networkd: various fixes for the IPv6 privacy extensions support

- Make sure that the IPv6PrivacyExtensions=yes results in
  prefer-temporary, not prefer-public.

- Introduce special enum value "kernel" to leave setting unset, similar
  how we have it for the IP forwarding settings.

- Bring the enum values in sync with the the strings we parse for them,
  to the level this makes sense (specifically, rename "disabled" to
  "no", and "prefer-temporary" to "yes").

- Make sure we really set the value to to "no" by default, the way it is
  already documented in the man page.

- Fix whitespace error.

- Make sure link_ipv6_privacy_extensions() actually returns the correct
  enum type, rather than implicitly casting it to "bool".

- properly size formatting buffer for ipv6 sysctl value

- Don't complain if /proc/sys isn't writable

- Document that the enum follows the kernel's own values (0 = off, 1 =
  prefer-public, 2 = prefer-temporary)

- Drop redundant negating of error code passed to log_syntax()

- Manpage fixes

This fixes a number of issues from PR #417

1 files changed, 13 insertions, 6 deletions

diff --git a/man/systemd.network.xml b/man/systemd.network.xml
index 85867e7f66..ff01da6249 100644
--- a/man/systemd.network.xml
+++ b/man/systemd.network.xml
@@ -393,12 +393,19 @@
         </varlistentry>
         <varlistentry>
           <term><varname>IPv6PrivacyExtensions=</varname></term>
-          <listitem><para>Configures use of stateless temporary addresses that change over time
-          (see <ulink url="https://tools.ietf.org/html/rfc4941">RFC 4941</ulink>, Privacy Extensions for Stateless Address
-          Autoconfiguration in IPv6). A boolean or <literal>prefer-temporary</literal>. When true or <literal>prefer-public</literal>
-          enables privacy extensions, but prefer public addresses over temporary addresses.
-          <literal>prefer-temporary</literal> prefers temporary addresses over public addresses.
-          Defaults to
+          <listitem><para>Configures use of stateless temporary
+          addresses that change over time (see <ulink
+          url="https://tools.ietf.org/html/rfc4941">RFC 4941</ulink>,
+          Privacy Extensions for Stateless Address Autoconfiguration
+          in IPv6). Takes a boolean or the special values
+          <literal>prefer-public</literal> and
+          <literal>kernel</literal>. When true enables the privacy
+          extensions and prefers temporary addresses over public
+          addresses. When <literal>prefer-public</literal> enables the
+          privacy extensions, but prefers public addresses over
+          temporary addresses. When false, the privacy extensions
+          remain disabled. When <literal>kernel</literal> the kernel's
+          default setting will be left in place.  Defaults to
           <literal>no</literal>.</para></listitem>
         </varlistentry>
         <varlistentry>