La seguridad al navegar en Internet es muy importante peor aún si nos conectamos en una de tantas redes gratis que nos ofrecen muchos establecimientos o en eventos de tecnología, estas redes pueden ser utilizadas para el robo de información valiosa de nosotros como información de entrada a correos electrónicos (usuario y contraseña), para lo cual habrían dos o mas soluciones una podría ser no utilizar esas redes o ninguna red en la que confiemos pero para la mayoría tener una red inalambrica mientras tomamos un café es muy valiosa y si vivos en un país en donde se intercepta o se monitorea el trafico web no podemos confiar en ninguna red, por lo que lo único que nos queda es utilizar una comunicación encriptada para nuestra salida a Internet y una de esas es el usar un VPN, en vista de esto me he dado a la tarea de instalar y configurar un servidor de VPN con OpenVPN.

Primero empecemos con el concepto de VPN obtenida de Wikipedia.

OpenVPN, es un producto de software creado por James Yonan en el año 2001 y que ha estado siendo mejorado desde entonces.

Ninguna otra solución ofrece una mezcla semejante de seguridad a nivel empresarial, seguridad, facilidad de uso y riqueza de características.

Es una solución multiplataforma que ha simplificado mucho la configuración de VPN’s dejando atrás los tiempos de otras soluciones difíciles de configurar como IPsec y haciéndola más accesible para gente inexperta en este tipo de tecnología.

Ahora procederemos a la instalación de OpenVPN sobre Debian Lenny para lo cual debemos instalar los siguientes paquetes:

apt-get update

apt-get upgrade

apt-get install openvpn

También necesitaremos instalar los siguientes paquetes «openssl» «liblzo1» si aún no los tenemos instalados:

apt-get install openssl

apt-get install liblzo1

Ahora nos toca entrar en materia y empezar con la configuración del OpenVPN comenzando con la generación de los certificados para la conexión los scripts para la generación de estos los encontramos en el siguiente PATH «/usr/share/doc/openvpn/examples/easy-rsa/2.0/» en este directorio tenemos que modificar el archivo «vars» con la información de nuestro certificado, luego ejecutamos «source vars» ya con esto podemos comenzar a ejecutar los ejecutables para generar los certificados:

./vars

./clean-all # para borrar los certificados viejos

./build-ca

Ahora ejecutaremos los siguientes scripts para crear el certificado para el servidor:

./build-key-server server

Ya creado el del servidor proseguimos con el certificado para los clientes, vale la pena decir que lo mejor es que le demos un certificado a cada cliente por lo cual el siguiente script lo debemos ejecutar para cada cliente:

./build-key [nombre del cliente]

Ahora vamos a construir el diffie hellman que es un protocolo que permite el intercambio secreto de claves el tamaño de encriptación esta dado por el valor que pongamos en el archivo «vars» en la variable «KEY_SIZE» por defecto viene en 1024.

./build-dh

Ya creadas los certificados los tenemos que copiar al directorio de configuración del OpenVpn que se encuentra en «/etc/openvpn», pero primero debemos cambiarnos al directorio en donde se crearon los certificados.

cd keys

y copiaremos los siguientes certificados mas todos los certificados extras que hallamos creado para los demás clientes.

cp ca.key ca.crt server.key server.csr server.crt client.key client.crt client.csr dh1024.pem /etc/openvpn

De los archivos que copiamos aquí están en donde deben estar cada uno:

ca.key server.crt server.key dh1024.pem #deben estar solo debe estar en el servidor
client.crt client.key #solo deben estar en el cliente
ca.crt # debe estar tanto en el servidor como en cada cliente

El siguiente paso es crear el archivo de configuración del lado del servidor para que funcione nuestro servidor con lo cual podemos crear el archivo con el nombre server.conf o podemos copiar el ejemplo que tenemos en «/usr/share/doc/openvpn/examples/sample-config-files» con cualquiera de las 2 opciones que escojan aquí están los parámetros:

#################################################
# 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 ca.crt
cert server.crt
key server.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 dh1024.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 #esta es el rango de ip asignado a los clientes

# 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

# 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
# the TUN/TAP interface to the internet in
# order for this to work properly).
# CAVEAT: May break client’s network config if
# client’s local DHCP server packets get routed
# through the tunnel. Solution: make sure
# client’s local DHCP server is reachable via
# a more specific route than the default route
# of 0.0.0.0/0.0.0.0.
;push «redirect-gateway def1» #opcion activa todo el trafico por el VPN

# Certain Windows-specific network settings
# can be pushed to clients, such as DNS
# or WINS server addresses. CAVEAT:
# http://openvpn.net/faq.html#dhcpcaveats
;push «dhcp-option DNS 10.8.0.1» #estas 2 hay que activarlas si queremos todo el trafico salga por el vpn
;push «dhcp-option WINS 10.8.0.1»

# 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 #hay que quitar comentario si queremos utilizar el mismo certificado cliente para todos

# 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 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 #para compresión

# 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 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

Lo anterior es para el Servidor ahora vamos a ir del lado del cliente:

##############################################
# 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 my-server-1 1194 #sustituir my-server con el ip o el nombre del host del servidor
;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 ca.crt
cert client.crt
key client.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

# If a tls-auth key is used on the server
# then every client must also have the key.
;tls-auth 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 #para compresión

# Set log file verbosity.
verb 3

# Silence repeating messages
;mute 20

Lo unico que nos falta si todo el trafico va a pasar por el servidor es activar el «ip_forward» y activar el «iptables» para lo cual haremos lo siguiente.

Para activar el «ip_forward» debemos cambiar el 0 por 1en el siguiente archivo «/etc/sysctl.conf» debemos buscar el siguiente parametro:

net.ipv4.ip_forward=1

Y el iptables con el siguiente comando:

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

Ahora solo nos falta probar las configuraciones en el servidor ejecutando en la carpeta «/etc/openvpn/»:

openvpn –config server.conf

Y en el cliente en el mismo directorio:

openvpn –config client.conf

Nota: yo he utilizado 2 linux tanto como servidor como cliente si desean utilizar clientes windows debe descargar el software de OpenVPN.

He utilizado muchos manuales entre algunos el dejo estos

OpenVPN
cihan.me
google.com