Thursday, September 21, 2017

Solaris Multicast Deafness Bug

Once again, the mighty Dave Zabel (of two different fames) has found another Multicast-related bug, this time in Solaris.  I think that recent versions of Solaris fix it, and I don't have the energy to track down *when* they fixed it, but if you have Solaris servers that you haven't kept updated for a while, you might have this bug.


You'll need Informatica's "mtools" package for Solaris.  These are great tools offered for free in both binary and source form at

And you'll need two hosts: A and B.  Host B should be Solaris 6.10 that hasn't been updated in a long time.  Host A can be anything.

1. On host A, run this:
    msend 12000 15

2. On host B, open two windows.  In the first, enter this:
    mdump 12000

Admire the printouts of the multicast packets for a while.  Isn't technology wonderful?  :-)

3. In a second host B window, enter:
    mdump 12000

Note that the first window continues to print, but the second window is silent.  No surprise; it is listening to a different and unused multicast group!  Of course it is silent.

4. Kill that second mdump.

WHOA!  The first mdump stops printing!  It went deaf to  When trying this same experiment on Linux, or on our Solaris 5.11 machines, it does not go deaf.  But we have several old, non-updated 5.10 machines where the first mdump does go deaf on this step.

5. Enter:
    netstat -g

The OS still thinks it is listening to the multicast group.

6. Enter:
    snoop -P host

The packets are still being received!  But they aren't being delivered to the first mdump.

7. Enter:
    mdump 12000

WHOA!  The first mdump starts printing again!  The second mdump is still silent since there still isn't any traffic on its multicast group.


Maybe PEBKAC?  Or a bug in mtools?

Nope.  Let's start over and try it again with a small change in step 3:

1. On host A, run this:
    msend 12000 15

2. On host B, open two windows.  In the first, enter this:
    mdump 12000

3. In a second host B window, enter:
    mdump 12000

See what I did there?  I changed the 128 to 64.  As before, the first window continues to print, but the second window is silent.

4. Kill that second mdump.

Lookie there!  The first mdump continues to print the messages.  No deafness.


Well, I'm not sure, but I think it's got to be related to multicast group aliasing.  Remember that there are 2**28 different IP multicast groups.  But what about Ethernet?  There are only 2**23 Ethernet multicast MAC addresses allocated for use by IP multicast.  It turns out that and map to the same Ethernet multicast MAC address: 01-00-5E-00-03-13.

The IGMP protocol doesn't care about that; host B still tells the switch which multicast groups are subscribed, and it treats and as different.  But when the IP layer interfaces with Ethernet, it needs to program the NIC with the same multicast MAC address for those two IP groups.  And apparently older versions of Solaris didn't do the book keeping right.

I've tried this experiment on other OSes and they all work as you would expect (no deafness).  Our Solaris 5.11 machine does it right.  And even a recently-installed 5.10 system works right.  But older systems that haven't been updated in a while all have this problem.


The obvious moral is to update your systems.

But even then, you should avoid using multicast groups that alias on top of each other.  The whole point of multicast is that you don't receive packets that you aren't interested in.  But if you have traffic published to both and,  a host subscribing to only one of them will get data for both.  The IP layer will do the right thing (discard the undesired packets), but it still produces an unnecessary load.


Sure.  Watch out for well-known and ad-hoc multicast protocols in the range -  Are any of those in use anywhere on your network?  No?  Are you sure they never will be?

Look at the multicast group we tested with:  That aliases on top of, which is in an ad-hoc1 range labeled "RFE Generic Service".  I don't know what that is (and Google doesn't seem to know either), but I'm thinking I want to avoid aliasing, even if low probability.

You should be fine if you use multicast groups between -

Oh, and update your systems too.  Good hygiene and all that.


We've upgraded one of our "problem servers" to the latest Solaris 5.11 and it fixed the deafness problem.

I'm not interested in figuring out exactly which minor release they fixed it in.

No comments: