Christian Biere wrote:
> Have you checked your ntp.conf? It seems one of the listed hosts 
resolves
> to an IPv6 address but you have no non-local IPv6 connectivity.

yes, it is right:
ntp.via.ecp.fr is an alias for krishna.via.ecp.fr.
krishna.via.ecp.fr has address 138.195.130.71
krishna.via.ecp.fr has IPv6 address 
2002:8ac3:802d:1243:204:e2ff:fe10:5c44

It is just confusing that those messages appear in ntpd logs only when 
ntpd is started _before_ dhclient sets interface ex0, and pppd 
establishes ppp0.  If I start or restart ntpd after ex0 and ppp0 are 
established, then those error messages do not appear.  This ipv6 address 
is unaccessible in any case.