OpenWRTDNSUpstreamChanger

This script changes the upstream DNS resolver of OpenWRT dnsmasq in case e.g. a local default DNS resolver (e.g. pi-hole) used as upstream resolver is unavailable or switching back to the default DNS upstream resolver in case it becomes available again. The scripts sends a mail if the DNS config is switched.

The script might be helpful if you deployed a local DNS resolver such as pi-hole. In this case, you will probably enable dnsmasq's option 'strict order' to have all your DNS request be processed by pi-hole. However, by enabling the strict order setting, the availability of your local pi-hole DNS resolver becomes crucial. In case the local pi-hole service is unavailable, DNS queries will time out before being send by OpenWRT to a secondary DNS defined in the config. Thus, your clients will experience DNS queries to be long running. In this scenario the provided script will regularly check if the local pi-hole service is available. If it is not, the script changes OpenWRT's upstream DNS resolver to a defined backup. The script also reverts OpenWRT upstream DNS resolver setting back to your local pi-hole DNS resolver once it becomes available again.

Dependencies

Install 'bind-dig' and 'msmtp' package.

opkg update
opkg install bind-dig msmtp

Copy files

Move DNS change script to /root and resolv configurations to /etc.

mv -r DNSChanger /root
mv resolvbackup.conf resolvdefault.conf /etc

Enable dnsmasq 'strict order'

Enable dnsmasq's option 'strict order' to ensure that always the defined primary upstream DNS resolver is used. This is a reasonable setting in case you want all DNS queries to be processed by your primary DNS i.e. if you deployed a pi-hole service which shall process all DNS queries.

uci set dhcp.@dnsmasq[0].strictorder=1
uci commit dhcp
/etc/init.d/dnsmasq restart

Change settings

Change the resolv configurations to your setting.

nano /etc/resolvbackup.conf
nano /etc/resolvdefault.conf

Add your mail credentials to msmtprc configuration.

nano /etc/msmtprc

Change email recipient and sender in script.

nano /root/DNSChanger/ChangeUpstreamDNS.sh

Make DNS change script executable for cron

sudo chmod +x /root/DNSChanger/ChangeUpstreamDNS.sh

Add cronjob

For example, add a cronjob checking the default DNS every 5 minutes..

crontab -e

Add the follwing line.

*/5 * * * * sh /root/DNSChanger/ChangeUpstreamDNS.sh

Start and enable cron

/etc/init.d/cron start
/etc/init.d/cron enable