[OpenWrt-Devel] Unblockable mutlicast

Daniel Dickinson openwrt at daniel.thecshore.com
Sat Sep 6 10:27:18 EDT 2014

On 06/09/14 08:07 AM, Felix Fietkau wrote:
> On 2014-09-06 08:25, Daniel Dickinson wrote:
>> If I posted from the right account, it would help for asking asking a
>> question...
>> At the risk of seeming a complete n00b despite being far from it, why am
>> I seeing this in iftop on BB-rc3 using iftop on the WAN?
>> all-systems.mcast.net       =>
>> all-systems.mcast.net      <=
>> Which whois says is an arin.net IP (which is early arpanet DoD network
>> IIRC).
>> It cannot be blocked via iptables which has me curious as to exactly
>> what kernel subsystem is doing this and why.
> It's probably IGMP traffic, used to join/leave multicast groups. No need
> to worry about this at all.
> - Felix

Since AFAIK I don't have anything doing IPv4 multicast on the wan of the
router (unless openwrt does some such) then I presume you mean that it's
random hosts broadcasting subscribe/unsubcribe to multicast groups to
anyone who will listen (and send multicast for the group to the
subscribed hosts) but is really irrelevant because nothing is doing
multicast on the wan (unless there is actual multicast usage that I
don't know about,e.g. if ipv6 stack means ipv4 multicast does get used
even though there is no ipv6 given out upstream);

I do know that the multicast forwarding is not an issue (since it's
turned off in the kernel unless you run a multicast routing daemon AIUI
(or is it now possible to do some type of minimal config via iproute?)),

I'm just was surprised to see the traffic, and haven't been able find
much (especially current) documentation of multicast on linux (and don't
hack on linux kernel networking code though I have been a consumer of
netlink when necessary).


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