packet parameter changing on fly

Hi all,
does anybody knows about a software which can change the packets parameters on the fly. Changing packet fields according to the users consent.

Thanks

What fields of what kind of packets? Ethernet? Tokenring? TCP/IP? IPX/SPX?

etherenet and tcp/ip.

It is scene dependent. To elaborate, what you want to achieve is important here. Sure we can understand you need to modify the packets on the fly. But it will be good if you can state what is the purpose of doing so.
Sometimes, you may need to modify an existing tool, or under certain circumstances you may need to create your own as well.

Alongwith this you should state the operating system in question, on which you want to run the tool.

I think ettercap can server the purpose on *nix
http://www.networkchemistry.com/products/packetyzer/ for windows.

guys, thanks for the replies,
i was just curious that is there any piece of software that changes the packets parameters on the fly except iptables and ettercap. Suppose u r doing a port scan on a network using nmap e.g. 'nmap -sS -P0' and when the packets are sent from the nmap, (as the packets are made thru raw sockets so they will bypass the tcp/ip stack) so i wannt to trap the packets just before the device driver and then there parameters are changed and then given to the device driver. I m just curious is that possible using any available tools.


Thanks

mmkhan it is perfectly possible.
You will have to write a linux kernel module , which will use netfilter hooks. Hook Pre Routing and you can acheve what you want.
documentation regarding netfilters and linux kernel modules on tldp.org and netfilter.org might prove to be helpful