Zeroconfig and Multicast DNS

[Pat Lashley]

> >> Howevery, your statement above brings up a question, do you assume
> >> that a system configured with lla should be able to communicate
> >> with a system configured via dhcp?
> >
> > Yes, of course. The question is basically the same as whether hosts on
> > the same link but different IP (sub)net ranges should be able to
> > communicate with each other. The answer is that either both hosts must
> > implement ARP/RARP functionality, or that there be at least one
> > additional host with addresses in both ranges that is willing to act as
> > a router.
>
> Of course it's possible with a router, but what I was after, was the
> situation when a host is configured with LLA but without a default route
> , should such host be able to communicate with other hosts on the
> same link that has addresses configured in other ranges (obtained by
> other utilities, dhcp, static etc).

That's one of those policy decisions for which there is no clear correct answer.

> If the answer to that is yes, how is it supposed to work?
> Should the routing code be change to always issue a ARP request when
> the source is from 169.254/16 ?

If the source is from an LLA, then the destination machine should already have 
the ARP info, extracted from the packet itself.  The problem crops up when a 
non-LLA host wants to initiate a connection to an LLA host.

> The responding host will have to
> implement the same algorithm or it will just send the packet to its
> default router which probably wouldn't know what to do with the packet
> and throw it away. Could be pretty ugly.

The default router is probably also the default router for LLA hosts; so it 
would send it to the right machine. (Unless local administrative policy 
prevents LLA hosts from having a default router...) But that extra hop is a bit 
ugly.


But the whole 'direct communication between LLA and non-LLA on the same link' 
discussion is really a side issue. It should only come up for us in a scenario 
where we want to have a completely zeroconf FreeBSD machine (using LLA) in an 
environment with non-LLA machines. If the FreeBSD machine is getting a static 
or DHCP address and wants to talk to LLA machines; then it should also also 
obtain an LLA address as an alias so that it can readily talk to both subnets. 
(This is one of the reasons that I'm campaigning so strongly for doing the LLA 
addresses as an alias even if there is some other address available.)


The difficulty, even for a pure LLA-scenario, is the case of a multi-homed host 
where more than one interface is using LLA, and the interfaces are on separate 
links.  The problem is that you may have different hosts with the same link 
local IP address on each link.  That raises some really ugly issues.  One of 
the simplest is that for a multi-homed host, we can't just add '169/254/16 
LINK#x' for each interface to the routing table and expect the right thing to 
happen.


> I also don't see how RARP would help because that would require the
> host to have knowledge of the other hosts MAC-address.

Yep, definitely less helpful than ARP. But setups that use ARP often also 
enable RARP; so I tend to mention them together.



-Pat