Multicast group memberships lost if eth0 brought down and up

[Brian Fahrlander]

    OK, I've been working a lot a sleeping a little; I think my head is
now clear enough to respond to this:


On Sat, 2004-10-02 at 06:49, Ted Kaczmarek wrote:
> On Fri, 2004-10-01 at 12:25 -0400, Deron Meranda wrote:
> > Is this the correct behavior, or a bug?  (seeing this in both FC2
> > as well as EL3)
> > 
> > I wrote an application which registers itself in a couple IPv6
> > multicast groups on an ethernet interface.  A "netstat -A inet6 -g"
> > then shows the group membership, such as
> > 
> > IPv6/IPv4 Group Memberships
> > Interface       RefCnt Group
> > --------------- ------ ---------------------
> > lo              1      ff02::1
> > eth0            1      ff02::eb42:8740
> > eth0            1      ff02::f6b6:d980
> > eth0            1      ff02::1:ff13:7276
> > eth0            1      ff02::1
> > 
> > If I then leave the application running, but do
> > an "ifdown eth0" then "ifup eth0", I get this,
> > 
> > IPv6/IPv4 Group Memberships
> > Interface       RefCnt Group
> > --------------- ------ ---------------------
> > lo              1      ff02::1
> > eth0            1      ff02::1:ff13:7276
> > eth0            1      ff02::1
> > 
> > Are group memberships supposed to be preserved across
> > up/down cycling?  If not, then what's the proper way for an
> > application to detect that this has occurred so it can re-register?
> > Note that the app can still successfully SEND packets to the
> > group, but obviously, it no longer receives any packets.
> > 

> up/down cycling of what?
> Most switches will and should flush all tables on a port state change.
> I am assuming you are in a switch that is doing igmp snooping as well.
> What you probably need is for your app to detect link loss and send a
> new join when the link comes back. I have seen what I consider many
> broken applications that have this same issue. You may want to read up
> on how igmp and Pim work, and how the network equipment you are using
> implements it. Some vendors allow for you to manually join a group, this
> will force that port to always get the multicast data for that group.
> 
> Also if come up with a good negative ack model , which is really the
> proper approach, it should recover in a relatively fast time on any
> up/down cycling assuming the network gear is set up right and working
> properly.


     CAUGHT YA!  You're using Multicast on Linux.  After all these
years, I've finally found the 'one guy' actually using what, to me, is
the coolest feature of Linux: multicast with no artificial limitations.

    1. What are ya using it for? (Hope it's not just NTP)

    2. How can I get involved?

    3. Why does the topic seem all-but-dead?

    4. Does any software really _work_ with multicast other than NTP?

-- 
------------------------------------------------------------------------
Brian Fahrländer                  Christian, Conservative, and Technomad
Evansville, IN                                 http://www.fahrlander.net
ICQ 5119262
AIM: WheelDweller
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