[Bug 1721092] Re: systemd-resolved and dns-masq make CPU 100% when using lxc name resolution

[Alex Garel]

Hello,

I tried to get DNS packets using :

$ sudo tcpdump -i lxcbr0 -l -vvv dst host 10.0.3.1 and dst port 53 |tee
/tmp/tcpdump-dns

I then wait until systemd-resolved ate 100% of CPU. But at this time
(unfortunately when I noticed it, it may have been eaten CPU for quite a
while). However I can't see any strange exchange between systemd-
resolved and dnsmasq.

I attach all the packets captured by tcpdump.

I'm not an expert in networks topics but I'm at ease with linux and cli
commands, so feel free to tell me which experiment you would like me to
run.

** Attachment added: "tcpdump session between dnsmasq and systemd-resolved"
   https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1721092/+attachment/5044944/+files/tcpdump-dns

-- 
You received this bug notification because you are a member of Ubuntu
Foundations Bugs, which is subscribed to systemd in Ubuntu.
https://bugs.launchpad.net/bugs/1721092

Title:
  systemd-resolved and dns-masq make CPU 100% when using lxc name
  resolution

Status in systemd package in Ubuntu:
  Triaged

Bug description:
  This bug may be a duplicate of
  https://bugs.launchpad.net/ubuntu/+source/dnsmasq/+bug/1688364 but I'm
  not sure, hence this new entry.

  It hits while I wanted systemd to resolve the lxc domain to my
  internal lxc hosts.

  Step to (maybe) reproduce:

  - having lxc installed
  - make some user space lxc container
  - uncomment the line LXC_DOMAIN="lxc" in /etc/default/lxc-net
  - restart lxc-net service

  Now the dnsmasq on 10.0.3.1 should be a dns resolving lxc names.

  - add a file /etc/systemd/resolved.conf.d/lxc.conf

    [Resolve]
    DNS=10.0.3.1
    Domains=~.lxc
    DNSSEC=false

  - restart systemd-resolved service

  Now systemd knows it should ask .lxc names to dnsmasq

  - start an lxc container, let assume it's called my-container
  - ping it using my-container.lxc it should work

  After some times, systemd-resolved should use 100% CPU.

  Commenting the /etc/systemd/resolved.conf.d/lxc.conf file and
  restarting it makes systemd-resolved never eating resources again.

  ProblemType: Bug
  DistroRelease: Ubuntu 17.04
  Package: systemd 232-21ubuntu5
  Uname: Linux 4.10.16-041016-generic x86_64
  ApportVersion: 2.20.4-0ubuntu4.5
  Architecture: amd64
  CurrentDesktop: GNOME
  Date: Tue Oct  3 18:18:56 2017
  InstallationDate: Installed on 2015-11-10 (692 days ago)
  InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Release amd64 (20151021)
  MachineType: Intel Corporation Skylake Platform
  ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.10.16-041016-generic root=UUID=6814e3c1-8cea-4ecc-964d-535fd18782e9 ro quiet splash crashkernel=384M-:128M vt.handoff=7
  SourcePackage: systemd
  UpgradeStatus: Upgraded to zesty on 2017-02-25 (219 days ago)
  dmi.bios.date: 11/06/2015
  dmi.bios.vendor: American Megatrends Inc.
  dmi.bios.version: 5.11
  dmi.board.asset.tag: Base Board Asset Tag
  dmi.board.name: WhiteTip Mountain1 Fab2
  dmi.board.vendor: Topstar
  dmi.board.version: RVP7
  dmi.chassis.asset.tag: Default string
  dmi.chassis.type: 9
  dmi.chassis.vendor: Default string
  dmi.chassis.version: Default string
  dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr5.11:bd11/06/2015:svnIntelCorporation:pnSkylakePlatform:pvr0.1:rvnTopstar:rnWhiteTipMountain1Fab2:rvrRVP7:cvnDefaultstring:ct9:cvrDefaultstring:
  dmi.product.name: Skylake Platform
  dmi.product.version: 0.1
  dmi.sys.vendor: Intel Corporation

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1721092/+subscriptions