vpnc is a VPN client compatible with cisco3000 VPN Concentrator (also known as Cisco's EasyVPN equipment). vpnc runs entirely in userspace and does not require kernel modules except of the tun driver to communicate with the network layer.
It supports most of the features needed to establish connection to the VPN concentrator: MD5 and SHA1 hashes, 3DES and AES ciphers, PFS and various IKE DH group settings.
Thorsten R. suggested this alternate way to secure network links:
Virtual private networking in cisco vpns under linux? vpnc is easy to set up and only requires a module from the standard kernel. If you use wlan you should investigate vpn, it's the standard choice for making wlan waterproof.
More information on this package can be found on the Debian web site.
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Now available in RSS and ATOM flavors too.