resolv: Implement trust-ad option for /etc/resolv.conf [BZ #20358]

This introduces a concept of trusted name servers, for which the
AD bit is passed through to applications.  For untrusted name
servers (the default), the AD bit in responses are cleared, to
provide a safe default.

This approach is very similar to the one suggested by Pavel Šimerda
in <https://bugzilla.redhat.com/show_bug.cgi?id=1164339#c15>.

The DNS test framework in support/ is enhanced with support for
setting the AD bit in responses.

Tested on x86_64-linux-gnu.

Change-Id: Ibfe0f7c73ea221c35979842c5c3b6ed486495ccc

1 files changed, 9 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 4e28dc473c..12b239c1fb 100644
--- a/NEWS
+++ b/NEWS
@@ -40,6 +40,15 @@ Major new features:
 
 * New locale added: mnw_MM (Mon language spoken in Myanmar).
 
+* The DNS stub resolver will optionally send the AD (authenticated data) bit
+  in queries if the trust-ad option is set via the options directive in
+  /etc/resolv.conf (or if RES_TRUSTAD is set in _res.options).  In this
+  mode, the AD bit, as provided by the name server, is available to
+  applications which call res_search and related functions.  In the default
+  mode, the AD bit is not set in queries, and it is automatically cleared in
+  responses, indicating a lack of DNSSEC validation.  (Therefore, the name
+  servers and the network path to them are treated as untrusted.)
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * The totalorder and totalordermag functions, and the corresponding