[Sugar-devel] Notes on service discovery XS/XO

[Jonas Smedegaard]

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On Mon, Apr 20, 2009 at 04:08:30PM +0200, Martin Langhoff wrote:
>On Mon, Apr 20, 2009 at 3:53 PM, Jonas Smedegaard <dr at jones.dk> wrote:
>> DNS-SD using unicast DNS seems reasonable to me too.
>
>If we can do without the avahi gunk, and use it in a way that is not 
>optimised for user driven browsing but for automated selection of 
>services, then it might work.
>
>> Looking closer at the RFC, the initial service queries do have an 
>> added overhead in that a layer of indirection is used (not SRV -> A, 
>> but instead PTR -> SRV + TXT -> A).  But standard DNS optimizations 
>> apply, so SOA record should allow clients to preserve bandwidth 
>> through caching.
>
>Can we teach dnsmasq to push all the relevant records with the SOA 
>record?

I don't understand your question.  Sounds like prefetching that isn't 
part of dns (id you perhaps think of DHCP here?)

As I understand dns, if you request a SOA record, then you get a SOA 
record and only that.  You can request the whole domain, but that does 
not seem bandwidth saving to me.

The main bandwidth saver, I believe, is that of sane query caching and 
knowledge of the segments of the domain structure.


>> In other words: Install dnsmasq on the XOs, use plain standard DNS 
>> internally and on the wire, setup DNS-SD entries in a standard 
>> nameserver on the XS, and extend Sugar to support DNS-SD.
>>
>> I'd be happy to help compose standard BIND9 files, if that is what 
>> will be used on the XS.
>
>If we have a dnsmasq resident expert, I rather use your help 
>transitioning to dnsmasq (note - with several bits of weird dhcp 
>rules). There is no upside to BIND and plenty of downsides, starting 
>with the >25MB memory footprint.

If that's me you titulate so nicely, then you are way too kind:

I have only little experience with dnsmasq used as a caching-only name 
server, i.e. zero experience with dnsmasq as a "real" name server 
containing local data.

You are right that BIND9 is a bastard with memory consumption, and it 
makes sense to use dnsmasq on the XS.  I just didn't think of that - I 
suggested it as a caching-only on the XOs.  ISC DHCPd has a complex 
macro language, however, which might be the upsight you cannot live 
without. Beware that it is DHCP, not DNS ;-)

It seems from its changelog that dnsmasq supports DNS-SD since version 
2.36.


Tell me more specifically what you fear can be tricky to handle in 
dnsmasq, either DHCP or DNS parts, and I shall have a look if I am any 
good at helping out.


  - Jonas


[1] 
http://twistedmatrix.com/pipermail/twisted-python/2003-July/004909.html

- -- 
* Jonas Smedegaard - idealist og Internet-arkitekt
* Tlf.: +45 40843136  Website: http://dr.jones.dk/

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