1This script can be used to implement persistent leases on openWRT, DD-WRT
2etc. Persistent leases are good: if the lease database is lost on a
3reboot, then it will eventually be restored as hosts renew their
4leases. Until a host renews (which may take hours/days) it will
5not exist in the DNS if dnsmasq's DDNS function is in use.
6
7*WRT systems remount all non-volatile fileystems read-only after boot,
8so the normal leasefile will not work. They do, however have NV
9storage, accessed with the nvram command:
10
11/usr/lib #  nvram
12usage: nvram [get name] [set name=value] [unset name] [show]
13
14The principle is that leases are kept in NV variable with data
15corresponding to the line in a leasefile:
16
17dnsmasq_lease_192.168.1.56=3600 00:41:4a:05:80:74 192.168.1.56 * *
18
19By giving dnsmasq the leasefile-ro command, it no longer creates or writes a
20leasefile; responsibility for maintaining the lease database transfers
21to the lease change script. At startup, in leasefile-ro mode,
22dnsmasq will run
23
24"<lease_change_script> init" 
25
26and read whatever that command spits out, expecting it to
27be in dnsmasq leasefile format.
28
29So the lease change script, given "init" as argv[1] will 
30suck existing leases out of the NVRAM and emit them from
31stdout in the correct format.
32
33The second part of the problem is keeping the NVRAM up-to-date: this
34is done by the lease-change script which dnsmasq runs when a lease is
35updated. When it is called with argv[1] as "old", "add", or "del"
36it updates the relevant nvram entry.
37
38So, dnsmasq should be run as :
39
40dnsmasq --leasefile-ro --dhcp-script=/path/to/lease_update.sh
41
42or the same flags added to /etc/dnsmasq.conf
43
44
45
46Notes: 
47
48This needs dnsmasq-2.33 or later to work.
49
50This technique will work with, or without, compilation with
51HAVE_BROKEN_RTC. Compiling with HAVE_BROKEN_RTC is
52_highly_recommended_ for this application since is avoids problems
53with the system clock being warped by NTP, and it vastly reduces the
54number of writes to the NVRAM. With HAVE_BROKEN_RTC, NVRAM is updated
55only when a lease is created or destroyed; without it, a write occurs
56every time a lease is renewed.
57
58It probably makes sense to restrict the number of active DHCP leases
59to an appropriate number using dhcp-lease-max. On a new DD_WRT system,
60there are about 10K bytes free in the NVRAM. Each lease record is
61about 100 bytes, so restricting the number of leases to 50 will limit
62use to half that. (The default limit in the distributed source is 150)
63
64Any UI script which reads the dnsmasq leasefile will have to be
65ammended, probably by changing it to read the output of 
66`lease_update init` instead.
67 
68
69Thanks:
70
71To Steve Horbachuk for checks on the script and debugging beyond the 
72call of duty.
73
74
75Simon Kelley
76Fri Jul 28 11:51:13 BST 2006
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