[avahi] getaddrinfo (Avahi) returning unreachable IP address for .local hostname

[Martin Nyolt]

Hi Jared.

(First, please note that I'm also quite new to avahi and this mailing list. I might not have all necessary information.)

As far as I understand from that old thread, avahi publishes only one address per interface - i.e. the machine decides which of its addresses it will publish.
If the machine has two addresses in two networks (say 169.254.0.0/16 and 192.16.0.0/24), it wouldn't know which to prefer:
* there may be machines which are *only* in the 169.254.0.0/16 net (because no DHCP was available) and
*there may also be machines which are *only* in the 192.16.0.0/24 net (because DHCP was available when setting up the interface).
For this reason, mixing self-configured IPv4 169.254.0.0/16 addresses with addresses in other networks is not recommended [1]. Therefore, you should probably fix the multiple addresses.

Yet, in contrast to that old post from 2009, RFC 6762 says that a response MUST include all addresses for the interface [2]. Section 6.4 also recommends response aggregation, which aleviates the problem of waiting for multiple responses to "guarantee" an exhaustive answer.
So maybe avahi needs some re-design with regard to that RFC?

Of course, it might be possible to determine which address to return based on the source address of the querying machine. But this is not how avahi publishing and mDNS works, I think.


Best regards,
Martin


[1]: RFC 3972, Section 1.9: https://tools.ietf.org/html/rfc3927#section-1.9
[2]: RFC 6762, Section 6.2: https://tools.ietf.org/html/rfc6762#section-6.2

On 31/12/15 19:29, Jared Albers wrote:
> Hi Guys,
>
> I'm using 'getaddrinfo' to resolve .local hostnames in an application.
> However I've encountered a scenario in which the address being returned for
> a device is a self-assigned IPv4 address (169.254.X.X) that can't be
> reached on any interface by the host machine. I'm assuming this because the
> host machine has received an IPv4 address via the DHCP server and therefore
> there is no reason to also have a self-assigned address?
>
> It is my understanding that 'getaddrinfo' will ultimately use Avahi to
> resolve .local hostnames as defined in the 'hosts:' line of the
> 'nsswitch.conf' file. It is also my understanding according to this old
> thread (http://lists.freedesktop.org/archives/avahi/2009-May/001644.html),
> that by design Avahi returns only one IP address when resolving.
>
> As such, I'd like to better understand why 'getaddrinfo' is returning an
> unreachable address for a host. Is this a bug in Avahi?
>
> The device in question has three addresses (1 self-assigned IPv4, 1
> DHCP-assigned IPv4, and 1 IPv6). If by design Avahi will only return a
> single address to 'getaddrinfo', why isn't it returning a reachable address?
>
> If I force binding a self-assigned address via 'avahi-autoipd --force-bind
> eth0', then I'm able to communicate with the device. Alternatively, if I
> force communication over IPv6, I'm also able to communicate with the device.
>
> However, both of the above mentioned techniques aren't really suitable
> solutions. It seems inappropriate and potentially problematic for an
> application to bind a self-assigned IP address to an interface, and relying
> solely on IPv6 to communicate won't work if the user has disabled IPv6
> addressing (perhaps for compatibility reasons).
>
> How do I go about fixing this issue?
>
> -Jared
>
>
>
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