[CentOS] how to increase DNS reliability?

Thu Jul 25 23:31:26 UTC 2019
Nataraj <incoming-centos at rjl.com>

On 7/25/19 1:10 PM, hw wrote:
>> Configure all dns servers as primary slaves (plus 1 primary master) for
>> your own domains.  I have never seen problems with resolution of local
>> dns domains when the Internet was down.
> It seemed to have to do with the TTL for the local names being too
> short and DNS being designed to generally query root servers rather
> than sticking to their local information.

It has nothing to do with the ttl. The TTL does cause expiration in an
authoritative server.  TTLs only affect  caching servers.  The primary
master gets changed when you edit the local zone database.  The
secondary slave gets updated when the serial number in the SOA record on
the primary master gets bumped.   You must either do that manually or
use a zone database management tool that does it for you.

If a dns server is configured as a primary master or a secondary slave
for a domain, then it is authoritative for that domain and does not
require queries to any other server on your network or on the Internet. 
The difference between a primary master and a secondary slave is the
primary master is where you edit the zone records and the secondary
slave replicates the zone database from the primary master.  Even if the
primary master goes down, the secondary slave still has a copy of the
zone files in it's disk files (or other database format that you
configure) and will server them flawlessly.

One way to see if a server is properly configured as authoritative for a
domain is:

nataraj at pygeum:~$ dig mydomain.com. soa @

; <<>> DiG 9.11.3-1ubuntu1.8-Ubuntu <<>> mydomain.com. soa at
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 52104
;; flags: qr *aa* rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 4

; EDNS: version: 0, flags:; udp: 4096
; COOKIE: 64f402c0c22d57aa2bbb10fc5d3a340d8c19377b924d01c2 (good)
;mydomain.com.            IN    SOA

Mydomain.Com.        14400    IN    SOA    ns1.mydomain.com.
postmaster.Mydomain.COM. 2019072505 1200 600 15552000 14400

Mydomain.Com.        14400    IN    NS    ns1.Mydomain.Com.
Mydomain.Com.        14400    IN    NS    ns2.Mydomain.Com.
Mydomain.Com.        14400    IN    NS    ns3.Mydomain.com.

ns1.mydomain.com.        14400    IN    A

;; Query time: 1 msec
;; WHEN: Thu Jul 25 15:58:21 PDT 2019
;; MSG SIZE  rcvd: 243

The AA flag in the flags section tells you that you have queried a dns
server that is authoritative for the domain that you queried.  If it
doesn't have the AA flag then you have not properly set up the primary
master or secondary slave for that domain.

If your masters and slaves are all configured correctly for a domain
then they will all have the same serial number  in the SOA record (and
same results for any query in that domain).  If they don't then
something is wrong and your zone transfers are not occuring properly.

>> Depending on the size of your network, you can run a caching server on
>> each host (configured as a primary slave for your own domains) and  then
>> configure that local server to use forwarders.  When you use multiple
>> forwarders the local server does not have to wait for timeouts before
>> querying another server.  Then you just run 2 or more servers to use for
>> forwarding.  Use forward-only to force all local servers to use only
>> forwarding (for security and caching reasons).  Much simpler than using
>> keepalived.
> Hm.  I thought about something like that, but without the separation
> into local slaves using forwarders and the forwarders.  I will
> probably do that; it seems like the most reasonable solution, and I
> should have at least one forwarder anyway so as not to leak
> information to the internet-only VLANs.  It would be an improvement in
> several ways and give better reliability.

The local server can have forward-only either on or off.  If off, It
will go out directly to the Internet if it does not receive a response
from a forwarder.  Using forward only and putting your forwarders on a
seperate network away from your inside network means if there is a
security hole in the nameserver, your inside hosts are less likely to be
compromised.    You could also configure your ISP's or google or other
public recursive name servers as forwarders if you don't want to run
your own.

> It doesn't really help those clients I can not run name servers on,
> though.
> > In recent years I *have not had any* problems with bind9 or
>> powerdns crashing.
>> As far as using the ISC server vs powerdns, you may want to check on
>> peoples recent experiences.  There was a time when many thought powerdns
>> had much better performance and fewer security issues.  For various
>> reasons  I've seen some people including myself, switch back to ISC
>> bind9.  I switched about 1.5 years ago because I was getting better
>> performance from bind9.  You may want to check out other peoples
>> experience before switching to powerdns.
> Bind has been around for ages, and I've never had any issues with it
> for the last 25 years or so.  Just set it up and let it do its thing,
> and it does.
> If there were performance problems, I imagine they would be more
> likely due to insufficient internet bandwidth.  Perhaps it would take
> all the DNS queries that come up during a week or even a month to
> arrive within a second before any performance considerations become
> relevant ...

Exactly, a simple bind9 configuration is adequate unless you run an
application with huge numbers of DNS queries.

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