[Samba] Authentication to Secondary Domain Controller initially fails when PDC is offline

Thu Nov 19 17:22:32 UTC 2015

Here to avoid issue when some DC is down I rely on MS Windows client
behaviour. Windows clients are asking DNS for list of all DC (according to
their sites, relying on client IP to define on which AD site this client
is). Once the client has this list of all DC it launch some LDAP requests
(I believe) and will use as its own DC for that session one of the first
DCs which were able to reply the LDAP request.
If your client is not in default-site this chosen DC will stay in cache as
default DC during 24h if I'm not wrong. If your client is in default-site I
believe the cache is 15min, so after 15min it will relaunch the whole
discovery process.

At least, from that, sites are important : )

Now to be back on resilience question, *you must always have several DCs*
to be sure at any moment your client will always find at least one DC
available. Use VMs to lower cost, but still, build several DCs.

I have also added two DNS servers which forward all DNS queries to AD when
the requeset is about AD and if the request is not about AD zone, this
request is sent to normal DNS (those internal of the company I work for).
These two DNS share a virtual IP and I have an awful hand-writed-script to
move that VIP from on DNS to the other.

Why these two DNS?
I don't want to change clients configuration, they will continue to use
main DNS servers (those from the company I work for). These main DNS
servers will forward all requeset for AD zone to AD DNS servers (in fact to
my two little DNS servers and these little DNS servers will forward request
to AD DNS servers)
I don't want (and I won't be able) to change the list of DC up and running
on main DNS servers. And that list is bound to change, so I need a way to
change it.

This is a bit heavy and I'm not yet sure to keep on the long run. But for
now it seems to work...

Cheers,

mathias

2015-11-19 16:25 GMT+01:00 Ole Traupe <ole.traupe at tu-berlin.de>:

> Ok, I see. Nevertheless, thank you very much for your effort!
>
> I must say that I can't actually believe that no one knows an answer to
> this problem. It must affect MANY people using Samba DCs. According to all
> the tests on the wiki, everything is working fine. Then I pull the plug on
> my first DC and no one can log on. And this time I waited far longer than
> the suggested "refresh interval" of 15 min - even longer than the value
> called "TTL" in the GUI of 1h. I also tried "ipconfig /flushdns" on my
> windows client. Does not improve the situation. Only a reboot solves the
> issue. But that would be no acceptable practice for Linux member servers.
> And it doesn't seem to help, anyway (just tried this).
>
> It is one of the first and most important tests for a domain to see what
> happens if the first DC is down. Without a working take-over, other DCs are
> nothing more than backup (replication) targets, and the domain is not
> fail-safe.
>
> This can't be the end of the story, right?
>
> Ole
>
>
>
>
> Am 19.11.2015 um 14:04 schrieb mathias dufresne:
>
> No idea about your main issue, I was merely answering to your last
> question about changing SOA record.
>
> Here is another view of that command:
> samba-tool dns update <server> <zone> <name> SOA \
> 'OLDnameserver email serial refresh retry expire minimumttl' \
> 'NEWnameserver email serial refresh retry expire minimumttl'
>
> I'm not too confident with DNS internals so I'm not sure if the TTL you
> mentioned is or isn't "expire" or "minimumttl".
>
> After digging a little bit it seems previous line is completely wrong,
> neither "expire" nor "minimumttl" are "TTL".
> This because :
> dig -t SOA SAMBADOMAIN.TLD
> ...
> samba.domain.tld. 1715 IN      SOA     DC1.samba.domain.tld. 62 900 600
> 86400 3600
> ...
>
> And from what I just read in dig "ANSWER SECTION" the second field is the
> TTL, so 1715 in my case, which as nothing to do with "expire" (86400) or
> "minimumtll" (3600).
>
> And that makes me wondering how TTL can be less than "minimumttl"...
>
> So, the short way: the command I gave do not seem to be designed to help
> you changing TTL. Sorry : )
>
> Cheers,
>
> mathias
>
> 2015-11-19 13:43 GMT+01:00 Ole Traupe <ole.traupe at tu-berlin.de>:
>
>> Mathias, thank you very much for your comprehensive instructions!
>>
>> Just one question: Harry suggested that, in order to overcome the below
>> DNS related problems, the TTL would have to be adjusted (lowered). However,
>> the TTL seems to be the only time value not covered by the command provided
>> by you.
>>
>> Is it really the TTL that is the culprit or is it rather the first time
>> value (something like "Refresh value" in english)?
>>
>> Do you know this?
>>
>> Ole
>>
>>
>>
>> Am 19.11.2015 um 11:19 schrieb mathias dufresne:
>>
>> Hi Ole,
>>
>> You want to change SOA of your AD domain?
>>
>> Here some working command:
>> samba-tool dns update <working DC> samba.domain.tld \
>> samba.domain.tld SOA \
>> 'oldSOA.samba.domain.tld. hostmaster.samba.domain.tld. 58 900 600 86400
>> 3600' \
>> 'newSOA.samba.domain.tld. hostmaster.saba.domain.tld. 59 900 600 86400
>> 3600' -k yes
>>
>> This needs you performed some kinit before using an account able to
>> modify this entry (by default only administrator is able to that I expect).
>>
>> This must be done for the two DNS zones of your domain:
>> samba.domain.tld + _msdcs.samba.domain.tld
>>
>> First number of replacement record (here "59") is serial number.
>> Replication of change seemed to work without changing that serial number
>> but as DNS love to rely on it, changing that serial should be a good idea.
>>
>> Hoping this helps...
>>
>> Cheers,
>>
>> mathias
>>
>>
>> 2015-11-18 16:44 GMT+01:00 Ole Traupe < <ole.traupe at tu-berlin.de>
>> ole.traupe at tu-berlin.de>:
>>
>>>
>>> It is DNS related.
>>>>
>>>> What is the best way of dealing with this?
>>>>>
>>>> The *best way* is a HA solution for your DNS Servers, but its expensive.
>>>>
>>>> The DNS client (resolver) caches the srv records for 15 minutes aka 900
>>>> seconds.
>>>>
>>>> ipconfig /flushdns drops the cache. Reboot does the same.
>>>>
>>>> On server side you may set shorter TTL for the server records, but then
>>>> you have more DNS traffic. On small netwoks (sites up to 20 clients, no
>>>> wifi) I have good experience with a TTL of 180.
>>>>
>>>
>>> Harry, I tried this - unsuccessfully.
>>>
>>> I have TTL settings in a) the SOA and b) the NS record of the FQDN and
>>> the _msdcs.FQDN sections in my Windows RSAT DNS console. None of these 4
>>> entries I can change: I get something like "The Source Of Authority (SOA)
>>> cannot be updated. The record already exists."
>>>
>>> Do you have an idea how to accomplish this? Currently the setting is 1h,
>>> which is pretty long.
>>>
>>> Ole
>>>
>>>
>>>
>>>
>>> --
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>>>
>>
>>
>>
>
>