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

Ole Traupe ole.traupe at tu-berlin.de
Mon Jan 4 17:23:50 UTC 2016

Hi all,

Wish you a happy new year altogether!

Mathias, James, let me first say that I highly appreciate your help with 
all your testing and writing up your thoughts.

Here are my responses:

A. I have no different sites, no various subnets; so I don't really know 
what to do.

B. I don't understand the purpose of setting my domain up with different 
sites with associated networks, if on failure the client looks into 
Default-First-Site-Name, where all DCs are known, and is able to pick a 
working one. Because all my DCs already are in one place (i.e. 

C. After I followed Rowland's advice to add an NS record for the 2nd DC, 
and after manually creating all the other DNS records for my 2nd DC, 
logon from Windows clients works with the 1st DC offline (as I stated 
earlier). "set" shows logon to 2nd DC, and I get no type 11 login event 
(I get type 2: interactive). This works "right away" - if that term 
allows for ignoring the fact that the first login took 74 seconds 
(according to Windows logs).

D. My remaining problem is that while Windows (7) clients appear to be 
somewhat flexible, this clearly is not the case for my Linux member 
servers. They insist on failing to ssh-login after some 20 or more 
seconds. However, I can 'kinit' now - thanks to the created DNS records, 
I suppose.

I wonder, whether this might be due to TTL (time-to-live) settings of 
DNS records, or DNS request timeouts accumulating (and hitting an ssh 
login time threshold?). I noticed that I can change the default TTL 
settings of an SOA - with the effect the the actually TTL of *newly 
created* DNS records is set accordingly. However, to test this, I would 
have to recreate all the DNS records for my DCs again. It is likely I'll 
try this sooner or later.


Am 23.12.2015 um 16:51 schrieb mathias dufresne:
> Hi all,
> Firs I apologize I did not manage to find time to reply earlier.
> The initial issue was about how Samba AD react when one DC is out and 
> more specifically about what happen when FSMO ower is unreachable 
> (poweroff in Ole tests).
> This issue is solved using a correct AD Sites configuration.
> Here I kept 3 DCs in my domain.
> Sites:
> I set up a second site named "authentication" and I've added in that 
> site 2 DCs, including FSMO owner.
> On that "authentication" site I've added the network on which my 
> clients are.
> On "Default-First-Site-Name" I do not configured any network 
> addresses. All 3 DCs are declared in "Default-First-Site-Name".
> All DC up behaviour:
> The windows client when connecting ask to AD for a DC, AD answer this 
> client it depends of "authentication" site and this client re-launch 
> DNS requests taking in account AD Site information to retrieve DC list 
> for that site.
> Then the client connects to AD.
> To check which AD DC was used to connected on: launch "cmd" then in 
> that window type "set". In "set" result there is a line
> This is the DC used for connection (and later normally).
> The test:
> I powered off both DC in "authentication" site. So only one DC is up 
> and running, the one *outside* of that site.
> Login in Windows works.
> Using cmd -> set -> LOGONSERVER=<the last remaining DC>
> I waited something like one hour then I rebooted my MS Windows client. 
> I can still log in using differnt accounts (administrator then my own 
> account).
> This MS Windows client is still using the only running DC, the one 
> outside of client's site, this because this DC is in 
> Default-First-Site-Name.
> This behaviour is normal. It is Site behaviour. It is AD Sites purpose.
> So configuring AD Sites correctly would solve the issue about failover.
> DNS issue:
> SOA and NS records are used when DNS servers are discussing together 
> but not when a DNS client is asking for records (execpt for SOA or NS 
> obvisouly).
> SOA and NS records are, in my understanding, used when the DNS server 
> receive a request for a zone this server does not know. In that 
> specific case the DNS server must find a name server for that zone, so 
> our DNS server is asking to upper level for a NS in the mentioned zone.
> So, if my understanding is correct, client never uses NS record (not 
> in normal mode). And so there is no much issue that Samba DNS has only 
> one SOA and one NS.
> Regarding NS records, I create them manually because it matches our 
> real configuration (each DC is DNS, each DNS as NS record).
> The point to make Samba AD works with internal is to work around the 
> issue of samba_dnsupdate. At least it is all I have to do on all 
> domains I tested, even with the 4.3.3.
> samba_dnsupdate could work with some option in smb.conf to grant 
> unsecure update of DNS zones.
> If you don't want to allow unsecure update, use the awk script I 
> provided in that thread days (or weeks) ago.
> I use Samba AD with Internal DNS with no issue except for 
> samba_dnsupdate which not able to create internal record. To solve 
> that issue I provided here a awk script which extract from 
> samba_dnsupdate needed information to force recrods creation using 
> samba-tool. This awk script is not dangerous: you can run it as much 
> you want, it just tries to create entries. If entry exists, an error 
> is displayed:
> ERROR: Record already exists
> If you are afraid of that script you can modify its behaviour replacing:
> cmd = "samba-tool dns add "
> with
> cmd = "echo samba-tool dns add "
> Then the script will do nothing except display what command could be 
> run to force DNS records creation.
> With all needed DNS records and a well configured AD Sites failover 
> works. Nicely.
> A last note: 3 DCs means 3 DNS servers. You want your DC can be down 
> so your clients MUST have all DC declared as "nameserver" in 
> /etc/resolv.conf.
> Another strategy is to build one Bind server with one zone configured 
> with the exact same name as AD domain, this zone will do forward only 
> and will forward to all DCs.
> Doing that your clients can have only one DNS configured: the one with 
> Bind forwarding to DCs.
> This bind zone config:
> ------------------
> zone "samba.domain.tld" IN {
>   type forward;
>   forward only;
>   forwarders {
>     IP_DC1;
>     IP_DC2;
>     IP_DC3;
>   };
> };
> -------------------
> I hope you will finally be able to have failover working Ole.
> 2015-12-22 11:44 GMT+01:00 Ole Traupe <ole.traupe at tu-berlin.de 
> <mailto:ole.traupe at tu-berlin.de>>:
>                 Can I suggest that you do what I did, create your own
>                 small test domain in VMs using Bind9
>             Yes, that is a good idea. However, from what I had read
>             before, much of it on the Samba wiki, I was expecting
>             Samba4 to just work with multiple DCs. I still wonder why
>             no one ever seems to have tested or questioned that
>             (publicly). And I don't feel that I have to question
>             something myself that is broadly recommended: use the
>             internal DNS unless you really have to do otherwise (even
>             by the developers, it seems). In addition, bind9 working
>             with multiple DC's does not necessarily mean that internal
>             DNS won't.
>         I am going to discuss this with Marc and the rest of the team,
>         like you, I am surprised that nobody has raised this before. I
>         have always used Samba with Bind9, so was unaware of this
>         possible problem, it only came to head for me when you
>         mentioned it. I then found I only had one NS  record in the
>         SOA and this lead to where we are now.
>     Hi Rowland,
>     Again: thanks a lot for your support.
>     Merry Christmas and good holidays to the list!
>     Ole
>             I also feel the need to would like to state that I am a
>             part-time admin and I can't test something for a year or
>             so (like others) before I go into production. With Samba 4
>             I was rather happy to find something that won't require so
>             much work (although it feels differently now, partially
>             due to me being more or less a newbee to unix-based
>             systems, I guess).
>         It doesn't need much looking after, once you have got it up
>         and running :-)
>         Rowland
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