How FreeBSD Handles a DNS that is Down

[Matthias Apitz]

El día Thursday, October 21, 2010 a las 06:22:15AM -0500, Martin McCormick escribió:

> 	This is an extremely novice question on my part, but
> after what I recently witnessed, I am not so sure I understand
> all I know.
> 
> 	The normal procedure on internet-connected systems is to
> set the resolv.conf file to include at least 2 domain name
> servers. Example:
> 
> nameserver	139.78.100.1
> nameserver	139.78.200.1
> 
> 	Last night, I had to take down our primary DNS for
> maintenance and lots of systems began having trouble of various
> kinds.
...

The man page of resolv.conf states that the DNS are queried in that
order and if one timed out the next is queried; and this is that way for
any new resolver request; I've put one which does not exist as first
entry (10.0.1.99) and the existing in 2nd place (10.0.1.201) and checked
with tcpdump what happened when I do 'ping www.muc.de' three times:

# tcpdump -n host 10.0.1.99 or host 10.0.1.201
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on wlan0, link-type EN10MB (Ethernet), capture size 96 bytes
13:37:43.401553 IP 10.49.96.52.44280 > 10.0.1.99.53: 13264+ A?  www.muc.de. (28)
13:37:48.403868 IP 10.49.96.52.15468 > 10.0.1.201.53: 13264+ A?  www.muc.de. (28)
13:37:48.430125 IP 10.0.1.201.53 > 10.49.96.52.15468: 13264 1/0/0 A 193.149.48.8 (44)


13:37:59.240499 IP 10.49.96.52.42369 > 10.0.1.99.53: 36140+ A?  www.muc.de. (28)
13:38:04.242653 IP 10.49.96.52.28001 > 10.0.1.201.53: 36140+ A?  www.muc.de. (28)
13:38:04.244321 IP 10.0.1.201.53 > 10.49.96.52.28001: 36140 1/0/0 A 193.149.48.8 (44)


13:38:14.964752 IP 10.49.96.52.24065 > 10.0.1.99.53: 39922+ A?  www.muc.de. (28)
13:38:19.967153 IP 10.49.96.52.19756 > 10.0.1.201.53: 39922+ A?  www.muc.de. (28)
13:38:19.968822 IP 10.0.1.201.53 > 10.49.96.52.19756: 39922 1/0/0 A 193.149.48.8 (44)

This mean that it will at least slow down any new network connection

HIH

	matthias


-- 
Matthias Apitz
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