On OpenWRT routers configured to route all traffic through VPN by default, force NTP to be routed via the local default gateway

assign-ntp-servers-to-local-default-route.sh

#!/bin/sh
DEFAULTROUTE=`/sbin/route | /bin/grep -v '*' | /bin/grep eth0 | /usr/bin/awk '{print $2}' | /usr/bin/head -n1`
NTPSERVERS=`/bin/grep 'list server' /etc/config/system  | /usr/bin/awk '{print $3}' | /bin/sed "s/'//g"`

for HOST in $NTPSERVERS; do /sbin/route add $HOST gw $DEFAULTROUTE; done

For some reason, this doesn't work when run from /etc/rc.local, likely because no routes exist by the time that executes. Running it from cron every few minutes works, it doesn't matter if it runs multiple times.