[Techtalk] moving to iptables

Carla Schroder carla at bratgrrl.com
Mon Aug 4 22:07:40 EST 2003

On Monday 04 August 2003 9:21 pm, rocketgirl wrote:
> I take care of a RH 7.1 server at a school. The server was installed as
> a fire wall between the main school network and administration so that
> internet could be shared off of one adsl modem.  So the system looks
> like this:
> adsl---->school net------>RH firewall and internet (2 eth cards eth0
> towards school network and eth1 towards admin)------>admin
> I used ipchains at the time to set up a rudimentary firewall and to
> implement ipmasquerading.
> Now I want to put Mandrake 9.1 in place of RH 7.1 and I want to use
> iptables.  I'm looking to put together a much more secure system.  Is it
> possible to ipmasq with iptables like I could with ipchains?  Could
> someone point me in the direction of some good tutorials to help me with
> this migration and the names of some really clear--books for dummys?
> Thanks in advance.
> Wendy

iptables is ever so much better than ipchains, because it is stateful, and yes 
you can masquerade. In netfilter/iptables it is called NAT- network address 
translation. Here are three simple lines to enable masquerading:

# modprobe iptable_nat
# iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
# echo "1" > /proc/sys/net/ipv4/ip_forward 

You can run these from the command line, in this order, for testing. Once you 
get your rules figured out, put 'em in a script.

It's complex, because TCP/IP networking is complex. It can take some study to 
get up to speed, but it is definitely worth it. Here are some articles that 
may help you get started:

 Building Firewalls with iptables, Part 1 

 Building Firewalls with iptables, Part 2 

Linux IP Masquerade HOWTO

Carla Schroder
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