Base system: Ubuntu 12.04.2
Samba4 version: latest git

I just tried to add a second DC to an existing Samba4 domain using samba-tool
But the join failed, complaining about being unable to find a writeable DC:

root at elektron:~# /opt/samba4/bin/samba-tool  domain join linex.r00t.la DC
Finding a writeable DC for domain 'linex.r00t.la'
ERROR(exception): uncaught exception - Failed to find a writeable DC for domain 'linex.r00t.la'
  File "/opt/samba4/lib/python2.7/site-packages/samba/netcmd/__init__.py", line 175, in _run
    return self.run(*args, **kwargs)
  File "/opt/samba4/lib/python2.7/site-packages/samba/netcmd/domain.py", line 552, in run
    machinepass=machinepass, use_ntvfs=use_ntvfs, dns_backend=dns_backend)
  File "/opt/samba4/lib/python2.7/site-packages/samba/join.py", line 1082, in join_DC
    machinepass, use_ntvfs, dns_backend, promote_existing)
  File "/opt/samba4/lib/python2.7/site-packages/samba/join.py", line 73, in __init__
    ctx.server = ctx.find_dc(domain)
  File "/opt/samba4/lib/python2.7/site-packages/samba/join.py", line 246, in find_dc
    raise Exception("Failed to find a writeable DC for domain '%s'" % domain)


Looking a little closer, I think the problem is IPv4/IPv6 related:
The existing DC has both IPv4 and IPv6 address (and both are available
via Samba4's internal DNS.
Unfortunately even if the client does not own an IPv6 address (see
further down below) samba-tool still tries to address the DC's LDAP
server via IPv6 - and fails miserably:

root at elektron:~# strace -f -e trace=network /opt/samba4/bin/samba-tool  domain join linex.r00t.la DC
<...>
[pid  1640] socket(PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 5
[pid  1640] connect(5, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.6")}, 16) = 0
[pid  1640] send(5, "NQ\1\0\0\1\0\0\0\0\0\0\5venus\5linex\4r00t\2la"..., 37, MSG_NOSIGNAL) = 37
[pid  1640] recvfrom(5, "NQ\205\200\0\1\0\1\0\0\0\0\5venus\5linex\4r00t\2la"..., 1500, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("192.168.1.6")}, [16]) = 53
Process 1635 suspended
[pid  1640] +++ killed by SIGKILL +++
Process 1635 resumed
--- SIGCHLD (Child exited) @ 0 (0) ---
socket(PF_INET6, SOCK_DGRAM, IPPROTO_IP) = 5
setsockopt(5, SOL_IPV6, IPV6_V6ONLY, [1], 4) = 0
connect(5, {sa_family=AF_INET6, sin6_port=htons(389), inet_pton(AF_INET6, "2001:xxxx:xxxx:xxxx::2", &sin6_addr), sin6_flowinfo=0, sin6_scope_id=0}, 28) = -1 ENETUNREACH (Network is unreachable)
ERROR(exception): uncaught exception - Failed to find a writeable DC for domain 'linex.r00t.la'
  File "/opt/samba4/lib/python2.7/site-packages/samba/netcmd/__init__.py", line 175, in _run
    return self.run(*args, **kwargs)
  File "/opt/samba4/lib/python2.7/site-packages/samba/netcmd/domain.py", line 552, in run
    machinepass=machinepass, use_ntvfs=use_ntvfs, dns_backend=dns_backend)
  File "/opt/samba4/lib/python2.7/site-packages/samba/join.py", line 1082, in join_DC
    machinepass, use_ntvfs, dns_backend, promote_existing)
  File "/opt/samba4/lib/python2.7/site-packages/samba/join.py", line 73, in __init__
    ctx.server = ctx.find_dc(domain)
  File "/opt/samba4/lib/python2.7/site-packages/samba/join.py", line 246, in find_dc
    raise Exception("Failed to find a writeable DC for domain '%s'" % domain)

root at elektron:~# ip a s
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 08:00:27:9e:df:48 brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.10/24 brd 192.168.1.255 scope global eth0