Creating VPNs with IPsec and SSL/TLS

OpenVPN is an open-source project founded by James Yonan. It provides a VPN solution based on SSL/TLS. Transport Layer Security (TLS) and its predecessor, Secure Sockets Layer (SSL), are cryptographic protocols that provide secure communications data transfer on the Internet. SSL has been in existence since the early '90s.

The OpenVPN networking model is based on TUN/TAP virtual devices; TUN/TAP is part of the Linux kernel. The first TUN driver in Linux was developed by Maxim Krasnyansky.

OpenVPN installation and configuration is simpler in comparison with IPsec. OpenVPN supports RSA authentication, Diffie-Hellman key agreement, HMAC-SHA1 integrity checks and more. When running in server mode, it supports multiple clients (up tp 128) to connect to a VPN server over the same port. You can set up your own Certificate Authority (CA) and generate certificates and keys for an OpenVPN server and multiple clients.

OpenVPN operates in user-space mode; this makes it easy to port OpenVPN to other operating systems.

First, download and install the ipsec-tools package and the Openswan package (most distros have these packages).

The VPN tunnel has two participants on its ends, called left and right, and which participant is considered left or right is arbitrary. You have to configure various parameters for these two ends in /etc/ipsec.conf (see man 5 ipsec.conf). The /etc/ipsec.conf file is divided into sections. The conn section contains a connection specification, defining a network connection to be made using IPsec.

An example of a conn section in /etc/ipsec.conf, which defines a tunnel between two nodes on the same LAN, with the left one as 192.168.0.89 and the right one as 192.168.0.92, is as follows:

...
conn linux-to-linux
        #
        # Simply use raw RSA keys
        # After starting openswan, run:
        # ipsec showhostkey --left (or --right)
        # and fill in the connection similarly
        # to the example below.
        left=192.168.0.89
        leftrsasigkey=0sAQPP...
        # The remote user.
        #
        right=192.168.0.92
        rightrsasigkey=0sAQON...
        type=tunnel
        auto=start
...

You can generate the leftrsasigkey and rightrsasigkey on both participants by running:

ipsec rsasigkey --verbose 2048 > rsa.key

Then, copy and paste the contents of rsa.key into /etc/ipsec.secrets.

In some cases, IPsec clients are roaming clients (with a random IP address). This happens typically when the client is a laptop used from remote locations (such clients are called Roadwarriors). In this case, use the following in ipsec.conf:

right=%any

instead of:

right=ipAddress

The %any keyword is used to specify an unknown IP address.

The type parameter of the connection in this example is tunnel (which is the default). Other types can be transport, signifying host-to-host transport mode; passthrough, signifying that no IPsec processing should be done at all; drop, signifying that packets should be discarded; and reject, signifying that packets should be discarded and a diagnostic ICMP should be returned.

The auto parameter of the connection tells which operation should be done automatically at IPsec startup. For example, auto=start tells it to load and initiate the connection; whereas auto=ignore (which is the default) signifies no automatic startup operation. Other values for the auto parameter can be add, manual or route.

After configuring /etc/ipsec.conf, start the service with:

service ipsec start

You can perform a series of checks to get info about IPsec on your machine by typing ipsec verify. And, output of ipsec verify might look like this:

Checking your system to see if IPsec has installed and started correctly:
Version check and ipsec on-path                                 [OK]
Linux Openswan U2.4.7/K2.6.21-rc7 (netkey)
Checking for IPsec support in kernel                            [OK]
NETKEY detected, testing for disabled ICMP send_redirects       [OK]
NETKEY detected, testing for disabled ICMP accept_redirects     [OK]
Checking for RSA private key (/etc/ipsec.d/hostkey.secrets)     [OK]
Checking that pluto is running                                  [OK]
Checking for 'ip' command                                       [OK]
Checking for 'iptables' command                                 [OK]
Opportunistic Encryption Support                                [DISABLED]

You can get information about the tunnel you created by running:

ipsec auto --status

You also can view various low-level IPSec messages in the kernel syslog.

You can test and verify that the packets flowing between the two participants are indeed esp frames by opening an FTP connection (for example), between the two participants and running:

tcpdump -f esp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes

You should see something like this:

IP 192.168.0.92 > 192.168.0.89: ESP(spi=0xd514eed9,seq=0x7)
IP 192.168.0.89 > 192.168.0.92: ESP(spi=0x3a1563b9,seq=0x6)
IP 192.168.0.89 > 192.168.0.92: ESP(spi=0x3a1563b9,seq=0x7)
IP 192.168.0.92 > 192.168.0.89: ESP(spi=0xd514eed9,seq=0x8)

Note that the spi (Security Parameter Index) header is the same for all packets; this is an identifier of the connection.

If you need to support NAT traversal, add nat_traversal=yes in ipsec.conf; nat_traversal=no is the default.

The Linux IPsec stack can work with pluto from Openswan, racoon from the KAME Project (which is included in ipsec-tools) or isakmpd from OpenBSD.