Posts Tagged ‘Linux’

Linux Internet Connection Sharing

September 24, 2008 Leave a comment

Iptables is a well known program for its firewall and traffic filtering capabilities. But it also can be used to share our beloved Internet connection. And amazingly it’s relatively easy to setup a shared connection in Linux.

For this example I will be using a Fedora 9 x86_64 system with a wireless Ethernet device named wlan0

First you have to enable IP forwarding. Log in as root and type:

# echo "net.ipv4.ip_forward=1" >> /etc/sysctl.conf
# /sbin/iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE
# /etc/init.d/iptables save

Now a little bit of theory on what the previous commands do.

The first command is mostly self explanatory; it just tells the system to activate the IP forwarding feature.
Next, the iptables command line defines a Routing Table of NAT (Network Address Translation) type which is used to hide (masquerade) an IP address space behind a single IP address in another address space.
Finally, the last command just saves the configuration.

At this point, the configuration is ready and the only thing left is to actually modify the machines network configuration:

Example Router Address:

First Computer (Connected to Internet):

Second Computer:

That’s it, now both computers share the same Internet connection… fast and simple.

Be aware that this is a very simple example of network sharing, not an Enterprise Level solution. There are many alternative ways… more “elegant” and more secure ways to share a connection (proxy servers, SSH encrypted tunnels, etc)… you might want to try different methods and go for the one that fits your needs.

Categories: networking Tags: , ,

x86_64 Kernel IOMMU Error on Intel Quad Core

July 28, 2008 1 comment

For those of you that have tried installing a 64 bit kernel on a new intel box, the machine wont boot from media if the processor makes use of DMAR graphics engine.

When trying to boot Linux from the media (DVD/CD-ROM), upon loading the system files and standard drivers, the system hangs after the loading kernel screen.

Late quad core processors present this issue becoming a royal pain in the ass:

IOMMU: Setting identity map for device 0000:00:1a.1 [xdefd6000 - 0xdefd7000]

To solve this issue, you have to disable the graphics mapping Intel engine. On the initial GRUB installation screen lets edit the kernel parameters by pressing E; Then we just append the following phrase:


Press B to boot and the system will start normally.

OK, but we don’t want to do this every time we power on the machine. There are two easy ways around this, the former being the recommended choice:

a) After the first successful booting, upgrade to the latest version of the kernel.
b) If you refuse to update your kernel, edit grub.conf to boot without loading DMAR.

Its up to you to decide the best way.

Categories: Kernel Tags: