semi OT: default route

semi OT: default route

[Payal Rathod]

Hi,
I have a question which has haunted me for many months. If I have 2 ISP
connections with me, with default gw 1.2.3.4 and 4.5.6.7 and if I add 
both as default routes on my Linux gateway as,
route add -net default gw 1.2.3.4
route add -net default gw 4.5.6.7
and if I send a packet from a windows client to internet, which ISP will it
go through?

Thanks a lot for the answer in advance and waiting eagerly for any
replies.

With warm regards,
-Payal

Re: semi OT: default route

[Jason Opperisano]

On Wed, Oct 20, 2004 at 09:25:51AM -0400, Payal Rathod wrote:
> Hi,
> I have a question which has haunted me for many months. If I have 2 ISP
> connections with me, with default gw 1.2.3.4 and 4.5.6.7 and if I add 
> both as default routes on my Linux gateway as,
> route add -net default gw 1.2.3.4
> route add -net default gw 4.5.6.7
> and if I send a packet from a windows client to internet, which ISP will it
> go through?

this will sound like a stupid answer, but it will probably always use
the route that you added first.  the 'route' command will allow you to
added multiple default routes, but the ones you add after the first one
end up getting ignored.  the 'ip' command won't let you add a default
route once you have one (it uses teq and multipath for this):

	$ ip route list | grep default
	default via 10.2.1.1 dev eth0

	$ sudo ip route add default via 10.2.1.2
	RTNETLINK answers: File exists

if you want to load-balance outbound traffic over multiple ISP links,
you'll want to use something along the lines of the 'nth' patch from POM:

	http://netfilter.org/documentation/HOWTO/netfilter-extensions-HOWTO-3.html#ss3.9

if you want to split outbound traffic over multiple links by source IP
or protocol, etc--you can use the concepts from:

	http://lartc.org/howto/lartc.netfilter.html

-j

-- 
Jason Opperisano <opie@817west.com>

Re: semi OT: default route

[Nick Drage]

On Wed, Oct 20, 2004 at 12:17:25PM -0400, Jason Opperisano wrote:
> On Wed, Oct 20, 2004 at 09:25:51AM -0400, Payal Rathod wrote:
> > Hi,
> > I have a question which has haunted me for many months. If I have 2 ISP
> > connections with me, with default gw 1.2.3.4 and 4.5.6.7 and if I add 
> > both as default routes on my Linux gateway as,
> > route add -net default gw 1.2.3.4
> > route add -net default gw 4.5.6.7
> > and if I send a packet from a windows client to internet, which ISP will it
> > go through?

Looking at this from a slightly different, well, simpler, point of view
than Jason...

> this will sound like a stupid answer, but it will probably always use
> the route that you added first.

On the host I've just tried this on - admittedly just the once, it tried
the route I added *last*.  But this is about thirty seconds worth of
testing :)

> the 'route' command will allow you to added multiple default routes,
> but the ones you add after the first one end up getting ignored.  the
> 'ip' command won't let you add a default route once you have one (it
> uses teq and multipath for this):
> 
> 	$ ip route list | grep default
> 	default via 10.2.1.1 dev eth0
> 
> 	$ sudo ip route add default via 10.2.1.2
> 	RTNETLINK answers: File exists

However if you want to give the routes different metrics....

ip route add default via 1.2.3.4 metric 1
ip route add default via 4.5.6.7 metric 2

"ip" will accept that input.

That should mean if the host can't send the traffic via 1.2.3.4 it will
realise this ( I presume solely if it gets no arp-reply for 1.2.3.4) it
will try and send the traffic via 4.5.6.7 instead.

That seems to be the way it should work, however on a test box my host
is happily trying to arp for 1.2.3.4 continuosly.

Anyone help me finish off this answer ;)

-- 
We are the Willing, led by the Unknowing,
Are doing the Impossible, for the Ungrateful.
We have done so much, for so long, with so little,
We are now qualified to do anything with nothing.

Re: semi OT: default route

[Jason Opperisano]

On Wed, Oct 20, 2004 at 05:37:34PM +0100, Nick Drage wrote:
> However if you want to give the routes different metrics....
> 
> ip route add default via 1.2.3.4 metric 1
> ip route add default via 4.5.6.7 metric 2
> 
> "ip" will accept that input.
> 
> That should mean if the host can't send the traffic via 1.2.3.4 it will
> realise this ( I presume solely if it gets no arp-reply for 1.2.3.4) it
> will try and send the traffic via 4.5.6.7 instead.
> 
> That seems to be the way it should work, however on a test box my host
> is happily trying to arp for 1.2.3.4 continuosly.
> 
> Anyone help me finish off this answer ;)

yes--the linux routing code will do dead gateway detection and fall
back to a lower metric route in the event of failure, and also go back
to the higher metric route upon resurrection.  this can be useful for
an active-standby setup.

as for which route added by 'route' actually gets used--i've never spent
the time to figure out how it picks the one it uses, but it certainly
only appears to ever use one--maybe it picks the one with the lowest
numerical value...  or maybe it uses one of those magic 8-ball things...

-j

-- 
Jason Opperisano <opie@817west.com>

Re: semi OT: default route

[Payal Rathod]

On Wed, Oct 20, 2004 at 12:48:30PM -0400, Jason Opperisano wrote:
> yes--the linux routing code will do dead gateway detection and fall
> back to a lower metric route in the event of failure, and also go back
> to the higher metric route upon resurrection.  this can be useful for
> an active-standby setup.

routed might help here. Right?
Can anyone with 2 ISP setup please check the load balance thing?
I vaguely remember seeing something about multiple default routes
in early chapters of TCP/IP Illustrated Vol. 1 (I am not at all sure).
Thanks.
With warm regards,
-Payal

-- 

Re: semi OT: default route

[Jason Opperisano]

On Wed, Oct 20, 2004 at 12:57:37PM -0400, Payal Rathod wrote:
> On Wed, Oct 20, 2004 at 12:48:30PM -0400, Jason Opperisano wrote:
> > yes--the linux routing code will do dead gateway detection and fall
> > back to a lower metric route in the event of failure, and also go back
> > to the higher metric route upon resurrection.  this can be useful for
> > an active-standby setup.
> 
> routed might help here. Right?

no--routed is a method for learning routes.  you already know what the
routes are.  you're looking for a method to balance traffic over two
physical links.  the solution to that goal is more complicated than just
adding two default gateways.

-j

-- 
Jason Opperisano <opie@817west.com>

multiple external interface ?

[Faisal]

Is it possible in iptables to have more then one external interfaces ?
I am going to run a webproxy server with iptables enabled with NAT & for
that i want to increase my bandwith in doing so ...

thanks