OpenWRT DNS Watch

dns_watch.sh

#!/bin/ash
# https://gist.github.com/levid0s/2cb9f64234aab3b3a400b57d5d92f840

set -eu

usage="
Suggested crontab:
*/5 * * * *  /root/dns_watch.sh 2>> /tmp/dns_watch.log
"

PREFERRED_DNS="192.168.1.2" # eg. Adguard Home
FALLBACK_DNS="8.8.8.8"
ROUTER_IP=$(ip addr show br-lan | grep 'inet ' | awk '{print $2}' | cut -d'/' -f1)
TEST_QUERY="www.example.com"
RESOLVER="/usr/bin/nslookup"

CHECK_INTERVAL=20
FALLBACK_THRESHOLD="${1-120}" # $1 or default
PREFERRED_THRESHOLD="${1-120}" # $1 or default

: ${debug:=false}
[ "$debug" = 1 ] && debug=true
command -v $debug >/dev/null || { echo "ERROR: Incorrect value for \$debug" >&2; exit 1; }

echo "$ROUTER_IP" | grep -qE '^((10\.([0-9]{1,2}|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.([0-9]{1,2}|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.([0-9]{1,2}|1[0-9]{2}|2[0-4][0-9]|25[0-5]))|(172\.(1[6-9]|2[0-9]|3[0-1])\.([0-9]{1,2}|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.([0-9]{1,2}|1[0-9]{2}|2[0-4][0-9]|25[0-5]))|(192\.168\.([0-9]{1,2}|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.([0-9]{1,2}|1[0-9]{2}|2[0-4][0-9]|25[0-5])))$' || { echo "ERROR: Error fetching the router's LAN IP: $ROUTER_IP" >&2; exit 1; }

check_preferred_dns_alive() {
    $RESOLVER $TEST_QUERY $PREFERRED_DNS >/dev/null 2>&1
}

update_to_failover() {
    dnsfw_base=$(uci get dhcp.@dnsmasq[0].server | sed "s~\s\?$PREFERRED_DNS~~g")
    dnsfw_new="$dnsfw_base $FALLBACK_DNS"
    $debug && echo "DEBUG: uci set dhcp.@dnsmasq[0].server=\"$dnsfw_new\"" >&2 
    $debug || uci set dhcp.@dnsmasq[0].server="$dnsfw_new"
    
    dhcp_base=$(uci get dhcp.lan.dhcp_option | sed "s~\s\?6,$PREFERRED_DNS~~g")
    dhcp_new="$dhcp_base 6,$ROUTER_IP"
    $debug && echo "DEBUG: uci set dhcp.lan.dhcp_option=\"$dhcp_new\"" >&2
    $debug || uci set dhcp.lan.dhcp_option="$dhcp_new"
    
    $debug && echo "DEBUG: restarting dnsmasq.." >&2
    $debug || /etc/init.d/dnsmasq restart
    echo "INFO: DNS FAILED OVER to: $ROUTER_IP -> $FALLBACK_DNS" >&2
}

update_to_preferred() {
    dnsfw_base=$(uci get dhcp.@dnsmasq[0].server | sed "s~\s\?$FALLBACK_DNS~~g")
    dnsfw_new="$dnsfw_base $PREFERRED_DNS"
    $debug && echo "DEBUG: uci set dhcp.@dnsmasq[0].server="$dnsfw_new"" >&2
    $debug || uci set dhcp.@dnsmasq[0].server="$dnsfw_new"
    
    dhcp_base=$(uci get dhcp.lan.dhcp_option | sed "s~\s\?6,$ROUTER_IP~~g")
    dhcp_new="$dhcp_base 6,$PREFERRED_DNS"
    $debug && echo "DEBUG: uci set dhcp.lan.dhcp_option="$dhcp_new"" >&2
    $debug || uci set dhcp.lan.dhcp_option="$dhcp_new"
    
    $debug || /etc/init.d/dnsmasq restart
    echo "INFO: DNS RESTORED to: $PREFERRED_DNS" >&2
}

wait_for_dns() {
    local target_status="$1" duration="$2"
    local elapsed=0 last_status
    
    $debug && echo "DEBUG: Waiting ${duration}s for preferred_dns_status to keep the status $target_status" >&2
    
    while [ $elapsed -lt $duration ]; do
        
        check_preferred_dns_alive && last_status=0 || last_status=1
        $debug && echo "DEBUG: check_preferred_dns_alive status: $last_status" >&2
        
        if [ "$last_status" -ne "$target_status" ]; then
            echo "WARN: Got DNS server status: $last_status, cancelling operation." >&2
            exit 0
        fi
        
        sleep $CHECK_INTERVAL
        elapsed=$((elapsed + CHECK_INTERVAL))
    done
}

get_current_mode() {
    if uci get dhcp.@dnsmasq[0].server | grep -qE "[^/]$PREFERRED_DNS"; then
        echo PREFERRED
    elif uci get dhcp.@dnsmasq[0].server | grep -qE "[^/]$FALLBACK_DNS"; then
        echo FALLBACK
    else
        echo "ERROR: Error determining fallback mode" >&2
        exit 1
    fi
}

CURRENT_MODE=$(get_current_mode)
DESIRED_MODE=$(check_preferred_dns_alive && echo "PREFERRED" || echo "FALLBACK")
if $debug; then
    echo -e "CURRENT=$CURRENT_MODE\nDESIRED=$DESIRED_MODE" >&2
    echo "uci get dhcp.@dnsmasq[0].server=$(uci get dhcp.@dnsmasq[0].server)" >&2
    echo "uci get dhcp.lan.dhcp_option=$(uci get dhcp.lan.dhcp_option)" >&2
fi

if [ "$CURRENT_MODE" = "$DESIRED_MODE" ]; then
    $debug && echo "DEBUG: Current mode already matches desired mode, exiting. ($CURRENT_MODE)" >&2
    exit 0
fi

if [ "$DESIRED_MODE" = FALLBACK ]; then
    echo "INFO: DNS server unreachable: $PREFERRED_DNS, waiting ${FALLBACK_THRESHOLD}s before failover.." >&2
    wait_for_dns 1 $FALLBACK_THRESHOLD && update_to_failover
    exit 0
fi

if [ "$DESIRED_MODE" = PREFERRED ]; then
    echo "INFO: DNS server back online: $PREFERRED_DNS, waiting ${PREFERRED_THRESHOLD}s before restore.." >&2
    wait_for_dns 0 $PREFERRED_THRESHOLD && update_to_preferred
    exit 0
fi