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Making the new commands sticky

Once the new routing table and the new rule are in place, packets coming in on the new DSL line will be routed back out the same line and all is well. But a reboot now will reset things to the previous state. On Trustix, I put the commands in a script in /etc/sysconfig/iptables-ipv4.d/ called "routing.sh" and make it executable; any executable script in this directory is run at boot time. I think this will work on other relatives of Redhat including Fedora and SUSE. On Ubuntu and its Debian-based cousins, you can put networking scripts in the /etc/network/ifup.d directory, but you create a separate script in /etc/init.d that can set up routing and firewalling and set it to run after /etc/init.d/networking. I haven't done much work with complex routing setups on Debian, as I prefer to use Trustix on servers. Refer to the update-rc.d man page on Debian systems.

Load balancing

When I originally embarked on learning advanced routing, it was to balance traffic from a LAN with around 300 nodes over several DSL lines. I am still working on it. If you want more information on load balancing see the iproute manual, especially section 4.2. (See "resources" section below.)

If you are thinking you will get twice the bandwidth on your desktop system if you add a second line, think again. Load balancing won't work very well. You can do a form of round-robin load balancing; each new outbound connection is routed to the next available line. The problem with this approach is that the route for each connection becomes associated with that one line (because the routes are cached). If you start a download from a site, all traffic will come over that one line; your download speed will be exactly the same whether you have one connection or 50. If you start a second simultaneous download from the same site, it too will use the same DSL line.

The only way this form of balancing can help is if you have many users all accessing many different sites through a central router. This is the approach that I am working on now.

I hope this article helps you to better understand the relationship between the ip rules and the ip routing tables. It seems simple to me now but it took Dr. Un*x many hours to figure it out.