Chain traversal with multiple internal IP subnets.

Chain traversal with multiple internal IP subnets.

[Jared Brick]

Hello everybody,

I have (for various reasons) a LAN with two gateways that internally use
three different IP subnets (don't ask). Most nodes within the LAN have a
single IP address and use one of the two gateways.

One gateway is a router with an IP address in each subnet that does no
filtering. Most hosts use this as there default gateway, and it of
course has no problem routing between the three subnets.

However the other gateway is a Linux box with two IP addresses (it only
acts as a gateway for two of the subnets)  using Iptables. The gateway
itself has static routes so that it can route between the different
subnets, including the subnet for which it has no IP address. My
question in which chain will a packet traverse when it is traveling
between two different internal subnets. I think it is the FORWARD chain
but my colleague feels it would be the OUTPUT chain. I ask because I am
having trouble routing between the internal networks.

Also note that I am well aware that connection tracking is useless in
this environment (because of the different default gateways) and all the
chains are set up before any connection tracking is going on.

If anyone has any experience with a similar set up, any help would be
much appreciated.

Thanks,

Jared

Re: Chain traversal with multiple internal IP subnets.

[Antony Stone]

On Sunday 22 September 2002 8:54 pm, Jared Brick wrote:

> I have (for various reasons) a LAN with two gateways that internally use
> three different IP subnets (don't ask).

Okay ...  :-)

> However the other gateway is a Linux box with two IP addresses (it only
> acts as a gateway for two of the subnets)  using Iptables. The gateway
> itself has static routes so that it can route between the different
> subnets, including the subnet for which it has no IP address. My
> question in which chain will a packet traverse when it is traveling
> between two different internal subnets. I think it is the FORWARD chain
> but my colleague feels it would be the OUTPUT chain. I ask because I am
> having trouble routing between the internal networks.

It is the FORWARD chain.   The reasoning is simple:

Only packets which are destined for the machine running netfilter traverse 
the INPUT chain.

Only packets which are originated on the machine running netfilter traverse 
the OUTPUT chain.

Packets which originate on another machine and are destined for another 
machine only traverse the FORWARD chain.

Of course, the above statements are for the filter tables only - all packets 
coming in to a machine will traverse the PREROUTING chain (nat table) and all 
packets leaving a machine will traverse the POSTROUTING chain (nat table).

The PREROUTING chain in particular may change the selection about whether the 
packet terminates on this machine and enters the INPUT chain, or whether it 
goes on to some other machine and therefore enters the FORWARD chain.

Netfilter does not even care about packets which enter through an interface 
and then exit by the same interface - they still go through the FORWARD chain.

Hope this helps,

Antony.

-- 

Which part of 'apt-get dist-upgrade' do you not understand ???

Re: Chain traversal with multiple internal IP subnets.

[Anders Fugmann]

Jared Brick wrote:
> Hello everybody,
> 
> However the other gateway is a Linux box with two IP addresses (it only
> acts as a gateway for two of the subnets)  using Iptables. 
I'm not sure of your setup. Do you have one network interface per ip, or 
two ip's for the same interface?

> The gateway
> itself has static routes so that it can route between the different
> subnets, including the subnet for which it has no IP address. My
> question in which chain will a packet traverse when it is traveling
> between two different internal subnets. I think it is the FORWARD chain
> but my colleague feels it would be the OUTPUT chain. I ask because I am
> having trouble routing between the internal networks.
If you hare only using one network interface, then you will have 
troubles routing between the two subnets. Please see the recent thread 
in which this was discussed:
"Internal ip exiting network on firewall external nic despight rule" 
(http://lists.netfilter.org/pipermail/netfilter/2002-September/038510.html)

> If anyone has any experience with a similar set up, any help would be
> much appreciated.
It seems to me that you do not need netfilter at all, as netfilter does 
not route packets. I recommend that you setup your linux router and make 
it work without netfilter. Then, if you want to restrict communication 
between the subnets, use netfilter.

Hope it helps.
Anders Fugmann