multicast arp entry

[Ruslan Ermilov]

On Tue, May 18, 2004 at 04:55:14PM +0800, Eugene Grosbein wrote:
> Hi!
> 
> route -n monitor shows me:
> 
> got message of size 236 on Tue May 18 16:42:26 2004 
> RTM_ADD: Add Route: len 236, pid: 0, seq 0, errno 0,
> flags:<UP,HOST,DONE,LLINFO,WASCLONED,MULTICAST>
> locks:  inits:
> sockaddrs: <DST,GATEWAY,IFP,IFA>
>  224.0.0.9 1.0.5e.0.0.9 em3:0.7.e9.1f.f1.de 172.20.2.75
> 
> After that arp -an shows:
> (224.0.0.9) at 01:00:5e:00:00:09 on em3 permanent [ethernet]
> 
> Then tcpdump shows that multicast packets with source IP of interface em1
> (those must be directed via em1) go out through em3. I run quagga/ripd
> (same effect for zebra) on FreeBSD 4.9-STABLE.
> 
> How do I find who installs this route?
> 
The short answer is: the kernel adds it for you, automatically,
just as it does this for you for normal unicast destinations.

The long answer could be: you could join to a single multicast
group on multiple interfaces, and you will be able to receive
multicast on all of them, but if you don't have multicast
forwarding enabled, only one interface will be used for sending.
Which one gets used will be determined by a normal routing
lookup, i.e., ``route -vn get -host 224.0.0.9'' where no entry
yet exists.  Then, when the actual packet gets delivered, the
kernel will insert the corresponding ARP entry, mapping the
multicast group address to a MAC address.


Cheers,
-- 
Ruslan Ermilov
ru at FreeBSD.org
FreeBSD committer
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