two default routes

two default routes

[ratheesh k]

Hi ,

 i have two default routes  , I used below two commands to add routes .

 route add default gw 192.168.1.1  dev eth0 metric 0
 route add default gw 10.232.18.5  dev eth1 metric 1

So  10.232.18.5 will become the default route since it is added
secondly . All packets are routed to this address . Suppose
10.232.18.5 machine is down  , will the packets will use the next
default route ? . Could you please mention any case in which packets
will use the route 192.168.1.1 .?

Thanks,
Ratheesh

Re: two default routes

[Richard Horton]

On 22 March 2010 10:35, ratheesh k <ratheesh.ksz@gmail.com> wrote:
> Hi ,
>
>  i have two default routes  , I used below two commands to add routes .
>
>  route add default gw 192.168.1.1  dev eth0 metric 0
>  route add default gw 10.232.18.5  dev eth1 metric 1
>

Without using rule based routing (part of iproute2) I don't believe
this will do anything - only one of the two default routes will ever
be used unless the eth0/eth1 interface itself changes to the down
state in which case its associated route should be removed.


-- 
Richard Horton
Users are like a virus: Each causing a thousand tiny crises until the
host finally dies.
http://www.solstans.co.uk - Solstans Japanese Bobtails and Norwegian Forest Cats
http://www.pbase.com/arimus - My online photogallery

Re: two default routes

[Pascal Hambourg]

Hello,

ratheesh k a écrit :
> 
>  i have two default routes  , I used below two commands to add routes .
> 
>  route add default gw 192.168.1.1  dev eth0 metric 0
>  route add default gw 10.232.18.5  dev eth1 metric 1
> 
> So  10.232.18.5 will become the default route since it is added
> secondly . All packets are routed to this address.

No. Lower metric wins regardless of route order, so the default gateway
is 192.168.1.1. You can check with "ip route get <destination>".

> Suppose
> 10.232.18.5 machine is down  , will the packets will use the next
> default route ?

No, the kernel has no dead gateway detection AFAIK.

Re: two default routes

[ratheesh k]

On Mon, Mar 22, 2010 at 8:24 PM, Pascal Hambourg
<pascal.mail@plouf.fr.eu.org> wrote:
> Hello,
>
> ratheesh k a écrit :
>>
>>  i have two default routes  , I used below two commands to add routes .
>>
>>  route add default gw 192.168.1.1  dev eth0 metric 0
>>  route add default gw 10.232.18.5  dev eth1 metric 1
>>
>> So  10.232.18.5 will become the default route since it is added
>> secondly . All packets are routed to this address.
>
> No. Lower metric wins regardless of route order, so the default gateway
> is 192.168.1.1. You can check with "ip route get <destination>".
>
>> Suppose
>> 10.232.18.5 machine is down  , will the packets will use the next
>> default route ?
>
> No, the kernel has no dead gateway detection AFAIK.
> --
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>


Thanks a lot . I have a related question .
suppose ip  8.8.8.8  is accessible thru gateway 10.232.18.5 and we
configured the linux machine to accept
icmp redirect messages ( if we set NET_IPV4_CONF_ACCEPT_REDIRECTS )  .
Linux will send packet to 8.8.8.8  thru 192.168.1.1 as per default
route . The gateway machine  ( ip 192.168.1.1 ) will respond back with
icmp redirect . (I think so ) . So our linux machine can use the other
route ?

thanks,
ratheesh

Re: two default routes

[Pascal Hambourg]

ratheesh k a écrit :
> 
> suppose ip  8.8.8.8  is accessible thru gateway 10.232.18.5 and we
> configured the linux machine to accept
> icmp redirect messages ( if we set NET_IPV4_CONF_ACCEPT_REDIRECTS )  .

Do you mean sysctl net.ipv4.conf.<interface>.accept_redirects ?
Note that this settings is ignored (and so are ICMP redirects) when
forwarding is enabled, thus when Linux acts as a router.

> Linux will send packet to 8.8.8.8  thru 192.168.1.1 as per default
> route . The gateway machine  ( ip 192.168.1.1 ) will respond back with
> icmp redirect . (I think so )

If it is configured to do so. If it runs a Linux kernel, see sysctl
net.ipv4.conf.<interface>.send_redirects.

> So our linux machine can use the other route ?

Yes, a temporary redirect route to 8.8.8.8 will be created in the
routing cache. You can see it with "ip route show cache".

Note : This does not seem to be much related to netfilter.

Re: two default routes

[ratheesh k]

On Mon, Mar 22, 2010 at 9:17 PM, Pascal Hambourg
<pascal.mail@plouf.fr.eu.org> wrote:
> ratheesh k a écrit :
>>
>> suppose ip  8.8.8.8  is accessible thru gateway 10.232.18.5 and we
>> configured the linux machine to accept
>> icmp redirect messages ( if we set NET_IPV4_CONF_ACCEPT_REDIRECTS )  .
>
> Do you mean sysctl net.ipv4.conf.<interface>.accept_redirects ?
> Note that this settings is ignored (and so are ICMP redirects) when
> forwarding is enabled, thus when Linux acts as a router.
>
>> Linux will send packet to 8.8.8.8  thru 192.168.1.1 as per default
>> route . The gateway machine  ( ip 192.168.1.1 ) will respond back with
>> icmp redirect . (I think so )
>
> If it is configured to do so. If it runs a Linux kernel, see sysctl
> net.ipv4.conf.<interface>.send_redirects.
>
>> So our linux machine can use the other route ?
>
> Yes, a temporary redirect route to 8.8.8.8 will be created in the
> routing cache. You can see it with "ip route show cache".
>
> Note : This does not seem to be much related to netfilter.
> --
> To unsubscribe from this list: send the line "unsubscribe netfilter" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>

thanks a ton .

>>>>>>> Note : This does not seem to be much related to netfilter

I agree . I should not ask this kind of questions here ( any thing
other than netfilter )  ?

thanks,
ratheesh

Re: two default routes

[Richard Horton]

On 22 March 2010 15:47, Pascal Hambourg <pascal.mail@plouf.fr.eu.org> wrote:
> Note : This does not seem to be much related to netfilter.


IP Routing is about the one area of linux which lacks a decent mailing
list - the iproute/lartc mailing lists are dead/defunct/ex-mailing
lists :(

-- 
Richard Horton
Users are like a virus: Each causing a thousand tiny crises until the
host finally dies.
http://www.solstans.co.uk - Solstans Japanese Bobtails and Norwegian Forest Cats
http://www.pbase.com/arimus - My online photogallery

Re: two default routes

[Jan Engelhardt]

On Monday 2010-03-22 17:21, Richard Horton wrote:

>On 22 March 2010 15:47, Pascal Hambourg <pascal.mail@plouf.fr.eu.org> wrote:
>> Note : This does not seem to be much related to netfilter.
>
>
>IP Routing is about the one area of linux which lacks a decent mailing
>list - the iproute/lartc mailing lists are dead/defunct/ex-mailing
>lists :(

NF will happily jump in :)

Re: two default routes

[Pascal Hambourg]

ratheesh k a écrit :
> On Mon, Mar 22, 2010 at 9:17 PM, Pascal Hambourg
>
> I agree . I should not ask this kind of questions here ( any thing
> other than netfilter )  ?

It's fine with me if no one else complains. There is a more general
Linux networking list, linux-net, which does not seem to receive much
traffic though.