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Top Forums UNIX for Advanced & Expert Users One of the two DNS server going down causes impacts Post 303043094 by Peasant on Saturday 18th of January 2020 02:07:05 AM
Old 01-18-2020
I've seen this mostly related to DNS query timeouts setup from client side.
The defaults are quite high on most linux/unix operating system, from AIX man page online :
Quote:
timeout:n Enables you to specify the initial timeout for a query to a nameserver. The default value is five seconds. The maximum value is 30 seconds. For the second and successive rounds of queries, the resolver doubles the initial timeout and is divided by the number of nameservers in the resolv.conf file.
attempts:n Enables you to specify how many queries the resolver should send to each nameserver in the resolv.conf file before it stops execution. The default value is 4. The maximum value is 5.
In practice if you have, for instance, two dns servers, and first one /etc/resolv.conf goes down...
The system will try to query first with timeout of 5 seconds and 4 attempts, totaling 20 seconds, until second is tried.

This will for sure hit some timeouts from application side, e.g application will timeout before system returns valid DNS entry.

As for nslookup working, i'm unsure. It this from the same box ?

Suggestion is to change to defaults to lower values and/or implement DNS caching mechanism locally on AIX box.

Hope that helps
Regards
Peasant.
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RESOLVER(5)							File Formats Manual						       RESOLVER(5)

NAME
resolver - resolver configuration file SYNOPSIS
/etc/resolv.conf DESCRIPTION
The resolver configuration file contains information that is read by the resolver routines the first time they are invoked by a process. The file is designed to be human readable and contains a list of name-value pairs that provide various types of resolver information. On a normally configured system this file should not be necessary. The only name server to be queried will be on the local machine and the domain name is retrieved from the system. The different configuration options are: nameserver followed by the Internet address (in dot notation) of a name server that the resolver should query. At least one name server should be listed. Up to MAXNS (currently 3) name servers may be listed, in that case the resolver library queries tries them in the order listed. If no nameserver entries are present, the default is to use the name server on the local machine. (The algorithm used is to try a name server, and if the query times out, try the next, until out of name servers, then repeat trying all the name servers until a maximum number of retries are made). domain followed by a domain name, that is the default domain to append to names that do not have a dot in them. If no domain entries are present, the domain returned by gethostname(2) is used (everything after the first `.'). Finally, if the host name does not contain a domain part, the root domain is assumed. The name value pair must appear on a single line, and the keyword (e.g. nameserver) must start the line. The value follows the keyword, separated by white space. FILES
/etc/resolv.conf SEE ALSO
gethostbyname(3N), resolver(3), named(8) Name Server Operations Guide for BIND 4th Berkeley Distribution September 14, 1987 RESOLVER(5)
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