1 This script can be used to implement persistent leases on openWRT, DD-WRT 2 etc. Persistent leases are good: if the lease database is lost on a 3 reboot, then it will eventually be restored as hosts renew their 4 leases. Until a host renews (which may take hours/days) it will 5 not 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, 8 so the normal leasefile will not work. They do, however have NV 9 storage, accessed with the nvram command: 10 11 /usr/lib # nvram 12 usage: nvram [get name] [set name=value] [unset name] [show] 13 14 The principle is that leases are kept in NV variable with data 15 corresponding to the line in a leasefile: 16 17 dnsmasq_lease_192.168.1.56=3600 00:41:4a:05:80:74 192.168.1.56 * * 18 19 By giving dnsmasq the leasefile-ro command, it no longer creates or writes a 20 leasefile; responsibility for maintaining the lease database transfers 21 to the lease change script. At startup, in leasefile-ro mode, 22 dnsmasq will run 23 24 "<lease_change_script> init" 25 26 and read whatever that command spits out, expecting it to 27 be in dnsmasq leasefile format. 28 29 So the lease change script, given "init" as argv[1] will 30 suck existing leases out of the NVRAM and emit them from 31 stdout in the correct format. 32 33 The second part of the problem is keeping the NVRAM up-to-date: this 34 is done by the lease-change script which dnsmasq runs when a lease is 35 updated. When it is called with argv[1] as "old", "add", or "del" 36 it updates the relevant nvram entry. 37 38 So, dnsmasq should be run as : 39 40 dnsmasq --leasefile-ro --dhcp-script=/path/to/lease_update.sh 41 42 or the same flags added to /etc/dnsmasq.conf 43 44 45 46 Notes: 47 48 This needs dnsmasq-2.33 or later to work. 49 50 This technique will work with, or without, compilation with 51 HAVE_BROKEN_RTC. Compiling with HAVE_BROKEN_RTC is 52 _highly_recommended_ for this application since is avoids problems 53 with the system clock being warped by NTP, and it vastly reduces the 54 number of writes to the NVRAM. With HAVE_BROKEN_RTC, NVRAM is updated 55 only when a lease is created or destroyed; without it, a write occurs 56 every time a lease is renewed. 57 58 It probably makes sense to restrict the number of active DHCP leases 59 to an appropriate number using dhcp-lease-max. On a new DD_WRT system, 60 there are about 10K bytes free in the NVRAM. Each lease record is 61 about 100 bytes, so restricting the number of leases to 50 will limit 62 use to half that. (The default limit in the distributed source is 150) 63 64 Any UI script which reads the dnsmasq leasefile will have to be 65 ammended, probably by changing it to read the output of 66 `lease_update init` instead. 67 68 69 Thanks: 70 71 To Steve Horbachuk for checks on the script and debugging beyond the 72 call of duty. 73 74 75 Simon Kelley 76 Fri Jul 28 11:51:13 BST 2006 77 78 79 80 81 82
