How can I change my IP info on a daily basis?

Posted on 2002-05-17
Last Modified: 2013-12-15
Currently, I have to do the following process manually, using the Mandrake
Config tools in X-Windows.  It takes me about 20 minutes.  Can you guys help
me figure out a script for the following:

- flush all IPtables rules (so the packet filters are not active)
- switch eth0 from a static IP to DHCP
- receive a DHCP IP address
- run a Perl Script that I have already
- switch eth0 back to a static IP
- restart my script to setup my packet filters again

I want to do the above on a nightly basis.  In Windows 2K, I would write a
batch file (using the netsh.exe command) and have the Scheduler service run
the batch file nightly.

In Linux, the scripting and scheduling look much different.

BTW, this is a Mandrake 8.1 box, P][-266, 64mb RAM, 4gb HD, running no
server services (no apache or whatnot).  It has 2 ethernet cards (eth0 and


Question by:cdub1234
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Accepted Solution

jlevie earned 38 total points
ID: 7020691
That's the hard way to do a firewall. I'd guess that you probably have a statement something like:

iptables -t nat -A POSTROUTING -o eth0 -j SNAT --to

where is the IP you get from the DHCP assignment and then configure eth0 to use. When you have a dynamic IP you should use:

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

so that iptables will track the changing IP that DHCP assigns to your outside interface. Obviously in that case you want to leave the outside NIC configured to use DHCP. There's no reason to go through the routine ao changing to DCHP to get the IP then configuring the system to use that IP statically when you can have iptables automatically adjust things as necessary to follow the changing IP.

Assisted Solution

st_steve earned 37 total points
ID: 7036444
And the scheduler service on Linux is "crond" and the "scheduler config file" is at /etc/crontab

Add your commands, scripts (and commands posted by jlevie) in this file and it should fire up when the time is right. If you need to change runlevels to do the tasks you've posted above, make sure the crond is running in that runlevel (the easiest way is to use chkconfig to configure whether crond runs in a particular runlevel).

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