build-sys: warn if people don't change the default NTP servers when building systemd

Also, explain the situation in the docs.

Relates to #437

1 files changed, 19 insertions, 0 deletions

diff --git a/DISTRO_PORTING b/DISTRO_PORTING
index d8e9ded943..07aea865be 100644
--- a/DISTRO_PORTING
+++ b/DISTRO_PORTING
@@ -14,6 +14,7 @@ HOWTO:
             --with-kbd-loadkeys=
             --with-kbd-setfont=
             --with-tty-gid=
+            --with-ntp-servers=
 
         2) Try it out. Play around (as an ordinary user) with
         '/usr/lib/systemd/systemd --test --system' for a test run
@@ -21,6 +22,24 @@ HOWTO:
         print the initial transaction it would execute during boot-up.
         This will also inform you about ordering loops and suchlike
 
+NTP POOL:
+
+        By default, timesyncd uses the Google NTP servers
+        time[1-4].google.com. They serve time that is not standards
+        compliant, and can be up to .5s off. Google does not
+        officially support these servers for the broader
+        audience. Distributions and vendors really should not ship
+        OSes or devices with these NTP servers configured. Instead,
+        please register your own vendor pool at ntp.org and make it
+        the built-in default by passing --with-ntp-servers= to
+        configure. Registering vendor pools is free:
+
+        http://www.pool.ntp.org/en/vendors.html
+
+        Again, if you ship your software or device with the default
+        NTP servers, then you will get served wrong time, and will
+        rely on services that might not be supported for long.
+
 CONTRIBUTING UPSTREAM:
 
         We generally do no longer accept distribution-specific