Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions
Privacy Policy




NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.

6.5. IPsec Network-to-Network configuration

IPsec can also be configured to connect an entire network (such as a LAN or WAN) to a remote network by way of a network-to-network connection. A network-to-network connection requires the setup of IPsec routers on each side of the connecting networks to transparently process and route information from one node on a LAN to a node on a remote LAN. Figure 6-1 shows a network-to-network IPsec tunneled connection.

Figure 6-1. A Network-to-network IPsec tunneled connection

This diagram shows two separate LANs separated by the Internet. These LANs use IPsec routers to authenticate and initiate a connection using a secure tunnel through the Internet. Packets that are intercepted in transit would require brute-force decryption in order to crack the cipher protecting the packets between these LANs. The process of communicating from one node on the IP range to another on is completely transparent to the nodes as the processing, encryption/decryption, and routing of the IPsec packets are completely handled by the IPsec router.

The information needed for a network-to-network connection include:

  • The externally-accessible IP addresses of the dedicated IPsec routers

  • The network address ranges of the LAN/WAN served by the IPsec routers (such as or

  • The IP addresses of the gateway devices that route the data from the network nodes to the Internet

  • A unique name to identify the IPsec connection and distinguish it from other devices or connections (for example, ipsec0)

  • A fixed encryption key or one automatically generated by racoon

  • A pre-shared authentication key that initiates the connection and exchange encryption keys during the session

For example, suppose LAN A ( and LAN B ( want to connect to each other through an IPsec tunnel. The network address for LAN A is in the range, while LAN B uses the range. The gateway IP address is for LAN A and for LAN B. The IPsec routers are separate from each LAN gateway and uses two network devices: eth0 is assigned to an externally-accessible static IP address which accesses the Internet, while eth1 acts as a routing point to process and transmit LAN packets from one network node to the remote network nodes.

The IPsec connection between each network uses a pre-shared key with the value of r3dh4tl1nux, and the administrators of A and B agree to let racoon automatically generate and share an authentication key between each IPsec router. The administrator of LAN A decides to name the IPsec connection ipsec0, while the administrator of LAN B names the IPsec connection ipsec1..

The following example are the contents the ifcfg file for a network-to-network IPsec connection for LAN A. The unique name to identify the connection in this example is ipsec1, so the resulting file is named /etc/sysconfig/network-scripts/ifcfg-ipsec1.


The connection is set to initiate upon boot-up (ONBOOT=yes) and uses the pre-shared key method of authentication (IKE_METHOD=PSK). The administrator for LAN A enters the destination gateway, which is the gateway for LAN B (DSTGW= as well as the source gateway, which is the gateway IP address for LAN A (SRCGW= The administrator then enters the destination network, which is the network range for LAN B (DSTNET= as well as the source network (SRCNET= Finally, the administrator enters the destination IP address, which is the externally-accessible IP address for LAN B (X.X.X.X).

The following example is the content of the pre-shared key file called /etc/sysconfig/network-scripts/keys-ipsecX (where X is 0 for LAN A and 1 for LAN B) that both networks use to authenticate each other. The contents of this file should be identical and only the root user should be able to read or write this file.

Important Important

To change the keys-ipsecX file so that only the root user can read or edit the file, perform the following command after creating the file:

chmod 600 /etc/sysconfig/network-scripts/keys-ipsec1

To change the authentication key at any time, edit the keys-ipsecX file on both IPsec routers. Both keys must be identical for proper connectivity.

The following example is the contents of the /etc/racoon/racoon.conf configuration file for the IPsec connection. Note that the include line at the bottom of the file is automatically generated and only appears if the IPsec tunnel is running.

# Racoon IKE daemon configuration file.
# See 'man racoon.conf' for a description of the format and entries.

path include "/etc/racoon";
path pre_shared_key "/etc/racoon/psk.txt";
path certificate "/etc/racoon/certs";

sainfo anonymous
        pfs_group 2;
        lifetime time 1 hour ;
        encryption_algorithm 3des, blowfish 448, rijndael ;
        authentication_algorithm hmac_sha1, hmac_md5 ;
        compression_algorithm deflate ;
include "/etc/racoon/X.X.X.X.conf"

The following is the specific configuration for the connection to the remote network. The file is named X.X.X.X.conf (replace X.X.X.X with the IP address of the remote IPsec router). Note that this file is automatically generated once the IPsec tunnel is activated and should not be edited directly.

remote X.X.X.X
        exchange_mode aggressive, main;
        my_identifier address;
        proposal {
                encryption_algorithm 3des;
                hash_algorithm sha1;
                authentication_method pre_shared_key;
                dh_group 2 ;

Prior to starting the IPsec connection, IP forwarding should be enabled in the kernel. As root at a shell prompt, enable IP forwarding:

  1. Edit /etc/sysctl.conf and set net.ipv4.ip_forward to 1.

  2. Execute the following command to enable the change:

    sysctl -p /etc/sysctl.conf

To start the IPsec connection, either reboot the IPsec routers or execute the following command as root on each router:

/sbin/ifup ipsec0

The connections are activated, and both LAN A and B are able to communicate with each other. The routes are created automatically via the initialization script called by running ifup on the IPsec connection. To show a list of routes for the network, run the following command:

/sbin/ip route list

To test the IPsec connection, run the tcpdump utility on the externally-routable device (eth0 in this example) to view the network packets being transfered between the hosts (or networks) and verify that they are encrypted via IPsec. For example, to check the IPsec connectivity of LAN A, type the following:

tcpdump -n -i eth0 host

The packet should include an AH header and should be shown as ESP packets. ESP means it is encrypted. For example (back slashes denote a continuation of one line):

12:24:26.155529 > AH(spi=0x021c9834,seq=0x358): \
 > ESP(spi=0x00c887ad,seq=0x358) (DF) \

  Published under the terms of the GNU General Public License Design by Interspire