how to forward packets to another gateway, if i'm one

how to forward packets to another gateway, if i'm one

[Saulius Menkevicius]

Hi,
  I've spend some time searching Google on the topic, however
eventually gave up. So..

  Is there any way of forwarding packets to another gateway, which,
besides, is on the same network. Say, I'm on IP 10.0.0.2, and I act
as a gateway (ADSL, really) for a number of machines on the same LAN.
And there's another such gateway, hooked to another ADSL, which
handles connections for another bunch of machines. Essentially the
hosts 10.0.0.1 and 10.0.0.2 are identic and perform the same
function.

  Now, I want to make some traffic that comes from some client and
that should go through gateway 10.0.0.1 to be forwarded to 10.0.0.2
(without any changes). The need is to make the forwarding transparent
to both 10.0.0.2 and the client.

  So it's the way I suppose it should be done:
	a) No changes to be made 10.0.0.2's firewalling rules (in fact, it
runs FreeBSD);
	b) Make 10.0.0.1 to forward them to 10.0.0.2 transparently.

_This_is_the_culprit_. I don't know how to make them (packets) go
that way. DNAT/SNAT doesn't suit, neither does REDIRECT, as they
modify packets they filter.

	In FreeBSD, there's ipfw rule fwd, e.g. if 10.0.0.1 ran bsd, i'd
type "ipfw add fwd 10.0.0.2 ip from some_client to any". I'm looking
for the a way on how to make the same effect on Linux/iptables.
	Wrote to much of blurb. Guess I didn't mess it up too much :)

	Thanks.
--
Saulius Menkevicius, razzmatazz@mail.lt on 12.06.2002

Re: how to forward packets to another gateway, if i'm one

[Marcello Scacchetti]

Hi Saulius,
i think your solution is to use advanced kernel routing options using
the ip tool. This will allow you to use source based routing. route
command allows only setup destination based routes.
I think that http://lartc.org/ will come really in help to you, look for
Linux Advanced Routing & Traffic Control HOWTO.
Hope this helps,

Marcello

Il ven, 2002-12-06 alle 18:28, Saulius Menkevicius ha scritto:
> Hi,
>   I've spend some time searching Google on the topic, however 
> eventually gave up. So..
> 
>   Is there any way of forwarding packets to another gateway, which, 
> besides, is on the same network. Say, I'm on IP 10.0.0.2, and I act 
> as a gateway (ADSL, really) for a number of machines on the same LAN. 
> And there's another such gateway, hooked to another ADSL, which 
> handles connections for another bunch of machines. Essentially the 
> hosts 10.0.0.1 and 10.0.0.2 are identic and perform the same 
> function.
> 
>   Now, I want to make some traffic that comes from some client and 
> that should go through gateway 10.0.0.1 to be forwarded to 10.0.0.2 
> (without any changes). The need is to make the forwarding transparent 
> to both 10.0.0.2 and the client.
> 
>   So it's the way I suppose it should be done:
> 	a) No changes to be made 10.0.0.2's firewalling rules (in fact, it 
> runs FreeBSD);
> 	b) Make 10.0.0.1 to forward them to 10.0.0.2 transparently. 
> 
> _This_is_the_culprit_. I don't know how to make them (packets) go 
> that way. DNAT/SNAT doesn't suit, neither does REDIRECT, as they 
> modify packets they filter.
> 
> 	In FreeBSD, there's ipfw rule fwd, e.g. if 10.0.0.1 ran bsd, i'd 
> type "ipfw add fwd 10.0.0.2 ip from some_client to any". I'm looking 
> for the a way on how to make the same effect on Linux/iptables.
> 	Wrote to much of blurb. Guess I didn't mess it up too much :)
> 
> 	Thanks. 
-- 
Marcello Scacchetti <marcello.scacchetti@nextrem.it>

Re: how to forward packets to another gateway, if i'm one

[Ben Russo]

On Fri, 2002-12-06 at 12:28, Saulius Menkevicius wrote:
>   Is there any way of forwarding packets to another gateway, which, 
> besides, is on the same network. Say, I'm on IP 10.0.0.2, and I act 
> as a gateway (ADSL, really) for a number of machines on the same LAN. 
> And there's another such gateway, hooked to another ADSL, which 
> handles connections for another bunch of machines. Essentially the 
> hosts 10.0.0.1 and 10.0.0.2 are identic and perform the same 
> function.
> 
If both 10.0.0.1 and 10.0.0.2 are on the same subnet then just find 
the client machines that you want to use 10.0.0.1 as their gateway
and change their default route. No changes at all would be required
on either gateway.

Re: how to forward packets to another gateway, if i'm one

[Andrea Rossato]

Saulius Menkevicius wrote:
>   Now, I want to make some traffic that comes from some client and 
> that should go through gateway 10.0.0.1 to be forwarded to 10.0.0.2 
> (without any changes). The need is to make the forwarding transparent 
> to both 10.0.0.2 and the client.

in 10.0.0.1

ip rule add from YOUR_CLIENT [to DESTINATION] table fbsd.out
ip route add default via 10.0.0.2 dev $DEV table fbsd.out

I think this should do the job (for more information http://lartc.org/)
you create a routing table (fbsd.out) that will use 10.0.0.2 as gateaway 
for packets comming from your client.

I did not test this config, since I'm marking packets in the FORWARD 
chain of the mangle table and use
ip rule add fwmark to route them.

hope this helps.
andrea