[ovs-discuss] find controller using DNS Server-Discovery?

Ben Pfaff blp at nicira.com
Fri Jun 18 20:05:09 UTC 2010

On Thu, Jun 17, 2010 at 06:18:30PM -0700, Neil McKee wrote:
> Was DNS Server-Discovery ever considered as a way for an Open VSwitch
> to find it's controller?
> We just implemented this for the host-sflow daemon and it seems to
> work well.  The daemon is identical on every server, but you can
> change the settings for the whole data-center just by tweaking one
> file on the DNS server.  For example, if /var/named/mycompany.com.zone
> is given:
> _sflow._udp   300  SRV     0 0 6343  collector1
> _sflow._udp   300  SRV     0 0 6343  collector2
> _sflow._udp   300 TXT     (
> "txtvers=1"
> "polling=20"
> )

I'm not sure whether you know that OVS already has a way to do
controller discovery.  It does.  OVS can use a vendor extension to DHCP
to discover its controller.  The protocol is described in the
ovs-openflowd manpage:

The original motivation for controller discovery was the idea of
allowing configuration of individual OpenFlow switch to be completely
automatic and hands-off.  You just plug Ethernet and power cables into
the switch and let it find its controller across the network
automatically, with no user interface at the switch and nothing else to
do.  This goal meant that we needed the switch to figure out its own IP
address, which means DHCP.  Adding the controller configuration to the
DHCP reply seemed like an obvious approach.  Also, DHCP already had
provisions for relaying across L2 boundaries, which enterprise switches
support, so it was nice to be able to piggyback on that.

SRV records sound like another reasonable approach.  You do need to
figure out what SRV record to look up.  How do you do that for sflow?

But the elephant in the room here, in my opinion, is that, as far as I
know, no one is using discovery for OVS anyhow.  If they were, then we'd
be getting bug reports or questions about it, and I don't think I've
ever received one.  I use discovery myself in my own test setup, so I
know that it works, but I doubt that anyone else uses it.  So I'm not
too enthused about enhancing or reworking discovery.

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