[Bug 3553] Retain lease information for udhcpc across restarts

[bugzilla at busybox.net]

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

--- Comment #6 from Denys Vlasenko <vda.linux at googlemail.com>  ---
(In reply to comment #3)
> 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.

I see. I think this functionality can be implemented by dhcp script:

>        When  a  new  lease  is  acquired,  it  is  appended  to the end of the
>        dhclient.leases file...

This can be implemented when "$script bound" is called, by recording values of
$ip, $lease etc.

>        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...

This can be implemented when "$script leasefail" is called, by analyzing
previously-recorded lease(s), and activating it.

>        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

Also can be implemented by script's "leasefail" handler.

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