OpenVPN 2

General Information

This guide will help you set up OpenVPN to allow remote users to securely connect to the internal LAN or use the VPN tunnel as an endpoint when on insecure wireless access points, allowing safe transmission of data without worries of being sniffed or intercepted.

Requirements

You will need the following items to be able to complete this guide:

  1. Root access to a FreeBSD machine
  2. FreeBSD install with ports up-to-date
  3. Bash installed

Installation

We only have one thing to compile.

# cd /usr/ports/security/openvpn
# make install clean

Server Configuration

First thing’s first, we need a directory to store the configuration files and keys, so let’s create that directory structure

# cd /usr/local/etc/
# mkdir openvpn
# cd openvpn
# mkdir easy-rsa
# mkdir cert

We need to copy over the reference files for easy-rsa, since OpenVPN is very picky about how it wants its certificate authority, and other such items.

# cd /usr/local/etc/openvpn/easy-rsa
# cp -r /usr/local/share/doc/openvpn/easy-rsa/2.0/ .

Next up, the file vars that was moved into the directory has to be created/modified to suite your needs. You may copy the text below and paste it into a completely new file overwriting the old one (/usr/local/openvpn/easy-rsa/vars)

export EASY_RSA="`pwd`"
export KEY_CONFIG="$EASY_RSA/openssl.cnf"
export KEY_DIR="/usr/local/etc/openvpn/cert"
# Issue rm -rf warning
echo NOTE: If you run ./clean-all, I will be doing a rm -rf on $KEY_DIR
export KEY_SIZE=2048
export CA_EXPIRE=3650
export KEY_EXPIRE=3650
export KEY_COUNTRY="US"
export KEY_PROVINCE="NY"
export KEY_CITY="New York"
export KEY_ORG="OrgName"
export KEY_EMAIL="email@example.net"

At this point in time, if you are not using bash, you will need to switch to bash as your shell

# bash

Now load the vars file you just created. It will give you a warning you may want to heed. Only do clean-all when you are certain you need it.

# . ./vars

Now we can go ahead and build the required Certificate Authority and the server keys. Follow the instructions and fill in the required fields, or just hit enter if they are correct (you did set the vars file up, right?)

# bash build-ca
# bash build-key-server server
# bash build-dh

Next up we create the beast that is the openvpn.conf (/usr/local/etc/openvpn/openvpn.conf) file. The comments have been removed, for brevity. The OpenVPN Howto contains an example config file, which also explains all the options available and what they do.

;local a.b.c.d
port 1194
;proto tcp
proto udp
dev tun
ca cert/ca.crt
cert cert/server.crt
key cert/server.key  # This file should be kept secret
dh cert/dh2048.pem
server 10.8.0.0 255.255.255.0
ifconfig-pool-persist ipp.txt
;push "route 192.168.20.0 255.255.255.0"
push "redirect-gateway"
push "dhcp-option DNS 4.2.2.2"
;push "dhcp-option WINS 10.8.0.1"
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

comp-lzo
max-clients 20

user nobody
group nobody
persist-key
persist-tun
status openvpn-status.log
verb 3
mute 10

Next up, we need to add it to rc.conf so that the service will be started upon boot.

# echo 'openvpn_enable="YES"' >> /etc/rc.conf

This should now allow us to start the OpenVPN service by using it’s rc.d script. It won’t do us much good until we generate some private/public key pairs on a client to allow them to connect. The command is included for demonstration purposes only.

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

For the OpenVPN client to be able to connect to the server, you need to open port 1149 protocol UDP and allow it inbound. For clients to be able to use your new OpenVPN server as their gateway (config file as created above does just that) you will need to provide some nat rules. The following included rules are for pf (/etc/pf.conf).

vpn_if="tun0"

nat on $ext_if from 10.8.0.0/24 to any -> ($ext_if)

pass in quick proto udp from any to port 1194 keep state label "openvpn"

# Pass stuff on the VPN interface
pass quick on $vpn_if keep state

Client Configuration

On the client we need to generate a private key, as well as a certificate signing request, or CSR. Create a new directory .openvpn to hold all the files. When OpenSSL asks you for the Organization name, you need to fill in the same value as you used to create the server key, which was also set as KEY_ORG in the vars file. When it requests a common name, use your full name, or a derivation thereof, all common names have to be unique, and if yours is not you will not be allowed to connect to the OpenVPN server.

# mkdir ~/.openvpn
# cd ~/.openvpn
# openssl req -days 3650 -new -keyout openvpn.key -out openvpn.csr

Now you need to transfer the openvpn.csr to the server and put it in the cert directory (/usr/local/etc/openvpn/cert/), give it a meaningful name, and sign the CSR with the Certificate Authority. Use bash again. For the purposes of this guide, I have named the file example.csr

# bash

And now we load vars again

# cd /usr/local/etc/openvpn/easy-rsa/
# . ./vars

Sign it

# bash sign-req ../cert/example

After you follow the instructions, you want to grab two things from the server and transfer them back to the client.

  1. ca.crt – Required so that your newly signed cert can be verified as being real
  2. example.crt – Was created by signing the CSR
  3. ta.key – Transfer this securely, if this falls in the wrong hands it could lead to easier breaking into encrypted streams

Rename example.crt back to openvpn.crt, so that you won’t need to modify the client side config file.

File: ~/.openvpn/client.conf

client
dev tun
proto udp

remote [IP address or hostname] 1194
resolv-retry infinite
nobind
persist-key
persist-tun

ca [homedir]/.openvpn/ca.crt
cert [homedir]/.openvpn/openvpn.crt
key [homedir]/.openvpn/openvpn.key

ns-cert-type server

tls-auth [homedir]/.openvpn/ta.key 1
comp-lzo
verb 3
mute 10

You will need to replace [homedir] with your full path to your home directory, and change the IP address and or hostname to something that makes sense for you. If the server is publicly available over the internet and has a domain name assigned to it, you may use that instead.

The client is all set to go. All you have to do now is become root (so you can dynamically allocate a tun/tap device. You can statically set a device if you want. Read the OpenVPN examples)

# openvpn --config [homedir]/.openvpn/client.conf

If you are connected to anything like instant messaging clients, you will be disconnected as the OS starts to send packets over the new gateway.

License: Creative Commons Attribution-NonCommercial-ShareAlike 2.5

Original located at: http://0×58.com/guides/openvpn.xml

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