[Babel-users] atomic updates and ipv6 cache flushing issues

[Leen Besselink]

Hi,

On Sun, May 05, 2013 at 06:53:50PM +0100, Robert Bradley wrote:
> On 05/05/13 16:59, Juliusz Chroboczek wrote:
> >>Robert bradley has been working on making atomic route updates work
> >>and possibly better route cache flushing.
> >[...]
> >>get the patchset from: git://github.com/rb12345/babeld.git
> >While it hasn't rained yet, I've looked at the code.
> >
> >1. The commit "Atomic route modification in kernel_netlink" contains
> >both the atomic route modification code and code for flushing the
> >routing cache.  I'd like the two to be separated.  (I can do that if
> >you want, Robert, and leave your name as the original author of the
> >commit.)
> 
> Yes, that's fine.  I think that the cache-flushing part may not be
> needed anyway.
> 
> >4. Is the cache flushing still needed with Baptiste's patch?  I'd
> >rather not be tweaking /proc files at runtime.
> >
> >-- Juliusz
> 
> I think the cache flushing is a misunderstanding on my part.  I
> added the cache flushing since
> http://linux-ip.net/html/linux-ip.html and
> http://lartc.org/howto/lartc.rpdb.html appeared to imply it might be
> needed after local route updates.  Looking at the iproute2 manpages
> closely though, it appears that it is only needed if source-policy
> rules ("ip rule" equivalents) are involved.  Without source-policy
> rules, cache flushing looks to be completely unnecessary.
> 
> The only interaction between this and Baptiste's patch is that the
> emptier caches mean it has fewer routes to filter out.
> 

Today I had some time to play with this code and 1.4.0.

In my simple/stupid/silly ? test case it performed worse.

The Babel 1.4.0 running on Ubuntu 13.04 / Linux 3.8.0 generic amd64 can
detect when 2 way communication is not possible.

But the one at https://github.com/rb12345/babeld does not do this.

It will just keep sending traffic to a gateway that can't receive any traffic.

I looked if there is any other change between 1.4.0 and the other code and there
was one, but it didn't improve things.

___

Ignore IPv6 routing cache on linux

Recent linux kernels (roughly 3.x) keep a routing cache for IPv6: each
time a host is contacted, a temporary /128 route is added. These
routes are visible using "ip -6 route show cache".

Asking babeld to redistribute all IPv6 routes known to the host, using
"redistribute ip ::/0", causes it to announce and de-announce these
/128 routes frequently, leading other nodes to mark them as
unreachable (thus preventing communication with the host described by
the /128 route, even though less specific routes are available).

https://github.com/jech/babeld/commit/73d8f6621425c624864e63df98c7e49893da09a3

___

My test-case is:

I create 2 VMs with 2 NICs eth1 and eth2 (eth0 is the 3rd NIC for SSH) and connect them to 2 networks.

Then I run these commands:

(i'm impatient so my intervals are really low)

On both VMs:

ifconfig eth1 up
ifconfig eth2 up
./babeld - -u -d 1 -H 0.3 -C redistribute proto 4 eth1 eth2

(proto 4 is static)

on VM1:
ip -6 addr add 2001:200::1/64 dev lo
ping6 -i 0.1 -n 2001:200::2

on VM2:
ip -6 addr add 2001:200::2/64 dev lo

Then with tcpdump I check on what interface traffic is coming in.

If it's eth2 I drop incoming traffic on eth2:

ip6tables -I INPUT -i eth2 -j DROP

It detects it as a failed 2-way communication and the routes in the VMs change to reflect that.

Then I run:

iptables -F INPUT #flush all
ip6tables -I INPUT -i eth1 -j DROP

Again it detects it as a failed 2-way communication and the routes in the VMs change to reflect that.

I hope this is useful to you.

Have a nice weekend,
	Leen.

> -- 
> Robert Bradley
> 
> 
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