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Firewalling support for Linux has gone through many changes. Unlike other aspects of the kernel's evolution that have been methodical, each change to the firewalling modules has been radical compared to prior versions. The downside of this is each change to the way Linux deals with firewalling has a new learning curve associated. Like other areas of Linux development, however, each release has been very stable and has provided support for backward compatibility. Linux is evolving into a mature, enterprise-strength firewalling solution, as evidenced by companies like Watchguard.
In previous releases of the kernel, the firewall configuration tool was called ipchains. Now, with the 2.4 release, this has once again changed to a tool called iptables. Both of these tools run on the command line but hook into the kernel to configure firewall settings at a very low level. The iptables arrangement offers significant advantages over the ipchains way of kernel organization, but ipchains will be supported for some time as a kernel loadable module. For more information, see kernel.org and IPCHAINS-HOWTO.
For my purposes, ipchains offered more than adequate support for my needs. My major concerns regarding firewalling were:
Lock down all extraneous/unused ports and services to have a safe box. Since I would potentially be housing sensitive information while developing software for my clients, it was important that the box be adequately protected.
Provide NAT (network address translation) services to my internal network, often called IP masquerading. Since I only had one valid IP address and a number of workstations needing access to the Internet, I needed to provide a way for to manage multiple connections through my single (real) internet connection.
Provide port tunneling services, where appropriate, to other internal boxes. In some cases, I had other machines running internet protocols internally that I wanted to allow access to from the Internet.
The ASCII diagram below shows our network/firewall setup:
|------| ------- 192.168.1.0 | | | | W1 |----------------| | 11 | | |------| | | |---------| |------| | | | | | | eth0|200 D|eth1 Internet | W2 |----------------|--------------| Linux |--------------------- | 12 | | | | |------| | |---| | | | |---------| |------| | | | | | | | VM | | W3 |----------------| | 201 | | 13 | | | | |------| -------- |---------|
The internal network is 192.168.1.0. Each machine on the diagram above shows the last number in its IP address. The Linux box has two physical adapters. The internal one (eth0) has an address on the internal network (200), while the external adapter (eth1) obtains its address dynamically via DHCP. This is why it is marked with a D on the diagram.
It is ipchains that allows NAT between the internal network and the Internet. All outgoing connections are allowed. All incoming connections are disallowed, with the exception of port 80 (HTTP), port 22 (SSH) and port 8080 (for an HTTP server running on the VM machine). These services will be explained in more detail below.
Notice the VM machine is shown "behind" the Linux box. This will be explained in more detail in the VMware section below.
A simple program named redir, found at sammy.net, allows services running on an internal machine to be accessed from the outside (Internet) network. This program accepts connections on a certain port and proxies those connections to another port on a different machine.
First of all, external connections must be allowed to the port on the Linux box. In my case, this port is 8080. The redir command we execute looks like this:
redir --lport=8080 --caddr=192.168.1.101 --cport=8080 --syslog &
The lport is the listen port of the local machine. The caddr and cport settings are the connection address and port, respectively. The syslog parameter instructs redir to log all connections to the standard system log.
As a result of this command, any connections to the Linux box on port 8080 will be served by the web server running on the VM box (also from port 8080).
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