Brett Sheffield . com

You’ll often come across docs and how-tos that say things like “to enable forwarding issue the following command”:

echo 1 > /proc/sys/net/ipv4/ip_forward

Ever wondered what all that stuff in /proc/sys/net actually does? Ok, a lot of it is pretty logical, but sometimes it’s nice to actually know with a bit more certainty. Today I broke something on a server because I assumed, instead of looking it up. Oops.

There’s a lot of documentation in the kernel sources which is surprisingly accessible to the non kernel hackers among us. First, get yourself a copy of the kernel source if you don’t have one. Take a look in /usr/src. If you don’t see a directory called something like linux-2.6.18, you probably don’t have the kernel source available. If you’re on a debian, ubuntu or other apt-based distro, you can apt-get the source for your kernel:

cd /usr/src
apt-get source linux-image-2.6.18-3-k7

Once your kernel source has downloaded and unpacked, cd into the source directory. You’ll find a directory called Documentation, and inside that a subdirectory called networking. The document we’re looking for in this case is ip-sysctl.txt. Open it in your favorite text editor.

  
/proc/sys/net/ipv4/* Variables:

ip_forward - BOOLEAN
        0 - disabled (default)
        not 0 - enabled

        Forward Packets between interfaces.

        This variable is special, its change resets all configuration
        parameters to their default state (RFC1122 for hosts, RFC1812
        for routers)

  

Have a browse around - there’s quite a bit of other doco. There’s an index file, 00-INDEX that lists what’s what.