Re: NAT Box

["John A. Sullivan III" <john.sullivan@nexusmgmt.com>]

On Wed, 2004-06-16 at 09:07, Simone Sestini wrote:
> Hi all..
> 
> I'm developing a nat box for some special Internet Value Added Services.
> I have a lot of NAS boxes that receives calls and then the default 
> router for all is the nat box.
> The box has 2 ethernet interfaces, one public for internet acces and one 
> over my intranet lan.
> I have just done a lot of special configuration but right now i'm in 
> front of a problem..
> Normaly a /23 subnet is always natted with a public ip address.
> Now.. i need to resolve a problem..
> 
> A special /23 need to browse only on some specific destination.. but if 
> one client with the ip into the /23 try to
> browse out of the range of the ip permitted i need to redirect the 
> request automatically to 1 or more specific
> ip address.
> 
> 
> This is the example that is working correct without the automatically 
> redirection.
> The 10.30.14.0 /23 can browse some public ip addresses but if it try to 
> go out of the range the REJECT rules
> block the packet and the final client will not browse..
> 
> /usr/local/sbin/iptables -t nat -A POSTROUTING -s 10.30.14.0/23 -o eth0 
> -j SNAT --to PUBLIC_IP_FOR_NAT
> 
> /usr/local/sbin/iptables -N PERSONAL_RULES
> /usr/local/sbin/iptables -A PERSONAL_RULES -j ACCEPT -s 10.30.14.0/23 -d 
> First_Public_IP
> /usr/local/sbin/iptables -A PERSONAL_RULES -j ACCEPT -s 10.30.14.0/23 -d 
> Second_Public_IP
> /usr/local/sbin/iptables -A PERSONAL_RULES -j ACCEPT -s 10.30.14.0/23 -d 
> Third_Public_IP
> /usr/local/sbin/iptables -A PERSONAL_RULES -j ACCEPT -s 10.30.14.0/23 -d 
> Other_Public_IP
> /usr/local/sbin/iptables -A PERSONAL_RULES -j ACCEPT -s 10.30.14.0/23 -d 
> Other_Public_IP
> /usr/local/sbin/iptables -A PERSONAL_RULES -j REJECT -s 10.30.14.0/23 -d 0/0
> 
> /usr/local/sbin/iptables -A FORWARD -s 10.30.14.0/23 -p tcp --dport 80 
> -j PERSONAL_RULES
> 
> Now i have to do something that permit me to add a new feature..
> I have to remove the REJECT rules and have to find the way to redirect 
> the request to a specific ip.. maybe with a DNAT ??
> 
> Any idea ? all is appreciated..
> 
> Best Regards.
Let me see if I understand you correctly.  If a station from
10.30.14.0/23 sends a TCP packet on port 80 to any of the allowed
destinations listed in PERSONAL_RULES, it should be ACCEPTed.  If it
sends to anything else, it should be redirected to a single IP address.

I can think of two approaches off the top of my head:

1) Use DNAT as you suggest.  You could do something like:

iptables -t nat -N RedirectChain
iptables -t nat -A PREROUTING -s 10.30.14.0/23 -p 6 --dport 80 -j
RedirectChain
iptables -t nat RedirectChain -d FirstPublicIP -j ACCEPT (or RETURN)
iptables -t nat RedirectChain -d SecondPublicIP -j ACCEPT . . . etc.
iptables -t nat RedirectChain -j DNAT --to-destination $REDIR_IP
iptables -A PERSONAL_RULES -s 10.30.14.0/23 -d $REDIR_IP -p 6 --dport 80
-j ACCEPT (before REJECT rule)

2) If it is all http traffic and you want to get a little fancy, use a
transparent proxy like Squid and a redirector like SquidGuard - that
will give you a lot of flexibility and control at the application layer.

Hope it helps - John

-- 
John A. Sullivan III
Chief Technology Officer
Nexus Management
+1 207-985-7880
john.sullivan@nexusmgmt.com
---
If you are interested in helping to develop a GPL enterprise class
VPN/Firewall/Security device management console, please visit
http://iscs.sourceforge.net