[Bug 1629797] Re: resolve service in nsswitch.conf adds 25 seconds to failed lookups before systemd-resolved is up

[Dan Watkins]

Adding After=systemd-resolved.service to /lib/systemd/system/cloud-
init.service causes the following in journalctl:

Oct 03 12:05:04 yakkety-161003-0945 systemd[1]: basic.target: Found ordering cycle on basic.target/start
Oct 03 12:05:04 yakkety-161003-0945 systemd[1]: basic.target: Found dependency on cloud-init.service/start
Oct 03 12:05:04 yakkety-161003-0945 systemd[1]: basic.target: Found dependency on systemd-resolved.service/start
Oct 03 12:05:04 yakkety-161003-0945 systemd[1]: basic.target: Found dependency on basic.target/start
Oct 03 12:05:04 yakkety-161003-0945 systemd[1]: basic.target: Breaking ordering cycle by deleting job cloud-init.service/start
Oct 03 12:05:04 yakkety-161003-0945 systemd[1]: cloud-init.service: Job cloud-init.service/start deleted to break ordering cycle starting with basic.target/start

** Description changed:

- Seems yakkety is consistently taking 2-3 minutes to boot on EC2 and GCE,
- compared to the ~30 seconds of the first boot and ~10 seconds thereafter
- in xenial.
+ During boot, cloud-init does DNS resolution checks to if particular
+ metadata services are available (in order to determine which cloud it is
+ running on).  These checks happen before systemd-resolved is up[0] and
+ if they resolve unsuccessfully they take 25 seconds to complete.
+ 
+ This has substantial impact on boot time in all contexts, because cloud-
+ init attempts to resolve three known-invalid addresses ("does-not-
+ exist.example.com.", "example.invalid." and a random string) to enable
+ it to detect when it's running in an environment where a DNS server will
+ always return some sort of redirect.  As such, we're talking a minimum
+ impact of 75 seconds in all environments.  This increases when cloud-
+ init is configured to check for multiple environments.
+ 
+ This means that yakkety is consistently taking 2-3 minutes to boot on
+ EC2 and GCE, compared to the ~30 seconds of the first boot and ~10
+ seconds thereafter in xenial.

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https://bugs.launchpad.net/bugs/1629797

Title:
  resolve service in nsswitch.conf adds 25 seconds to failed lookups
  before systemd-resolved is up

Status in cloud-init package in Ubuntu:
  Invalid
Status in systemd package in Ubuntu:
  New

Bug description:
  During boot, cloud-init does DNS resolution checks to if particular
  metadata services are available (in order to determine which cloud it
  is running on).  These checks happen before systemd-resolved is up[0]
  and if they resolve unsuccessfully they take 25 seconds to complete.

  This has substantial impact on boot time in all contexts, because
  cloud-init attempts to resolve three known-invalid addresses ("does-
  not-exist.example.com.", "example.invalid." and a random string) to
  enable it to detect when it's running in an environment where a DNS
  server will always return some sort of redirect.  As such, we're
  talking a minimum impact of 75 seconds in all environments.  This
  increases when cloud-init is configured to check for multiple
  environments.

  This means that yakkety is consistently taking 2-3 minutes to boot on
  EC2 and GCE, compared to the ~30 seconds of the first boot and ~10
  seconds thereafter in xenial.

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