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dnsmasq 2.84

    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 filesystems 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: amended, 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: 
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   79: 
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