Configuring Linux VPNs

VPN Terms and Methods

A virtual private network (VPN) provides security for transmission of sensitive information over unprotected networks such as the Internet. VPN relationships are established between trusted sites on the Internet making the public network appear to be virtually the same as a private network to the VPN members. VPNs, however, have their own language. Once you know the terms, you'll be ready to get to work.

  • Authentication: The process of ensuring that the VPN data received is both unchanged and from the expected source.
  • Encryption: The use of special mathematical equations to scramble the bits in a data stream, rendering the data stream unreadable to anyone who does not have access to the corresponding decryption equation. The process is usually made even harder through the use of an encryption key, which modifies the way the equations do the scrambling. Only persons with access to this key and the corresponding programs are able to recover the original data. Data encryption helps to prevent unauthorized persons from having access to the data.
  • IPSec: The name given to a number of data communications protocols designed to authenticate and encrypt VPN data to protect it from unauthorized viewing or modification as it is transmitted across a network.
  • Authentication header (AH): One of two IPSec security protocols. Provides authentication and antireplay services, without encryption, by adding its own security header to the original IP packet

Encapsulating security protocol (ESP): The other IPSec security protocol. Provides authentication, encryption, and antireplay services. It encrypts the data within the packet and then adds its own security header to the original IP packet. Because ESP headers don't authenticate the outer IP header as AH headers do, AH and an ESP are often used in combination with each other. This is called transport adjacency.

Transport mode VPN: A style of VPN in which the original source and destination address of the data sent over the VPN is unchanged. Figures I.1 and I.2 provide examples of transport mode VPN IP packets.

Figure I.1 Transport mode AH packet format

Original
IP Header / Inserted
AH Header / Original
TCP Header / DATA

Figure I.2 Transport mode AH / ESP packet format

Original
IP Header / Inserted
AH Header / Inserted
ESP Header / Original
TCP Header / DATA
  • Tunnel mode VPN: A style of VPN in which the original source and destination address of the data sent over the VPN is changed by encapsulating the original IP packet within another IP packet. The original packet is frequently encrypted, header and all, in an effort to provide an additional layer of security by not revealing the true identities of the servers communicating with each other. Figures I.3 and I.4 show examples of tunnel mode VPN IP packets.

Figure I.3 Tunnel mode AH packet format

New
IP Header / Inserted
AH Header / Original
IP Header / Original
TCP Header / DATA

Figure I.4 Tunnel mode AH / ESP packet format

New
IP Header / Inserted
AH Header / Inserted
ESP Header / Original
IP Header / Original
TCP Header / DATA

Authentication and Encryption methods

IPSec data integrity is usually provided by one of two hashed message authentication code (HMAC) methods:

  • Message Digest 5 (MD5)
  • Secure Hash Algorithm (SHA-1).

IPSec usually uses one of two methods to encrypt data:

  • Data Encryption Standard (DES) using a 56-bit encryption key
  • Triple DES using a 168-bit encryption key.

Internet Key Exchange (IKE)

IKE provides authentication of the IPSec peers, negotiates IPSec security associations, and establishes IPSec keys. There are two main methods of establishing a trusted relationship between two devices that want to create a VPN between themselves.

Public Encrypted Keys

The first method is public key cryptography using RSA encryption pioneered by RSA Data Security Inc. Each VPN device has its own public and private keys. Anything encrypted with one of the keys can only be decrypted with the other. This allows you to create a signature when the message is encrypted with a sender's private key. The receiver verifies the signature by decrypting the message with the sender's public key.

A successful exchange requires the receiver to have a copy of the sender's public key and to know with a high degree of certainty that it really does belong to the sender, and not to someone pretending to be the sender. This certainty is assured using certificates and Certification Authorities.

A digital certificate contains information that identifies a user or device, such as a name, serial number, company, department, or IP address. It also contains a copy of the entity's public key. Certificates are managed and issued by Certification Authorities (CAs). A CA can either be a trusted public third party, such as VeriSign, or an in-house private server that you establish within your organization.

Prior to installing a certificate-based VPN, each VPN device must be preconfigured with the certificate generated for them by the CA. The VPN devices are also be preconfigured with the CA's certificate.

During the key exchange, the VPN peers authenticate by sending each other the certificate issued to them by the CA, but encrypted using their private key.

Each peer then uses the pre-installed CA certificate they have to authenticate with the CA and securely receive the other peer's certificate from the CA using public key cryptography.

Each peer then extracts the public key from the certificate they receive from the CA and uses it to decrypt the certificate they just received from the other peer.

Once the certificates received from the CA and the other peer match, authentication is complete.

Private Shared keys

The second method of establishing a trusted relationship is to have the devices at each end of the VPN use a shared key or password. The disadvantage is that each pair of VPN connections needs set of keys, making it difficult for large scale implementations. Unlike the RSA method, there is no CA to provide an impartial audit trail of VPN connection initiations.

IKE's role in creating Security Associations

Once authentication is complete, the VPN peers use IKE to negotiate the security associations (SAs) to be used at each end point. IKE, in turn, uses special ISAKMP IP packets using IP protocol 50 to establish a security association. SAs are comprised of transforms and shared keys. Transforms describe how the data will be transformed by the VPN and between which pre-defined networks at each end of the tunnel to provide the desired security, for example:

  • Packet encryption methods
  • Packet authentication methods
  • Transport versus tunnel mode
  • AH and or ESP usage
  • SA lifetime before it is renegotiated. (SAs are permanent for when manually established)

Shared keys are the actual keywords used by the encryption and authentication processes to protect the data.

VPN Security And Firewalls

All security devices in the path of a VPN connection have to allow IP protocol 50 between the two VPN devices to ensure that IKE works properly.

VPNs also use a separate channel through which the encrypted data passes as UDP packets through port 500. Unusually, the source and destination port is 500.

For the VPN to function correctly these protocols must also be allowed to pass through unimpeded. In certificate based VPNs, you may have to open up these ports and protocols to the CA as well.

Note: VPN tunnels frequently don't operate correctly if they have to pass through a firewall that uses NAT. They will tend to work if the NAT firewall is the termination point of the VPN.

VPN User Authentication Methods For Temporary Connections

The discussion so far has been slanted towards a permanent connection between purpose built VPN devices. Frequently, however, the device at the other end of the connection is a PC. Table I.2 shows some authentication methods used in such cases.

Table I-2 Types Of Dial Up VPN Authentication

Method / Description
IKE-XAUTH secured RADIUS / Usernames and passwords entered into the VPN remote login software are relayed by the VPN device at the remote end to a trusted RADIUS server. If the username/password combination is valid for remote login, then the RADIUS server authorizes the VPN device to continue with the IKE interchange.
ACE/SecurID / Software uses a username and password in conjunction with a small key-ring token with a digital display, also known as a fob, whose authentication serial number changes every few minutes for a login to occur. To log in, the user not only has to enter the username and password, but also the PIN tied to the fob plus the fob's dynamic serial number which is synchronized with the authentication server at the other end of the VPN.
Windows Domain / Remote home user authentication relies on the same username/password combination of the Windows Domain Controller that the user would normally use to log in when they are at work.
Local user database / Valid usernames and passwords are configured into the VPN device at the other end of the VPN

VPN Guidelines

Here are some recommended guidelines that I suggest you consider before attempting a simple SOHO Linux VPN.

  • The IPSec protocol on which VPNs are based will not tolerate its data packets being network address translated. If your firewall does NAT, then you'll have to disable it specifically for the packets that will traverse the VPN.
  • Life will be much easier if you make your Linux VPN box also function as a firewall. Configure and test the firewall first and then configure the VPN.
  • The networks at both ends of the VPN tunnel must use different IP address ranges. Many company networks operate using 192.168.0.x or 192.168.1.x addresses, you may have to reassign IP addresses to your network if overlaps occur.
  • Permanent site-to-site VPNs require firewalls at both ends that use static, DHCP IP addresses.

Scenario

Figure 35-1 illustrates the topology of a VPN between two SOHO environments. Here's the scenario.

  • The two sites need a VPN so that they can communicate with each other without the fear of eavesdropping.
  • The network administrators at both sites are aware that permanent site-to-site VPNs require fixed Internet IP addresses and have upgraded from their basic DHCP services originally provided by their ISPs. The sites' IP addressing schemes do not overlap.
  • Neither site wants to invest in a CA certificate service or infrastructure. The RSA key encryption methodology will be used for key exchange. (At the end of the chapter, I'll discuss an alternative Cisco-compatible method called alternately shared secret, pre-shared, or symmetric key.)
  • Site 1 uses a private network of 172.168.1.0 /24 and has a Linux VPN/firewall device default gateway with an external Internet IP address of 97.158.253.25.
  • Site 2 uses a private network of 10.0.0.0 /24 and has a Linux VPN/firewall device default gateway with an external Internet IP address of 6.25.232.1.

Figure 35-1 Openswan Topolology Diagram

Download And Install The Openswan Package

You can download the Openswan RPM at . The site has good instructions on how to install the product on Fedora and other versions of Linux. Be aware that to download the RPM version of Openswan you must have the ipsec-tools RPM package installed on your system.

How to get Openswan Started

You can configure Openswan to start at boot time using the chkconfig command:

[root@bigboy tmp]# chkconfig ipsec on

You can start, stop, and restart Openswan after booting using the ipsec initialization script as shown:

[root@bigboy tmp]# service ipsec start

[root@bigboy tmp]# service ipsec stop

[root@bigboy tmp]# service ipsec restart

Remember to restart the ipsec process every time you make a change to the ipsec.conf file for the changes to take effect on the running process.

Get The Status Of The Openswan Installation

Immediately after installing Openswan, run the ipsec verify command. It should give an [OK] status for most of its checks:

[root@vpn2 tmp]# ipsec verify

Checking your system to see if IPsec got installed and started correctly

Version check and ipsec on-path [OK]

Linux Openswan U2.2.0/K2.6.8-1.521 (native)

Checking for IPsec support in kernel [OK]

Checking for RSA private key (/etc/ipsec.secrets) [OK]

Checking that pluto is running [OK]

Two or more interfaces found, checking IP forwarding [OK]

Checking NAT and MASQUERADEing [N/A]

Checking for 'ip' command [OK]

Checking for 'iptables' command [OK]

Checking for 'setkey' command for native IPsec stack support [OK]

[root@vpn2 tmp]#

How to fix common Status errors

The status check may provide some errors. The most common ones are related to IP forwarding and opportunistic encryption.

IP forwarding

Each Linux VPN device needs to have routing or IP forwarding enabled. To enable it, simply add an ip_forward entry to the /etc/sysctl.conf file.

#

# File: /etc/sysctl.conf

#

#------

# Enable routing (IP forwarding)

#------

net/ipv4/ip_forward = 1

Now use the sysctl -p command to activate the settings.

[root@bigboy tmp]# sysctl -p

...

...

net.ipv4.ip_forward = 1

[root@bigboy tmp]#

You can find more details on the /etc/sysctl.conf, in Appendix I, "Miscellaneous Linux Topics."

Opportunistic Encryption DNS Checks:

The opportunistic encryption DNS checks feature of Openswan allows gateways to encrypt their traffic, even if the two gateway administrators have had no prior contact and neither system has any preset information about the other. The rationale behind this is to make all connections to servers behind the VPN device automatically be encrypted using IPSec in the same way that HTTP traffic can be seamlessly encrypted to become HTTPS traffic.

This feature should be disabled by default in your configuration file; simply ignore the errors related to it.

VPN Configuration Steps (Using RSA Keys)

One of the more secure ways of setting up a VPN tunnel is to encrypt the data using certificate-based (RSA) keys. There are other VPN parameters too, but Openswan is very forgiving when it establishes a tunnel. It automatically goes through all the various combinations of IKE and IPSec settings with the remote VPN box until it finds a match. You don't have to configure most of these settings explicitly as you often have to do in the case of routers and firewall/VPN appliances.

The /etc/ipsec.conf file

Preparation work requires you to draw a basic network diagram such as Figure 35.1. The VPN box on the left is called the left-hand side and the one on the right is called the right-hand side. Left and right parameters must be configured in the /etc/ipsec.conf configuration file. Table 35-1 explains each parameter.

Table 35-1 Parameters of the /etc/ipsec.conf file
Parameter / Description
Left / Internet IP address of the left-hand side VPN device.
Leftsubnet / The network protected by the left-hand side VPN device.
Leftid / Fully qualified domain name in DNS of the left-hand side VPN device, which is preceded by an "@" sign. If DNS is set up for the IP addresses, remove this entry, because names that don't resolve correctly cause the VPN initialization to fail.
Leftrsasigkey / The entire left RSA sig public key for the left-hand side VPN device. This can be obtained by using the ipsec showhostkey --left command.
Leftnexthop / The next hop router from the left-hand side VPN device when trying to reach the right-hand side VPN device. You may use an auto-generated variable %defaultroute, which will be valid in most cases, or the actual IP address of the next hop router in cases where the next hop is not the default router.
Right / Internet IP address of the right-hand side VPN device.
Rightsubnet / The network protected by the right-hand side VPN device.
Rightid / Fully qualified domain name in DNS of the right-hand side VPN device, which is preceded by an @ sign. If DNS isn't set up for the IP addresses, remove this entry, because names that don't resolve correctly cause the VPN initialization to fail.
Rightrsasigkey / The entire right RSA sig public key for the right-hand side VPN device. This can be obtained by using the ipsec showhostkey --right command.
Rightnexthop / The next hop router from the right-hand side VPN device when trying to reach the right-hand side VPN device. You may use an auto-generated variable %defaultroute, which will be valid in most cases, or the actual IP address of the next hop router in cases where the next hop is not the default router.

First you must gather all this information, then you have to enter it in the /etc/ipsec.conf configuration file.

Obtaining RSA Keys

To configure the /etc/ipsec.conf file, you need to get the left RSA public key for the left VPN device and the right key for the right VPN device. You need to generate these and insert them in the /etc/ipsec.conf file of the VPN peer device.