Date: Thu, 21 Oct 2010 13:44:17 +0200 From: Matthias Apitz <guru@unixarea.de> To: Martin McCormick <martin@dc.cis.okstate.edu> Cc: freebsd-questions@freebsd.org Subject: Re: How FreeBSD Handles a DNS that is Down Message-ID: <20101021114417.GA4059@current.Sisis.de> In-Reply-To: <201010211122.o9LBMFa2063759@x.it.okstate.edu> References: <201010211122.o9LBMFa2063759@x.it.okstate.edu>
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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 t +49-89-61308 351 - f +49-89-61308 399 - m +49-170-4527211 e <guru@unixarea.de> - w http://www.unixarea.de/
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