lost and found ( for me ? ): unbound : prefetch

[ what timing does unbound perform DNS prefetch ? ]

Seen from source files , prefetching algorithm is :

If TTL in the cache reaches 90% of original TTL , unbound seems to perform prefetch

# find . -name "*" | xargs grep -i prefetch
./util/data/msgreply.c: rep->prefetch_ttl = prettl;
./util/data/msgreply.c: rep->prefetch_ttl = PREFETCH_TTL_CALC(rep->ttl);
./util/data/msgreply.c: rep->prefetch_ttl += timenow;
./util/data/msgreply.c: rep->ttl, rep->prefetch_ttl, rep->an_numrrsets,
./util/data/msgreply.h:/** calculate the prefetch TTL as 90% of original. Calculation
./util/data/msgreply.h:#define PREFETCH_TTL_CALC(ttl) ((ttl) - (ttl)/10)

For example :

original TTL : 30 seconds
90% of original TTL : 30 - 30/10 = 27
In this example , unbound will perform prefetch when the remaining of TTL reaches less 3 seconds.
30(100%) - 27(90%) = 3

Please note that unbound will not perform prefetch if there are no requests of this query , even though TTL is less 3 seconds.

[ when prefetch enabled ]

unbound.conf ( default is no )

prefetch: yes

name resolution for www.facebook.com.
original TTL is 30 seconds.

repeat dig command.

Seen from TTL and Query time , you could find whether unbound performes prefetch or not.

debug logs

unbound: [1385:0] info: server stats for thread 0: 19 queries, 18 answers from cache, 1 recursions, 1 prefetch

[ when prefetch disabled ]

unbound.conf ( default is no )

prefetch: no

unbound does not perform prefetch.

root@hat1:~/unbound-1.4.13# dig @127.1 www.facebook.com.

; <<>> DiG 9.9.0a2 <<>> @127.1 www.facebook.com.
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 65369
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;www.facebook.com. IN A

;; ANSWER SECTION:
www.facebook.com. 0 IN A 69.171.228.12

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Thu Oct 13 01:33:40 2011
;; MSG SIZE rcvd: 50

root@hat1:~/unbound-1.4.13# dig @127.1 www.facebook.com.

; <<>> DiG 9.9.0a2 <<>> @127.1 www.facebook.com.
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 25443
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;www.facebook.com. IN A

;; ANSWER SECTION:
www.facebook.com. 0 IN A 69.171.228.12

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; MSG SIZE rcvd: 50

root@hat1:~/unbound-1.4.13# dig @127.1 www.facebook.com.

; <<>> DiG 9.9.0a2 <<>> @127.1 www.facebook.com.
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49418
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;www.facebook.com. IN A

;; ANSWER SECTION:
www.facebook.com. 30 IN A 66.220.149.18

;; Query time: 116 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; MSG SIZE rcvd: 50

When enabling prefetch , common sites ( FQDNs ) will be cached before expiring its data.
Benefits of this technique are low latency and high caching hit rate.

lost and found ( for me ? )

lost and found ( for me ? )

unbound : prefetch

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