IPsec can be configured to connect one desktop or workstation to
another by way of a host-to-host connection. This type of
connection uses the network to which each host is connected to
create the secure tunnel to each other. The requirements of a
host-to-host connection are minimal, as is the configuration of
IPsec on each host. The hosts need only a dedicated connection to a
carrier network (such as the Internet) and Red Hat Enterprise Linux
to create the IPsec connection.
The first step in creating a connection is to gather system and
network information from each workstation. For a host-to-host
connection, you need the following information:
The IP address for both hosts
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
A pre-shared authentication key that is used to initiate the
connection and exchange encryption keys during the session
For example, suppose Workstation A and Workstation B want to
connect to each other through an IPsec tunnel. They want to connect
using a pre-shared key with the value of foobarbaz and the users agree to let
racoon automatically generate and share an
authentication key between each host. Both host users decide to
name their connections ipsec0.
The following is the ifcfg file for
Workstation A for a host-to-host IPsec connection with Workstation
B (the unique name to identify the connection in this example is
ipsec0, so the resulting file is
Workstation A would replace X.X.X.X with the IP address of Workstation B,
while Workstation B replaces X.X.X.X
with the IP address of Workstation A. The connection is set to
initiate upon boot-up (ONBOOT=yes) and uses the pre-shared key
method of authentication (IKE_METHOD=PSK).
The following is the content of the pre-shared key file (called
both workstations need to authenticate each other. The contents of
this file should be identical on both workstations and only the
root user should be able to read or write this file.
To change the keys-ipsec0 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-ipsec0
To change the authentication key at any time, edit the
keys-ipsec0 file on both workstations.
Both keys must be identical for proper
The next example shows the specific configuration for the phase
1 connection to the remote host. The file is named X.X.X.X.conf
(X.X.X.X is replaced 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.
exchange_mode aggressive, main;
dh_group 2 ;
The default phase 1 configuration file created when an IPsec
connection is initialized contains the following statements used by
the Red Hat Enterprise Linux implementation of IPsec:
- remote X.X.X.X
Specifies that the subsequent stanzas of this configuration file
applies only to the remote node identified by the X.X.X.X IP address.
The default configuration for IPsec on Red Hat Enterprise Linux
uses an aggressive authentication mode, which lowers the connection
overhead while allowing configuration of several IPsec connections
with multiple hosts.
- my_identifier address
Defines the identification method to be used when authenticating
nodes. Red Hat Enterprise Linux uses IP addresses to identify
Defines the encryption cipher used during authentication. By
default, Triple Data Encryption Standard
(3DES) is used.
- hash_algorithm sha1;
Specifies the hash algorithm used during phase 1 negotiation
between nodes. By default, Secure Hash Algorithm version 1 is
Defines the authentication method used during node negotiation.
Red Hat Enterprise Linux by default uses pre-shared keys for
- dh_group 2
Specifies the Diffie-Hellman group number for establishing
dynamically-generated session keys. By default, the 1024-bit group
The /etc/racoon/racoon.conf files
should be identical on all IPsec nodes except for the include
statement. This statement (and the file it references) is generated
when the IPsec tunnel is activated. For Workstation A, the
X.X.X.X in the include statement is Workstation B's IP address. The
opposite is true of Workstation B. The following shows a typical
racoon.conf file when IPsec connection is
# 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";
lifetime time 1 hour ;
encryption_algorithm 3des, blowfish 448, rijndael ;
authentication_algorithm hmac_sha1, hmac_md5 ;
compression_algorithm deflate ;
This default racoon.conf file includes
defined paths for IPsec configuration, pre-shard key files, and
certificates. The fields in sainfo
anonymous describe the phase 2 SA between the IPsec nodes
— the nature of the IPsec connection (including the supported
encryption algorithms used) and the method of exchanging keys. The
following list defines the fields of phase 2:
- sainfo anonymous
Denotes that SA can anonymously initialize with any peer insofar
as the IPsec credentials match.
- pfs_group 2
Defines the Diffie-Hellman key exchange protocol, which
determines the method in which the IPsec nodes establish a mutual
temporary session key for the second phase of IPsec connectivity.
By default, the Red Hat Enterprise Linux implementation of IPsec
uses group 2 (or modp1024) of
the Diffie-Hellman cryptographic key exchange groups. Group 2 uses
a 1024-bit modular exponentiation that prevents attackers from
decrypting previous IPsec transmissions even if a private key is
- lifetime time 1 hour
This parameter specifies the life cycle of an SA and can be
quantified either by time or by bytes of data. The Red Hat
Enterprise Linux implementation of IPsec specifies a one hour
- encryption_algorithm 3des,
blowfish 448, rijndael
Specifies the supported encryption ciphers for phase 2. Red Hat
Enterprise Linux supports 3DES, 448-bit Blowfish, and Rijndael (the
cipher used in the Advanced Encryption
Standard, or AES).
Lists the supported hash algorithms for authentication.
Supported modes are sha1 and md5 hashed message authentication
Defines the Deflate compression algorithm for IP Payload
Compression (IPCOMP) support, which allows for potentially faster
transmission of IP datagrams over slow connections.
To start the connection, either reboot the workstation or
execute the following command as root on each host:
To test the IPsec connection, run the tcpdump utility to view the network packets being
transfered between the hosts (or networks) and verify that they are
encrypted via IPsec. The packet should include an AH header and
should be shown as ESP packets. ESP means it is encrypted. For
17:13:20.617872 pinky.example.com > ijin.example.com: \
AH(spi=0x0aaa749f,seq=0x335): ESP(spi=0x0ec0441e,seq=0x335) (DF)