Book Image

OpenVPN: Building and Integrating Virtual Private Networks

Book Image

OpenVPN: Building and Integrating Virtual Private Networks

Overview of this book

OpenVPN is a powerful, open source SSL VPN application. It can secure site-to-site connections, WiFi and enterprise-scale remote connections. While being a full-featured VPN solution, OpenVPN is easy to use and does not suffer from the complexity that characterizes other IPSec VPN implementations. It uses the secure and stable TLS/SSL mechanisms for authentication and encryption. This book is an easy introduction to this popular VPN application. After introducing the basics of security and VPN, the book moves on to cover using OpenVPN, from installing it on various platforms, through configuring basic tunnels, to more advanced features, such as using the application with firewalls, routers, proxy servers, and OpenVPN scripting. While providing only necessary theoretical background, the book takes a practical approach, presenting plenty of examples.
Table of Contents (17 chapters)
OpenVPN
Credits
About the Author
About the Reviewers
Preface
Index

Scanning Servers with Nmap


Nmap is a port scanner that can be used to determine whether a UDP or TCP port on a machine is open, and whether there is a server process accepting connections. Nmap can also find out if a firewall is protecting the machine scanned, and Nmap can scan whole networks. Let's scan the local client PC (which is obviously not protected by a firewall...):

opensuse01:~ # nmap 172.16.76.128
Starting nmap 3.81 ( http://www.insecure.org/nmap/ ) at 2005-12-02 18:02 CET
Interesting ports on localhost (172.16.76.128):
(The 1661 ports scanned but not shown below are in state: closed)
PORT   STATE SERVICE
22/tcp open  ssh
68/tcp open  dhcpclient
MAC Address: 00:0C:29:21:07:FC
Nmap finished: 1 IP address (1 host up) scanned in 1.773 seconds

There are two ports open on this system; port 1661 and other scanned ports are closed. If there were a firewall on this system, then scanning would not be that easy, because most firewalls detect scans and can prevent them. But there are many...