multipath routing doesn't work with netlink

multipath routing doesn't work with netlink

[Rik van Riel]

Hi,

Am I doing something wrong or is there a kernel bug that
prevents multipath routing from working with NETLINK ?

# ip route add default via 10.0.16.1
# ip route add default via 10.0.17.2
RTNETLINK answers: File exists
# route add default gw 10.0.17.2
#

... of course, this last command just works and results in
a routing table like this:

# ip route
10.0.16.0/21 dev eth0  proto kernel  scope link  src 10.0.17.3
127.0.0.0/8 dev lo  scope link
default via 10.0.17.2 dev eth0
default via 10.0.16.1 dev eth0

Is there a fundamental reason that iproute2 (using NETLINK) should
fail here, or is this a bug for which patches will be accepted ?

regards,

Rik
-- 
A: No.
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Re: multipath routing doesn't work with netlink

[Julian Anastasov]

	Hello,

On Mon, 28 Oct 2002, Rik van Riel wrote:

> Am I doing something wrong or is there a kernel bug that
> prevents multipath routing from working with NETLINK ?

	Not bug but a hidden feature :) You can define
alternative default routes with "ip route append".

> # ip route add default via 10.0.16.1
> # ip route add default via 10.0.17.2
> RTNETLINK answers: File exists

	And this is "ip route prepend" (NLM_F_REQUEST|NLM_F_CREATE
without NLM_F_APPEND):

> # route add default gw 10.0.17.2
> #

> Is there a fundamental reason that iproute2 (using NETLINK) should
> fail here, or is this a bug for which patches will be accepted ?

	You can reach exactly the text explaining these features
by searching for "prepend" in ip-cref.tex. Note that the paragraph
recommends to avoid them, may be something that Alexey will change
one day :)

> regards,
>
> Rik

Regards

--
Julian Anastasov <ja@ssi.bg>

Re: multipath routing doesn't work with netlink

[Rik van Riel]

On Mon, 28 Oct 2002, Julian Anastasov wrote:
> On Mon, 28 Oct 2002, Rik van Riel wrote:
> > # ip route add default via 10.0.16.1
> > # ip route add default via 10.0.17.2
> > RTNETLINK answers: File exists
>
> 	And this is "ip route prepend" (NLM_F_REQUEST|NLM_F_CREATE
> without NLM_F_APPEND):

"ip route prepend" doesn't work here, but "ip route append" does
exactly what is wanted.

Thank you,

Rik
-- 
Bravely reimplemented by the knights who say "NIH".
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Re: multipath routing doesn't work with netlink

[Julian Anastasov]

On Mon, 28 Oct 2002, Rik van Riel wrote:

> "ip route prepend" doesn't work here, but "ip route append" does
> exactly what is wanted.

	Try harder, it should work. Of course, append/prepend
fail too if the same route exists, they simply use different
keys for route uniqueness.

Regards

--
Julian Anastasov <ja@ssi.bg>

Re: multipath routing doesn't work with netlink

[Andi Kleen]

There are two kinds of multipath routing in the kernel (which are differently
implemented in the kernel and do different things): 

- default route failover. You configured it. It doesn't do any load balancing,
but when your default route stops working it'll eventually try the other
one. You configured that.

- real multipath routing.
It is configured by setting multiple nexthops to a single route
(ip route add ... nexthop ... )
The kernel will do load balancing by route (=srcip/dstip/tos tripple)

-Andi

Re: multipath routing doesn't work with netlink

[jamal]

On Wed, 30 Oct 2002, Andi Kleen wrote:

> - real multipath routing.
> It is configured by setting multiple nexthops to a single route
> (ip route add ... nexthop ... )
> The kernel will do load balancing by route (=srcip/dstip/tos tripple)
>

The only caveat is of course you have to wait for the cache entry to
expire ;-> (or ARPs to fail etc). I have a patch that will force specific
cache entries to be deleted via netlink; if theres a fireman/woman with
some time in their hand and willing to write something in user space,
contact me ;->

cheers,
jamal