This is a mini-firewall that completely isolates a target device from the local network. This is for allowing infected machines Internet access, but without endangering the local network.

This project depends upon libpcap, and of course a C compiler.

On Debian, the following should work:

# apt-get install git gcc make libpcap-dev
# git clone https://github.com/robertdavidgraham/isowall
# cd isowall
# make

This will put the binary isowall in the local isowall/bin directory.

This should also work on Windows, Mac OS X, xBSD, and pretty much any operating system that supports libpcap.

First, setup a machine with three network interfaces.

The first network interface (like eth0) will be configured as normal, with a TCP/IP stack, so that you can SSH to it.

The other two network interfaces should have no TCP/IP stack, no IP address, no anything. This is the most important configuration step, and the most common thing you'll get wrong. For example, the DHCP software on the box may be configured to automatically send out DHCP requests on these additional interfaces. You have to go fix that so nothing is bound to these interfaces.

To run, simply type:

# ./bin/isowall --internal eth1 --external eth2 -c xxxx.conf

where xxxx.conf contains your configuration, which is described below.

The following shows a typical configuration file

internal = eth1
internal.target.ip = 10.0.0.129
internal.target.mac = 02:60:8c:37:87:f3

external = eth2
external.router.ip = 10.0.0.1
external.router.mac = 66:55:44:33:22:11

allow = 0.0.0.0/0
block = 192.168.0.0/16
block = 10.0.0.0/8
block = 224.0.0.0-255.255.255.255

The target device we are isolating has the indicated IP and MAC address.

Only IPv4 and ARP packets are passed.

Outbound packets must have the following conditions:

• source MAC address equal to internal.target.mac
• destination MAC address equal to external.router.mac
• EtherType of 0x800 or 0x806
• source IPv4 address equal to internal.target.ip
• destination IPv4 address within an allow range, but not in a block range
• if an ARP packet, then the destination IPv4 address must equal that external.router.ip
• if an ARP packet, must be a "request"

Inbound packets must have the following conditions:

• destination MAC address equal to internal.target.mac
• source MAC address equal to external.router.mac
• EtherType of 0x800 or 0x806
• destination IPv4 address equal to internal.target.ip
• source IPv4 address within an allow range, but not in a block range
• if an ARP packet, then the source IPv4 address must equal that external.router.ip
• if an ARP packet, then must be a "reply"

There is no guarantee, of course, but this program has pretty good security.

The security rests on the fact that there is no IP stack bound to adapters. What that means is that the infected targetted cannot touch the firewall machine in any way, except as allowed within the is_allowed() function. That function represents the majority of the attack surface for the firewall machine. And, as you can tell from reading the function, it contains almost no functionality, meaning that the attack surface is very small indeed.

There are a few theoretical attacks that might happen at the physical layer, but for the most part, we don't have to worry about them.