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OPENVPN - The Easy Tutorial - Advanced Settings

openmaniak little square blue OpenVPN openmaniak little arrow right blue Advanced Settings
Last Change : Dec 07 2010


Tool
Install
Ergonomy
Forum



Details What is OpenVPN?
Screenshots
Prerequisites & Installation
Tutorial OpenVPN
-----SECURITY MODE-----------
Transparent Tunnel
Static key
SSL & PKI (certificates)
-----CASE STUDY-----------
IP VPN (TUN)
Ethernet VPN (TAP)
VPN Advanced Settings
----------------
Bridging
Routing




If you like our tutorials, don't hesitate to support us and visit our sponsors!
Si vous aimez nos tutoriaux, n'hésitez pas à nous supporter et visiter nos sponsors!



1. PRESENTATION
4. START OPENPVN
7. ROUTING
2. INTERFACES SETTING
5. TCP/UDP PORTS
8. CHECKS
3. CONFIGURATION
6. IPTABLES
9. STARTUP SCRIPT



openmaniak little square blue 1. PRESENTATION:

In this advanced case study two sites are connected together through an OpenVPN tunnel in SSL/TLS mode with some optional and very useful OpenVPN settings.

The major goals of our case study are the following:

-




-


-
Establish a full protocol connectivity between the two LANs (networks 10.0.1.0/24 and 10.0.2.0/24) through an OpenVPN tunnel on Linux boxes.
This means that the two local networks will see each other as they were in the same physical network and just separated by a router.

Permit the LANs users to surf on the Internet (TCP port HTTP,80 and HTTPS,443). Source Address translation (NAT) will be needed on the Linux Boxes.

Secure the Linux boxes with Netfilter, the Linux Firewall kernel module.
openmaniak openvpn advanced settings

Top of the page



openmaniak little square blue 2. INTERFACES SETTING:

First thing to do is to configure the IP settings on the OpenVPN devices and the LAN desktops.
See the OpenManiak tutorial for more details.

openmaniak little arrow right blue OpenVPN Server

Set the interfaces' IP addresses:

#ifconfig eth0 50.0.0.1 netmask 255.255.255.0
#ifconfig eth1 10.0.1.1 netmask 255.255.255.0
Set the default gateway:

#route add default gateway 50.0.0.100
If you want to keep your IP settings when the system reboots, edit the /etc/network/interfaces file:

#vim /etc/network/interfaces
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 50.0.0.1
netmask 255.255.255.0
gateway 50.0.0.100

auto eth1
iface eth1 inet static
address 10.0.1.1
netmask 255.255.255.0
openmaniak little arrow right blue OpenVPN Client

Set the interfaces IP addresses:

#ifconfig eth0 100.0.0.1 netmask 255.255.255.0
#ifconfig eth1 10.0.2.1 netmask 255.255.255.0
Set the default gateway:

#route add default gateway 100.0.0.100
If you want to keep your IP settings when the system reboots, edit the /etc/network/interfaces file:

#vim /etc/network/interfaces
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
address 100.0.0.1
netmask 255.255.255.0
gateway 100.0.0.100

auto eth1
iface eth1 inet static
address 10.0.2.1
netmask 255.255.255.0
openmaniak little arrow right blue Local Site A Desktop

IP address:
Netmask:
Gateway:
10.0.1.100
255.255.255.0
10.0.1.1
openmaniak little arrow right blue Local Site B Desktop

IP address:
Netmask:
Gateway:
10.0.2.100
255.255.255.0
10.0.2.1

Top of the page



openmaniak little square blue 3. CONFIGURATIONS:

The files used by OpenVPN are put in the /etc/openvpn directory
The OpenVPN machines will have the following files:

openvpn.conf - the OpenVPN configuration file.
ipp.txt (server only) - a IP reservation file used for dynamic IP assignment.
route.txt (server only)- script to add a route on the server when the tunnel is up.

Files created by the certificate authority (CA), see the SSL/PKI tutorial to know how to create them.

ca.crt - the certificate authority certificate (CA public key).
server.crt or client.crt - the server or client certificate (server or client public key).
server.key or client.key - the server or client private key.
dh1024pem (server only) - the Diffie-Hellman (DH) settings.

openmaniak little arrow right blue openvpn.conf file:

SERVER CLIENT
# Listening server IP address
local 50.0.0.1
# Local port
lport 2000
# Remote port
rport 2001
# Tunnel mode
dev tap
# CA certificate
ca ca.crt
# Server certificate
cert server.crt
# Server private key
key server.key
# Diffie-Hellman
dh dh1024.pem
# DHCP range, server will take
# 10.7.0.1
server 10.7.0.0 255.255.255.248
# IP reservation file
ifconfig-pool-persist ipp.txt
# Push a route to the client
push "route 10.0.1.0 255.255.255.0"
# Encryption
ciper AES-256-CBC
# Authentication
auth MD5
# Compression is activated
comp-lzo
# Allowed clients
max-clients 10
# Reduce the OpenVPN daemon's
# privileges after initialization
# (Linux only)
user nobody
# OpenVPN statistics
status openvpn-status.log
# Verbosity level
verb 2
# Script launched when the tunnel is
# up
up "./route.txt"
# After initialization, OpenVPN can
# only access a directory
# (Linux only)
chroot /etc/openvpn/
 
 
 
# Server WAN IP address
remote 50.0.0.1
# Local port
lport 2001
# Remote port
rport 2000
# Tunnel mode
dev tap
# CA certificate
ca ca.crt
# Client certificate
cert client.crt
# Client private key
key client.key
 

 


 

 

# Encryption
ciper AES-256-CBC
# Authentication
auth MD5
# Compression is activated
comp-lzo
 

# Reduce the OpenVPN daemon's
# privileges after initialization
# (Linux only)
user nobody
# OpenVPN statistics
status openvpn-status.log
# Verbosity level
verb 2
 

 
# After initialization, OpenVPN can only
# access a directory
# (Linux only)
chroot /etc/openvpn/
# The client accepts options pushed
# by the server
pull
Here is a summary of the OpenVPN functionnalities used in our tutorial compared to their default value.

Optional settings:
Tunnel mode
DHCP server
Persistant addresses
route push
cipher
auth
compression
Server source port
Server destination port
User
rights
Max-user
"up" script
IP assignment
Verbosity level
Case study
dev tap
server 10.7.0.0 255.255.255.248
ifconfig-pool-persist ipp.txt
push "10.0.1.0 255.255.255.0"
cipher AES
auth md5
comp-lzo
lport 2000
rport 2001
user nodody
chroot /etc/openvpn
10
up "route.txt"
ifconfig-pool-persist file_name
verb 2
Default settings:
dev tun
-
-
-
cipher BF-CBC
auth sha1
-
lport 1194
rport 1194
user root
-
- (Unlimited)
-
-
verb 0
Be sure to add the ".conf" file extension to your configuration file. This is required to use the /etc/init.d/openvpn script to start OpenVPN automatically.

openmaniak little arrow right blue ipp.txt file:

The file /etc/openvpn/ipp.txt is used to store the IP reservations when the OpenVPN server dynamically assigns IP addresses to clients.

The syntax is the following:
certificate_client_name,ip_address

The certificate_client_name is the name chosen when creating the client certificate, see the OpenVPN PKI tutorial. In our case study, we chose client1 as the certificate_client_name.

client1,10.7.0.6
openmaniak little arrow right blue route.txt file:

When the server is up, it will launch the "route.txt" bash script which will add a route on it to reach the client local network.

#!/bin/bash
route add -net 10.0.2.0/24 gw 10.7.0.6
-------------------------------------

Last thing to do is to set the files permissions inside the /etc/openvpn directory.

The user teddy has read/write/execute permissions. None else has any permissions at all.
The most important file inside the directory are the private keys (server.key or client.key depending on which system you are using). If your server public key is captured, this will mean that all your Open VPN architecture is compromised!

#chmod 700 /etc/openvpn/*
The user teddy is the owner of the files inside the /etc/openvpn directory.

#chown teddy /etc/openvpn/*
Top of the page



openmaniak little square blue 4. START OPENVPN

Let's start OpenVPN on the client and the server and check the logs:

#openvpn /etc/openvpn/openvpn.conf
Server log:

1

16





22
21
10
8/11

15

14

3
4


2


17
18


5
6

12
OpenVPN 2.0.9 i486-pc-linux-gnu [SSL] [LZO] [EPOLL] built on Mar 2 2007
WARNING: No server certificate verification method has been enabled.
   See http://openvpn.net/howto.html#mitm for more info.
LZO compression initialized
Control Channel MTU parms [ L:1586 D:138 EF:38 EB:0 ET:0 EL:0 ]
Data Channel MTU parms [ L:1586 D:1450 EF:54 EB:135 ET:32 EL:0 AF:3/1 ]
Local Options hash (VER=V4): '579db898'
Expected Remote Options hash (VER=V4): 'a0883d96'
chroot to '/etc/openvpn' and cd to '/' succeeded
UID set to nobody
UDPv4 link local (bound): [undef]:2001
UDPv4 link remote: 50.0.0.1:2000
TLS: Initial packet from 50.0.0.1:2000, sid=100aa16d 662ac586
VERIFY OK: depth=1, /C=US/ST=CA/L=SanFrancisco/O=OpenManiak/
   CN=OpenManiak_CA/emailAddress=opensource@openmaniak.com
VERIFY OK: depth=0, /C=US/ST=CA/L=SanFrancisco/O=OpenManiak/
   CN=server/emailAddress=opensource@openmaniak.com
Data Channel Encrypt: Cipher 'AES-256-CBC' initialized with 256 bit key
Data Channel Encrypt: Using 128 bit message hash 'MD5' for HMAC authentication
Data Channel Decrypt: Cipher 'AES-256-CBC' initialized with 256 bit key
Data Channel Decrypt: Using 128 bit message hash 'MD5' for HMAC authentication
Control Channel: TLSv1, cipher TLSv1/SSLv3 DHE-RSA-AES256-SHA, 1024 bit RSA
[server] Peer Connection Initiated with 50.0.0.1:2000
SENT CONTROL [server]: 'PUSH_REQUEST' (status=1)
PUSH: Received control message: 'PUSH_REPLY,route 10.0.1.0 255.255.255.0,
   route-gateway 10.7.0.1,ifconfig 10.7.0.6 255.255.255.248'

OPTIONS IMPORT: --ifconfig/up options modified
OPTIONS IMPORT: route options modified
TUN/TAP device tap0 opened
ifconfig tap0 10.7.0.6 netmask 255.255.255.248 mtu 1500 broadcast 10.7.0.7
route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.7.0.1
Initialization Sequence Completed
Legend:

1.
2.
3.
4.
OpenVPN version
Security mode
Cipher algorithm
Hash algorithm
5.
6.
7.
8.
TUN/TAP drivers
Tunnel MTU
Tunnel Local IP
Tunnel Remote IP
9.
10.
11.
12.
OpenVPN server
Local port
Remote port
OpenVPN status
13.
14.
15.
16.
Client certificat
Server certificat
CA certificat
compression
17.
18.
19.
 
Route pushed to client
IP pushed to client
IP reservation
 
21.
22.
23
 
User ID
Process limitation
"up" script
 

Client log:

1



5
6/7
23

22
21
9
10


19

12

8
16
11





15

13

3
4




2



17
18
 
OpenVPN 2.0.9 i486-pc-linux-gnu [SSL] [LZO] [EPOLL] built on Mar 2 2007
WARNING: --keepalive option is missing from server config
Diffie-Hellman initialized with 1024 bit key
TLS-Auth MTU parms [ L:1586 D:138 EF:38 EB:0 ET:0 EL:0 ]
TUN/TAP device tap0 opened
ifconfig tap0 10.7.0.1 netmask 255.255.255.248 mtu 1500 broadcast 10.7.0.7
./echo.txt tap0 1500 1586 10.7.0.1 255.255.255.248 init
Data Channel MTU parms [ L:1586 D:1450 EF:54 EB:135 ET:32 EL:0 AF:3/1 ]
chroot to '/etc/openvpn' and cd to '/' succeeded
UID set to nobody
UDPv4 link local (bound): 50.0.0.1:2000
UDPv4 link remote: [undef]
MULTI: multi_init called, r=256 v=256
IFCONFIG POOL: base=10.7.0.2 size=5
IFCONFIG POOL LIST
client1,10.7.0.6
Initialization Sequence Completed
MULTI: multi_create_instance called
100.0.0.1:2001 Re-using SSL/TLS context
100.0.0.1:2001 LZO compression initialized
100.0.0.1:2001 Control Channel MTU parms [ L:1586 D:138 EF:38 EB:0 ET:0 EL:0 ]
100.0.0.1:2001 Data Channel MTU parms [ L:1586 D:1450 EF:54 EB:135 ET:32 EL:0
   AF:3/1 ]
100.0.0.1:2001 Local Options hash (VER=V4): 'a0883d96'
100.0.0.1:2001 Expected Remote Options hash (VER=V4): '579db898'
100.0.0.1:2001 TLS: Initial packet from 100.0.0.1:2001, sid=85abe7b5 a5dcafc0
100.0.0.1:2001 VERIFY OK: depth=1, /C=US/ST=CA/L=SanFrancisco/O=OpenManiak/
   CN=OpenManiak_CA/emailAddress=opensource@openmaniak.com
100.0.0.1:2001 VERIFY OK: depth=0, /C=US/ST=CA/L=SanFrancisco/O=OpenManiak/
   CN=client1/emailAddress=opensource@openmaniak.com
100.0.0.1:2001 Data Channel Encrypt: Cipher 'AES-256-CBC' initialized with 256 bit key
100.0.0.1:2001 Data Channel Encrypt: Using 128 bit message hash 'MD5' for
   HMAC authentication
100.0.0.1:2001 Data Channel Decrypt: Cipher 'AES-256-CBC' initialized with 256 bit key
100.0.0.1:2001 Data Channel Decrypt: Using 128 bit message hash 'MD5' for
   HMAC authentication
100.0.0.1:2001 Control Channel: TLSv1, cipher TLSv1/SSLv3 DHE-RSA-AES256-SHA,
   1024 bit RSA
100.0.0.1:2001 [client1] Peer Connection Initiated with 100.0.0.1:2001
client1/100.0.0.1:2001 PUSH: Received control message: 'PUSH_REQUEST'
client1/100.0.0.1:2001 SENT CONTROL [client1]: 'PUSH_REPLY,route 10.0.1.0
   255.255.255.0,route-gateway 10.7.0.1,ifconfig 10.7.0.6 255.255.255.248'
(status=1)
client1/100.0.0.1:2001 MULTI: Learn: 12:5a:a3:22:f7:11 -> client1/100.0.0.1:2001
The ping utility is very useful to test if the tunnel is up.
The server (10.7.0.1) should be able to ping the client (10.7.0.6) and vice versa.

server#ping 10.7.0.6
An init.d script is available to start or stop OpenVPN. Be sure, as indicated in the Configurations section, that your configuration file are located in the /etc/openvpn directory and have a ".conf" extension file.

#/etc/init.d/openvpn stop
Stopping virtual private network daemon: openvpn.

#/etc/init.d/openvpn start
Starting virtual private network daemon: openvpn.

#/etc/init.d/openvpn restart
Stopping virtual private network daemon: openvpn.
Starting virtual private network daemon: openvpn(OK).


The logs will be written by default in the /var/log/syslog file.

You can check the OpenVPN process status:

#ps -ef | grep openvpn
UID PID PPID C STIME TTY TIME CMD
nobody
 
 
 
 
2792
 
 
 
 
1
 
 
 
 
0
 
 
 
 
09:59
 
 
 
 
?
 
 
 
 
00:00:00
 
 
 
 
/usr/sbin/openvpn
-- writepid /var/run/openvpn.conf.pid
--daemon ovpn-conf
--cd /etc/openvpn
--config /etc/openvpn/openvpn.conf
Check the routing table on the OpenVPN server:

#route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.7.0.0 0.0.0.0 255.255.255.248 U 0 0 0 tap0
10.0.2.0 10.7.0.6 255.255.255.0 UG 0 0 0 tap0
50.0.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
0.0.0.0 50.0.0.100 0.0.0.0 UG 0 0 0 eth0
Top of the page



openmaniak little square blue 5. TCP or UDP OPENED PORTS:

Let's check which UDP or TCP ports are opened:

The goal is to close all the unnecessary opened ports to improve the security by preventing potential attacks.
Let's identify the TCP and UDP opened ports on our test Linux which is an Ubuntu 7.10 Desktop Edition.

openmaniak little arrow right blue UDP ports:

#netstat -uaen
-u: UDP
-t: TCP
-a: all
-e: extended
-n: numeric


Active Internet connections (servers and established)
Proto
Recv-Q
Send-Q
Local Address Foreign Address State User Inode
udp
0
0
0.0.0.0:1025 0.0.0.0:*   105 15129
udp
0
0
50.0.0.1:2000 0.0.0.0:*   0 17810
udp
0
0
0.0.0.0:5353 0.0.0.0:*   105 15128
Let's see which users own the ID "0" and "105".

#cat /etc/passwd
user: status: userid: groupid: description: home_directory: shell
root: x: 0: 0: root: /root: /bin/bash
avahi: x: 105: 105: Avahi mDNS daemon,,,: /var/run/avahi-daemon: /bin/false
Avahi is a daemon that allows programs to publish and discover services and hosts running on a local network with no specific configuration.
This daemon is not needed in our case study so we can either deactivate or uninstall it.
Note that the two avihi ports are already closed on the Ubuntu server edition.

Stop the daemon:

#/etc/init.d/avihi-daemon stop
Deactivate the avahi at startup:

In the /etc/defaut/avahi-daemon file, set the AVAHI_DAEMON_START from "1" to "0":

AVAHI_DAEMON_START=0
Uninstall the daemon:

#apt-get remove avihi-daemon
The unnecessary Avihi daemon is no longer running, so we can now check the UDP ports status again and see that the OpenVPN port is the only opened UDP port.

#netstat -uaen
Active Internet connections (servers and established)
Proto
Recv-Q
Send-Q
Local Address Foreign Address State User Inode
udp
0
0
50.0.0.1:2000 0.0.0.0:*   0 17810
openmaniak little arrow right blue TCP ports:

#netstat -taen
Active Internet connections (servers and established)
Proto
Recv-Q
Send-Q
Local Address Foreign Address State User Inode
tcp
0
0
127.0.0.0.1:631 0.0.0.0:* LISTEN 0 15395
tcp
0
0
127.0.0.0.1:2207 0.0.0.0:* LISTEN 108 15469
tcp
0
0
127.0.0.0.1:2208 0.0.0.0:* LISTEN 0 15426
tcp6
0
0
:::22 :::* LISTEN 0 15688
TCP ports 631 (cupsys), 2207 and 2208 (hplip) are printer ports and not needed in our case study. So we can uninstall the associated services and thus close the ports.
Note that the three printer ports are already closed on the Ubuntu server edition.

#apt-get remove hplip
#apt-get remove cupsys
The unneeded printer daemons are no longer running, we can now check the TCP ports status again and see that the SSH port is the only opened TCP port.

#netstat -taen
Active Internet connections (servers and established)
Proto
Recv-Q
Send-Q
Local Address Foreign Address State User Inode
tcp6
0
0
:::22 :::* LISTEN 0 15688
Top of the page



openmaniak little square blue 6. IPTABLES:

IPtables is a tool needed to configure Netfilter and must be launched as root.
NetFilter is a Linux kernel module available since the kernel version 2.4. It provides three main functionalities:

- Packet filtering - Accepts or drops packets
- NAT - Changes the source or destination IP address of network packets
- Packet mangling - Modifies packets (as for Quality of Service, QoS)

The goal for us is to open only the needed ports and to close all the other to limit potential attacks on our Linux systems.

Our case study security strategy is the following:

Filter rules:

- Open the ports used by OpenVPN to generate the tunnel between the two Linux systems.
- Open the 80 and 443 ports to the outside to let the LAN machines surf on the Internet.
- Accept all traffic inside the tunnel.
- Drop all the rest.

openmaniak little arrow right blue OpenVPN Server Linux configuration:

- RESET YOUR IPTABLES SETTINGS:

#iptables -F
--------------------------------------------------------

- DEFAULT POLICIES:
Set the rules to deny by default all the incoming and outgoing traffics and accept the forward traffic (inter-interface routing):

#iptables -P OUTPUT DROP
#iptables -P INPUT DROP
#iptables -P FORWARD ACCEPT
--------------------------------------------------------

- OPENVPN RULES:
Authorize the OpenVPN tunnel:

#iptables -A INPUT -i eth0 -p udp -s 100.0.0.1 -d 50.0.0.1 --sport 2001 --dport 2000 -j ACCEPT
#iptables -A OUTPUT -o eth0 -p udp -s 50.0.0.1 -d 100.0.0.1 --sport 2000 --dport 2001 -j ACCEPT
Authorize all the traffic inside the tunnel:

#iptables -A INPUT -i tap+ -p all -j ACCEPT
#iptables -A OUTPUT -o tap+ -p all -j ACCEPT
--------------------------------------------------------

- LAN INTERFACE
All the traffic to and from the LAN interface (eth1) is accepted:

#iptables -A INPUT -i eth1 -p all -s 10.0.1.0/24 -j ACCEPT
#iptables -A OUTPUT -o eth1 -p all -d 10.0.1.0/24 -j ACCEPT
--------------------------------------------------------

- INTERNET ACCESS:
NAT rule:
Users from the site A network must be able to surf on the Internet, this will require NAT settings.
For example, when the Desktop located in Site A wants to access a web page on the Internet, its source IP address is translated and takes the OpenVPN server WAN IP address. In other words, 10.0.1.100 is translated to 50.0.0.1 and vice versa when the packets come back to the Desktop.
This kind of NAT is called 'masquerade'.

#iptables -t nat -A POSTROUTING -j MASQUERADE
Internet access authorization:
LAN users are allowed to access only HTTP and HTTPS resources:

#iptables -A OUTPUT -p tcp -m multiport --dports 80,443 -o eth0 -m state --state NEW,ESTABLISHED -j ACCEPT
#iptables -A INPUT -p tcp -m multiport --sports 80,443 -i eth0 -m state --state ESTABLISHED -j ACCEPT
--------------------------------------------------------

- FACULTATIVE RULES:
If you have SSH servers on your OpenVPN machines, you can set the rules to accept the SSH traffic.
To install a SSH server, you just have to use the following command: "apt-get install openssh-server"

Rules to permit the local SSH client to access a remote SSH server.

#iptables -A OUTPUT -p tcp -o eth0 --dport 22 -s 50.0.0.1 -j ACCEPT
#iptables -A INPUT -p tcp --sport 22 -i eth0 -d 50.0.0.1 -m state --state ESTABLISHED -j ACCEPT
Rules to permit remote SSH clients to access the local SSH server.

#iptables -A INPUT -p tcp --dport 22 -i eth0 -d 50.0.0.1 -j ACCEPT
#iptables -A OUTPUT -p tcp --sport 22 -o eth0 -s 50.0.0.1 -m state --state ESTABLISHED -j ACCEPT
For connectivity checks, 50.0.0.1 and 100.0.0.1 can ping each other.

#iptables -A INPUT -p icmp -i eth0 -s 100.0.0.1 -d 50.0.0.1 -j ACCEPT
#iptables -A OUTPUT -p icmp -o eth0 -d 100.0.0.1 -s 50.0.0.1 -j ACCEPT
--------------------------------------------------------

- CHECKS
Check the FireWall table:

#iptables -v -L
Chain INPUT (policy DROP 13 packets, 683 bytes)
pkts bytes target prot opt in out source destination  
0 0 ACCEPT udp -- eth0 * 100.0.0.1 50.0.0.1 udp spt:2001 dpt:2000
4 272 ACCEPT 0 -- tap+ * 0.0.0.0/0 0.0.0.0/0  
0 0 ACCEPT 0 -- eth0 * 10.0.1.0/24 0.0.0.0/0  
0
 
0
 
ACCEPT
 
tcp
 
--
 
eth0
 
*
 
0.0.0.0/0
 
0.0.0.0/0
 
multiport sports 80,443 state ESTABLISHED
4 336 ACCEPT icmp -- eth0 * 100.0.0.1 50.0.0.1  
0
 
0
 
ACCEPT
 
tcp
 
--
 
eth0
 
*
 
0.0.0.0/0
 
50.0.0.1
 
tcp spt:22 state ESTABLISHED
157 10884 ACCEPT tcp -- eth0 * 0.0.0.0/0 50.0.0.1 tcp dpt:22
                   
Chain FORWARD (policy ACCEPT 5 packets, 217 bytes)
pkts bytes target prot opt in out source destination  
                   
Chain OUTPUT (policy DROP 339 packets, 110K bytes)
pkts bytes target prot opt in out source destination  
0 0 ACCEPT udp -- * eth0 50.0.0.1 100.0.0.1 udp spt:2000 dpt:2001
    ACCEPT 0 -- * tap+ 0.0.0.0/0 0.0.0.0/0  
0 0 ACCEPT 0 -- * eth0 0.0.0.0/0 10.0.1.0/24  
0 0 ACCEPT tcp -- * eth0 0.0.0.0/0 0.0.0.0/0 multiport dports 80,443
4 336 ACCEPT icmp -- * eth0 50.0.0.1 100.0.0.1  
0 0 ACCEPT tcp -- * eth0 50.0.0.1 0.0.0.0/0 tcp dpt:22
173
 
22594
 
ACCEPT
 
tcp
 
--
 
*
 
eth0
 
50.0.0.1
 
0.0.0.0/0
 
tcp spt:22 state ESTABLISHED
Check the NAT table:

#iptables -L -t nat
Chain INPUT (policy DROP 13 packets, 683 bytes)
pkts bytes target prot opt in out source destination  
                   
Chain FORWARD (policy ACCEPT 5 packets, 217 bytes)
pkts bytes target prot opt in out source destination  
108 9273 MASQUERADE 0 - - any eth0 anywhere    
                   
Chain OUTPUT (policy DROP 339 packets, 110K bytes)
pkts bytes target prot opt in out source destination  
openmaniak little arrow right blue OpenVPN Client Linux Configuration:

- RESET YOUR IPTABLES SETTINGS:

#iptables -F
--------------------------------------------------------

- DEFAULT POLICIES:
Set the rules to deny by default all the incoming and outgoing traffics and accept the forward traffic (inter-interface routing):

#iptables -P OUTPUT DROP
#iptables -P INPUT DROP
#iptables -P FORWARD ACCEPT
--------------------------------------------------------

- OPENVPN RULES:
Authorize the OpenVPN tunnel:

#iptables -A INPUT -i eth0 -p udp -s 50.0.0.1 -d 100.0.0.1 --sport 2000 --dport 2001 -j ACCEPT
#iptables -A OUTPUT -o eth0 -p udp -s 100.0.0.1 -d 50.0.0.1 --sport 2001 --dport 2000 -j ACCEPT
Authorize all the traffic inside the tunnel:

#iptables -A INPUT -i tap+ -p all -j ACCEPT
#iptables -A OUTPUT -o tap+ -p all -j ACCEPT
--------------------------------------------------------

- LAN INTERFACE
All the traffic to and from the LAN interface (eth1) is accepted:

#iptables -A INPUT -i eth1 -p all -s 10.0.2.0/24 -j ACCEPT
#iptables -A OUTPUT -o eth1 -p all -d 10.0.2.0/24 -j ACCEPT
--------------------------------------------------------

- INTERNET ACCESS:
NAT rules:

Users from the sites B networks must be able to surf on the Internet, this will imply NAT settings.
For example, when the Desktop located in site B wants to access a web page on the Internet, its source IP address is translated and takes the OpenVPN client WAN IP address. In other words, 10.0.2.100 is translated to 100.0.0.1 and vice versa when the packets come back to the Desktop.
This kind of NAT is called masquerade.

#iptables -t nat -A POSTROUTING -j MASQUERADE
Internet access authorization:
LAN users are allowed to access only HTTP and HTTPS resources:

#iptables -A OUTPUT -p tcp -m multiport --dports 80,443 -o eth0 -j ACCEPT
#iptables -A INPUT -p tcp -m multiport --sports 80,443 -i eth0 -m state --state ESTABLISHED -j ACCEPT
--------------------------------------------------------

- FACULTATIVE RULES:
If you have SSH servers on your OpenVPN machines, you can set the rules to accept the SSH traffic.
To install a SSH server, you just have to use the following command: "apt-get install openssh-server"

Rules to permit the local SSH client to access a remote SSH server.

#iptables -A OUTPUT -p tcp -o eth0 --dport 22 -s 100.0.0.1 -j ACCEPT
#iptables -A INPUT -p tcp --sport 22 -i eth0 -d 100.0.0.1 -m state --state ESTABLISHED -j ACCEPT
Rules to permit remote SSH clients to access the local SSH server.

#iptables -A INPUT -p tcp --dport 22 -i eth0 -d 100.0.0.1 -j ACCEPT
#iptables -A OUTPUT -p tcp --sport 22 -o eth0 -s 100.0.0.1 -m state --state ESTABLISHED -j ACCEPT
For connectivity checks, 50.0.0.1 and 100.0.0.1 can ping each other.

#iptables -A INPUT -p icmp -i eth0 -s 50.0.0.1 -d 100.0.0.1 -j ACCEPT
#iptables -A OUTPUT -p icmp -o eth0 -d 50.0.0.1 -s 100.0.0.1 -j ACCEPT
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openmaniak little square blue 7. ROUTING

openmaniak little arrow right blue Network routes

To establish the link between machines inside the LANs of site A and site B, the following routes need to be added on the Linux VPN devices.

On OpenVPN server: destination network 10.0.2.0 mask 255.255.255.0 gateway 10.7.0.6
On OpenVPN client: destination network 10.0.1.0 mask 255.255.255.0 gateway 10.7.0.1

The two routes are automatically added with our server configuration. The OpenVPN server route is added through the "route.txt" script and the OpenVPN client route is pushed by the OpenVPN server.

openmaniak little arrow right blue IP forwarding

IP forwarding is required to transfer packets between the network interfaces of a Linux system.

#echo "1" > /proc/sys/net/ipv4/ip_forward
The command above will add the "1" value inside the /proc/sys/net/ipv4/ip_forward file and thus activate the IP forwarding.
If you want to keep the IP forwarding after a Linux reboot:

#echo "net.ipv4.ip_forward = 1" >> /etc/sysctl.conf
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openmaniak little square blue 8. CHECKS:

The clients (10.0.1.100 and 10.0.2.100) should be able to see each other and access HTTTP or HTTPS resources on the Internet.

openmaniak little arrow right blue LAN to LAN connectivity checks:

The clients (10.0.1.100 and 10.0.2.100) should be able to see each other. The ping and traceroute commands can be used for this purpose.
From the 10.0.1.100 client which is a Linux machine:

#ping 10.0.2.100
#traceroute 10.0.2.100
traceroute to 10.0.2.100 (10.0.2.100), 30 hops max, 40 byte packets
1 10.0.1.1 (10.0.1.1)  0.521 ms  0.848 ms  1.011 ms
2 10.7.0.6 (10.7.0.6)  0.420 ms  0.472 ms  0505 ms
3 10.0.2.100 (10.0.2.100)  0.538 ms  *  *

openmaniak little arrow right blue HTTTP Internet access check:

A way to test if the IPtables firewall is set to accept HTTP (TCP port 80) and HTTPS (TCP port 443) traffic is to use a browser or simpler, the Command Line interface (CLI) is to check if the TCP 80 and TCP 443 ports are opened with the telnet client.

#telnet 100.0.0.100 80
Trying 100.0.0.100...
Connected to 100.0.0.100.
Escape character is '^]'.
Of course, we will see only a banner (Escape character is '^]'.) but this is enough to indicate that the port is opened.

Here is the result you would obtain when you test a closed port, for instance the FTP 21 port:

#telnet 100.0.0.100 21
Trying 100.0.0.100...
openmaniak little arrow right blue Route Tables:

Server:

Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.7.0.0 0.0.0.0 255.255.255.248 U 0 0 0 tap0
10.0.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
10.0.2.0 10.7.0.6 255.255.255.0 UG 0 0 0 tap0
50.0.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
0.0.0.0 50.0.0.100 0.0.0.0 UG 0 0 0 eth0
Client:

Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
10.7.0.0 0.0.0.0 255.255.255.248 U 0 0 0 tap0
10.0.2.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1
10.0.1.0 10.7.0.1 255.255.255.0 UG 0 0 0 tap0
100.0.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
0.0.0.0 100.0.0.100 0.0.0.0 UG 0 0 0 eth0
openmaniak little arrow right blue TCPdump

TCPdump is used below to check the traffic inside the OpenVPN tunnel in the first example and on the port 2000 of the Linux WAN interface in the second example. Both examples are performed on the OpenVPN server.

#tcpdump -i tap0 -n
- n: numeric
- i: interface


tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on tap0, link-type EN10MB (Ethernet), capture size 96 bytes
IP 10.7.0.1 > 10.7.0.6: ICMP echo request, id 1824, seq 60, length 64
IP 10.7.0.6 > 10.7.0.1: ICMP echo reply, id 1824, seq 60, length 64
IP 10.7.0.1 > 10.7.0.6: ICMP echo request, id 1824, seq 61, length 64
#tcpdump -i eth0 port 2000 -n
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
IP 50.0.0.1.2000 > 100.0.0.1.2001: UDP, length 145
IP 100.0.0.1.2001 > 50.0.0.1.2000: UDP, length 145
IP 50.0.0.1.2000 > 100.0.0.1.2001: UDP, length 145
IP 100.0.0.1.2001 > 50.0.0.1.2000: UDP, length 145
IP 50.0.0.1.2000 > 100.0.0.1.2001: UDP, length 145
IP 100.0.0.1.2001 > 50.0.0.1.2000: UDP, length 145
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openmaniak little square blue 9. STARTUP SCRIPT

openmaniak little arrow right blue OpenVPN

The OpenVPN software is set to be launched automatically when the linux system boots.

To manually set OpenVPN to start automatically at bootup:

#update-rc.d openvpn defaults
To prevent OpenVPN to start automatically at bootup:

#update-rc.d -f openvpn remove
openmaniak little arrow right blue IPtables

The IPtables commands need to be added in a file called "iptables.sh" which will be executed when the Linux system boots.
The file is stored in the /root directory.

Add a line inside the /etc/crontab file to start the IPtables commands automatically after a reboot:

#vim /etc/crontab
@reboot root /root/iptables.sh >> /dev/null
- OpenVPN server file.
/home/root/iptables.sh

# OpenVPN server IPtables settings
#
#RESET your IPtables settings
iptables -F
#
#DEFAULT POLICIES:
#
iptables -P OUTPUT DROP
iptables -P INPUT DROP
iptables -P FORWARD ACCEPT
#
#Authorize the OpenVPN tunnel:
#
iptables -A INPUT -i eth0 -p udp -s 100.0.0.1 -d 50.0.0.1 --sport 2001 --dport 2000 -j ACCEPT
iptables -A OUTPUT -o eth0 -p udp -s 50.0.0.1 -d 100.0.0.1 --sport 2000 --dport 2001 -j ACCEPT
#
#Authorize all the traffic inside the tunnel:
#
iptables -A INPUT -i tap+ -p all -j ACCEPT
iptables -A OUTPUT -o tap+ -p all -j ACCEPT
#
#All the traffic to and from the LAN interface (eth1) is accepted:
#
iptables -A INPUT -i eth1 -p all -s 10.0.1.0/24 -j ACCEPT
iptables -A OUTPUT -o eth1 -p all -d 10.0.1.0/24 -j ACCEPT
#
#NAT rules:
#
iptables -t nat -A POSTROUTING -j MASQUERADE
#
#LAN users are allowed to access only Internet HTTP and HTTPS resources:
#
iptables -A OUTPUT -p tcp -m multiport --dports 80,443 -o eth0 -j ACCEPT
iptables -A INPUT -p tcp -m multiport --sports 80,443 -i eth0 -m state --state ESTABLISHED -j ACCEPT
#
# FACULTATIVE RULES:
#
#Rules to permit the local SSH client to access a remote SSH server
#
iptables -A OUTPUT -p tcp -o eth0 --dport 22 -s 50.0.0.1 -j ACCEPT
iptables -A INPUT -p tcp --sport 22 -i eth0 -d 50.0.0.1 -m state --state ESTABLISHED -j ACCEPT
#
# Rules to permit remote SSH clients to access the local SSH server
#
iptables -A INPUT -p tcp --dport 22 -i eth0 -d 50.0.0.1 -j ACCEPT
iptables -A OUTPUT -p tcp --sport 22 -o eth0 -s 50.0.0.1 -m state --state ESTABLISHED -j ACCEPT
#
#For connectivity checks, 50.0.0.1 and 100.0.0.1 can ping each other.
#
iptables -A INPUT -p icmp -i eth0 -s 100.0.0.1 -d 50.0.0.1 -j ACCEPT
iptables -A OUTPUT -p icmp -o eth0 -d 100.0.0.1 -s 50.0.0.1 -j ACCEPT
- OpenVPN client file.
/home/root/iptables.sh

# OpenVPN client IPtables settings
#
#RESET your IPtables settings
iptables -F
#
#DEFAULT POLICIES:
#
iptables -P OUTPUT DROP
iptables -P INPUT DROP
iptables -P FORWARD ACCEPT
#
#Authorize the OpenVPN tunnel:
#
iptables -A INPUT -i eth0 -p udp -s 50.0.0.1 -d 100.0.0.1 --sport 2000 --dport 2001 -j ACCEPT
iptables -A OUTPUT -o eth0 -p udp -s 100.0.0.1 -d 50.0.0.1 --sport 2001 --dport 2000 -j ACCEPT
#
#Authorize all the traffic inside the tunnel:
#
iptables -A INPUT -i tap+ -p all -j ACCEPT
iptables -A OUTPUT -o tap+ -p all -j ACCEPT
#
#All the traffic to and from the LAN interface (eth1) is accepted:
#
iptables -A INPUT -i eth1 -p all -s 10.0.2.0/24 -j ACCEPT
iptables -A OUTPUT -o eth1 -p all -d 10.0.2.0/24 -j ACCEPT
#
#NAT rules:
#
iptables -t nat -A POSTROUTING -j MASQUERADE
#
#LAN users are allowed to access only Internet HTTP and HTTPS resources:
#
iptables -A OUTPUT -p tcp -m multiport --dports 80,443 -o eth0 -j ACCEPT
iptables -A INPUT -p tcp -m multiport --sports 80,443 -i eth0 -m state --state ESTABLISHED -j ACCEPT
#
# FACULTATIVE RULES:
#
#Rules to permit the local SSH client to access a remote SSH server
#
iptables -A OUTPUT -p tcp -o eth0 --dport 22 -s 100.0.0.1 -j ACCEPT
iptables -A INPUT -p tcp --sport 22 -i eth0 -d 100.0.0.1 -m state --state ESTABLISHED -j ACCEPT
#
# Rules to permit remote SSH clients to access the local SSH server
#
iptables -A INPUT -p tcp --dport 22 -i eth0 -d 100.0.0.1 -j ACCEPT
iptables -A OUTPUT -p tcp --sport 22 -o eth0 -s 100.0.0.1 -m state --state ESTABLISHED -j ACCEPT
#
#For connectivity checks, 50.0.0.1 and 100.0.0.1 can ping each other.
#
iptables -A INPUT -p icmp -i eth0 -s 50.0.0.1 -d 100.0.0.1 -j ACCEPT
iptables -A OUTPUT -p icmp -o eth0 -d 50.0.0.1 -s 100.0.0.1 -j ACCEPT
-------------------------------------

The very last thing to do is to set the /root/iptables.sh file permission.

The user root has read/write/execute permissions. None else has any permissions at all.

#chmod 700 /root/iptables.sh
The user root is the owner of the /root/iptables.sh file.

#chown root /root/iptables.sh

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