DHCP limit can be set over safe value, causing router to not boot up

[okias]

Luci allow setting DHCP out-of-range, which leads to state, where DHCP won't start next boot. (since 100+250 > 255).

For many routers, there is definition of DHCP range by start and end address, which can be easily overlooked / mistaken on luci.

I propose to check with some easy math JS, which will restrict start + limit < 255, otherwise changes field color to red.

[jow-]

Works fine for me, ipcalc.sh, which is used by the dnsmasq init script, is capping the value to the maximum allowed value within the subnet mask:

root@jj:~# ipcalc.sh 10.11.12.13 255.255.255.0 100 250
IP=10.11.12.13
NETMASK=255.255.255.0
BROADCAST=10.11.12.255
NETWORK=10.11.12.0
PREFIX=24
START=10.11.12.100
END=10.11.12.254

I also did put option limit 250 into /etc/config/dhcp and restarted dnsmasq which worked fine.
The generated config also contained a sane value:

root@jj:~# grep dhcp-range /var/etc/dnsmasq.conf.cfg01411c 
dhcp-range=set:lan,10.11.12.100,10.11.12.254,255.255.255.0,12h

I propose to check with some easy math JS, which will restrict start + limit < 255, otherwise changes field color to red.

Such a solution would be wrong as the limit is not 255 but actually depending on the interface subnet size. This is also the reason why OpenWrt uses start, limit notation and not "last digit of IP" since we do not presume fixed /24 subnets.

[okias]

Maybe my "not working DHCP" issue was caused by something else, since it was production router, when I changed value back and it booted, I haven't tried again. I'll try investigate more.