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Annotation of embedaddon/dnsmasq/contrib/wrt/README, revision 1.1

1.1     ! misho       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: