IPTables issue

[Hassnain Badami]

Dear all
I am learning IPTables and have been given a problem on our network to diagnose and solve. 
Our network infrastructure contains an internet provider line from Colt that feeds into a bandwidth router (provided by our building management) and then Ubuntu 10.04 box running iptables. This firewall is then connected to a switch and we run a local area network of around 20 computers (both Linux and windows).
Our firewall has a certain set of rules enabled. When I try to download a file on the firewall itself everything seems fine. But when I try to download the same file from a windows box behind the firewall, it starts well, downloads upto 5 MB, but then interrupts or enormously slows down.
To solve this problem I wrote a small script, first to clean my iptables rules and then to create a few rules that only allow basic configuration. The first script is
Code:echo "Stopping firewall and allowing everyone..."iptables -Fiptables -Xiptables -t nat -Fiptables -t nat -Xiptables -t mangle -Fiptables -t mangle -Xiptables -P INPUT ACCEPTiptables -P FORWARD ACCEPTiptables -P OUTPUT ACCEPT
The second script only allows for basic rules to be setup and is as follows (eth0 is lan and eth5 is WAN)
Code:iptables -A FORWARD -i eth0 -o eth5 -j ACCEPTiptables -t nat -A POSTROUTING -o eth5 -j MASQUERADEiptables -A FORWARD -i eth5 -o eth0 -m state --state RELATED,ESTABLISHED -j ACCEPT
Using the 2nd script I can browse fine, but any downloads on the lan box again slow down or interrupt.
I am very confused as this firewall was operational for more than a year and all of a sudden it has started to fail (on March 7). Secondly the building management claims they havent done anything at their end. I am kind of lost. Any help will be highly appreciated.
many thanks
Hassnain. 		 	   		  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.ubuntu.com/archives/ubuntu-users/attachments/20120314/0e5f9fab/attachment.html>