multi-uplink and link state judgement

Hi all,

I have three NICs, eth0, eth1, eth2 in a linux box, eth0 connects
a internal network with private IP addreses, while eth1 and eth2
connect to two ISPs. I set SNAT on both eth1 and eth2 for purpose of
NATing the internal private-ip hosts. Now I hope to realize this:
1. When the two uplinks are good, internal packets go from eth2 to internet.
2. When uplink of eth2 is down, internal packets can be automatically
switched to go from eth1 and
3. The system can automatically detect the link state of eth2's uplink.
When it finds it becomes normal from down, it will let the internal
packets automatically to use eth2's uplink again.

Can anybody give me a solution or any clues ?

Bow to you in advance.

On 3 Nov 2003 07:12:05 -0800, alfasun <(E-Mail Removed)> wrote:
> I have three NICs, eth0, eth1, eth2 in a linux box, eth0 connects
> a internal network with private IP addreses, while eth1 and eth2
> connect to two ISPs. I set SNAT on both eth1 and eth2 for purpose of
> NATing the internal private-ip hosts. Now I hope to realize this:
> 1. When the two uplinks are good, internal packets go from eth2 to internet.
> 2. When uplink of eth2 is down, internal packets can be automatically
> switched to go from eth1 and
> 3. The system can automatically detect the link state of eth2's uplink.
> When it finds it becomes normal from down, it will let the internal
> packets automatically to use eth2's uplink again.
>
> Can anybody give me a solution or any clues ?

Not sure if you can simply set a higher metric for the backup default
route. But you may want to read the Adv-Routing HOWTO.

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(E-Mail Removed) (David Efflandt) wrote in message news:<(E-Mail Removed)>...
> On 3 Nov 2003 07:12:05 -0800, alfasun <(E-Mail Removed)> wrote:
> > I have three NICs, eth0, eth1, eth2 in a linux box, eth0 connects
> > a internal network with private IP addreses, while eth1 and eth2
> > connect to two ISPs. I set SNAT on both eth1 and eth2 for purpose of
> > NATing the internal private-ip hosts. Now I hope to realize this:
> > 1. When the two uplinks are good, internal packets go from eth2 to internet.
> > 2. When uplink of eth2 is down, internal packets can be automatically
> > switched to go from eth1 and
> > 3. The system can automatically detect the link state of eth2's uplink.
> > When it finds it becomes normal from down, it will let the internal
> > packets automatically to use eth2's uplink again.
> >
> > Can anybody give me a solution or any clues ?
>
> Not sure if you can simply set a higher metric for the backup default
> route. But you may want to read the Adv-Routing HOWTO.
Hi,
Thank you for your reply. Yes, I want to have a backup route to internet
and I hope when eth2's uplink line become normal from break, all packets
can automatically to switch to use this route again.
By mentioning "higher metric" do you mean to let me add to default routes
with diffrent metric? I tried to set eth2 route metric to 20 and
eth1 40. In the begining, when both route are good, I can see packets
coming into linux box from eth0 indeed go out from eth2 route. But
when the uplink of eth2 is off, hosts connected to eth0 can not get to
internet. The packets coming into linux box from eth0 won't chose eth1
route to go through to internet. I suppose the packets are dropped
somewhere in the kernel. My linux box is RH-8.0 and from internet
to access eth1's ip and eth2's ip got no any problems.
And I have read Adv-Routing HOWTO, not find any stuff mentioned to
satisfy the purpose of mine.
So would you pls to give me any other clues or a bit more detailed
instructions?
Thank you and bow.