nsfailover

Together with EC2 Premium support I've established that:

"Unfortunately the Linux DNS resolver doesn't seem to have direct support for detecting and doing failovers for DNS servers so you may need to write your own solution as you mentioned. " - Amazon Web Services Jan 22, 2013 01:13 AM PST

Read a longer introduction on my blog which was featured on Hacker News.

This simple program makes DNS outages suck less.

nsfailover

Every minute (or whatever), nsfailover.sh checks to see if the primary configured nameserver can resolve google.com. If it cannot, it writes the secondary, or even tertary server to function as the primary server in /etc/resolv.conf.

This way, requests are stalled for max a minute, and then all following requests are fast, even if the primary stays down.

Install

sudo curl -q https://raw.github.com/kvz/nsfailover/master/nsfailover.sh -o /usr/bin/nsfailover.sh && sudo chmod +x $_

Example

crontab -e
* * * * * NS_1=172.16.0.23 nsfailover.sh 2>&1 |logger -t cron-nsfailover

Config

nsfailover is configured through environment variables. Here they are with their defaults:

LOG_LEVEL="6" # 7 = debug, 0 = emergency
NS_1="" # Primary Nameserver (172.16.0.23 for Amazon EC2). You need to set this yourself
NS_2="8.8.8.8" # Secundary Nameserver: Google
NS_3="4.2.2.2" # Tertiary Nameserver: Level3
NS_ATTEMPTS="1" # http://linux.die.net/man/5/resolv.conf
NS_ENABLE="no" # Set to no to disable
NS_FILE="/etc/resolv.conf" # Where to write resolving conf
NS_SEARCH="" # Domain to search hosts in (compute-1.internal for Amazon EC2)
NS_TESTDOMAIN="google.com" # Use this to determine if NS is healthy
NS_TIMEOUT="3" # http://linux.die.net/man/5/resolv.conf
NS_WRITEPROTECT="no" # Use this to write-protect /etc/resolv.conf

You can use environment variables in many ways: at the top of a script or crontab, export from another script, or pass them straight to the program:

NS_ENABLE="no" ./nsfailover.sh # <-- silly, but works :)

Notes

nsfailover

• only rewrites /etc/resolv.conf if it has changes
• makes a backup to e.g. /etc/resolv.conf.bak-20130327114321
• needs to run as root

Tips

• Prefix your cronjob with timeout -s 9 50s so there can never be an overlap. More tips in my Lock your Cronjobs article.

Versioning

This project implements the Semantic Versioning guidelines.

Releases will be numbered with the following format:

<major>.<minor>.<patch>

And constructed with the following guidelines:

• Breaking backward compatibility bumps the major (and resets the minor and patch)
• New additions without breaking backward compatibility bumps the minor (and resets the patch)
• Bug fixes and misc changes bumps the patch

For more information on SemVer, please visit http://semver.org.

License

Copyright (c) 2013 Kevin van Zonneveld, http://kvz.io
Licensed under MIT: http://kvz.io/licenses/LICENSE-MIT