-is it possible for iptables configuration to be written on a txt file like squid?
- also is there a iptables generator and simulator?

iptables-save and iptables-restore are one way.

But I just have all my iptables rules in a bash script, and then run that script at startup.

Most "Linux firewalls" could be called an "iptables generator", but I've never used any to comment on their capabilities/useability. I write all my rules manually.

I use FireHOL - an iptables firewall generator. But there's a lot of others front-ends for iptables and rules generators.

Even with GUI interface.

-how do you run scripts on startup?

Here's a similar thread I posted to a while back. See post #2 in the thread:

http://www.linuxquestions.org/questi...light=firewall

- i have checked that linked...can you pls explain to me how does this work.

Those are "System V style" init scripts. I would recommend you do some LQ.org forum searches (or Google searches) on "init", "runlevel", "startup", "automatically execute a script", "boot sequence", and things like that. There have been many discussions and threads regarding what you are asking about. You can probably find more information, more quickly, searching these forums than anything I might be able to provide in a single post here. And discussion of startup scripting would fit better in it's own thread. It does not fit well under the current "iptables.conf" thread. People who know about runlevels and such may not be reading and responding to iptables threads.