|This script can be used to implement persistent leases on openWRT, DD-WRT
|etc. Persistent leases are good: if the lease database is lost on a
|reboot, then it will eventually be restored as hosts renew their
|leases. Until a host renews (which may take hours/days) it will
|not exist in the DNS if dnsmasq's DDNS function is in use.
|*WRT systems remount all non-volatile filesystems read-only after boot,
|so the normal leasefile will not work. They do, however have NV
|storage, accessed with the nvram command:
|/usr/lib # nvram
|usage: nvram [get name] [set name=value] [unset name] [show]
|The principle is that leases are kept in NV variable with data
|corresponding to the line in a leasefile:
|dnsmasq_lease_192.168.1.56=3600 00:41:4a:05:80:74 192.168.1.56 * *
|By giving dnsmasq the leasefile-ro command, it no longer creates or writes a
|leasefile; responsibility for maintaining the lease database transfers
|to the lease change script. At startup, in leasefile-ro mode,
|dnsmasq will run
|and read whatever that command spits out, expecting it to
|be in dnsmasq leasefile format.
|So the lease change script, given "init" as argv will
|suck existing leases out of the NVRAM and emit them from
|stdout in the correct format.
|The second part of the problem is keeping the NVRAM up-to-date: this
|is done by the lease-change script which dnsmasq runs when a lease is
|updated. When it is called with argv as "old", "add", or "del"
|it updates the relevant nvram entry.
|So, dnsmasq should be run as :
|dnsmasq --leasefile-ro --dhcp-script=/path/to/lease_update.sh
|or the same flags added to /etc/dnsmasq.conf
|This needs dnsmasq-2.33 or later to work.
|This technique will work with, or without, compilation with
|HAVE_BROKEN_RTC. Compiling with HAVE_BROKEN_RTC is
|_highly_recommended_ for this application since is avoids problems
|with the system clock being warped by NTP, and it vastly reduces the
|number of writes to the NVRAM. With HAVE_BROKEN_RTC, NVRAM is updated
|only when a lease is created or destroyed; without it, a write occurs
|every time a lease is renewed.
|It probably makes sense to restrict the number of active DHCP leases
|to an appropriate number using dhcp-lease-max. On a new DD_WRT system,
|there are about 10K bytes free in the NVRAM. Each lease record is
|about 100 bytes, so restricting the number of leases to 50 will limit
|use to half that. (The default limit in the distributed source is 150)
|Any UI script which reads the dnsmasq leasefile will have to be
|amended, probably by changing it to read the output of
|`lease_update init` instead.
|To Steve Horbachuk for checks on the script and debugging beyond the
|call of duty.
|Fri Jul 28 11:51:13 BST 2006