This bug was fixed in the package dnsmasq - 2.76-5ubuntu0.2
---------------
dnsmasq (2.76-5ubuntu0.2) zesty; urgency=medium
* Fix replying prematurely if one of many servers replies REFUSED
(LP: #1726017) by adding an upstream patche.
- 2.77: 68f6312d4b: Stop treating SERVFAIL as a successful response from
upstream servers.
-- Christian Ehrhardt <christian.ehrha...@canonical.com> Mon, 23 Oct
2017 08:48:44 +0200
** Changed in: dnsmasq (Ubuntu Zesty)
Status: Fix Committed => Fix Released
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https://bugs.launchpad.net/bugs/1726017
Title:
dnsmasq prematurely returns REFUSED, breaking resolver
Status in dnsmasq package in Ubuntu:
Fix Released
Status in dnsmasq source package in Xenial:
Fix Released
Status in dnsmasq source package in Zesty:
Fix Released
Bug description:
[Impact]
* DNS name resolution fails in certain network configurations, where
different DNS servers are responsible for different domains and one or
more servers reply REFUSED to queries that regard other domains than
their own. Without the patch, dnsmasq returns a negative reply to
if only one such negative answer is received from a forwarder, even
if other forwarders return valid responses.
This breaks
the resolver and practically all internet connectivity, including web
browsing, email, and receiving updates.
* This should be backported to stable to fix internet connectivity
for users.
* The patch fixes the problem by querying all servers and only returning
a negative reply to the requestor only if *all* forwarders return negative
responses.
[Test Case]
* It should be possible to test this in a virtual network. One DNS server
should be responsible for queries to the outside world, and the other one
could be a DHCP/DNS instance (perhaps dnsmasq, also) that handles internal
IP addresses and names. It's important that at least one of these servers
return REFUSED to queries that don't belong into its realm (assuming the
domain name is "my.net", the server for "my.net" would reply REFUSED to
"ubuntu.com" and every other domain. I am not sure if this is normally the
case, all I can say is that my Linux-based ASUS router does it.
Connect an Ubuntu VM to this network.
To aggravate the problem, the DHCP server would put the internal DNS
server first in the nameservers field. If that's the case, the problem
would also occur if the client used "strict-order" in dnsmasq.conf.
[Regression Potential]
* I don't see any. Would there be networks where admins rely upon getting
NXDOMAIN back if just one server fails for a DNS query? I don't know.
* [racb] As the behaviour in the area of REFUSED and SERVFAIL is
being changed, it's probably worth checking during SRU verification
that dnsmasq correctly passes back successful, REFUSED, SERVFAIL,
zero-answer and 1+ answer responses in the simple, single upstream DNS
server case. If there is a regression introduced by these patches, it
is likely to be in the area of handling SERVFAIL, REFUSED and
successful replies.
[Other Info]
Original bug description follows.
Seen with dnsmasq 2.75-1ubuntu0.16.04.3, after Trusty->Xenial update.
In my local network, I have two DNS servers; 192.168.1.1 is the local
DHCP/DNS server configured to reply to queries inside the local
network, and 192.168.1.4 is the forwarder in my DSL Router,
responsible to answer queries about the outside world. THe DHCP server
returns these in the order 192.168.1.4,192.168.1.1. The internal
server replies REFUSED to queries about external domains.
This configuration has worked well with Ubuntu 14.04 and other Linux
Distros (using Fedora and OpenSUSE internally here), as well as
various other OSes.
It does not work with Ubuntu 16.04. NetworkManager's dnsmasq instance
pushes the REFUSED reply from 192.168.1.1 to applications and ignores
the successful reply from 2.168.1.4. This causes all DNS queries to
external servers to fail.
I believe this is fixed in dnsmasq 2.76 and related to
http://lists.thekelleys.org.uk/pipermail/dnsmasq-
discuss/2016q1/010263.html
http://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=commitdiff;h=68f6312d4bae30b78daafcd6f51dc441b8685b1e
http://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=object;h=4ace25c5d6
According to these sources, the bug was introduced with
http://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=object;h=51967f9807665dae403f1497b827165c5fa1084b
In my local setup at least, I can work around the problem by using the
"strict-order" option to dnsmasq.
echo strict-order >/etc/NetworkManager/dnsmasq.d/order.conf
But that's not a general solution. If dnsmasq has several forwarders,
and some return SERVFAIL or REFUSED and others return SUCCESS, the
successful answer should be returned to clients, independent of the
strict-order setting.
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