it
# to reach other private subnets behind
# the server.  Remember that these
# private subnets will also need
# to know to route the OpenVPN client
# address pool (10.8.0.0/255.255.255.0)
# back to the OpenVPN server.
;push "route 192.168.10.0 255.255.255.0"
;push "route 192.168.20.0 255.255.255.0"

# To assign specific IP addresses to specific
# clients or if a connecting client has a private
# subnet behind it that should also have VPN access,
# use the subdirectory "ccd" for client-specific
# configuration files (see man page for more info).

# EXAMPLE: Suppose the client
# having the certificate common name "Thelonious"
# also has a small subnet behind his connecting
# machine, such as 192.168.40.128/255.255.255.248.
# First, uncomment out these lines:
;client-config-dir ccd
;route 192.168.40.128 255.255.255.248
# Then create a file ccd/Thelonious with this line:
#   iroute 192.168.40.128 255.255.255.248
# This will allow Thelonious' private subnet to
# access the VPN.  This example will only work
# if you are routing, not bridging, i.e. you are
# using "dev tun" and "server" directives.

# EXAMPLE: Suppose you want to give
# Thelonious a fixed VPN IP address of 10.9.0.1.
# First uncomment out these lines:
;client-config-dir ccd
;route 10.9.0.0 255.255.255.252
# Then add this line to ccd/Thelonious:
#   ifconfig-push 10.9.0.1 10.9.0.2

# Suppose that you want to enable different
# firewall access policies for different groups
# of clients.  There are two methods:
# (1) Run multiple OpenVPN daemons, one for each
#     group, and firewall the TUN/TAP interface
#     for each group/daemon appropriately.
# (2) (Advanced) Create a script to dynamically
#     modify the firewall in response to access
#     from different clients.  See man
#     page for more info on learn-address script.
;learn-address ./script

# If enabled, this directive will configure
# all clients to redirect their default
# network gateway through the VPN, causing
# all IP traffic such as web browsing and
# and DNS lookups to go through the VPN
# (The OpenVPN server machine may need to NAT
# or bridge the TUN/TAP interface to the internet
# in order for this to work properly).
;push "redirect-gateway def1 bypass-dhcp"

# Certain Windows-specific network settings
# can be pushed to clients, such as DNS
# or WINS server addresses.  CAVEAT:
# http://openvpn.net/faq.html#dhcpcaveats
# The addresses below refer to the public
# DNS servers provided by opendns.com.
;push "dhcp-option DNS 208.67.222.222"
;push "dhcp-option DNS 208.67.220.220"

# Uncomment this directive to allow different
# clients to be able to "see" each other.
# By default, clients will only see the server.
# To force clients to only see the server, you
# will also need to appropriately firewall the
# server's TUN/TAP interface.
;client-to-client

# Uncomment this directive if multiple clients
# might connect with the same certificate/key
# files or common names.  This is recommended
# only for testing purposes.  For production use,
# each client should have its own certificate/key
# pair.
#
# IF YOU HAVE NOT GENERATED INDIVIDUAL
# CERTIFICATE/KEY PAIRS FOR EACH CLIENT,
# EACH HAVING ITS OWN UNIQUE "COMMON NAME",
# UNCOMMENT THIS LINE OUT.
;duplicate-cn

# The keepalive directive causes ping-like
# messages to be sent back and forth over
# the link so that each side knows when
# the other side has gone down.
# Ping every 10 seconds, assume that remote
# peer is down if no ping received during
# a 120 second time period.
keepalive 10 120

# For extra security beyond that provided
# by SSL/TLS, create an "HMAC firewall"
# to help block DoS attacks and UDP port flooding.
#
# Generate with:
#   openvpn --genkey --secret ta.key
#
# The server and each client must have
# a copy of this key.
# The second parameter should be '0'
# on the server and '1' on the clients.
tls-auth /etc/openvpn/keys/ta.key 0 # This file is secret

# Select a cryptographic cipher.
# This config item must be copied to
# the client config file as well.
;cipher BF-CBC        # Blowfish (default)
;cipher AES-128-CBC   # AES
;cipher DES-EDE3-CBC  # Triple-DES

# Enable compression on the VPN link.
# If you enable it here, you must also
# enable it in the client config file.
comp-lzo

# The maximum number of concurrently connected
# clients we want to allow.
;max-clients 100

# It's a good idea to reduce the OpenVPN
# daemon's privileges after initialization.
#
# You can uncomment this out on
# non-Windows systems.
user nobody
group nobody

# The persist options will try to avoid
# accessing certain resources on restart
# that may no longer be accessible because
# of the privilege downgrade.
persist-key
persist-tun

# Output a short status file showing
# current connections, truncated
# and rewritten every minute.
status openvpn-status.log

# By default, log messages will go to the syslog (or
# on Windows, if running as a service, they will go to
# the "\Program Files\OpenVPN\log" directory).
# Use log or log-append to override this default.
# "log" will truncate the log file on OpenVPN startup,
# while "log-append" will append to it.  Use one
# or the other (but not both).
;log         openvpn.log
log-append  /var/log/openvpn.log

# Set the appropriate level of log
# file verbosity.
#
# 0 is silent, except for fatal errors
# 4 is reasonable for general usage
# 5 and 6 can help to debug connection problems
# 9 is extremely verbose
verb 3

# Silence repeating messages.  At most 20
# sequential messages of the same message
# category will be output to the log.
;mute 20

# If you want to use OpenVPN as a daemon, uncomment this line.
# Generally speaking, servers should run OpenVPN as a daemon
daemon

Note that comments in server.conf may be either start with # or ; In order to help you with entering parameters, the former are used to comment out text while the latter are for commented out configuration lines.

Copy the rc.openvpn listed hereunder and place under /etc/rc.d/

#!/bin/sh 
# 
# /etc/rc.d/rc.openvpn 
# 
# Start/stop/restart the openvpn server. 
#
 
ovpn_start() { 
  if [ -x /usr/sbin/openvpn -a -r /etc/openvpn/server.conf ]; then 
    echo "Starting OpenVPN:  /usr/sbin/openvpn server.conf" 
    /usr/sbin/openvpn /etc/openvpn/server.conf
  fi 
} 

ovpn_stop() { 
  killall openvpn 
} 

ovpn_restart() { 
  ovpn_stop 
  sleep 2 
  ovpn_start 
} 

case "$1" in 
'start') 
  ovpn_start 
  ;; 
'stop') 
  ovpn_stop 
  ;; 
'restart') 
  ovpn_restart 
  ;; 
*) 
  echo "Usage: $0 {start|stop|restart}" 
esac

Then give it executable permissions:

# chmod 755 /etc/rc.d/rc.openvpn

Eventually when you start openvpn on the server, you may read /var/log/openvpn.log to verify that your work has been successful:

# cat /var/log/openvpn.log

7. Port Forwarding

You will need to forward traffic from the port you have chosen for Openvpn to be routed to the Server. To accomplish this you will need to provide your Server with a fixed IP and you will need to configure your router. You may use netconfig, wicd or network-manager to set the fixed IP on Slackware. Then you also need to consult the documentation provided with your router to set up the selected IP address reserved for the Server, and the port forwarding. For our default Openvpn set up, the UDP Port would be 1194.

In case if you have misplaced such documentation, you may search on the Internet on how this may be achieved. A good place to start is http://portforward.com/.

8. Setting up the Client

On the Client machine perform the following instructions to set it up.

Download the openvpn source tarball and extracted it as explained in Chapter 6, then proceed to copy the included configuration file for clients:

# cp /usr/src/openvpn-*/sample/sample-config-files/client.conf \
> /etc/openvpn/

Edit the following lines of /etc/openvpn/client.conf

remote my-server-1 1194

;user nobody 
;group nobody 

ca ca.crt 
cert client.crt 
key client.key 

;tls-auth ta.key 1

to the following lines:

remote servervpn.no-ip.org 1194

user nobody 
group nobody 

ca /etc/openvpn/certs/ca.crt 
cert /etc/openvpn/certs/client1.crt 
key /etc/openvpn/keys/client1.key 

tls-auth /etc/openvpn/keys/ta.key 1

Note that comments in client.conf may be either # or ; The former are used to comment out text while the latter are for commented out configuration lines. This should help you a lot in the configuration process.

You will need this file that were generated by the Client's easy-rsa scripts:

$HOME/easy-rsa/easyrsa3/pki/private/client1.key

and the following from the Server's easy-rsa scripts:

ca.crt
ta.key

You will also need client1.crt generated by the Server as explained above in Chapter 5.2.1.

Place these files as indicated in client.conf. So ca.crt and client1.crt go under /etc/openvpn/certs/ while client1.key and ta.key go under /etc/openvpn/keys/

Also in client.conf, comment out the line:

ns-cert-type server

and insert the line:

remote-cert-tls server

My full client.conf is the following:

##############################################
# Sample client-side OpenVPN 2.0 config file #
# for connecting to multi-client server.     #
#                                            #
# This configuration can be used by multiple #
# clients, however each client should have   #
# its own cert and key files.                #
#                                            #
# On Windows, you might want to rename this  #
# file so it has a .ovpn extension           #
##############################################

# Specify that we are a client and that we
# will be pulling certain config file directives
# from the server.
client

# Use the same setting as you are using on
# the server.
# On most systems, the VPN will not function
# unless you partially or fully disable
# the firewall for the TUN/TAP interface.
;dev tap
dev tun

# Windows needs the TAP-Win32 adapter name
# from the Network Connections panel
# if you have more than one.  On XP SP2,
# you may need to disable the firewall
# for the TAP adapter.
;dev-node MyTap

# Are we connecting to a TCP or
# UDP server?  Use the same setting as
# on the server.
;proto tcp
proto udp

# The hostname/IP and port of the server.
# You can have multiple remote entries
# to load balance between the servers.
remote servervpn.no-ip.org 1194
;remote my-server-2 1194

# Choose a random host from the remote
# list for load-balancing.  Otherwise
# try hosts in the order specified.
;remote-random

# Keep trying indefinitely to resolve the
# host name of the OpenVPN server.  Very useful
# on machines which are not permanently connected
# to the internet such as laptops.
resolv-retry infinite

# Most clients don't need to bind to
# a specific local port number.
nobind

# Downgrade privileges after initialization (non-Windows only)
user nobody
group nobody

# Try to preserve some state across restarts.
persist-key
persist-tun

# If you are connecting through an
# HTTP proxy to reach the actual OpenVPN
# server, put the proxy server/IP and
# port number here.  See the man page
# if your proxy server requires
# authentication.
;http-proxy-retry # retry on connection failures
;http-proxy [proxy server] [proxy port #]

# Wireless networks often produce a lot
# of duplicate packets.  Set this flag
# to silence duplicate packet warnings.
;mute-replay-warnings

# SSL/TLS parms.
# See the server config file for more
# description.  It's best to use
# a separate .crt/.key file pair
# for each client.  A single ca
# file can be used for all clients.
ca /etc/openvpn/certs/ca.crt
cert /etc/openvpn/certs/client1.crt
key /etc/openvpn/keys/client1.key

# Verify server certificate by checking
# that the certicate has the nsCertType
# field set to "server".  This is an
# important precaution to protect against
# a potential attack discussed here:
#  http://openvpn.net/howto.html#mitm
#
# To use this feature, you will need to generate
# your server certificates with the nsCertType
# field set to "server".  The build-key-server
# script in the easy-rsa folder will do this.
;ns-cert-type server
remote-cert-tls server

# If a tls-auth key is used on the server
# then every client must also have the key.
tls-auth /etc/openvpn/keys/ta.key 1

# Select a cryptographic cipher.
# If the cipher option is used on the server
# then you must also specify it here.
;cipher x

# Enable compression on the VPN link.
# Don't enable this unless it is also
# enabled in the server config file.
comp-lzo

# Set log file verbosity.
verb 3

# Silence repeating messages
;mute 20
#

9. Testing the VPN

On the Server:

# /etc/rc.d/rc.openvpn start

Enter the Server PEM pass phrase when prompted.

On the Client:

# /usr/sbin/openvpn /etc/openvpn/client.conf

Enter the Client PEM pass phrase when prompted. To stop openVPN on the Client just hit CTRL+C

On both you should see a new network interface called tun0. On the Server, I obtained the following:

# ifconfig tun0 
tun0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST>  mtu 1500 
        inet 10.8.0.1  netmask 255.255.255.255  destination 10.8.0.2 
        unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  txqueuelen 100  (UNSPEC) 
        RX packets 0  bytes 0 (0.0 B) 
        RX errors 0  dropped 0  overruns 0  frame 0 
        TX packets 0  bytes 0 (0.0 B) 
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0 

Similarly on the Client:

# ifconfig tun0 
tun0: flags=4305<UP,POINTOPOINT,RUNNING,NOARP,MULTICAST>  mtu 1500 
        inet 10.8.0.6  netmask 255.255.255.255  destination 10.8.0.5 
        unspec 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  txqueuelen 100  (UNSPEC) 
        RX packets 0  bytes 0 (0.0 B) 
        RX errors 0  dropped 0  overruns 0  frame 0 
        TX packets 0  bytes 0 (0.0 B) 
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

Naturally you can ping the Server from Client (or vice versa):

For example, from the Client:

# ping -c 3 10.8.0.1
PING 10.8.0.1 (10.8.0.1) 56(84) bytes of data. 
64 bytes from 10.8.0.1: icmp_req=1 ttl=64 time=2888 ms 
64 bytes from 10.8.0.1: icmp_req=2 ttl=64 time=1997 ms 
64 bytes from 10.8.0.1: icmp_req=3 ttl=64 time=1324 ms 

--- 10.8.0.1 ping statistics --- 
3 packets transmitted, 3 received, 0% packet loss, time 1999ms 
rtt min/avg/max/mdev = 1324.475/2070.293/2888.429/640.527 ms, pipe 3

10. Storing the PEM pass phrase in a secure file and Automatic start of service after booting

To start the Openvpn service on boot, an entry in /etc/rc.d/rc.local is needed, but you would have to enter the server PEM pass phrase every time. This might be undesirable if the Server is unreachable. If this is the case, create a file containing your PEM pass phrase in a secure location; e.g. /root/password.ovpn which contains only this pass phrase. Then restrict its permission:

# chmod 600 /root/password.ovpn

On the Server, edit /etc/openvpn/server.conf with the following lines:

askpass /root/password.ovpn
auth-nocache

This may be repeated also on the Client, just edit /etc/openvpn/client.conf instead of /etc/openvpn/server.conf.

To start the Openvpn service automatically on boot-up from the Server, include these lines in /etc/rc.d/rc.local

# Start the OpenVPN Service
if [ -x /etc/rc.d/rc.openvpn ]; then 
  /etc/rc.d/rc.openvpn start 
fi

11. IP Routing

Up to now we have created a tunnel device on both the Server and the Client called tun0 which is visible only to these two machines. However more work is needed to route the Client's connection via tun0 and then to the WAN that is connected to the Server.

11.1 Server Configuration

Enable IP forwarding:

# chmod +x /etc/rc.d/rc.ip_forward
# /etc/rc.d/rc.ip_forward start

IP forwarding is now enabled and will be enabled also after you reboot.

Make a directory called ccd in /etc/openvpn

# mkdir /etc/openvpn/ccd/

Create a file with the same name of the client (in this case client1) and enter the following line in /etc/openvpn/ccd/client1

iroute 192.168.1.0 255.255.255.0

Replace 192.168.1.0 255.255.255.0 by the Network Route of your Client.

Similarly edit /etc/openvpn/server.conf with the following lines:

push "route 192.168.200.0 255.255.255.0"

client-config-dir /etc/openvpn/ccd 
route 192.168.1.0 255.255.255.0 

push "redirect-gateway def1 bypass-dhcp"

push "dhcp-option DNS 208.67.222.222" 
push "dhcp-option DNS 208.67.220.220"

Naturally replace 192.168.200.0 255.255.255.0 with the Server's Network Route, and 192.168.1.0 255.255.255.0 with the Client's Network Route. 208.67.222.222 and 208.67.220.220 are the OpenDNS IP addresses.

Up to now the DNS push configuration has not been successful.

You can either use the original Client DNS servers or else you may rewrite /etc/resolv.conf manually:

# OpenDNS Servers
nameserver 208.67.222.222
nameserver 208.67.220.220

According to your routing table however, it is still worth trying to use the DNS servers listed by the Client, I find that they are generally still available, so you would not need to do anything. However do be aware of possible DNS leaks if you are concerned about your privacy.

Some users have reported that their Client's Network Manager, (or any other similar application) re-wrote the original /etc/resolv.conf back after their manual editing. This could not be reproduced by the author of this article (yet), but you may consider installing and configuring openresolv(5) if this actually happens to you. A SlackBuild for openresolv may be found on http://slackbuilds.org. Openresolv is currently out of the scope of this article.

Next you will have to configure some iptables NAT forwarding on the Server (only). You can do this by first flushing the iptables:

# iptables -F

And then:

# iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE

On Slackware, such a line may be included in /etc/rc.d/rc.firewall and /etc/rc.d/rc.inet2 will run it each time you reboot the Server if the former has executable permissions. You do not have to include anything in /etc/rc.d/rc.local.

The exact lines which you need to include depend on whether you already entered your own iptables filter chains and rules, but I will assume that that this is not the case.

As already explained, as a minimum you only need to enter the following lines in /etc/rc.d/rc.firewall

#!/bin/sh
iptables -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE

If on the other hand you would like a better firewall and you are at least moderately confident with iptables, I propose the following script to be included in your /etc/rc.d/rc.firewall. The comments in the script should help you understand the impact they will have on the Server.

#!/bin/bash                                                                                                                                          
# Start/stop/restart/status the firewall                                                                                                             
                                                                                                                                                     
IPT=/usr/sbin/iptables # This will provide some portability                                                                                          
                                                                                                                                                     
firewall_start() {                                                                                                                                   
                                                                                                                                                     
  # flush the iptables                                                                                                                               
  echo -e "Starting the firewall ....\c"                                                                                                             
  $IPT -F                                                                                                                                            
                                                                                                                                                     
  # policies                                                                                                                                         
  $IPT -P OUTPUT DROP                                                                                                                                
  $IPT -P INPUT DROP                                                                                                                                 
  $IPT -P FORWARD DROP 

  $IPT -N SERVICES # services is a custom chain 

  # allowed output
  $IPT -A OUTPUT -o lo -j ACCEPT
  $IPT -A OUTPUT -o eth0 -j ACCEPT
  $IPT -A OUTPUT -o tun0 -j ACCEPT

  # allowed inputs 
  #$IPT -A INPUT -i lo -j ACCEPT # uncomment if the host is a desktop 
  $IPT -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT # allow responses
  $IPT -A INPUT -j SERVICES # append the services chain to the input 

  # allowed forwarding for openVPN
  $IPT -A FORWARD -i eth0 -o tun0 -m state --state ESTABLISHED,RELATED -j ACCEPT
  $IPT -A FORWARD -s 10.8.0.0/24 -o eth0 -j ACCEPT

  # masquerade the openvpn network
  $IPT -t nat -A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE

  # allow sshd on the default tcp port 22
  #$IPT -A SERVICES -p tcp --dport 22 -j ACCEPT # Uncomment to allow sshd

  # allow openvpn for the default udp port 1194
  $IPT -A SERVICES -p udp --dport 1194 -j ACCEPT
  
  echo "done."
} 

firewall_stop() { 
  echo -e "Stopping the firewall ....\c"

  # polcies (permissive)
  $IPT -P OUTPUT ACCEPT
  $IPT -P INPUT ACCEPT 
  $IPT -P FORWARD ACCEPT 

  # flush the iptables 
  $IPT -F 

  # delete the services custom chain 
  $IPT -X SERVICES
  echo "done."
}

firewall_status() {
  $IPT -vL
}

case "$1" in 
'start') 
  firewall_start 
  ;; 
'stop') 
  firewall_stop 
  ;; 
'restart') 
  firewall_stop 
  firewall_start 
  ;; 
'status')
  firewall_status
  ;;
*) 
  echo "Usage $0 start|stop|restart|status" 
esac

Give the firewall rc script executable permission:

# chmod +x /etc/rc.d/rc.firewall

and start it:

# /etc/rc.d/rc.firewall start

Restart the Openvpn service on the Server:

# /etc/rc.d/rc.openvpn restart

and reconnect from the Client:

# /usr/sbin/openvpn /etc/openvpn/client.conf

12. Firewalls

In the previous chapter we referred to a firewall you may include to protect your Openvpn Server. However this chapter refers to firewalls on the Client LAN that may block the VPN connection by blocking traffic on UDP port 1194.

In order to penetrate through the Client firewall your may want to try changing the port to 443 - normally reserved for https. Using TCP instead of UDP will also help. To make these change you will need to amend /etc/openvpn/server.conf of the Server, from

port 1194
proto udp

to:

port 443
proto tcp

and /etc/openvpn/client.conf of the Client, from

proto udp

remote servervpn.no-ip.org 1194

to:

proto tcp

remote servervpn.no-ip.org 443

The Server's firewall script would also need to be modified. Change these lines:

# allow vpn on the default udp port 1194 
$IPT -A SERVICES -p udp --dport 1194 -j ACCEPT 

to:

# allow vpn on the custom tcp port 443 
$IPT -A SERVICES -p tcp --dport 443 -j ACCEPT 

You also have to modify your Router's port forwarding to TCP port 443.

13. Sources


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