Re: [BUG] net/ipv4: inconsistent routing table

IMO, the routing decision is determined, given a specific routing
table and local network the result MUST be determined, independence of
how/what order the routing entry is added.

Now there are two ways to configure the system resulting EXACTLY the
same routing table and local addresses, but the routing decision is
totally different.

SAME routing table, DIFFERENT routing decision, there MUST be bugs in kernel.

On Thu, Aug 6, 2015 at 3:43 PM, Alexander Duyck
[off-list ref] wrote:

On 08/06/2015 03:13 AM, Zang MingJie wrote:

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On Thu, Aug 6, 2015 at 1:45 AM, Alexander Duyck
[off-list ref] wrote:

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On 08/05/2015 02:06 AM, Daniel Borkmann wrote:

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[ please cc netdev ]

On 08/05/2015 10:56 AM, Zang MingJie wrote:

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Hi:

I found a bug when remove an ip address which is referenced by a
routing
entry.

step to reproduce:

ip li add type dummy
ip li set dummy0 up
ip ad add 10.0.0.1/24 dev dummy0
ip ad add 10.0.0.2/24 dev dummy0

Okay, so up to this point you have 2 addresses on the same subnet that
are
now on dummy0.

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ip ro add default via 10.0.0.2/24

This makes the default route go through 10.0.0.2.

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ip ad del 10.0.0.2/24 dev dummy0

Then you remove 10.0.0.2 from the local system, however since 10.0.0.1 is
on
the same subnet dummy0 would still be the correct interface to access
10.0.0.2 it is just no longer local to the system.

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after deleting the secondary ip address, the routing entry still
pointing to 10.0.0.2

You didn't delete the default routing entry so why would you expect it to
change?  All you did is remove 10.0.0.2 from the local system.  I believe
the assumption is that 10.0.0.2 is still out there somewhere, it just
isn't
on the local system anymore.

Yes, 10.0.0.2 is migrated to somewhere else

The address might have migrated, but the interface is still up and 10.0.0.1
is still present on the same subnet.  Because you made a local address the
default gateway the assumption is any routes not specifically called out on
other interfaces are directly accessible to this interface.

The bug indicates that the kernel is doing something to make the table
inconsistent, but a default route that is a local interface address does
essentially the same thing.

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# ip ro
default via 10.0.0.2 dev dummy0
10.0.0.0/24 dev dummy0  proto kernel  scope link  src 10.0.0.1

This matches up with what I would expect.  10.0.0.2 is the default
gateway
and it is accessible from dummy0 since 10.0.0.0/24 is accessible from
dummy0.

This means 0.0.0.0/0 is accessible via 10.0.0.2 on the network of dummy0

Yes, but at the time you specified it 10.0.0.2 was a local address which
belonged to dummy0.  This means that dummy0 can access anything not
specified elsewhere via pretty much any address it wants.  So it is
perfectly valid if it wants to use a source address of 10.0.0.1 to send
packets to 1.1.1.1 over dummy0.

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but actually, kernel considers the default route is directly connected.

# ip ro get 1.1.1.1
1.1.1.1 dev dummy0  src 10.0.0.1
      cache

I'm not sure how you came to the "directly connected" conclusion. It is
still routing things out through 10.0.0.2 from 10.0.0.1.

Maybe your example would work better if you used 10.0.0.1 and 10.0.1.1
instead.  Then I think you might be able to better see that when you
delete
the second address the route would be broken.

No, it isn't. when ping 1.1.1.1, kernel will directly send arp request
braodcast to 1.1.1.1, this is not what I expect. it should send arp
request to 10.0.0.2, following should be the correct routing entry:

# ip ro get 1.1.1.1
1.1.1.1 via 10.0.0.2 dev dummy0  src 10.0.0.1
     cache

I see what you are trying to say, but the example provided is a bit lacking.
Assuming you could ping 1.1.1.1 via dummy0 before with 10.0.0.2 as your
default gateway, that shouldn't change if 10.0.0.2 is migrated to another
address.  That is, unless there is an issue on the system 10.0.0.2 was
migrated to.

Now if I move away from using dummy interface and instead using a real
network interface things can get a bit more interesting.  So if we follow
your example and use 2 different subnets on the two systems then pings
continue to work after we remove the addresses.  However if we flip things a
bit and add the default route, and then the local address for the gateway
they don't.  So something like below:
        ip li set eth0 up
        ip ad add 10.0.0.1/24 dev eth0
        ip ro add default via 10.0.0.2
        ip ad add 10.0.0.2/24 dev eth0

What you end up with is eth0 sending arp requests looking for 10.0.0.2 even
though it is a local address on the system.

My question would be what is the correct behavior for this?  If a local
address is removed or added that is being used as a gateway address should
we delete the route, or update the scope of the next hop?

- Alex