IPsec Host-to-Host Configuration

IPsec can be configured to connect one desktop or workstation (host) to another using a host-to-host connection. This type of connection uses the network to which each host is connected to create a secure tunnel between each host. 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 Community Enterprise Linux to create the IPsec connection.

Host-to-Host Connection

A host-to-host IPsec connection is an encrypted connection between two systems, both running IPsec with the same authentication key. With the IPsec connection active, any network traffic between the two hosts is encrypted.

To configure a host-to-host IPsec connection, use the following steps for each host:

You should perform the following procedures on the actual machine that you are configuring. Avoid attempting to configure and establish IPsec connections remotely.

  1. In a command shell, type system-config-network to start the Network Administration Tool.

If automatic encryption is selected, /etc/racoon/racoon.conf is also created.

When the interface is up, /etc/racoon/racoon.conf is modified to include <remote-ip>.conf.

Manual IPsec Host-to-Host Configuration

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:

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 Key_Value01, 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 ipsec1.

You should choose a PSK that uses a mixture of upper- and lower-case characters, numbers and punctuation. An easily-guessable PSK constitutes a security risk.

It is not necessary to use the same connection name for each host. You should choose a name that is convenient and meaningful for your installation.

The following is the IPsec configuration 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 ipsec1, so the resulting file is called /etc/sysconfig/network-scripts/ifcfg-ipsec1. 

DST=X.X.X.X
TYPE=IPSEC
ONBOOT=no
IKE_METHOD=PSK

For Workstation A, X.X.X.X is the IP address of Workstation B. For Workstation B, X.X.X.X is the IP address of Workstation A. This connection is not set to initiate on boot-up (ONBOOT=no) and it uses the pre-shared key method of authentication (IKE_METHOD=PSK).

The following is the content of the pre-shared key file (called /etc/sysconfig/network-scripts/keys-ipsec1) that 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. 

IKE_PSK=Key_Value01

To change the keys-ipsec1 file so that only the root user can read or edit the file, use 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-ipsec1 file on both workstations. Both authentication keys must be identical for proper connectivity.

The next example shows the specific configuration for the phase 1 connection to the remote host. The file is called X.X.X.X.conf, where X.X.X.X is the IP address of the remote IPsec host. Note that this file is automatically generated when 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 ;
 }
}

The default phase 1 configuration file that is created when an IPsec connection is initialized contains the following statements used by the Community Enterprise Linux implementation of IPsec:

remote X.X.X.X

Specifies that the subsequent stanzas of this configuration file apply only to the remote node identified by the X.X.X.X IP address.

exchange_mode aggressive

The default configuration for IPsec on Community 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

Specifies the identification method to use when authenticating nodes. Community Enterprise Linux uses IP addresses to identify nodes.

encryption_algorithm 3des

Specifies 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 used.

authentication_method pre_shared_key

Specifies the authentication method used during node negotiation. By default, Community Enterprise Linux uses pre-shared keys for authentication.

dh_group 2

Specifies the Diffie-Hellman group number for establishing dynamically-generated session keys. By default, modp1024 (group 2) is used.

The Racoon Configuration File

The /etc/racoon/racoon.conf files should be identical on all IPsec nodes except for the include "/etc/racoon/X.X.X.X.conf" 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 the IPsec connection is activated. 

# 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";

This default racoon.conf file includes defined paths for IPsec configuration, pre-shared 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 provided that the IPsec credentials match.

pfs_group 2

Defines the Diffie-Hellman key exchange protocol, which determines the method by which the IPsec nodes establish a mutual temporary session key for the second phase of IPsec connectivity. By default, the Community 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 compromised.

lifetime time 1 hour

This parameter specifies the lifetime of an SA and can be quantified either by time or by bytes of data. The default Community Enterprise Linux implementation of IPsec specifies a one hour lifetime.

encryption_algorithm 3des, blowfish 448, rijndael

Specifies the supported encryption ciphers for phase 2. Community Enterprise Linux supports 3DES, 448-bit Blowfish, and Rijndael (the cipher used in the Advanced Encryption Standard, or AES).

authentication_algorithm hmac_sha1, hmac_md5

Lists the supported hash algorithms for authentication. Supported modes are sha1 and md5 hashed message authentication codes (HMAC).

compression_algorithm deflate

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, use the following command on each host: 

ifup <nickname>

where <nickname> is the name you specified for the IPsec connection.

To test the IPsec connection, run the tcpdump utility to view the network packets being transferred between the hosts 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 example: 

~]# tcpdump -n -i eth0 host <targetSystem>
IP 172.16.45.107 > 172.16.44.192: AH(spi=0x0954ccb6,seq=0xbb): ESP(spi=0x0c9f2164,seq=0xbb)