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Problems with LINUX as a router - Masquerading, NAT, Internet connection sharing

Hello,

I have the following problem:
I want to use a linux box as a gateway to the internet. It is connected through a DSL (dialup) connection (ppp0 -> eth1).
I want to have access from clients connected to this machine through wireless connections. There is a notebook, running Windows2000. I can ping the linux box and vice versa. Trying to surf to e.g. www.google.com doesn't work.

What I have done so far:
Linux (fedora, kernel 2.6.8) IP on device atml0 (this is the wireless device): 192.168.0.1
Win2k IP: 192.168.0.99
DNS AND gateway on Win2K: 192.168.0.1

/etc/rc.d/rc.ipmasq:
#!/bin/sh
IPTABLES=/sbin/iptables
$IPTABLES -F -t nat
$IPTABLES -A POSTROUTING -t nat -o ppp0 -j MASQUERADE
echo 1 > /proc/sys/net/ipv4/ip_forward

Whats wrong? Can anybody help?

loony

jlevie

Unless you've set up Bind on your Linux box you'll need to configure windows to use the same name server IP(s) that you find in /etc/resolv.conf. Your firewall set up should work as configured and when you fix the nameserver the 2k box should be able to surf.

l00ny_tn1

ASKER

I thought it could have something to do with the DNS entry on the Win2k box, but even if i have set it to the one shown in /etc/resolv.conf it doesn't work. What else could it be? By the way: How do I set up a bind on the linux box?

loony

l00ny_tn1

ASKER

Some extra info:
Output of
/sbin/iptables -L
==============
Chain INPUT (policy DROP)
target prot opt source destination
ACCEPT tcp -- anywhere anywhere tcp dpt:domain
ACCEPT udp -- anywhere anywhere udp dpt:domain
ACCEPT tcp -- anywhere anywhere tcp dpt:netbios-ns
ACCEPT tcp -- anywhere anywhere tcp dpt:netbios-dgm
ACCEPT tcp -- anywhere anywhere tcp dpt:netbios-ssn
ACCEPT tcp -- anywhere anywhere tcp dpt:4662
ACCEPT udp -- anywhere anywhere udp dpt:4223
ACCEPT icmp -- anywhere anywhere icmp echo-request limit: avg 5/min burst 5
ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED
ACCEPT all -- anywhere localhost.localdomain
LOG all -- anywhere anywhere limit: avg 10/hour burst 5 LOG level warning prefix `IPTABLES: '

Chain FORWARD (policy DROP)
target prot opt source destination
ACCEPT all -- anywhere anywhere state NEW,RELATED,ESTABLISHED
ACCEPT all -- anywhere anywhere state RELATED,ESTABLISHED

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

/sbin/iptables -t NAT -L
===================
Chain PREROUTING (policy ACCEPT)
target prot opt source destination

Chain POSTROUTING (policy ACCEPT)
target prot opt source destination

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

Hm... Anybody?

l00ny_tn1

ASKER

Okay, I got it:

For anybody having the same probs:
Reset any firewall rules (for testing though <g>), then do the following:

iptables -F -t nat
iptables -A POSTROUTING -t nat -o ppp0 -j MASQUERADE
echo 1 > /proc/sys/net/ipv4/ip_forward

iptables -A FORWARD -t filter -i atml0 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -t filter -i ppp0 -m state --state ESTABLISHED,RELATED -j ACCEPT

(where atml0 is the interface connected to the lan (perhaps eth1 for you?!), ppp0 is the interface connected to the internet (perhaps something other like eth0 for you?!))
Don't forget to set the DNS server on the clients to the one given by DHCP of your isp (/etc/resolv.conf)...

For me, it seems it was a problem with the rules I set up to protect the router/gateway.
loony

kidoman

Hi,

Good that you found a solution but I can give u a small hint. When you want to chase IPTABLES buys, then use the utility "tcpdump". By seeing the packets arrriving and going, you can diagnose the problems easily.

Actually you could have jsut set the DNS server addresses in the clients to the one supplied by ur ISP and seen if it worked...?

Just my 2 cents,

Cheers,

karan

l00ny_tn1

ASKER

Thanks, a really cool tool this tcpdump is :)

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