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Network Security Hacks by Andrew Lockhart

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Create an Authenticated Gateway

Use PF to keep unauthorized users off the network.

Firewalling gateways have traditionally been used to block traffic from specific services or machines. Instead of watching IP addresses and port numbers, an authenticated gateway allows you to regulate traffic to or from machines based on a user’s credentials. With an authenticated gateway, a user will have to log in and authenticate himself to the gateway in order to gain access to the protected network. This can be useful in many situations, such as restricting Internet access or restricting a wireless segment to be used only by authorized users.

With the release of OpenBSD 3.1, you can implement this functionality through the use of PF and the authpf shell. Using authpf also provides an audit trail by logging usernames, originating IP addresses, and the time that they authenticated with the gateway, as well as when they logged off the network.

To set up authentication with authpf, you’ll first need to create an account on the gateway for each user. Specify /usr/sbin/authpf as the shell, and be sure to add authpf as a valid shell to /etc/shells. When a user logs in through SSH, authpf will obtain the user’s name and IP address through the environment. After doing this, a template file containing NAT and filter rules is read in, and the username and IP address are applied to it. The resulting rules are then added to the running configuration. When the user logs out (i.e., types ^C), the rules ...

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