[Pi-hole auto DNS switch] The script automatically switches the DNS servers between Pi-hole and Cloudflare based on Pi-hole DNS Server status. #pihole #openwrt

auto-dns.sh

#!/bin/sh

# The script automatically changed DNS servers advertised by DHCP based on Pi-hole DNS Server status.
# https://gist.github.com/flexoid/fb2cc01b3ff3ac19659269a9c4c9ae2d

TARGET=192.168.20.112 # Pi-hole
FALLBACK=8.8.8.8,8.8.4.4

function set_fallback_dns() {
    echo $(date)
    echo "Setting fallback DNS servers"
    echo $FALLBACK

    uci -q delete dhcp.lan.dhcp_option
    uci add_list dhcp.lan.dhcp_option="6,$FALLBACK"
    uci commit dhcp

    /etc/init.d/dnsmasq restart
    /etc/init.d/odhcpd restart
}

TARGET_PING_COUNT=$(ping -c 3 -w 3 $TARGET | grep seq | wc -l)
# check if pi is down
if [ $TARGET_PING_COUNT -eq 0 ]; then
    FALLBACK_DNS_COUNT=$(uci show dhcp.lan.dhcp_option | grep $FALLBACK | wc -l)
    # check if fallback is not set as a DNS server
    if [ $FALLBACK_DNS_COUNT -eq 0 ]; then
        set_fallback_dns
    fi
else
    TARGET_DNS_COUNT=$(uci show dhcp.lan.dhcp_option | grep $TARGET | wc -l)
    # check if target is not set as a DNS server
    if [ $TARGET_DNS_COUNT -eq 0 ]; then
        TARGET_DNS_INFO_COUNT=$(dig +short @$TARGET TXT CHAOS version.bind | grep "pi-hole" | wc -l)
        # check if pi-hole DNS is up
        if [ $TARGET_DNS_INFO_COUNT -eq 1 ]; then
            echo $(date)
            echo "Setting target DNS server"
            echo $TARGET
            uci -q delete dhcp.lan.dhcp_option
            uci add_list dhcp.lan.dhcp_option="6,$TARGET"
            uci commit dhcp

            /etc/init.d/dnsmasq restart
            /etc/init.d/odhcpd restart
        else
            set_fallback_dns
        fi
    fi
fi

crontab.md

crontab

Edit configuration

crontab -e

The following job runs the /root/auto-dns.sh script every minute and appends the output to /root/cron.log file.

* * * * * /root/auto-dns.sh >> /root/cron.log 2>&1

Show configuration

crontab -l

Apply changes

/etc/init.d/cron restart