DNS hostname resolution fails intermittently

[Horst Schirmeier]

Since upgrading to 17.10, hostname resolution (via, e.g., Firefox or the "host" command-line tool) does not work reliably anymore. Once hostname resolution fails (Firefox 57: "Hmm. We're having trouble finding that site."), manually resolving the host via "systemd-resolve" succeeds and temporarily allows Firefox to also resolve that host. After a few minutes, resolution of the same host fails again.

Example with command-line "host" utility:

$ host www.heise.de
;; connection timed out; no servers could be reached
$ cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
# 127.0.0.53 is the systemd-resolved stub resolver.
# run "systemd-resolve --status" to see details about the actual nameservers.

nameserver 127.0.0.53
search fritz.box
$ systemd-resolve www.heise.de
www.heise.de: 2a02:2e0:3fe:1001:7777:772e:2:85
              193.99.144.85

-- Information acquired via protocol DNS in 273.2ms.
-- Data is authenticated: no
$ host www.heise.de
www.heise.de has address 193.99.144.85
www.heise.de has IPv6 address 2a02:2e0:3fe:1001:7777:772e:2:85
$ systemd-resolve --status
Global
         DNS Servers: 192.168.178.1
          DNS Domain: fritz.box
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      d.f.ip6.arpa
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test

Link 4 (wlp3s0)
      Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
         DNS Servers: fd00::cece:1eff:fe61:2ece

Link 3 (enp0s20f0u5c2)
      Current Scopes: none
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no

Link 2 (enp0s31f6)
      Current Scopes: none
       LLMNR setting: yes
MulticastDNS setting: no
      DNSSEC setting: no
    DNSSEC supported: no

[... wait 2-3 minutes ...]

$ host www.heise.de
;; connection timed out; no servers could be reached
$ systemd-resolve www.heise.de
www.heise.de: 2a02:2e0:3fe:1001:7777:772e:2:85
              193.99.144.85

-- Information acquired via protocol DNS in 284.2ms.
-- Data is authenticated: no
$ host www.heise.de
www.heise.de has address 193.99.144.85
www.heise.de has IPv6 address 2a02:2e0:3fe:1001:7777:772e:2:85

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: systemd 234-2ubuntu12.1
ProcVersionSignature: Ubuntu 4.13.0-16.19-generic 4.13.4
Uname: Linux 4.13.0-16-generic x86_64
ApportVersion: 2.20.7-0ubuntu3.4
Architecture: amd64
CurrentDesktop: KDE
Date: Sat Nov 18 19:32:56 2017
InstallationDate: Installed on 2016-11-26 (357 days ago)
InstallationMedia: Kubuntu 16.10 "Yakkety Yak" - Release amd64 (20161012.1)
MachineType: LENOVO 20FXS05500
ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.13.0-16-generic root=/dev/mapper/vgubuntu-root ro kopt=root=/dev/mapper/vgubuntu-root
SourcePackage: systemd
UpgradeStatus: Upgraded to artful on 2017-10-21 (28 days ago)
dmi.bios.date: 09/04/2017
dmi.bios.vendor: LENOVO
dmi.bios.version: R07ET79W (2.19 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20FXS05500
dmi.board.vendor: LENOVO
dmi.board.version: Not Defined
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: None
dmi.modalias: dmi:bvnLENOVO:bvrR07ET79W(2.19):bd09/04/2017:svnLENOVO:pn20FXS05500:pvrThinkPadT460p:rvnLENOVO:rn20FXS05500:rvrNotDefined:cvnLENOVO:ct10:cvrNone:
dmi.product.family: ThinkPad T460p
dmi.product.name: 20FXS05500
dmi.product.version: ThinkPad T460p
dmi.sys.vendor: LENOVO

[Horst Schirmeier (horst)]

Note that on my machine (clean 16.10 install, upgraded to 17.04 and later 17.10), DNS resolution already partially broke in 17.04 (involving the switch to systemd-resolve). With Ubuntu 17.04, DNS resolution was broken after suspending, moving to a different WiFi, and resuming the machine. This could be manually fixed by manually running dhclient. This does not fix the abovementioned issue, though.

[Dan Streetman (ddstreet)]

Much has changed in systemd since 2017, please update this bug if this still happens.