About the prefetch function within bind 9.10.

[Hongyi Zhao]

Dear all,

I compilled and installed the BIND 9.10.0-P1 on my Debian Wheezy box.  See following for detail:

------------------
werner@debian:~$ sudo named -V
BIND 9.10.0-P1 <id:e94d8db1> built by make with '--prefix=/usr' '--mandir=/usr/share/man' '--infodir=/usr/share/info' '--sysconfdir=/etc/bind' '--localstatedir=/var' '--enable-threads' '--enable-largefile' '--with-libtool' '--enable-shared' '--enable-static' '--with-openssl=/usr' '--with-gssapi=/usr' '--with-gnu-ld' '--with-geoip=/usr' '--enable-ipv6' 'CFLAGS=-fno-strict-aliasing -DDIG_SIGCHASE -O2'
compiled by GCC 4.7.2
using OpenSSL version: OpenSSL 1.0.1e 11 Feb 2013
------------------

Now, I setting the prefetch in the named.conf.options by adding the follow line into it:

-----------

prefetch 2 9;

------------

Then restart the named and test it by using the following command:

---------------------
werner@debian:~$ dig ssh.sshcenter.info

; <<>> DiG 9.10.0-P1 <<>> ssh.sshcenter.info
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 54538
;; flags: qr rd ra; QUERY: 1, ANSWER: 15, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;ssh.sshcenter.info.        IN    A

;; ANSWER SECTION:
ssh.sshcenter.info.    60    IN    CNAME    c-ssh.cloudkvm.net.
c-ssh.cloudkvm.net.    60    IN    A    23.94.104.29
c-ssh.cloudkvm.net.    60    IN    A    199.119.224.224
c-ssh.cloudkvm.net.    60    IN    A    23.226.226.124
c-ssh.cloudkvm.net.    60    IN    A    204.44.87.100
c-ssh.cloudkvm.net.    60    IN    A    192.3.23.197
c-ssh.cloudkvm.net.    60    IN    A    173.254.237.179
c-ssh.cloudkvm.net.    60    IN    A    96.44.134.231
c-ssh.cloudkvm.net.    60    IN    A    23.90.4.124
c-ssh.cloudkvm.net.    60    IN    A    23.92.50.22
c-ssh.cloudkvm.net.    60    IN    A    107.150.4.170
c-ssh.cloudkvm.net.    60    IN    A    199.233.236.200
c-ssh.cloudkvm.net.    60    IN    A    69.197.147.93
c-ssh.cloudkvm.net.    60    IN    A    162.211.227.83
c-ssh.cloudkvm.net.    60    IN    A    192.227.161.20

;; Query time: 1034 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Sun May 18 13:13:03 CST 2014
;; MSG SIZE  rcvd: 303
-----------------------

As you can see, it takes 1034 msec for the first query.   According to my understanding of the prefetch option,  if I enabled it, then after the first query of a site, then it will always updated automatically into the cache of bind.  So I will always obtain a 0 msec query time for that site after the first query. 

But for my above case, after 60 seconds, if I do the same query, I will get the following results:

-----------------------------
werner@debian:~$ dig ssh.sshcenter.info

; <<>> DiG 9.10.0-P1 <<>> ssh.sshcenter.info
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 41322
;; flags: qr rd ra; QUERY: 1, ANSWER: 15, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;ssh.sshcenter.info.        IN    A

;; ANSWER SECTION:
ssh.sshcenter.info.    60    IN    CNAME    c-ssh.cloudkvm.net.
c-ssh.cloudkvm.net.    58    IN    A    23.90.4.124
c-ssh.cloudkvm.net.    58    IN    A    192.3.23.197
c-ssh.cloudkvm.net.    58    IN    A    192.227.161.20
c-ssh.cloudkvm.net.    58    IN    A    23.226.226.124
c-ssh.cloudkvm.net.    58    IN    A    23.92.50.22
c-ssh.cloudkvm.net.    58    IN    A    199.119.224.224
c-ssh.cloudkvm.net.    58    IN    A    23.94.104.29
c-ssh.cloudkvm.net.    58    IN    A    199.233.236.200
c-ssh.cloudkvm.net.    58    IN    A    96.44.134.231
c-ssh.cloudkvm.net.    58    IN    A    162.211.227.83
c-ssh.cloudkvm.net.    58    IN    A    107.150.4.170
c-ssh.cloudkvm.net.    58    IN    A    173.254.237.179
c-ssh.cloudkvm.net.    58    IN    A    204.44.87.100
c-ssh.cloudkvm.net.    58    IN    A    69.197.147.93

;; Query time: 2186 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Sun May 18 13:16:50 CST 2014
;; MSG SIZE  rcvd: 303
-----------------------------

As you can see, the query time is 2186 msec instead of 0 msec.  Any hints on this issue?

Regards
--
Hongyi Zhao <hongy...@gmail.com>
Xinjiang Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
GnuPG DSA: 0xD108493

[Leonard Mills]

Taking the CNAME line in the response, please notice that the published TTL is 60 seconds.  Prefetch does not cause named to ignore TTLs.

hth,

Len

[Hongyi Zhao]

What do you mean by saying that "Prefetch does not cause named to ignore TTLs"?

I think in my case, I have set the preftch option like this:

--------------
prefetch 2 9;
--------------

This will enable the prefetching for all of the entries with the TTL larger that 2 seconds, (in my case, the TTL is 60 seconds, so the prefetch will be enabled for this entry).  The digit 9 triggering condition for doing the prefetching.  In my case, it should mean that when 51 seconds elapsed for the entry, i.e., 9 = 60 - 51,  the bind will prefetching the record, and then the record will have a full TTL, i.e., 60 seconds for this case again.

Am I wrong about the meaning of the prefetching function of bind or not? 

Regards

Zhao   

[Mark Andrews]

If there is a query in that 9 second window then named will make a query to repopulate the cache. If there is not a query then the records will expire. You only want to prefetch records that are being queried for regularly. 

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[Hongyi Zhao]

Yes,  I want to let bind/named prefetch records that are being queried regularly.  In this way, I'll have a set of up-to-date cached records that I've been queried.  Can the prefetch function plus caching mode of bind/named do this for me?

Regards

[Phil Mayers]

On 18/05/14 09:26, Hongyi Zhao wrote:
> Yes, I want to let bind/named prefetch records that are being queried
> regularly. In this way, I'll have a set of up-to-date cached records
> that I've been queried. Can the prefetch function plus caching mode of
> bind/named do this for me?

Re-read Marks reply. "Queried regularly" is not enough. For prefetch to
work, a query must occur in the prefetch window.

With your settings if you do this:

dig
sleep 52
dig
sleep 10
dig

...you should see it work.