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

[Anders Kaseorg]

You’re right: glibc seems to treat the absence of libnss-resolve itself
as UNAVAIL, which is the same code returned on DNSSEC validation
failures when libnss-resolve is working.  I don’t see a way around this
other than patching libnss-resolve to return NOTFOUND (or TRYAGAIN?) on
validation failure.

It looks like there may be other ways for an active attacker to force a
TRYAGAIN code (with a response that doesn’t fit in the caller-provided
buffer), which suggests that the right configuration is
[!UNAVAIL=return], not merely [NOTFOUND=return].

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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:
  Triaged

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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