On Tue, 31 May 2016 11:34:41 +0200 Martin Pitt <martin.p...@ubuntu.com> wrote:
> Hello all, > > yesterday I landed [1] in Yakkety which changes how DNS resolution > works -- i. e. how names like "www.ubuntu.com" get translated to an IP > address like 1.2.3.4. > > Until now, we used two different approaches for this: > > * On desktops and touch, NetworkManager launched "dnsmasq" configured > as effectively a local DNS server which forwards requests to the > "real" DNS servers that get picked up usually via DHCP. Thus > /etc/resolv.conf said "nameserver 127.0.0.1" and it was rather > non-obvious to show the real DNS servers. (This was one of the > complaints/triggers that led to creating this blueprint). But > dnsmasq does proper rotation and fallback between multiple > nameservers, i. e. if one does not respond it uses the next one > without long timeouts. > > * On servers, cloud images etc. we did not have any local DNS server. > Configured DNS servers (via DHCP or static configuration in > /etc/network/interfaces) were put into /etc/resolv.conf, and > every program (via glibc's builtin resolver) directly contacted > those. > > This had the major drawback that if the first DNS server does not > respond (or is slow), then *every* DNS lookup suffers from a ~ 10s > timeout, which makes every network operation awfully slow. > Addressing this was the main motivation for the blueprint. On top > of that, there was no local caching, thus requesting the same name > again would do another lookup. > > As of today, we now have one local resolver service for all Ubuntu > products; we picked "resolved" as that is small and lightweight, > already present (part of the systemd package), does not require D-Bus > (unlike dnsmasq), supports DNSSEC, provides transparent fallback to > contacting the real DNS servers directly (in case anything goes wrong > with the local resolver), and avoids the first issue above that > /etc/resolv.conf always shows 127.0.0.1. > > Now DNS resolution goes via a new "libnss-resolve" NSS module which > talks to resolved [2]. /etc/resolv.conf has the "real" nameservers, > broken name servers are handled efficiently, and we have local DNS > caching. NetworkManager now stops launching a dnsmasq instance. > > I've had this running on my laptop for about three weeks now without > noticing problems, but there may well be some corner cases where this > causes problems. If you encounter a regression that causes DNS names > to not get resolved correctly, please do "ubuntu-bug libnss-resolve" > with the details. > > Thanks, > > Martin > > [1] > https://blueprints.launchpad.net/ubuntu/+spec/foundations-y-local-resolver > [2] This is configured in /etc/nsswitch.conf ("hosts: files ... > resolve dns") How will this work on the phone if it is only enabled in yakkety? Ho w will this affect landing phone silos? Have you tests with a 3g/4g dongle so you have 2 dns servers up at the same time, how is suspend, reboot and flight mode scenarios handled? I think that is all the questions I can think of :) -- You Make It, I'll Break It! I Love My Job :) http://www.canonical.com http://www.ubuntu.com
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