NAT overlaps with ports

NAT overlaps with ports

[Kapetanakis Giannis]

Hi,

I have the following setup
kernel: 2.6.30.5-43.fc11
iptables-1.4.3.1-1.fc11

192.168.1.0/24 is my public  IP range (eth0)
10.0.0.0/24 is my private IP range (eth1)
192.168.1.1 public IP of server
10.0.0.1 private IP of server


I'd like to add the following rules in the nat table:

[1] -A PREROUTING -i eth0 -d 192.168.1.1 -p tcp --dport 8080 -j DNAT 
--to-destination 10.0.0.1:8080
[2] -A POSTROUTING -o eth0 -s 10.0.0.1 -p tcp --sport 8080 -j SNAT 
--to-source 192.168.1.1:8080
[3] -A POSTROUTING -o eth0 -s 10.0.0.0/24 --to-source 
192.168.1.1-192.168.1.10

According to http://www.netfilter.org/documentation/HOWTO/NAT-HOWTO-6.html
iptables is clever enough to avoid overlaps and clashes.
Are we sure that there isn't any chance to map a random packet (not from 
the server)
to 192.168.1.1:8080 in rule [3]?

I mean, does rule [2] reserve port 8080 of 192.168.1.1 ?

best regards,

Giannis

Re: NAT overlaps with ports

[Pascal Hambourg]

Hello,

Kapetanakis Giannis a écrit :
> 
> 192.168.1.0/24 is my public  IP range (eth0)

This IP range is private, not public.
If you made it up, please use the 192.0.2.0/24 range reserved for
examples and documentation instead.

> 10.0.0.0/24 is my private IP range (eth1)
> 192.168.1.1 public IP of server
> 10.0.0.1 private IP of server
> 
> I'd like to add the following rules in the nat table:
> 
> [1] -A PREROUTING -i eth0 -d 192.168.1.1 -p tcp --dport 8080 -j DNAT 
> --to-destination 10.0.0.1:8080
> [2] -A POSTROUTING -o eth0 -s 10.0.0.1 -p tcp --sport 8080 -j SNAT 
> --to-source 192.168.1.1:8080
> [3] -A POSTROUTING -o eth0 -s 10.0.0.0/24 --to-source 
> 192.168.1.1-192.168.1.10

Rule [2] is pointless. Packets with source port 8080 are obviously
replies, and Netfilter NAT implicitly takes care of reply packets
packets. Actually, the 'nat' chains don't even see reply packets.

> According to http://www.netfilter.org/documentation/HOWTO/NAT-HOWTO-6.html
> iptables is clever enough to avoid overlaps and clashes.
> Are we sure that there isn't any chance to map a random packet (not from 
> the server)
> to 192.168.1.1:8080 in rule [3]?

No. That could happen as long as it does not create a collision with an
existing mapping. Why do you worry about it ?
The important point is that netfilter avoids collisions between existing
NAT mappings. Rules do not create mappings by themselves, a mapping is
created only for each new connection created by a packet.

> I mean, does rule [2] reserve port 8080 of 192.168.1.1 ?

No. NAT rules do not reserve anything.

Re: NAT overlaps with ports

[Kapetanakis Giannis]

On 23/09/09 13:46, Pascal Hambourg wrote:
> This IP range is private, not public.
> If you made it up, please use the 192.0.2.0/24 range reserved for
> examples and documentation instead.
>    
Yes I made it up.
>> 10.0.0.0/24 is my private IP range (eth1)
>> 192.168.1.1 public IP of server
>> 10.0.0.1 private IP of server
>>
>> I'd like to add the following rules in the nat table:
>>
>> [1] -A PREROUTING -i eth0 -d 192.168.1.1 -p tcp --dport 8080 -j DNAT
>> --to-destination 10.0.0.1:8080
>> [2] -A POSTROUTING -o eth0 -s 10.0.0.1 -p tcp --sport 8080 -j SNAT
>> --to-source 192.168.1.1:8080
>> [3] -A POSTROUTING -o eth0 -s 10.0.0.0/24 --to-source
>> 192.168.1.1-192.168.1.10
>>      
> Rule [2] is pointless. Packets with source port 8080 are obviously
> replies, and Netfilter NAT implicitly takes care of reply packets
> packets. Actually, the 'nat' chains don't even see reply packets.
>    
You're right, I will remove it.
>> According to http://www.netfilter.org/documentation/HOWTO/NAT-HOWTO-6.html
>> iptables is clever enough to avoid overlaps and clashes.
>> Are we sure that there isn't any chance to map a random packet (not from
>> the server)
>> to 192.168.1.1:8080 in rule [3]?
>>      
> No. That could happen as long as it does not create a collision with an
> existing mapping. Why do you worry about it ?
> The important point is that netfilter avoids collisions between existing
> NAT mappings. Rules do not create mappings by themselves, a mapping is
> created only for each new connection created by a packet.
>    

What I'm worried of is than a random connection could be created which uses
the mapping of port 8080 of 192.168.1.1 and then the internal server 
would not be available.
But I guess this is not a problem since a connection has 4 parameters 
src/dst ip/port.

thanks for answering

Giannis

Re: NAT overlaps with ports

[Pascal Hambourg]

Kapetanakis Giannis a écrit :
> 
> What I'm worried of is than a random connection could be created which uses
> the mapping of port 8080 of 192.168.1.1 and then the internal server 
> would not be available.
> But I guess this is not a problem since a connection has 4 parameters 
> src/dst ip/port.

Indeed, if some random outgoing connection to a remote host is mapped on
source port 8080 it is very unlikely to disrupt access to the internal
server, and it would only affect access from that remote host during a
short delay after the connection has been terminated (until the mapping
is deleted).