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Annotation of embedaddon/dnsmasq/contrib/wrt/README, revision 1.1.1.2

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: 
1.1.1.2 ! misho       7: *WRT systems remount all non-volatile filesystems read-only after boot,
1.1       misho       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
1.1.1.2 ! misho      65: amended, probably by changing it to read the output of 
1.1       misho      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: