[Bug 1624071] [NEW] libnss-resolve: Fallback from resolve to dns breaks DNSSEC validation

[Anders Kaseorg]

Public bug reported:

The libnss-resolve postinst script inserts ‘resolve’ before ‘dns’ in the
hosts line of /etc/nsswitch.conf.  This makes DNSSEC validation
impossible, even with DNSSEC=yes in /etc/systemd/resolved.conf, because
if libnss_resolve returns a validation failure, glibc will simply fall
back to libnss_dns.  It also makes NXDOMAIN lookups twice as slow.

The following syntax would preserve the fallback in the case that
systemd-resolved is not running at all, but allow systemd-resolved to
fail lookups that should fail when it is running:

hosts:          files resolve [!TRYAGAIN=return] dns

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1624071

Title:
  libnss-resolve: Fallback from resolve to dns breaks DNSSEC validation

Status in systemd package in Ubuntu:
  New

Bug description:
  The libnss-resolve postinst script inserts ‘resolve’ before ‘dns’ in
  the hosts line of /etc/nsswitch.conf.  This makes DNSSEC validation
  impossible, even with DNSSEC=yes in /etc/systemd/resolved.conf,
  because if libnss_resolve returns a validation failure, glibc will
  simply fall back to libnss_dns.  It also makes NXDOMAIN lookups twice
  as slow.

  The following syntax would preserve the fallback in the case that
  systemd-resolved is not running at all, but allow systemd-resolved to
  fail lookups that should fail when it is running:

  hosts:          files resolve [!TRYAGAIN=return] dns

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