Building a Transparent Firewall with Linux, Part IV
Listing 1. Corrected /etc/config/network
config 'switch' 'eth0' option 'enable' '1' config 'switch_vlan' 'eth0_1' option 'device' 'eth0' option 'vlan' '1' option 'ports' '4 5' config 'switch_vlan' 'eth0_0' option 'device' 'eth0' option 'vlan' '0' option 'ports' '0 1 2 3 5' config 'interface' 'loopback' option 'ifname' 'lo' option 'proto' 'static' option 'ipaddr' '127.0.0.1' option 'netmask' '255.0.0.0' config 'interface' 'lan' option 'type' 'bridge' option 'proto' 'static' option 'netmask' '255.255.255.0' option 'ipaddr' '10.0.0.253' option 'ifname' 'eth0.0 eth0.1'
Note that on your system, sections may be listed “out of order”, for example, with one VLAN section near the top and another near the bottom. Commands within a given section need to be in the correct order, but the sections themselves do not, so don't worry!
You also have to disable OpenWrt's native DHCP and iptables systems. The need for disabling DHCP services is obvious: acting as a DHCP server wouldn't be very “transparent” behavior! So, disable it with these two commands:
root@sugartongs# /etc/init.d/dnsmasq disable root@sugartongs# /etc/init.d/dnsmasq stop
OpenWrt's native iptables script (/etc/init.d/firewall) is fine if you want to use OpenWrt as a standard “Layer 3” (routing) firewall. Leaving this script enabled allows you to use the uci command and the file /etc/config/firewall to manage iptables in a manner very similar to how you manage network configuration and other OpenWrt system settings.
However, this system doesn't lend itself very well to running iptables in bridging mode—to use it that way, you'd need to hack the script extensively, which would be a bewildering task given the large number of custom tables it uses beyond “INPUT”, “OUTPUT” and “FORWARDING”. Therefore, disable it like this:
root@sugartongs# /etc/init.d/firewall disable root@sugartongs# /etc/init.d/firewall stop
Now you can create a custom iptables script more suitable for a transparent firewall.
In order to write a firewall script, you need to consider your network's topology and how the transparent firewall fits in. Figure 1 shows the example home network I sketched out in Part II of this series, with a firewall cabled between the network's Internet uplink (via DSL router or cable modem) and its backbone (which collapses back to a wireless broadband router configured with Internet uplink and LAN on the same logical subnet).
You could use a number of topologies instead. If you have only a few hosts on your internal network, and your Internet uplink device is already providing DHCP services, you could use your transparent firewall as your broadband router (though configuring WLAN on OpenWrt is outside this series' scope). If your cable modem or DSL router includes a switch and/or wireless LAN access point, you could connect some of your network nodes directly to that and use your transparent firewall to protect other devices.
I'm going to stick with the topology in Figure 1, however, for simplicity's sake. It should be clear enough how to customize my sample iptables script for whatever topology you choose. Let's take a closer look at Figure 1.
The first thing you should notice is that everything on this network resides on the same logical subnet (10.0.0.0/24) except, of course, for the cable/DSL modem's WAN interface (the one connected to the Internet), which has the Internet-routable address 220.127.116.11. That WAN address is strictly illustrative; in actual practice, WAN IP addresses in any residential Internet scenario are assigned by your Internet service provider, often automatically, so please don't attempt to set yours to 18.104.22.168!
Another important point is that on this example network, client PCs are assigned IP addresses via DHCP from the pool 10.0.0.2 through 10.0.0.100. My diagram doesn't indicate which host is providing DHCP services. Is it the cable/DSL modem, the broadband router or the Web proxy?
As a matter of fact, it doesn't matter! Because this entire network fabric is switched, DHCP requests will propagate freely, including through the transparent firewall. However, if the cable/DSL modem acts as the DHCP server, you will need to write rules on the firewall to allow DHCP through in both directions.
Practical Task Scheduling Deployment
July 20, 2016 12:00 pm CDT
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
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