Você pode criar um script if-up que verificará se o dnsmasq deve ser reiniciado.
/etc/network/if-up.d/dnsmasq:
#!/bin/sh
[ "$IFACE" != "lo" ] || exit 0
restartDnsMasq() {
if [ -d /run/systemd/system ]; then
systemctl reload --no-block dnsmasq >/dev/null 2>&1 || true
else
invoke-rc.d dnsmasq restart >/dev/null 2>&1 || true
fi
}
# Find out if dnsmasq is configured to run on a single interface
interface=$(cat /etc/dnsmasq.conf | grep interface | awk -F '=' '{print $2}')
if [ "x${interface}" = "x" ]; then
# all interfaces
logger DnsMasq not configured for any particular interface, restarting because $IFACE came up.
restartDnsMasq
else
if [ "${interface}" = "$IFACE" ]; then
# The interface that dnsmasq is running on is being brought up
logger DnsMasq configured for interface ${interface}, restarting because $IFACE came up.
restartDnsMasq
else
logger DnsMasq configured for interface ${interface}, not restarting because $IFACE came up.
fi
fi