Promote slave DNS server

[Grant Taylor]

On 08/06/2018 07:40 AM, Leroy Tennison wrote:
> If there is already an ISC document I didn't find it, please provide 
> the URL.

I'm not aware of any such best practices type document.  I too would be 
interested in reading it is it exists.

> I just added a slave of a master for disaster recovery and now need to 
> know how to promote it should the master be offline too long.

The first thing I would do is make sure the expiry timer is sufficiently 
high.  I'd likely go with something between multiple days and a month.

> What I have found so far is:
> 
> 1. For the zone definitions in /etc/named.conf  (or equivalent): 
> (a) Change the “type” statements from ”slave” to “master” 
> and remove the “masters” statement.  (b) Add “allow-update” 
> and “allow-transfer” statements as appropriate.  (c) Possibly add 
> “also-notify” statements as appropriate.  2. Add key definitions 
> if needed

I took a slightly different approach to this the last time I had this 
problem.

I created an intermediary file that was a list of the zones that I 
needed to be able to quickly switch between master and slave.  Then I 
used that file as data for a script (really different m4 macros) to 
create the proper configurations (at the same time) for both master and 
slave operating modes.  Each configuration was (effectively) stored in 
their own file; /etc/named/zones.master.conf and 
/etc/named/zones.slave.conf.  Then I would dynamically update a sym-link 
to point to the operating mode of the server.

Master:
    /etc/named/zones.conf -> /etc/named/zones.master.conf

Slave:
    /etc/named/zones.conf -> /etc/named/zones.slave.conf

My main named.conf file would then simply include the 
/etc/named/zones.conf file.

> 3. If “masterfile-format text” wasn't used in named.conf.local convert 
> the zone files to text using named-compilezone including the -j parameter.

I prefer to keep my zones in text format.  But I thought that the same 
binary file format could be used for master and slave zones.  -  I could 
be wrong about this.

> 4. If the server's name is different than the former master then the 
> SOA record for each (to be ) master zone must be updated.

IMHO the MNAME does not need to be changed.  (More details below.)

> Since rndc freeze/thaw doesn't work on slave zones the server probably 
> needs to be shut down.

You are going to need to reconfigure all the zones that you want to 
change role.  IMHO the easiest way to do that is to shut down BIND, 
change the config file (read: change where the /etc/named/zones.conf 
file points to) and restart BIND.

It may be possible to dynamically reconfigure BIND but I've never gone 
there as I found the method I'm describing to work satisfactorily well.

> 5. Change the MNAME to the new server name

I decided to go a different route with this.

I created a (bogus) record / hostname that was used specifically for the 
MNAME parameter of the SOA record.  I.e. soa.example.com

I then created (and properly delegated) the soa.example.com zone in the 
parent zone, example.com.  Then I would have different local versions of 
the soa.example.com zone on each DNS server.

That way, the parent zone would say that the MNAME was soa.example.com, 
which each local server would resolve from it's local specific version 
of the zone to itself.

> Anything I've missed?  Thanks for your help.  I also have a question 
> about DNS and keepalived but I'll make that another post.

I don't know that it applies to your example, but you may need to also 
allow dynamic updates on the new master.  This should be doable, but may 
be an added complication that you need to overcome.

The only other problem that I've seen people run into is what I call the 
"return to home problem".  How do you gracefully make the transition 
back the other way?  Especially if you have any updates that originated 
on the slave->master that need to be propagated back to the original master.



-- 
Grant. . . .
unix || die

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