[CentOS] SNAT question

Peter Peltonen peter.peltonen at gmail.com
Sat Nov 28 14:41:54 UTC 2009


Hi,

On Thu, Nov 26, 2009 at 1:17 AM, Tait Clarridge <tait at clarridge.ca> wrote:
>> >> <peter.peltonen at gmail.com> wrote:
>> >>> Hi,
>> >>>
>> >>> I am unable to get my LAN masqueraded using SNAT with CentOS 5.3 and iptables.
>> >>>
>> >>> I have the following setup:
>> >>>
>> >>> eth0: connects to internet with static public IP 1.2.3.1 (obscured
>> >>> here for privacy)
>> >>> eth1: connects to DMZ with static public IP 1.2.3.2 (obscured here for privacy)
>> >>> eth2: connects to LAN with static private IP 192.168.0.1
>> >>>
>> >>> Traffic to hosts in the DMZ/Internet through eth0/1 work fine.
>
> I had to get the VPN address range masqueraded on the LAN as the gateway
> address.. so for example:
>
> VPN Server LAN IP: 192.168.1.20 (not the real thing, but doesn't matter)
> VPN IP Range:   10.99.0.0
>
> So when I connect through OpenVPN, my tunnel adaptor is given an ip like
> 10.99.0.5 (basically like a LAN, or your eth2).
>
> What I did in IPTABLES is the following (eth0 is the LAN connection for
> the VPN server)
>
> iptables -t nat -A POSTROUTING -s 10.99.0.0/255.255.255.0 -o eth0 -j
> MASQUERADE
>
> After that it worked. All connections to anything on the LAN appear as
> if I am coming from 192.168.1.20. Just make sure that forwarding is
> enabled (I believe it is required for masquerade):
>
> cat /proc/sys/net/ipv4/ip_forward
>
> If it equals 0, change it to 1.
>
> You may want to remove all the other entries you tried to get
> LAN->Internet going to ensure there is nothing conflicting.

It appears my problems were somehow DNS related: I can't access my
ISPs DNS from LAN when masquerading is on (I can't understand why).
Using a nameserver in the DMZ solved my issues and everything seems to
work now ok.

Thanks for your help,
Peter


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