[Bug 1785383] Re: missing EDNS0 record confuses systemd-resolved

[Brian Murray]

The SRU of systemd for Hirsute (and all its accompanying bugs) looks
good to me, but I'd like to see this fix merged for Impish at least per
SRU policy.

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Title:
  missing EDNS0 record confuses systemd-resolved

Status in systemd:
  Fix Released
Status in dnsmasq package in Ubuntu:
  Fix Released
Status in systemd package in Ubuntu:
  Triaged
Status in dnsmasq source package in Bionic:
  Triaged
Status in systemd source package in Bionic:
  In Progress
Status in dnsmasq source package in Focal:
  Fix Released
Status in systemd source package in Focal:
  In Progress
Status in dnsmasq source package in Groovy:
  Fix Released
Status in systemd source package in Groovy:
  In Progress
Status in dnsmasq source package in Hirsute:
  Fix Released
Status in systemd source package in Hirsute:
  In Progress
Status in dnsmasq source package in Impish:
  Fix Released
Status in systemd source package in Impish:
  Triaged

Bug description:
  [Impact]
  dnsmasq 2.79 and below omits EDNS0 OPT records when returning an empty answer for a domain it is authoritative for. systemd-resolved seems to get confused by this in certain circumstances; when using the stub resolver and requesting an address for which there are no AAAA records, there can sometimes be a five second hang in resolution.

  [Fix]
  This is fixed by upstream commit http://thekelleys.org.uk/gitweb/?p=dnsmasq.git;a=commit;h=1682d15a744880b0398af75eadf68fe66128af78

  Not sure if it is worth cherry picking? I imagine the most likely
  trigger will be dnsmasq on routers which are not likely to be running
  Ubuntu, but maybe just in case.

  I also think there are some logic issues in systemd-resolved, upstream
  bug filed:

  https://github.com/systemd/systemd/issues/9785

  [Test Case]
  Simple-ish test case for bionic:

  ---
  IFACE=dummy0
  SUBNET=10.0.0

  ip link add $IFACE type dummy
  ifconfig $IFACE ${SUBNET}.1/24
  dnsmasq -h -R -d -C /dev/null -2 $IFACE -z -i $IFACE -I lo --host-record=test.test,${SUBNET}.1 --server=/test/ &

  dig -t a test.test @10.0.0.1 | grep EDNS
  # should return "; EDNS ..."
  dig -t aaaa test.test @10.0.0.1 | grep EDNS
  # again, should return "; EDNS ..." but doesn't
  ---

  To reproduce the systemd-resolved side of the problem

  ---
  # as above, but
  # now configure systemd-resolved to look at only 10.0.0.1, then

  systemd-resolve --reset-server-features
  # should exhibit five second delay then connect, assuming sshd is running :)
  ssh test.test
  ---

  
  More detailed test case for focal and later:

  install dnsmasq on a bionic system and start it, listening to an
  interface that is externally reachable, e.g. for a normal libvirt vm
  with interface name 'ens3':

  IFACE=ens3
  dnsmasq -h -R -d -C /dev/null -2 $IFACE -z -i $IFACE -I lo --host-record=test.test,1.2.3.4 --server=/test/

  note that the '1.2.3.4' address doesn't matter, any addr is ok.

  then setup a test system that can reach the dnsmasq system, and
  configure networkd to use the dnsmasq server, e.g. using config like:

  [Match]
  Name=ens3

  [Network]
  DHCP=yes
  DNS=DNSMASQ_IP_ADDRESS
  Domains=test

  [DHCPv4]
  UseDNS=no
  UseDomains=no

  replace 'DNSMASQ_IP_ADDRESS' with the addr of the bionic system where
  dnsmasq is running, and replace 'ens3' with whatever the test system
  interface name is. Then restart systemd-networkd, and test:

  systemd-resolve --reset-server-features
  systemd-resolve --flush-caches
  host test.test

  The lookup using 'host' should complete immediately;.

  [Discussion]
  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: dnsmasq-base 2.79-1
  ProcVersionSignature: Ubuntu 4.15.0-23.25-generic 4.15.18
  Uname: Linux 4.15.0-23-generic x86_64
  ApportVersion: 2.20.9-0ubuntu7.2
  Architecture: amd64
  Date: Sat Aug  4 11:33:56 2018
  InstallationDate: Installed on 2018-05-31 (64 days ago)
  InstallationMedia: Xubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
  ProcEnviron:
   TERM=xterm
   PATH=(custom, no user)
   LANG=en_GB.UTF-8
   SHELL=/bin/bash
  SourcePackage: dnsmasq
  UpgradeStatus: No upgrade log present (probably fresh install)

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