AIX DNS rotate, timeout and attempts options

Back in 2009/10, I wrote a couple of blog posts about some new (at the time) options for the resolv.conf file on AIX 6.1. Now, some 15 years later, things haven’t changed a lot since then, but an email I received from someone, asking about an issue with AIX DNS, prompted me to re-visit the options and provide a new example of how to (potentially) configure /etc/resolv.conf on your AIX systems.

The email said:

“We performed some maintenance on our primary DNS server (hosted on another OS, not AIX). We had hoped that the AIX resolver would automatically ‘fail over’ to the secondary DNS server during the outage. And then automatically ‘fall back’ to the primary DNS server when it was online again. We’d hoped that AIX would use the nameserver entries in our /etc/resolv.conf file to determine which name server to use. But this did not happen. And as a result, it impacted our application. To work around the issue we temporarily removed the first name server from the resolv.conf file. When the primary DNS server was online again, we added it back.

# cat /etc/resolv.conf

domain ibm.edu

search ibm.edu

nameserver      10.1.1.120

nameserver      10.1.1.121

nameserver      10.2.1.130

# tail /etc/netsvc.conf

hosts = local,bind4

# oslevel -s

7300-01-02-2320

Can you suggest how we could configure our resolv.conf to handle this type of situation better? Thanks!”

My response was:

Based on your current resolv.conf file, the DNS behaviour you described is working as expected, because you haven't configured name server rotation. Typically, the resolver sends the DNS queries to the first name server only, unless you explicitly configure the name server rotation option. For example:

domain ibm.edu

search ibm.edu

nameserver      10.1.1.120

nameserver      10.1.1.121

nameserver      10.2.1.130

options rotate

options timeout:1

options attempts:1

With the options above, the resolver waits for 1 second (timeout) for the DNS query before it moves to the next one in the list, after one DNS query attempt (attempts). The same behaviour repeats for the rest of the name servers (rotate). 

Here are some useful links that discuss AIX, DNS, resolv.conf, rotate, timeout and attempts options.

https://www.ibm.com/docs/en/aix/7.3?topic=formats-resolvconf-file-format-tcpip

http://gibsonnet.net/blog/cgaix/html/AIX%206.1%20resolv.conf%20and%20netcd..html

http://gibsonnet.net/blog/cgaix/html/New%20resolv.conf%20options%20in%20AIX%206.1.html

https://www.ibm.com/support/pages/ibm-aix-behavior-host-name-resolution

https://keymon.wordpress.com/2010/03/09/the-weird-domain-name-resolution-mechanism-on-aix/