As seen in bug 532367 / bug 579092, we need to call res_init() if resolv.conf changes, to notice the new nameserver addresses. (We need to do this at the GResolver level rather than just saying the apps should call res_init() themselves, because in the case of GUnixResolver, calling res_init() in the main process isn't going to do anything to the forked-off resolver processes.) (Also, I'm assuming this is a unix-only thing and we don't need to do anything special on Windows at all.)

Assuming we don't want to wait for D-Bus support in glib so we can listen for the NetworkManager signal, we could either stat() /etc/resolv.conf on each call, or else set up a GFileMonitor on it. Each is wasteful in its own way, but I think I prefer the former.

I'm working on a patch, though the GUnixResolver bits are a bit awkward and will require some testing. (We need to free the existing _g_asyncns_t and create a new one, but the old one may have queries pending on it, so we need to wait until they complete before freeing it.)