Openvpn on raspberry pi

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This page is fully copied from https://www.aaflalo.me/2015/01/openvpn-tap-bridge-mode/.

That page is probably kept up to date more, and has active comments. This is a copy I made in case I need access and that site is down or unavailable for other reasons.

Tutorial for OpenVPN TAP Bridge Mode

I’m using my raspberry pi as a NAS with a samba server to provide the content through my home network. I wanted to have that content directly accessible from the rest of the world but without the traffic to be visible to anybody else than me. I then decide to use OpenVPN TAP in bridged mode.

OpenVPN Tap: Setup

  • TL-841 as router
  • Raspberry Pi as NAS
  • Internal IP : 192.168.42.0/24
  • Range IP router: 192.168.42.10 – 192.168.42.120
  • Range IP OpenVPN: 192.168.42.128 – 192.168.42.254
  • OpenVPN version: 2.4

Preparation

First, important point, if you have a DHCP server on your router, be sure to configure it to not assign IP address for the whole subnet, but only a part (as I’ve done in the setup). If you don’t, you could encounter 2 devices sharing the same IP, and trust me you don’t want that.

Download scripts/conf

You need to download and extract my scripts and configuration. I’ll walk you through each of those. The following command will download the files and extract them into /tmp/openvpn-scripts:

wget -O /tmp/master.zip https://gist.github.com/Belphemur/3b03eaad96172b2159fc/archive/master.zip && mkdir /tmp/openvpn-scripts && unzip /tmp/master.zip -d /tmp/openvpn-scripts/


Installing OpenVPN

sudo aptitude install openvpn

Enable TLS

We need the easy-rsa to easily create our root certificate, the certificate of the server and the one for each client. Using those cert, the client will authenticate themselves to the server. No need for login/password. 

sudo aptitude install easy-rsa
sudo mkdir /etc/openvpn/easy-rsa
sudo cp -R /usr/share/easy-rsa/* /etc/openvpn/easy-rsa/
sudo ln -s /etc/openvpn/easy-rsa/openssl-1.0.0.cnf /etc/openvpn/easy-rsa/openssl.cnf

Configuring TLS

Edit /etc/openvpn/easy-rsa/vars bottom according to your organization.

Look for those in the file and change them for your needs. 

export KEY_COUNTRY="US"
export KEY_PROVINCE="CA"
export KEY_CITY="SanFrancisco"
export KEY_ORG="Fort-Funston"
export KEY_EMAIL="mail@domain"
export KEY_EMAIL=mail@domain
export KEY_SIZE=4096

Now execute those command to prepare for the generation of the certificate and keys: 

cd /etc/openvpn/easy-rsa/
sudo -s #because of the sourcing of the ./vars file you need to be root to do this
mkdir keys
touch keys/index.txt
echo 01 > keys/serial
. ./vars # set environment variables
./clean-all


From Debian wiki on OpenVPN: 

   Remember:
   
       only .key files should be kept confidential.
       .crt and .csr files can be sent over insecure channels such as plaintext email.
       do not need to copy a .key file between computers.
       each computer will have its own certificate/key pair.

Generating the certs and keys

Those commands will generate the root certificate, the server certificate and the DH keys. I give two choices to generate the DH Param, either you use your own machine or DHTool free service. 

cd /etc/openvpn/easy-rsa/
./build-ca
./build-key-server server
./build-dh #takes time

Disabling Current Systemd Service

By default when you’re installing the OpenVPN server, the installer take care of setting OpenVPN to start with your system.

We don’t want that since you’re installing another service configuration that takes care of the Bridge. 

sudo systemctl disable openvpn
sudo systemctl stop openvpn

Packet Forwarding

Because we’re going to create a bridge, we need to set the kernel to let the IP packet transit through it. To do this, we’ll use sysctl.

You’ll need to edit the file /etc/sysctl.conf to add or uncomment the following line: net.ipv4.ip_forward = 1

Then you reload the configuration: sysctl -p /etc/sysctl.conf

The packet switching is not set in your kernel and will stay after reboot.

Bridge Scripts

Following the OpenVPN tutorial on how to create a bridge and make it work with OpenVPN, I created my own scripts to do this. First, you need to install the bridge-utils, scripts used to create network bridge then create a directory to put my scripts into it. 

sudo aptitude install bridge-utils

My bridge scripts are in 3 parts: 

   Bridge-conf: to configure the scripts
   Bridge-start: to start the bridge
   Bridge-stop: to destroy the bridge

My scripts are made to restore the system to its previous state after destroying the bridge. I tested them and they work on Debian Jessie on a raspberry pi.

You’ll find all the following file in /tmp/openvpn-scripts/ if you used the command to download the gist.

Bridge-conf

You NEED to modify this file with your own configuration. 

sudo ip addr

or 

sudo ifconfig

Should give you the information you need to put here. The idea is to make your bridge act like your normal Ethernet interface with the same properties 

#!/bin/bash

# Define physical ethernet interface to be bridged
# with TAP interface(s) above.
eth="eth0"
eth_ip="192.168.42.2"
eth_netmask="255.255.255.0"
eth_broadcast="192.168.42.255"
eth_gateway="192.168.42.1"
eth_mac="XX:XX:XX:XX:XX:XX"

# Define Bridge Interface
br="br0"

# Define list of TAP interfaces to be bridged together
tap="tap0"

Bridge-start

If you’re using VMWare for your OpenVPN server, you have some extra steps to do (thanks Per Mejdal Rasmussen) 

   When using VMware, you need to set the port group in promiscuous mode and add “brctl setageing $br 0” to bridge-start.

   Source: StackOverflow

#!/bin/bash

. /etc/openvpn/bridge/bridge-conf 
#################################
# Set up Ethernet bridge on Linux
# Requires: bridge-utils
#################################

for t in $tap; do
    openvpn --mktun --dev $t
done

brctl addbr $br
brctl addif $br $eth

for t in $tap; do
    brctl addif $br $t
done

for t in $tap; do
    ifconfig $t 0.0.0.0 promisc up
    iptables -A INPUT -i $t -j ACCEPT
done

iptables -A INPUT -i $br -j ACCEPT
iptables -A FORWARD -i $br -j ACCEPT

ifconfig $eth 0.0.0.0 promisc up

ifconfig $br $eth_ip netmask $eth_netmask broadcast $eth_broadcast
ip link set $br address $eth_mac
route add default gw $eth_gateway $br

Bridge-stop

#!/bin/bash

. /etc/openvpn/bridge/bridge-conf
####################################
# Tear Down Ethernet bridge on Linux
####################################


iptables -D INPUT -i $br -j ACCEPT
iptables -D FORWARD -i $br -j ACCEPT

ifconfig $br down
brctl delbr $br

for t in $tap; do
    openvpn --rmtun --dev $t
    iptables -D INPUT -i $t -j ACCEPT
done

ifconfig $eth $eth_ip netmask $eth_netmask broadcast $eth_broadcast
route add default gw $eth_gateway $eth

Server Configuration

In the same folder, you find the server.conf. The file contains the configuration for OpenVPN in server mode.

You should update the port, protocol (proto) and the server-bridge with your network setting. 

insert my own example here.

You’ll find also this:

  1. set the dns servers

push "dhcp-option DNS 192.168.42.1"

  1. set the WINS server (SAMBA)

push "dhcp-option WINS 192.168.42.2"

It needs to be changed also, the first one to point at your router (that surely act as a DNS server) and the second one to your SAMBA server (NAS). In my case, it’s the same IP as the server because I’m hosting the Samba server and the OpenVPN server on the same device.

Client Configuration Script

Last file that needs to be changed for your needs, build-client. The script generates a configuration file for OpenVPN in the folder you launch it. Either you edit the configuration script with your variable or use the environment variable to set it (See Generate a client configuration)

You’ll find a section VARIABLES with : 

   PROTO: The protocol used on the server. (Need to be the same as you set on the server)
   REMOTE: The IP address or HOST of your server. It needs to be your external IP address or an hostname (like exmaple.com) that point to your external IP. (I’m using no-ip as a DDNS service). If you leave it empty, the script will retrieve your external IP for you.
   PORT: the port used by the server
   COMPRESS: the compression algorithm used. If you’re using OpenVPN >= 2.4, replace the default argument by compress. This will make the client ask the server for the compression algorithm.
   OPENVPN_CLIENT_DIRECTIVE: if you need to add another directive in the client configuration

#!/bin/bash
#This script generate the key for the wanted client and it's configuration file
#to be used with OpenVPN. If the key has already been generated it will only
#generate the configuration file for OpenVPN

#VARIABLES
#If you don't set a remote (the external IP of the server or the hostname)
#the script will try to gather it using dig
#You need to change the port to the one set in your server
#if you want to add new directive to client configuration use $OPENVPN_CLIENT_DIRECTIVE

PROTO="${PROTO:-udp}"
REMOTE="${REMOTE:-}"
PORT="${PORT:-5555}"
OPENVPN_EASY_RSA_PATH="/etc/openvpn/easy-rsa/"
OPENVPN_CLIENT_DIRECTIVE="${OPENVPN_CLIENT_DIRECTIVE:-}"
DEV_TYPE=${DEV_TYPE:-tap0}
COMPRESS=${COMPRESS:-compress}

#DO NOT MODIFY BELOW
function usage {
	echo "Usage: $0 clientName"
	echo "ENV Variables:"
	echo "PROTO:				protocol used"
	echo "REMOTE:				host or IP address of the server"
	echo "PORT:				port on the server"
	echo "DEV_TYPE:			device type (tun+/tap+)"
	echo "COMPRESS:			The compression algorithm used (comp-lzo, compress (if pushed by the server), compress snappy)"
	exit -1
}

function getIp {
	echo  `dig +short myip.opendns.com @resolver1.opendns.com`
}

function clientConfig {
cat <<CLIENT_CONF
client
dev $DEV_TYPE
proto $PROTO
remote $REMOTE $PORT
resolv-retry infinite
nobind	
persist-key
persist-tun
remote-cert-tls server
cipher AES-256-CBC
auth SHA512
$COMPRESS
verb 3
pull
$OPENVPN_CLIENT_DIRECTIVE
<ca>
$CA
</ca>
<cert>
$CERT
</cert>
<key>
$KEY
</key>
CLIENT_CONF
}
if [ $# -eq 0 ]; then
	echo "No arguments supplied"
	usage	
fi
if [ -z "$1" ]; then
	echo "Client name not supplied"
	usage
fi

if [ -z "$REMOTE" ]; then
	REMOTE=$(getIp)
fi
CLIENT=$1
if [ ! -f $OPENVPN_EASY_RSA_PATH/keys/${CLIENT}.crt ]; then	
	pushd $OPENVPN_EASY_RSA_PATH
	source ./vars
	./build-key $CLIENT 
	popd
fi


CA=`cat $OPENVPN_EASY_RSA_PATH/keys/ca.crt | grep -A 100 "BEGIN CERTIFICATE" | grep -B 100 "END CERTIFICATE"`

CERT=`cat $OPENVPN_EASY_RSA_PATH/keys/${CLIENT}.crt | grep -A 100 "BEGIN CERTIFICATE" | grep -B 100 "END CERTIFICATE"`

KEY=`cat $OPENVPN_EASY_RSA_PATH/keys/${CLIENT}.key | grep -A 100 "BEGIN PRIVATE KEY" | grep -B 100 "END PRIVATE KEY"`

clientConfig > $CLIENT.ovpn
exit 0

Installing the configuration and scripts

Now that the preparation is ready, let’s put all the script and configuration where they should be.

OpenVPN configuration

Move the server.conf into /etc/openvpn/

sudo mv /tmp/openvpn-scripts/server.conf /etc/openvpn/

Bridge scripts

sudo mkdir /etc/openvpn/bridge sudo mv /tmp/openvpn-scripts/bridge-* /etc/openvpn/bridge/ sudo chmod +x /etc/openvpn/bridge/bridge-*

Client Generator Script

sudo mv /tmp/openvpn-scripts/build-client /etc/openvpn/easy-rsa/ sudo chmod +x /etc/openvpn/easy-rsa/build-client

Service Script for systemd

If you are using Debian Jessie or your distribution is using systemd as an init do this to add service unit for OpenVPN Bridged.

sudo mv /tmp/openvpn-scripts/openvpn@.service /etc/systemd/system/ sudo systemctl daemon-reload

To make SystemD start the service at boot time

sudo systemctl enable openvpn@server

Generate a client configuration

Now we will generate a client configuration that will be used on your computer to connect to the OpenVPN server. Here we build for the client OnMyWay. 

cd /etc/openvpn/easy-rsa/
sudo ./build-client OnMyWay

or 

cd /etc/openvpn/easy-rsa/
sudo PROTO=udp REMOTE=example.com PORT=1255 COMPRESS=comp-lzo ./build-client OnMyWay


Using the environment variable is another way to generate a client configuration.

You need to say yes to the signing of the cert and the commit.

You should see something like this: 

/etc/openvpn/easy-rsa /home/pi
NOTE: If you run ./clean-all, I will be doing a rm -rf on /etc/openvpn/easy-rsa/keys
Generating a 2048 bit RSA private key
..................................................................................+++
...................................+++
writing new private key to 'OnMyWay.key'
-----
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.
-----
Country Name (2 letter code) [US]:
State or Province Name (full name) [NY]:
Locality Name (eg, city) [New-York]:
Organization Name (eg, company) [Raspy]:
Organizational Unit Name (eg, section) [RaspberryPiOrg]:
Common Name (eg, your name or your server's hostname) [OnMyWay]:
Name [RaspyKey]:
Email Address [raspy@raspy.net]:

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:
Using configuration from /etc/openvpn/easy-rsa/openssl-1.0.0.cnf
Check that the request matches the signature
Signature ok
The Subject's Distinguished Name is as follows
countryName                                             :PRINTABLE:'US'
stateOrProvinceName                                     :PRINTABLE:'NY'
localityName                                            :PRINTABLE:'New-York'
organizationName                                        :PRINTABLE:'Raspy'
organizationalUnitName                                  :PRINTABLE:'RaspberryPiOrg'
commonName                                              :PRINTABLE:'OnMyWay'
name                                                    :PRINTABLE:'RaspyKey'
emailAddress                                            :IA5STRING:'raspy@raspy.net'
Certificate is to be certified until Jan 18 17:49:16 2025 GMT (3650 days)
Sign the certificate? [y/n]:y

1 out of 1 certificate requests certified, commit? [y/n]y
Write out database with 1 new entries
Data Base Updated

You’ll find an OnMyWay.ovpn in the same folder you are. It’s the configuration with the certificate embedded into it.

Revoke a user

sudo -s
cd /etc/openvpn/easy-rsa
. ./vars
./revoke-full OnMyWay
exit

Start the server

If you used my service unit for systemd :

sudo service openvpn@server start

else

sudo /etc/openvpn/bridge/bridge-start sudo service openvpn start

Stop the server

If you used my service unit for systemd :

sudo service openvpn@server stop

else

sudo service openvpn stop sudo /etc/openvpn/bridge/bridge-stop

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