Linux in General > Linux Tutorials & How To's

Internet Sharing in Linux / NAT How to

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taiwo:
.pls does the explanation regarding the NAT also applicable to those using DSL. 'cos i saw ppp in the syntax.[/url]

Ricky:
when you are connectied to internet then run "ifconfig" and if it shows ur DSL as eth1 or eth0 then change ppp0 to that one and if it shows it to ppp0 then keep it ppp0 !

anybody1234:
Hello

I have requirement in iptables rules as follows

I would need a script for iptables with some configuration option such that
I should be able to make changes in the iptables script for allowing  or disallowing requests made by specifc clients on specifid ports
Suppose My cleints have their default gateway set as my linux box

What I would like is block/disallow requests made by clients on ports
80 while allow the requests made on port 25 ,110

But What I would like to have is some granularity ie
If the requests come from specific IP addresses  I should have the option of allowing them directly thru port 80  

Would anybody please give me step by step process assumimg that I am starting from scratch
I also remember I saw a simliar script in this forum which almost does the same but I can't locate where it is now

anybody1234:
To be more specififc I would use the following commands

Please Let me know If I go wrong anywhere

echo "1" > /proc/sys/net/ipv4/ip_forward

for forwarding packets
and Iptables masquerading rle as
#######################################
iptables -t nat -A POSTROUTING -j MASQUERADE
#####################################
/usr/sbin/iptables-save
##########very imp
iptables-save  > /etc/sysconfig/iptables.rules


Now all teh requests thru my gateway would be allowed
FOr blocking requests made on port 80 by all  would add folloing rule
###******** this rule blocks all requests coming on port 80 ******************
iptables -A FORWARD -i eth0 -p tcp -m tcp --dport 80 -j DROP
#*****************************************************

Now I would like to make an exception for IP addresses 192.168.0.82, 192.168.0.110

I would add rule

iptables -A FORWARD -i eth0 -p tcp -m tcp -s ! 192.168.0.82,192.168.0.110 --dport 80 -j DROP

Now at any stage I felt I ned to remove this rule what I would have done manually is

iptables -D FORWARD -i eth0 -p tcp -m tcp --dport 80 -j DROP
and
iptables -D FORWARD -i eth0 -p tcp -m tcp -s ! 192.168.0.82,192.168.0.110 --dport 80 -j DROP

Now I want a script to do the same thing with start stop  option
I think our experts can give me a simple script to do the same

Ricky:
Basically this thread is about discussing the NAT in linux. I request you to start a new thread in General networking section about this firewall stuff. The thing you have asked is simple but will take little time sO I think it will be better to discuss it separately to give it full preference. BTW.. you are right. I have already given so many solutions for it that you can find answer already posted in forum so better try to find once more else I m here always to solve the problems which I can solve.

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