I have no idea if something is even possible. dnsmasq is caching dns forwarder (with some support for also master authoritative dns server) and its responsibility is to forward every one valid dns query (A or MX or AAAA or SRV or TXT ...). Dropping or rejecting queries is strictly against what such software is doing.

That depends on application and how good/poor is written. Yes, nobody can prevent or reject application behaviour if it first send DNS packet to ask for AAAA, wait for big timeout and after that new packet for A. But I hope that such applications are not there... You can also send separate packets in parallel...

Do you have particular reason why to disable IPv6? I think now it is time to start enabling it and be ready for it as giant CGNs are more and more used in IPv4 world.

That is quite tricky in Maemo...

Normally it should be done into kernel cmdline, but on N900 you need either recompile kernel or boot via usb from computer (where you can specify cmdline). So no go.

Another option is to use sysctl.conf, which is not read on Maemo. So no go.

So last option is to call it manually and because Maemo does not support SysV init scripts (Maemo has removed this support from upstart init daemon) you cannot even use /etc/init.d/ and /etc/rc*.d/.

You need to create your own upstart script which will be executed on device bootup. You need to run sysctl or write to /proc/sys/... For that you need to already have mounted procfs. procfs is mounted by /etc/init.d/rcS called by upstart script /etc/event.d/rcS. So you need to create upstart script which will be called after rcS is finished. This can be achieved by specifying "start on stopped rcS" in your upstart script.

Something like this should work:

/etc/event.d/disable_ipv6
But it is untested. So you probably need to debug...

Thanks for all your answers.

As I said I don't understand it and if things tend to work better without something I don't understand, then I try to disable it.
Now just for one scenario I encountered : while being connected to a wlan which provided ipv6 addresses (which I didn't notice at first) I connected to my home network through vpn (ipv4 only) with all traffic being redirected through the vpn. But as I had an ipv6 address some traffic was going directly to the Internet (which was bad) and some (ipv4) through my vpn server. That was on my Debian laptop. Since then I completely disabled ipv6 on the laptop and would like to do the same on my N900 to not be in the same situation in the future.
Also ipv6 caused a lot of trouble on some webapps that I use on a Debian VM ; disabling ipv6 made all the trouble go away. So for me it is better to disable ipv6 than letting it active and try to debug a problem not knowing if ipv6 has something to do with it.

When I will not have a choice and have to use ipv6 in the future I will do my best to understand it, but it is not a priority for me right now.

Will try your /etc/event.d/disable_ipv6 script.

I activated log-queries on my router (as it runs dnsmasq too) and monitored queries from my devices.

From my laptop, only ipv4 queries are made (even if weirdly I have an ipv6 address on my wlan0 interface on Debian, but I guess it's a local address).

From the N900 there are ipv6 queries first (which gets a valid response or a nodata response) and then ipv4 queries. The N900 also queries some PTR records.....

I don't have an ipv6 network at hand (on which I know ipv6 works) to test stuff, so for now it will have to wait until I get access to one.

Thanks for your answers.