[avahi] Strange MDNS response from Axis cameras

[Iván Sánchez Ortega]

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El Miércoles 15 de Febrero de 2006 16:23, Lennart Poettering escribió:
> > - - Discard non locally routable addresses.
>
> This is not an option, since it's fine to refer to external IP
> addresses with local host names. (such as creating a local name
> "debian.local" pointing to your local Debian mirror, like
> ftp.de.debian.org)

I really meant "Discard non locally routable link-local addresses".

As the 169.254.0.0/16 IP addresses are reserverd by the ICANN for use with RFC 
3927, we can be absolutely sure that such an address will never be from a 
server outside our network (or, at least, we can safely assume so).

> The way to go is probably just to favor local IP addresses over non-locals
> if both are available.

Remember the scenario we were discussind in this thread: 50 Axis cameras in 
the same network, with mDNS (RFC 3927-compliant) and the *same* static IP 
address. Always preferring local (10.0.0.0/172.16.0.0/192.168.0.0) addresses 
over local-link (169.254.0.0) addresses is wrong.

> I guess I will implement something like this:
>
> 1. if a local/non-IPv4LL address exists, use it, quit
> 2. if a local/IPv4LL address exists, use it, quit
> 3. otherwise use what is left

IMHO the way to to is something like:

if (a 169.154.0.0/16 address exists), then
{
  if (my computer can route such addresses without going through a gateway)
  {
    return the 169.254.0.0/16 address, quit
  }
}
else if (a 10.0.0.0, 172.16.0.0 or a 196.168.0.0 address exists)
{
 return that address, quit
}
else
{
 use whatever address is left
}

> If I understand you correctly this algorithm is compatible with those
> AXIS cameras you were talking of, right?

That's right.

> > I agree with Marc: link-local addresses (169.254.0.0) are preferible than
> > static addresses.
>
> I don't think so. In my LAN I use IP addresses from the range
> 192.168.50.x and would prefer I they are used instead of the
> 169.254.x.x addresses.

But if your computer can route link-local traffic, and by any chance you have 
got two devices in the same static IP (due to hard resets to the factory 
default, for example), what should happen?

The key is to prefer link-local addresses, but *only* if your computer knows 
that link-local addresses are directly routable.

Let's say that I have a device with two addresses: 192.168.2.5 and 
169.254.5.5.

If my computer has zeroconf installed (or has a entry in the routing table 
according to http://developer.apple.com/qa/qa2004/qa1357.html ), then prefer 
the link-local address over local addresses over remote addresses.

If my computer has no zeroconf (and no such entry in the route table), then 
prefer local addresses over remote addresses over link-local addresses.

> If you want compliance with RFC 3927, you're probably better off
> installing an IPv4ll implemntation, such as Anand Kumrias "zeroconf"
> package. (yes, that package has a stupid name!)
[...]
> apt-get install zeroconf

Been there, done that, seems like a good solution to me.

- -- 
- ----------------------------------
Iván Sánchez Ortega <ivansanchez at escomposlinux.org> <i.sanchez at mirame.net>

http://acm.asoc.fi.upm.es/~mr/ 
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