(usagi-users 03566) Re: Can't remove route while used ???

On Wed, Dec 07, 2005 at 05:21:04PM +0100, thus spake Ignacy Gawedzki:
> Hi all,
> 
> I've just come across a strange behavior with route removal.

Okay, I've just figured out the answer by myself. =)

> Suppose you have two nodes A and B with addresses fec0::a and fec0::b.  Each
> one of them has a route to the other.
> 
>  A % ip -6 route add fec0::b dev eth0
>  B % ip -6 route add fec0::a dev eth0
> 
> Now suppose that you have some intense traffic from A to B (TCP or UDP, it
> doesn't matter) and you need to remove the route to fec0::b on A.
> 
>  A % ip -6 route del fec0::b dev eth0
> 
> Then it usually doesn't actually remove that route, as though it was kind of
> locked by the traffic (?!).  Of course ip doesn't return any error, but
> the traffic continues and the route is still present.

Each "used" route is put into the cache and that command (ip -6 route del
fec0::b dev eth0) does only remove the first route found (if a cached version
exists, it get removed instead of the actual route in the table).  To see all
the routes, including those cached, use "ip -6 route list table all".  To
remove a route along with its cached version, use "ip -6 route flush table all
exact fec0::b dev eth0".

>  A % ip -6 route
>  ...
>  fec0::b dev eth0 metric 1024 mtu 1500 advmss 1440 hoplimit 4294967295
>  ...
> 
> When the traffic ends and you try to remove that route again, it fails at the
> first attempt and works at the second one (as though the attempt to remove the
> route while it was "used" had put it into some other "state").

The first attempt removes the cached route, while the second removes the
actual route.

> If, instead of trying to remove it while the traffic is going, you try
> to remove that route while it isn't "used", it works perfectly as expected (at
> the first attempt).

Since the route doesn't get cached.

> If you try several times while the traffic is going, you may ultimately
> succeed in removing it, but the actual number of times you have to try is not
> predictable.
> 
>  A % a=0; while echo try $((a++)); do \
>      ip -6 route del fec0::b dev eth0 || break; \
>      ip -6 route; done
> 
>  ...

The removal may succeed if no packet triggered any route lookup and caching
between two attempts.

QED

Thanks anyway. =)

-- 
To err is human, to moo bovine.