CentOS - Apr 2008 - Problem with CentOS server: am I running a firewall?

I recently tried to swap server from an ancient Asus PIII machine
running Fedora-8 to a Dell PowerEdge T105 running Centos-5.1 .

Unfortunately, I have not been able to set it up
to allow local machines to access the internet.

I can access the internet directly from the server
(which connects to an ADSL modem)
but not from a laptop connected to the server 
through an ethernet hub or WiFi router.

I can ssh into the server from the laptop,
but cannot ping anything beyond the server.

It is as though there were a firewall running on the server
which will not allow packets on the LAN to pass to the internet.

This is probably a very ignorant question,
but is there such a default firewall running under CentOS?

I've gone back to my old server temporarily,
but would prefer to sort out the new one if I could.

Any advice or suggestions gratefully received.

On Tue, Apr 15, 2008 at 5:33 PM, Timothy Murphy <gayleard at eircom.net>
wrote:
>  Unfortunately, I have not been able to set it up
>  to allow local machines to access the internet.
Did you enable routing? The output of "cat
/proc/sys/net/ipv4/ip_forward" must be 1, otherwise the server won't
route packets between networks. In some cases (depending on your
Internet modem/router) it's possible that you'll need to configure the
server to do NAT as well.

You can see the firewall rules with the command "iptables -nL". You
can delete all rules by running "iptables -F" (beware, your server
will be unprotected if you do that).

If you give more details about your network topology, it would be
easier to help you. You may also look at the contents of
/etc/sysctl.conf and /etc/sysconfig/iptables on your old server,
probably the routing, NAT and firewalling configuration of that server
is there.

HTH,
Filipe