Dear Maintainer,

dhclient provides a --dad-wait-time option which suggests that the
dhclient-script should not return immediately after configuring a new
address, but should wait for Duplicate Address Detection (DAD) to
complete.  The upstream example dhclient-script does so [0], as does
the Fedora version based on it.  The Debian version, however does not
[1].

This means that dhclient -6 can easily return with the discovered
address in tentative state, and therefore unusable.  This can be
reproduced easily using the "pasta" tool (in 'passt' package) which
will create a network namespace and includes a built-in DHCPv6 server:

Note that the manual ifup and sleep here is to work around a different
bug [3].

$ pasta -I exampleif -- sh -c "ip link set exampleif up; sleep 2; dhclient -6 exampleif; ip -6 a show dev exampleif -dynamic scope global"
Multiple default IPv6 routes, picked first
2: exampleif: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 1000
    inet6 2001:4479:e300:7200:222:4dff:fea6:88f3/128 scope global tentative
       valid_lft forever preferred_lft forever

Note that the assigned address is tentative (and therefore not
usable).  If we add an additional sleep to wait for DAD to complete:

$ pasta -I exampleif -- sh -c "ip link set exampleif up; sleep 2; dhclient -6 exampleif; sleep 2; ip -6 a show dev exampleif -dynamic scope global"
Multiple default IPv6 routes, picked first
2: exampleif: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 1000
    inet6 2001:4479:e300:7200:222:4dff:fea6:88f3/128 scope global
       valid_lft forever preferred_lft forever

.. the address becomes non-tentative.

[0] https://github.com/isc-projects/dhcp/blob/572032cb0e514606559de3784e3f7ca8e1539d17/client/scripts/linux#L183
[1] https://salsa.debian.org/debian/isc-dhcp/-/blob/master/debian/dhclient-script.linux?ref_type=heads#L398
[3] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1085229

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Dear submitter,

as the package isc-dhcp has just been removed from the Debian archive
unstable we hereby close the associated bug reports.  We are sorry
that we couldn't deal with your issue properly.

For details on the removal, please see https://bugs.debian.org/1143544

The version of this package that was in Debian prior to this removal
can still be found using https://snapshot.debian.org/.

Please note that the changes have been done on the master archive and
will not propagate to any mirrors until the next dinstall run at the
earliest.

This message was generated automatically; if you believe that there is
a problem with it please contact the archive administrators by mailing
ftpmaster@ftp-master.debian.org.

Debian distribution maintenance software
pp.
Thorsten Alteholz (the ftpmaster behind the curtain)