DNAT and SNAT

DNAT and SNAT

[Fabrizio Regalli]

Hi list.

I have a router "closed" and I can't using PAT but I need to forward
the ssh port to another external port (50002)
My router is 192.168.1.1
My server is 192.168.1.2
My client is 192.168.1.3
In other words I need to forward the ssh port of 192.168.1.3 to server ip:50002

For PREROUTING rule I suppose this is right:

iptables -t nat -A PREROUTING -p tcp -m tcp --dport 50002 -j DNAT
--to-destination 192.168.1.3:22

For POSTROUTING rule, I need an help. I'm trying in this way:

iptables -t nat -A POSTROUTING -p tcp -m tcp -d 192.168.1.2 --dport 22
-j SNAT --to-source 192.168.1.2:50002

But unfortunately it doesn't work.

Could please someone give me a suggest for POSTROUTING rule?

Thanks in advance.

Re: DNAT and SNAT

[Fabrizio Regalli]

I've found the right rules:

iptables -t nat -A PREROUTING -p tcp -m tcp --dport 50002 -j DNAT
--to-destination 192.168.1.3:22

iptables -t nat -A POSTROUTING -p tcp -m tcp -d 192.168.1.3 --dport 22
-j SNAT --to-source 192.168.1.2:50002

iptables -P FORWARD ACCEPT

Now works.

Thanks.
Fabrizio

2008/9/3 Fabrizio Regalli <fabreg@gmail.com>:
> Hi list.
>
> I have a router "closed" and I can't using PAT but I need to forward
> the ssh port to another external port (50002)
> My router is 192.168.1.1
> My server is 192.168.1.2
> My client is 192.168.1.3
> In other words I need to forward the ssh port of 192.168.1.3 to server ip:50002
>
> For PREROUTING rule I suppose this is right:
>
> iptables -t nat -A PREROUTING -p tcp -m tcp --dport 50002 -j DNAT
> --to-destination 192.168.1.3:22
>
> For POSTROUTING rule, I need an help. I'm trying in this way:
>
> iptables -t nat -A POSTROUTING -p tcp -m tcp -d 192.168.1.2 --dport 22
> -j SNAT --to-source 192.168.1.2:50002
>
> But unfortunately it doesn't work.
>
> Could please someone give me a suggest for POSTROUTING rule?
>
> Thanks in advance.
>

Re: DNAT and SNAT

[ArcosCom Linux User]

The solution is not to "allow all" in "forwading" in your router.

Think how the tables work:
   * The nat is for the NAT operations.
   * The filter is for filter operations.

If you are not allowing all forwarding traffic, then you need to put
analogous rule to your NAT rule in your nat table into your filter table.

For example, if you want to redirect 50002 from external to internal, you
must allow this in filter table, something as:

iptables -t filter -A FORWARD -p tcp -m tcp --dport 50002 -d 192.168.1.3
-j ACCEPT

Perhaps you need more rules, for example one to delimite the input and
another for the output, because you are using diferent ports in external
interface and destination host.

The idea is only allow the things you need.

Regards

El Mie, 3 de Septiembre de 2008, 16:43, Fabrizio Regalli escribió:
> I've found the right rules:
>
> iptables -t nat -A PREROUTING -p tcp -m tcp --dport 50002 -j DNAT
> --to-destination 192.168.1.3:22
>
> iptables -t nat -A POSTROUTING -p tcp -m tcp -d 192.168.1.3 --dport 22
> -j SNAT --to-source 192.168.1.2:50002
>
> iptables -P FORWARD ACCEPT
>
> Now works.
>
> Thanks.
> Fabrizio
>
> 2008/9/3 Fabrizio Regalli <fabreg@gmail.com>:
>> Hi list.
>>
>> I have a router "closed" and I can't using PAT but I need to forward
>> the ssh port to another external port (50002)
>> My router is 192.168.1.1
>> My server is 192.168.1.2
>> My client is 192.168.1.3
>> In other words I need to forward the ssh port of 192.168.1.3 to server
>> ip:50002
>>
>> For PREROUTING rule I suppose this is right:
>>
>> iptables -t nat -A PREROUTING -p tcp -m tcp --dport 50002 -j DNAT
>> --to-destination 192.168.1.3:22
>>
>> For POSTROUTING rule, I need an help. I'm trying in this way:
>>
>> iptables -t nat -A POSTROUTING -p tcp -m tcp -d 192.168.1.2 --dport 22
>> -j SNAT --to-source 192.168.1.2:50002
>>
>> But unfortunately it doesn't work.
>>
>> Could please someone give me a suggest for POSTROUTING rule?
>>
>> Thanks in advance.
>>
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