[Techtalk] re: DNS and IP masquerading

[Malcolm Tredinnick]

On Wed, Mar 19, 2003 at 11:08:44PM -0800, Berenice wrote:
> Sorry to be a pest about this topic!  I've finally finished setting
> up my home network for internet connection sharing, but my sister's
> computer still can't access the internet.  Our network consists of 2
> computers.  Mine is the host and is a Win2000/RH 8 dual boot.  My
> sister's is a Win98.  ATM our internet connection sharing only works
> when I'm using Win2000.  When I use linux, we can still ping each
> other but only my machine can reach the internet.  My sister gets the
> "file not found" message if she opens Explorer. A friend suggested I
> check her ICS, but I can't find anything wrong it.  
> 
> I have no idea why this is happening, or where to start
> troubleshooting.  Does anyone have suggestions as to what could be
> causing this problem?

My guess would be that IP forwarding is not enabled on your machine.
Look at the contents of /proc/sys/net/ipv4/ip_forward. If it is '0',
rather than '1', then packets that your machine should be forwarding
will be dropped. Note that this option is turned off in a standard
install for security reasons. Enable it by echoing 1 into that file and
editing /etc/sysctl.conf and having a line that says

	# Controls IP packet forwarding
	net.ipv4.ip_forward = 1

(it will say '= 0' by default).

If forwarding is turned on, do you have any IP tables rules that are
stopping the data from flowing (e.g. a default DENY rule in the forward
chain)?

Failing all of this, can you post some more details, such as the IP
addresses of the machines involved, the routing table on your machine
(and relevant iptables rules)? Often these problems are easy to spot
when we can stare at the raw information a bit.

Cheers,
Malcolm

-- 
To succeed in politics, it is often necessary to rise above your
principles.