[Bug 3553] Retain lease information for udhcpc across restarts

[bugzilla at busybox.net]

https://bugs.busybox.net/show_bug.cgi?id=3553

--- Comment #3 from Philip Prindeville <philipp at redfish-solutions.com>  ---
To be clear, I'll quote the ISC dhclient man page, which already has this
functionality:

       In  order  to  keep  track  of  leases across system reboots and server
       restarts, dhclient keeps a list of leases it has been assigned  in  the
       dhclient.leases(5)  file.   On startup, after reading the dhclient.conf
       file, dhclient reads the dhclient.leases file  to  refresh  its  memory
       about what leases it has been assigned.

       When  a  new  lease  is  acquired,  it  is  appended  to the end of the
       dhclient.leases file.   In order to  prevent  the  file  from  becoming
       arbitrarily   large,   from   time  to  time  dhclient  creates  a  new
       dhclient.leases file from its in-core lease database.  The old  version
       of the dhclient.leases file is retained under the name dhclient.leases~
       until the next time dhclient rewrites the database.

       Old leases are kept around in case the DHCP server is unavailable  when
       dhclient  is  first  invoked  (generally during the initial system boot
       process).   In that event, old leases  from  the  dhclient.leases  file
       which have not yet expired are tested, and if they are determined to be
       valid, they are used until  either  they  expire  or  the  DHCP  server
       becomes available.

       A  mobile host which may sometimes need to access a network on which no
       DHCP server exists may be preloaded with a lease for a fixed address on
       that network.   When all attempts to contact a DHCP server have failed,
       dhclient will try to validate the static lease,  and  if  it  succeeds,
       will use that lease until it is restarted.

       A  mobile  host  may  also travel to some networks on which DHCP is not
       available but BOOTP is.   In that  case,  it  may  be  advantageous  to
       arrange  with the network administrator for an entry on the BOOTP data‐
       base, so that the host can boot quickly on  that  network  rather  than
       cycling through the list of old leases.

So what I'm talking about is the ability to statically maintain (and reuse)
unexpired leases across reboots.

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