Re: [CentOS] how to increase DNS reliability?

[Giles Coochey]

On 26/07/2019 17:35, Nataraj wrote:


If you administer the secondary slave servers, there is no reason not to
use a very large number, 30 days or more for the SOA expiration.  Only
reason to use a lower number would be if you don't have control over the
slave servers and don't want to have old zone files that you can't update.

Another alternative, which many people did for years in the early days
when zone transfers were unreliable, is to use a script which replicates
the entire DNS configuration to the secondaries and then run all the
servers as primary masters.  If the script is written cleanly, you can
then edit the zone on any server and rsync it to the other servers.
Main thing is to prevent multiple people applying updates simultaneously.

Nataraj
PowerDNS supports MySQL backends for the zone files, so one way that 
they can work is in Native mode, as an alternative to Master / Slave, in 
which the replication and information resilience is handled by the 
backend (e.g. a MySQL cluster), and the servers just read the zone from 
the database, with no need to perform zone transfers at all. The expire 
timer in the SOA record then becomes pretty defunct, although if you 
export your zones to non-PowerDNS servers, e.g. bind, then they take effect.

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Re: [CentOS] how to increase DNS reliability?

[Warren Young]

On Jul 25, 2019, at 5:42 PM, Nataraj  wrote:
> 
> On 7/25/19 4:31 PM, Nataraj wrote:
>> It doesn't really help those clients I can not run name servers on,
>> though.
> 
> Another alternative is to look at the multicast dns (mdns) protocol.

That’s for allowing a device to self-advertise its own name, along with other 
things, like available services.  If you have such devices, then configuring 
the other machines on the network to pay attention to such advertisements 
allows them to see the new names and services when they appear.

…And much more importantly, when they *disappear*, since many 
ZeroConf/Bonjour/Avahi/mDNS speaking devices are mobile and aren’t always 
available.

This protocol is one common way for network printers to advertise their 
services, for example.  (The other common way is SMB/CIFS.)

> I'm pretty sure it's inplemented in avahi daemon

Yes, that’s an implementation of mDNS for POSIX type systems.  

> If your client supports
> it then I would think that all you have to do is enable it.

I’m not sure how this is relevant here.  For mDNS to be the solution to the 
OP’s problems, he’d have to also have mDNS multicasts going out advertising 
services, so the Avahi daemon would have something to offer when a compatible 
program comes along looking for services to connect to.

I suppose you could use mDNS in datacenter type environments, but it’s a long 
way away from the protocol’s original intent.

You could imagine a load balancer that paid attention to mDNS advertisements to 
decide who’s available at the moment.  But I don’t know of any such 
implementation.
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Re: [CentOS] how to increase DNS reliability?

[Nataraj]

On 7/26/19 6:52 AM, Giles Coochey wrote:
>
> On 26/07/2019 14:45, Leroy Tennison wrote:
>> This brings up one of the caveats for (at least ISC) DNS, if the
>> master goes down the slaves will take over for a time but eventually
>> will stop serving for the domains of the master if it remains down
>> too long.  If my (sometimes faulty) memory serves me well it is in
>> the three day range (but configurable) which is ample time unless the
>> problem occurs early in a holiday weekend and and the
>> notification/escalation process isn't what it should be (Murphey's
>> Law)...
>
> The value you refer to is the SOA record _expire_ value for a zone, I
> believe is should be set to between 14 and 28 days.
>
> https://en.wikipedia.org/wiki/SOA_record
>
>
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If you administer the secondary slave servers, there is no reason not to
use a very large number, 30 days or more for the SOA expiration.  Only
reason to use a lower number would be if you don't have control over the
slave servers and don't want to have old zone files that you can't update.

Another alternative, which many people did for years in the early days
when zone transfers were unreliable, is to use a script which replicates
the entire DNS configuration to the secondaries and then run all the
servers as primary masters.  If the script is written cleanly, you can
then edit the zone on any server and rsync it to the other servers. 
Main thing is to prevent multiple people applying updates simultaneously.

Nataraj


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Re: [CentOS] how to increase DNS reliability?

[Giles Coochey]

On 26/07/2019 14:45, Leroy Tennison wrote:

This brings up one of the caveats for (at least ISC) DNS, if the master goes 
down the slaves will take over for a time but eventually will stop serving for 
the domains of the master if it remains down too long.  If my (sometimes 
faulty) memory serves me well it is in the three day range (but configurable) 
which is ample time unless the problem occurs early in a holiday weekend and 
and the notification/escalation process isn't what it should be (Murphey's 
Law)...


The value you refer to is the SOA record _expire_ value for a zone, I 
believe is should be set to between 14 and 28 days.


https://en.wikipedia.org/wiki/SOA_record


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Re: [CentOS] how to increase DNS reliability?

[Leroy Tennison]

This brings up one of the caveats for (at least ISC) DNS, if the master goes 
down the slaves will take over for a time but eventually will stop serving for 
the domains of the master if it remains down too long.  If my (sometimes 
faulty) memory serves me well it is in the three day range (but configurable) 
which is ample time unless the problem occurs early in a holiday weekend and 
and the notification/escalation process isn't what it should be (Murphey's 
Law)...


From: CentOS  on behalf of Nataraj 

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Subject: [EXTERNAL] Re: [CentOS] how to increase DNS reliability?

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@pygeum:~$ dig mydomain.com. soa @127.0.0.1

; <<>> DiG 9.11.3-1ubuntu1.8-Ubuntu <<>> mydomain.com. soa@127.0.0.1
;; 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

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; COOKIE: 64f402c0c22d57aa2bbb10fc5d3a340d8c19377b924d01c2 (good)
;; QUESTION SECTION:
;mydomain.com.INSOA

;; ANSWER SECTION:
Mydomain.Com.14400INSOAns1.mydomain.com.
postmaster.Mydomain.COM. 2019072505 1200 600 15552000 14400

;; AUTHORITY SECTION:
Mydomain.Com.14400INNSns1.Mydomain.Com.
Mydomain.Com.14400INNSns2.Mydomain.Com.
Mydomain.Com.14400INNSns3.Mydomain.com.

;; ADDITIONAL SECTION:
ns1.mydomain.com.14400INA8.8.8.8

;; Query time: 1 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; 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 d

Re: [CentOS] how to increase DNS reliability?

[Nataraj]

On 7/25/19 4:31 PM, Nataraj wrote:
> It doesn't really help those clients I can not run name servers on,
> though.

Another alternative is to look at the multicast dns (mdns) protocol.  I
have no experience with it, so I can't say very much, but I know it
exists.  I'm pretty sure it's inplemented in avahi daemon, so it may
just be an issue of enabling it on the client.  If your client supports
it then I would think that all you have to do is enable it.

Nataraj

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Re: [CentOS] how to increase DNS reliability?

[Nataraj]

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@pygeum:~$ dig mydomain.com. soa @127.0.0.1

; <<>> DiG 9.11.3-1ubuntu1.8-Ubuntu <<>> mydomain.com. soa@127.0.0.1
;; 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

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

;; ANSWER SECTION:
Mydomain.Com.        14400    IN    SOA    ns1.mydomain.com.
postmaster.Mydomain.COM. 2019072505 1200 600 15552000 14400

;; AUTHORITY SECTION:
Mydomain.Com.        14400    IN    NS    ns1.Mydomain.Com.
Mydomain.Com.        14400    IN    NS    ns2.Mydomain.Com.
Mydomain.Com.        14400    IN    NS    ns3.Mydomain.com.

;; ADDITIONAL SECTION:
ns1.mydomain.com.        14400    IN    A    8.8.8.8

;; Query time: 1 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; 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 

Re: [CentOS] how to increase DNS reliability?

[Giles Coochey]

On 25/07/2019 22:17, Giles Coochey wrote:


Separate DNS servers must be on a different subnet according to 
RFC2182 (https://tools.ietf.org/html/rfc2182):


Secondary servers must be placed at both topologically and
   geographically dispersed locations on the Internet, to minimise the
   likelihood of a single failure disabling all of them.

I know that UPSs are physical, and subnets are logical, but the 
reasoning behind the requirement is due to having to be on a different 
infrastructure.


__


Shock horror, replying to my own post, but in cloud cluster 
environments, you might consider anti-affinity rules to prevent multiple 
name servers going down at the same time due to a cluster node failure 
(i.e. rules to ensure that hypervisors keep different name servers on 
different hosts).


I know it doesn't help OP, who was looking for cluster based solutions, 
but the same applies if using load balancing virtual appliances, hosting 
IPs as name servers.



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Re: [CentOS] how to increase DNS reliability?

[Giles Coochey]

On 25/07/2019 20:39, John Pierce wrote:

On Thu, Jul 25, 2019 at 10:32 AM hw  wrote:


I can't help it when the primary name server goes down because the UPS
fails
the self test and tells the server it has 2 minutes or so left in wich case
the server figures it needs to shut down.  I wanted better UPSs ...


critical infrastructure servers should have redudant PSUs, on seperate UPSs.


Separate DNS servers must be on a different subnet according to RFC2182 
(https://tools.ietf.org/html/rfc2182):


Secondary servers must be placed at both topologically and
   geographically dispersed locations on the Internet, to minimise the
   likelihood of a single failure disabling all of them.

I know that UPSs are physical, and subnets are logical, but the 
reasoning behind the requirement is due to having to be on a different 
infrastructure.


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Re: [CentOS] how to increase DNS reliability?

[hw]

On 7/25/19 7:10 PM, Nataraj wrote:

[...]

I meant to say:

Configure all dns servers as secondary/slaves (one should be the primary 
master) for your own domains.  Thos means that all of your servers are 
authoritative for your own domains, so they cannot fail on local dns lookups 
due to Internet problems.


Ah!?

When I had it happen a couple years ago and wondered why even local 
names couldn't be resolved (which didn't make sense to me because the 
server would always know about them from the zone files), I was told 
that nothing could be done about it because DNS is designed to do 
lookups no matter what.


However, that was a server acting as both a local master and as a 
forwarder.  If what you say is true, I would now understand this much 
better --- and I'd need to change my setup.

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Re: [CentOS] how to increase DNS reliability?

[Paul Heinlein]

On Thu, 25 Jul 2019, hw wrote:


 On Linux systems, you can set the timeout in /etc/resolv.conf, e.g.,

#  I think the default nameserver timeout is 5; use rotate
#  option if you prefer round-robin queries rather than
#  always using the first-listed first
 nameserver 10.11.12.13 timeout:2 rotate
 nameserver 10.11.12.14 timeout:2 rotate

 I'll admit that I'm not sure if those options are configurable on
 Mac and/or Windows workstations.


It was those showing problems.

Only 5 seconds isn't long enough that I would expect any problems. 
What do I need to put into the ifcf files or tell nmcli to set these 
options?


If you're using dhclient to manage addresses, then you can add the 
RES_OPTIONS variable to /etc/sysconfig/network:


# /etc/sysconfig/network
RES_OPTIONS="timeout:2 rotate"

Or, with even less patience:

RES_OPTIONS="timeout:1 retries:1 rotate"


Grep for RES_OPTIONS in /sbin/dhclient-script for the gory details.

--
Paul Heinlein
heinl...@madboa.com
45°38' N, 122°6' W
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Re: [CentOS] how to increase DNS reliability?

[hw]

On 7/25/19 9:39 PM, John Pierce wrote:

On Thu, Jul 25, 2019 at 10:32 AM hw  wrote:


I can't help it when the primary name server goes down because the UPS
fails
the self test and tells the server it has 2 minutes or so left in wich case
the server figures it needs to shut down.  I wanted better UPSs ...



critical infrastructure servers should have redudant PSUs, on seperate UPSs.


right, with hot swappable batteries ...


my last rack builds, I had 2 Eaton PowerWare 7KVA 4U UPS's in the bottom of
each rack.  one fed the left side PDUs, the other fed the right side PDUs,
and each server had redundant PSU's, one plugged into each PDU.

those Eaton UPS's had hotswappable batteries, too.


... like this
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Re: [CentOS] how to increase DNS reliability?

[hw]

On 7/25/19 9:11 PM, mark wrote:

hw wrote:

On 7/25/19 4:07 PM, Giles Coochey wrote:



Sounds like you're performing maintenance on your servers


(a) too often
(b) during office / peak hours



I can't help it when the primary name server goes down because the UPS
fails the self test and tells the server it has 2 minutes or so left in
wich case the server figures it needs to shut down.  I wanted better UPSs
...


Change that. Are you using apcupsd? You can set the config from
SHUTDOWN=/sbin/shutdown to /bin/false. Then, the next time you see the
UPS, change the battery. If it's just started to complain, it's not dead
yet!

Works for me with all of our mostly APC SmartUPS 3000 rackmounts.


I don't remember which UPS it was, either the crappy one for which a 
replacement battery was already waiting to be put in, or the normal one 
that already had a new battery in it which is either broken or doesn't 
get charged ...


That's how I rather have not everything go dark even when Murphy comes 
along.  I have generally deprecated all non-rackmount UPSs, and being 
able to change batteries without outage has become a requirement.

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Re: [CentOS] how to increase DNS reliability?

[hw]

On 7/25/19 7:58 PM, Paul Heinlein wrote:

On Thu, 25 Jul 2019, hw wrote:


On 7/25/19 3:28 PM, Leroy Tennison wrote:

If you don't want multiple DNS server entries on the client


I'm ok with them, only the problem is that the clients take their 
timeouts

when a server is unreachable, and users panic.


On Linux systems, you can set the timeout in /etc/resolv.conf, e.g.,

# I think the default nameserver timeout is 5; use rotate
# option if you prefer round-robin queries rather than
# always using the first-listed first
nameserver 10.11.12.13 timeout:2 rotate
nameserver 10.11.12.14 timeout:2 rotate

I'll admit that I'm not sure if those options are configurable on Mac
and/or Windows workstations.



It was those showing problems.

Only 5 seconds isn't long enough that I would expect any problems.  What 
do I need to put into the ifcf files or tell nmcli to set these options?

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Re: [CentOS] how to increase DNS reliability?

[mark]

John Pierce wrote:
> On Thu, Jul 25, 2019 at 10:32 AM hw  wrote:
>
>> I can't help it when the primary name server goes down because the UPS
>> fails the self test and tells the server it has 2 minutes or so left in
>> wich case the server figures it needs to shut down.  I wanted better
>> UPSs ...
>>
> critical infrastructure servers should have redudant PSUs, on seperate
> UPSs.
>
> my last rack builds, I had 2 Eaton PowerWare 7KVA 4U UPS's in the bottom
> of each rack.  one fed the left side PDUs, the other fed the right side
> PDUs, and each server had redundant PSU's, one plugged into each PDU.
> those Eaton UPS's had hotswappable batteries, too.

*shrug* All UPSes have hot-swappable. Mine beep while you disconnect the
batteries, pull out the sled, replace all 8, shove it back in, and
reconnect, and it shuts up.

For those that haven't done it, though, DO NOT BELIEVE WHAT ANYONE SAYS,
DO NOT USE *ANYTHING* BUT HR (high rate) batteries in a UPS (maybe in a
home one, but...). APC, for example, simply stays red, and insists that
you still need to change them. *Good* battery vendors know this.

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Re: [CentOS] how to increase DNS reliability?

[hw]

On 7/25/19 5:14 PM, Nataraj wrote:

On 7/25/19 6:48 AM, rai...@ultra-secure.de wrote:

Am 2019-07-25 15:41, schrieb hw:

On 7/25/19 2:53 PM, rai...@ultra-secure.de wrote:

Am 2019-07-25 14:51, schrieb hw:

Hi,

how can DNS reliability, as experienced by clients on the LAN who are
sending queries, be increased?

Would I have to set up some sort of cluster consisting of several
servers all providing DNS services which is reachable under a single
IP address known to the clients?

Just setting up several name servers and making them known to the
clients
for the clients to automatically switch isn't a good solution because
the clients take their timeouts and users lacking even the most basic
knowledge inevitably panic when the first name server does not answer
queries.


Run a local cache (unbound) and enter all your local resolvers as
upstreams.


That can fail just as well --- or be even worse when the clients
can't switch
over anymore.  I have that and am avoiding to use it for some clients
because
it takes a while for the cache to get updated when I make changes.

However, if that cache fails, chances are that the internet
connection is also
down in which case it can be troublesome to even get local host names
resolved.
When that happens, trouble is to be expected.



Anything else is - IMHO - much more work, much more complicated and
much more likely to fail, in a more spectacular way.
Especially all those keepalive "solutions".

I have found that I need to restart unbound if all upstreams had failed. 



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.



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.


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 ...

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Re: [CentOS] how to increase DNS reliability?

[John Pierce]

On Thu, Jul 25, 2019 at 10:32 AM hw  wrote:

> I can't help it when the primary name server goes down because the UPS
> fails
> the self test and tells the server it has 2 minutes or so left in wich case
> the server figures it needs to shut down.  I wanted better UPSs ...
>

critical infrastructure servers should have redudant PSUs, on seperate UPSs.

my last rack builds, I had 2 Eaton PowerWare 7KVA 4U UPS's in the bottom of
each rack.  one fed the left side PDUs, the other fed the right side PDUs,
and each server had redundant PSU's, one plugged into each PDU.

those Eaton UPS's had hotswappable batteries, too.


-- 
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Re: [CentOS] how to increase DNS reliability?

[mark]

hw wrote:
> On 7/25/19 4:07 PM, Giles Coochey wrote:

>> Sounds like you're performing maintenance on your servers
>>
>>
>> (a) too often
>> (b) during office / peak hours
>>
>
> I can't help it when the primary name server goes down because the UPS
> fails the self test and tells the server it has 2 minutes or so left in
> wich case the server figures it needs to shut down.  I wanted better UPSs
> ...

Change that. Are you using apcupsd? You can set the config from
SHUTDOWN=/sbin/shutdown to /bin/false. Then, the next time you see the
UPS, change the battery. If it's just started to complain, it's not dead
yet!

Works for me with all of our mostly APC SmartUPS 3000 rackmounts.

   mark

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Re: [CentOS] how to increase DNS reliability?

[Leon Fauster via CentOS]

> Am 25.07.2019 um 19:58 schrieb Paul Heinlein :
> 
> On Thu, 25 Jul 2019, hw wrote:
> 
>> On 7/25/19 3:28 PM, Leroy Tennison wrote:
>>> If you don't want multiple DNS server entries on the client
>> 
>> I'm ok with them, only the problem is that the clients take their timeouts
>> when a server is unreachable, and users panic.
> 
> On Linux systems, you can set the timeout in /etc/resolv.conf, e.g.,
> 
> # I think the default nameserver timeout is 5; use rotate
> # option if you prefer round-robin queries rather than
> # always using the first-listed first
> nameserver 10.11.12.13 timeout:2 rotate
> nameserver 10.11.12.14 timeout:2 rotate

IMO such entries are done via "options" ... 

yum install man-pages ; man resolv.conf

--
LF

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Re: [CentOS] how to increase DNS reliability?

[John Pierce]

On Thu, Jul 25, 2019 at 11:00 AM Paul Heinlein  wrote:

> On Thu, 25 Jul 2019, hw wrote:
>
> > On 7/25/19 3:28 PM, Leroy Tennison wrote:
> >> If you don't want multiple DNS server entries on the client
> >
> > I'm ok with them, only the problem is that the clients take their
> timeouts
> > when a server is unreachable, and users panic.
>
> On Linux systems, you can set the timeout in /etc/resolv.conf, e.g.,...
>


Windows will 'rotate' the list of NS servers if the top one times out, so
next time it will use the first alternate and if that times out, it
will start using the next alternate, etc.


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Re: [CentOS] how to increase DNS reliability?

[Paul Heinlein]

On Thu, 25 Jul 2019, hw wrote:


On 7/25/19 3:28 PM, Leroy Tennison wrote:

If you don't want multiple DNS server entries on the client


I'm ok with them, only the problem is that the clients take their timeouts
when a server is unreachable, and users panic.


On Linux systems, you can set the timeout in /etc/resolv.conf, e.g.,

# I think the default nameserver timeout is 5; use rotate
# option if you prefer round-robin queries rather than
# always using the first-listed first
nameserver 10.11.12.13 timeout:2 rotate
nameserver 10.11.12.14 timeout:2 rotate

I'll admit that I'm not sure if those options are configurable on Mac
and/or Windows workstations.

--
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heinl...@madboa.com
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Re: [CentOS] how to increase DNS reliability?

[hw]

On 7/25/19 4:07 PM, Giles Coochey wrote:
> 
> On 25/07/2019 13:51, hw wrote:
>> Hi,
>>
>> how can DNS reliability, as experienced by clients on the LAN who are
>> sending queries, be increased?
>>
>> Would I have to set up some sort of cluster consisting of several
>> servers all providing DNS services which is reachable under a single
>> IP address known to the clients?
>>
>> Just setting up several name servers and making them known to the clients
>> for the clients to automatically switch isn't a good solution because
>> the clients take their timeouts and users lacking even the most basic
>> knowledge inevitably panic when the first name server does not answer
>> queries.
> 
> Sounds like you're performing maintenance on your servers
> 
> (a) too often
> (b) during office / peak hours

I can't help it when the primary name server goes down because the UPS fails
the self test and tells the server it has 2 minutes or so left in wich case
the server figures it needs to shut down.  I wanted better UPSs ...

> You could load balance multiple servers (using lots of available 
> load-balancing technologies) to allow you to perform maintenance at certain 
> times, but it has its own issues.

Load balancing or clustering?  At least clustering seems not entirely
trivial to do.

> I've recently been looking at PowerDNS, which separates the recursor and the 
> authoritative server into two distinct packages. I'm just running the 
> authoritative server as a master, and keeping my old bind/named servers as 
> recursors / slaves.

This can be done with bind, how does it require something called PowerDNS?

> It's a home
> office network, but I only have issues when I'm tinkering, and if I were to 
> be doing this kind of work in a larger commercial environment, then I would 
> not be doing DNS server maintenance while others were relying on them.

The maintenance didn't cause any problems.  You can edit the configuration
just fine and restart the server when done ... :)

> For much of the back end infrastructure I use IP addresses rather than DNS 
> names in their configuration, just to take DNS issues out of the equation 
> completely.

I think this is a very bad idea because it causes lots of work and is likely to
cause issues.  What if you, for example, migrate remote logging to another 
server?
All the time, you have to document every place where you put an IP address; you
have to keep the documentation always up to date and then change the address at
every place when you make a change.  Forget one place, and things break.

But when you use names instead of addresses, like 'log.example.com', you only
need to make a single change at a single place such as you alter the address
in your name server config.

DNS can be difficult to get right, though it's not all that difficult, and
once it's working, there aren't really any issues other than that a server can
become unreachable.
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Re: [CentOS] how to increase DNS reliability?

[hw]

On 7/25/19 4:49 PM, Nux! wrote:
> I'm about to do an overhaul of the DNS service at work and my plan is to use 
> powerdns recursor + dnsdist + keepalived.

I've more or less done the overhaul, only some sort of failover thing
is missing ...  I'll check those out, thanks!
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Re: [CentOS] how to increase DNS reliability?

[hw]

On 7/25/19 3:28 PM, Leroy Tennison wrote:
> If you don't want multiple DNS server entries on the client

I'm ok with them, only the problem is that the clients take their timeouts
when a server is unreachable, and users panic.

> then a master and (possibly multiple) slave server configuration can be set 
> up (I'm assuming ISC DNS - their solution to redundancy/failover is master 
> and slave servers, this may be the way it is with all DNS).

Yes, bind9, and I've set up a master and a slave.  The router uses them to
forward requests to on behalf of those clients that use the router as a name
server while other clients know master and slave but not the router as name
servers.

There was a failure a while ago (IIRC because of a UPS causing a server to
shut down when the battery failed the self test), and things didn't quite work
anymore with the master server being unreachable.

This is how I have a problem with the clients knowing multiple servers: The
very setup is intended to keep things working during an outage and yet it
doesn't help.

>  keepalived can be used for fail over and will present a single IP address 
> (which the clients would use) shared among the servers.  haproxy or 
> alternatives might be another fail over option.

Thanks, I'll look into that!  I've been searching for "dns proxy" and no useful
results came up ...

> Each technology has its own learning curve (and doing this will require at 
> least two) and caveats.  In particular systemd doesn't appear to play well 
> with technologies creating IP addresses it doesn't manage.  The version of 
> keepalived we're using also has its own nasty quirk as well where it comes up 
> assuming it is master until discovered otherwise, this is true even if it is 
> configured as backup.  In most cases this is probably either a non-issue (no 
> scripts being used) or a minor annoyance.  But if you're using scripts trigger
>  ed by keepalived which make significant (and possibly conflicting) changes 
> to the environment then you'll need to embed "intelligence" in them to wait 
> until final state is reached or test state before acting or some other option.

I consider myself warned :)
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Re: [CentOS] how to increase DNS reliability?

[Nataraj]

On 7/25/19 8:14 AM, Nataraj wrote:
> On 7/25/19 6:48 AM, rai...@ultra-secure.de wrote:
>> Am 2019-07-25 15:41, schrieb hw:
>>> On 7/25/19 2:53 PM, rai...@ultra-secure.de wrote:
 Am 2019-07-25 14:51, schrieb hw:
> Hi,
>
> how can DNS reliability, as experienced by clients on the LAN who are
> sending queries, be increased?
>
> Would I have to set up some sort of cluster consisting of several
> servers all providing DNS services which is reachable under a single
> IP address known to the clients?
>
> Just setting up several name servers and making them known to the
> clients
> for the clients to automatically switch isn't a good solution because
> the clients take their timeouts and users lacking even the most basic
> knowledge inevitably panic when the first name server does not answer
> queries.
 Run a local cache (unbound) and enter all your local resolvers as
 upstreams.
>>> That can fail just as well --- or be even worse when the clients
>>> can't switch
>>> over anymore.  I have that and am avoiding to use it for some clients
>>> because
>>> it takes a while for the cache to get updated when I make changes.
>>>
>>> However, if that cache fails, chances are that the internet
>>> connection is also
>>> down in which case it can be troublesome to even get local host names
>>> resolved.
>>> When that happens, trouble is to be expected.
>>
>> Anything else is - IMHO - much more work, much more complicated and
>> much more likely to fail, in a more spectacular way.
>> Especially all those keepalive "solutions".
>>
>> I have found that I need to restart unbound if all upstreams had failed. 
>
> 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.

I meant to say:

Configure all dns servers as secondary/slaves (one should be the primary 
master) for your own domains.  Thos means that all of your servers are 
authoritative for your own domains, so they cannot fail on local dns lookups 
due to Internet problems.

>
> 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.  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.
>
>
> Nataraj
>
>
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Re: [CentOS] how to increase DNS reliability?

[Nataraj]

On 7/25/19 6:48 AM, rai...@ultra-secure.de wrote:
> Am 2019-07-25 15:41, schrieb hw:
>> On 7/25/19 2:53 PM, rai...@ultra-secure.de wrote:
>>> Am 2019-07-25 14:51, schrieb hw:
 Hi,

 how can DNS reliability, as experienced by clients on the LAN who are
 sending queries, be increased?

 Would I have to set up some sort of cluster consisting of several
 servers all providing DNS services which is reachable under a single
 IP address known to the clients?

 Just setting up several name servers and making them known to the
 clients
 for the clients to automatically switch isn't a good solution because
 the clients take their timeouts and users lacking even the most basic
 knowledge inevitably panic when the first name server does not answer
 queries.
>>>
>>> Run a local cache (unbound) and enter all your local resolvers as
>>> upstreams.
>>
>> That can fail just as well --- or be even worse when the clients
>> can't switch
>> over anymore.  I have that and am avoiding to use it for some clients
>> because
>> it takes a while for the cache to get updated when I make changes.
>>
>> However, if that cache fails, chances are that the internet
>> connection is also
>> down in which case it can be troublesome to even get local host names
>> resolved.
>> When that happens, trouble is to be expected.
>
>
> Anything else is - IMHO - much more work, much more complicated and
> much more likely to fail, in a more spectacular way.
> Especially all those keepalive "solutions".
>
> I have found that I need to restart unbound if all upstreams had failed. 


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.

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.  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.


Nataraj


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Re: [CentOS] how to increase DNS reliability?

[Nux!]

I'm about to do an overhaul of the DNS service at work and my plan is to 
use powerdns recursor + dnsdist + keepalived.



---
Sent from the Delta quadrant using Borg technology!

On 2019-07-25 14:28, Leroy Tennison wrote:

If you don't want multiple DNS server entries on the client then a
master and (possibly multiple) slave server configuration can be set
up (I'm assuming ISC DNS - their solution to redundancy/failover is
master and slave servers, this may be the way it is with all DNS).
keepalived can be used for fail over and will present a single IP
address (which the clients would use) shared among the servers.
haproxy or alternatives might be another fail over option.  Each
technology has its own learning curve (and doing this will require at
least two) and caveats.  In particular systemd doesn't appear to play
well with technologies creating IP addresses it doesn't manage.  The
version of keepalived we're using also has its own nasty quirk as well
where it comes up assuming it is master until discovered otherwise,
this is true even if it is configured as backup.  In most cases this
is probably either a non-issue (no scripts being used) or a minor
annoyance.  But if you're using scripts trigger
 ed by keepalived which make significant (and possibly conflicting)
changes to the environment then you'll need to embed "intelligence" in
them to wait until final state is reached or test state before acting
or some other option.


From: CentOS  on behalf of hw 
Sent: Thursday, July 25, 2019 7:51:39 AM
To: centos@centos.org 
Subject: [EXTERNAL] [CentOS] how to increase DNS reliability?


Hi,

how can DNS reliability, as experienced by clients on the LAN who are
sending queries, be increased?

Would I have to set up some sort of cluster consisting of several
servers all providing DNS services which is reachable under a single
IP address known to the clients?

Just setting up several name servers and making them known to the 
clients

for the clients to automatically switch isn't a good solution because
the clients take their timeouts and users lacking even the most basic
knowledge inevitably panic when the first name server does not answer
queries.
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Re: [CentOS] how to increase DNS reliability?

[hw]

On 7/25/19 3:48 PM, rai...@ultra-secure.de wrote:
> Am 2019-07-25 15:41, schrieb hw:
>> On 7/25/19 2:53 PM, rai...@ultra-secure.de wrote:
>>> Am 2019-07-25 14:51, schrieb hw:
 Hi,

 how can DNS reliability, as experienced by clients on the LAN who are
 sending queries, be increased?

 Would I have to set up some sort of cluster consisting of several
 servers all providing DNS services which is reachable under a single
 IP address known to the clients?

 Just setting up several name servers and making them known to the clients
 for the clients to automatically switch isn't a good solution because
 the clients take their timeouts and users lacking even the most basic
 knowledge inevitably panic when the first name server does not answer
 queries.
>>>
>>> Run a local cache (unbound) and enter all your local resolvers as upstreams.
>>
>> That can fail just as well --- or be even worse when the clients can't switch
>> over anymore.  I have that and am avoiding to use it for some clients because
>> it takes a while for the cache to get updated when I make changes.
>>
>> However, if that cache fails, chances are that the internet connection is 
>> also
>> down in which case it can be troublesome to even get local host names 
>> resolved.
>> When that happens, trouble is to be expected.
> 
> 
> Anything else is - IMHO - much more work, much more complicated

That's what I was thinking.  Perhaps it is better to live with a main server and
one or two slaves so the clients can keep their alternatives.

But still ...  There's got to be a better way ...

> and much more likely to fail, in a more spectacular way.
> Especially all those keepalive "solutions".

You mean like probing if the DNS server is still responsive and somehow 
switching
over when it's not?  I never tried, though it is evident that more complicated
things may tend to be less reliable.

Yet it reminds me that I could actually check the name servers and dispatch a 
message
when one fails as I'm already doing for a couple other things.  That would 
suffice
and doesn't introduce more possibilites of failure to name resolution.

> I have found that I need to restart unbound if all upstreams had failed.
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Re: [CentOS] how to increase DNS reliability?

[Giles Coochey]

On 25/07/2019 13:51, hw wrote:

Hi,

how can DNS reliability, as experienced by clients on the LAN who are
sending queries, be increased?

Would I have to set up some sort of cluster consisting of several
servers all providing DNS services which is reachable under a single
IP address known to the clients?

Just setting up several name servers and making them known to the clients
for the clients to automatically switch isn't a good solution because
the clients take their timeouts and users lacking even the most basic
knowledge inevitably panic when the first name server does not answer
queries.


Sounds like you're performing maintenance on your servers

(a) too often
(b) during office / peak hours

You could load balance multiple servers (using lots of available 
load-balancing technologies) to allow you to perform maintenance at 
certain times, but it has its own issues.


I've recently been looking at PowerDNS, which separates the recursor and 
the authoritative server into two distinct packages. I'm just running 
the authoritative server as a master, and keeping my old bind/named 
servers as recursors / slaves. It's a home office network, but I only 
have issues when I'm tinkering, and if I were to be doing this kind of 
work in a larger commercial environment, then I would not be doing DNS 
server maintenance while others were relying on them.


For much of the back end infrastructure I use IP addresses rather than 
DNS names in their configuration, just to take DNS issues out of the 
equation completely.


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Re: [CentOS] how to increase DNS reliability?

[rainer]

Am 2019-07-25 15:41, schrieb hw:

On 7/25/19 2:53 PM, rai...@ultra-secure.de wrote:

Am 2019-07-25 14:51, schrieb hw:

Hi,

how can DNS reliability, as experienced by clients on the LAN who are
sending queries, be increased?

Would I have to set up some sort of cluster consisting of several
servers all providing DNS services which is reachable under a single
IP address known to the clients?

Just setting up several name servers and making them known to the 
clients

for the clients to automatically switch isn't a good solution because
the clients take their timeouts and users lacking even the most basic
knowledge inevitably panic when the first name server does not answer
queries.


Run a local cache (unbound) and enter all your local resolvers as 
upstreams.


That can fail just as well --- or be even worse when the clients can't 
switch
over anymore.  I have that and am avoiding to use it for some clients 
because

it takes a while for the cache to get updated when I make changes.

However, if that cache fails, chances are that the internet connection 
is also
down in which case it can be troublesome to even get local host names 
resolved.

When that happens, trouble is to be expected.



Anything else is - IMHO - much more work, much more complicated and much 
more likely to fail, in a more spectacular way.

Especially all those keepalive "solutions".

I have found that I need to restart unbound if all upstreams had failed.
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Re: [CentOS] how to increase DNS reliability?

[hw]

On 7/25/19 2:53 PM, rai...@ultra-secure.de wrote:
> Am 2019-07-25 14:51, schrieb hw:
>> Hi,
>>
>> how can DNS reliability, as experienced by clients on the LAN who are
>> sending queries, be increased?
>>
>> Would I have to set up some sort of cluster consisting of several
>> servers all providing DNS services which is reachable under a single
>> IP address known to the clients?
>>
>> Just setting up several name servers and making them known to the clients
>> for the clients to automatically switch isn't a good solution because
>> the clients take their timeouts and users lacking even the most basic
>> knowledge inevitably panic when the first name server does not answer
>> queries.
> 
> Run a local cache (unbound) and enter all your local resolvers as upstreams.

That can fail just as well --- or be even worse when the clients can't switch
over anymore.  I have that and am avoiding to use it for some clients because
it takes a while for the cache to get updated when I make changes.

However, if that cache fails, chances are that the internet connection is also
down in which case it can be troublesome to even get local host names resolved.
When that happens, trouble is to be expected.
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Re: [CentOS] how to increase DNS reliability?

[rainer]

Am 2019-07-25 14:51, schrieb hw:

Hi,

how can DNS reliability, as experienced by clients on the LAN who are
sending queries, be increased?

Would I have to set up some sort of cluster consisting of several
servers all providing DNS services which is reachable under a single
IP address known to the clients?

Just setting up several name servers and making them known to the 
clients

for the clients to automatically switch isn't a good solution because
the clients take their timeouts and users lacking even the most basic
knowledge inevitably panic when the first name server does not answer
queries.


Run a local cache (unbound) and enter all your local resolvers as 
upstreams.


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