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 @page libdhcpsrv libdhcpsrv - Server DHCP library

This library contains code useful for DHCPv4 and DHCPv6 server operations, like
Lease Manager that stores leases information, configuration manager that stores
configuration etc. The code here is server specific. For generic (useful in
server, client, relay and other tools like perfdhcp) code, please see
\ref libdhcp.

This library contains several crucial elements of the DHCP server operation:

- isc::dhcp::LeaseMgr - Lease Manager is a name for database backend that stores
- isc::dhcp::CfgMgr - Configuration Manager that holds DHCP specific
  configuration information (subnets, pools, options, timer values etc.) in
  easy to use format.
- AllocEngine - allocation engine that handles new requestes and allocates new

@section leasemgr Lease Manager

LeaseMgr provides a common, unified abstract API for all database backends. All
backends are derived from the base class isc::dhcp::LeaseMgr. Currently the
only available backend is MySQL (see \ref isc::dhcp::MySqlLeaseMgr).

@section cfgmgr Configuration Manager

Configuration Manager (\ref isc::dhcp::CfgMgr) stores configuration information
necessary for DHCPv4 and DHCPv6 server operation. In particular, it stores
subnets (\ref isc::dhcp::Subnet4 and \ref isc::dhcp::Subnet6) together with
their pools (\ref isc::dhcp::Pool4 and \ref isc::dhcp::Pool6), options and
other information specified by the used in BIND10 configuration.

@section allocengine Allocation Engine

Allocation Engine (\ref isc::dhcp::AllocEngine) is what its name say - an engine
that handles allocation of new leases. It takes parameters that the client
provided (client-id, DUID, subnet, a hint if the user provided one, etc.) and
then attempts to allocate a lease.

There is no single best soluction to the address assignment problem. Server
is expected to pick an address from its available pools is currently not used.
There are many possible algorithms that can do that, each with its own advantages
and drawbacks. This allocation engine must provide robust operation is radically
different scenarios, so there address selection problem was abstracted into
separate module, called allocator. Its sole purpose is to pick an address from
a pool. Allocation engine will then check if the picked address is free and if
it is not, then will ask allocator to pick again.

At lease 3 allocators will be implemented:

- Iterative - it iterates over all addresses in available pools, one
by one. The advantages of this approach are speed (typically it only needs to
increase last address), the guarantee to cover all addresses and predictability.
This allocator behaves very good in case of nearing depletion. Even when pools
are almost completely allocated, it still will be able to allocate outstanding
leases efficiently. Predictability can also be considered a serious flaw in
some environments, as prediction of the next address is trivial and can be
leveraged by an attacker. Another drawback of this allocator is that it does
not attempt to give the same address to returning clients (clients that released
or expired their leases and are requesting a new lease will likely to get a 
different lease). This allocator is implemented in \ref isc::dhcp::AllocEngine::IterativeAllocator.

- Hashed - ISC-DHCP uses hash of the client-id or DUID to determine, which
address is tried first. If that address is not available, the result is hashed
again. That procedure is repeated until available address is found or there
are no more addresses left. The benefit of that approach is that it provides
a relative lease stability, so returning old clients are likely to get the same
address again. The drawbacks are increased computation cost, as each iteration
requires use of a hashing function. That is especially difficult when the 
pools are almost depleted. It also may be difficult to guarantee that the
repeated hashing will iterate over all available addresses in all pools. Flawed
hash algorithm can go into cycles that iterate over only part of the addresses.
It is difficult to detect such issues as only some initial seed (client-id
or DUID) values may trigger short cycles. This allocator is currently not

- Random - Another possible approach to address selection is randomization. This
allocator can pick an address randomly from the configured pool. The benefit
of this approach is that it is easy to implement and makes attacks based on
address prediction more difficult. The drawback of this approach is that
returning clients are almost guaranteed to get a different address. Another
drawback is that with almost depleted pools it is increasingly difficult to
"guess" an address that is free. This allocator is currently not implemented.