Kernel Interface
****************

There are several different methods for reading kernel routing table
information, updating kernel routing tables, and for looking up
interfaces.

* ioctl
     This method is a very traditional way for reading or writing
     kernel information. *ioctl* can be used for looking up interfaces
     and for modifying interface addresses, flags, mtu settings and
     other types of information. Also, *ioctl* can insert and delete
     kernel routing table entries. It will soon be available on almost
     any platform which zebra supports, but it is a little bit ugly
     thus far, so if a better method is supported by the kernel, zebra
     will use that.

* sysctl
     This is a program that can lookup kernel information using MIB
     (Management Information Base) syntax. Normally, it only provides
     a way of getting information from the kernel. So one would
     usually want to change kernel information using another method
     such as *ioctl*.

* proc filesystem
     This is a special filesystem mount that provides an easy way of
     getting kernel information.

* routing socket / Netlink
     On recent Linux kernels (2.0.x and 2.2.x), there is a kernel/user
     communication support called *Netlink*. It makes asynchronous
     communication between kernel and FRR possible, similar to a
     routing socket on BSD systems.

     Before you use this feature, be sure to select (in kernel
     configuration) the kernel/Netlink support option 'Kernel/User
     network link driver' and 'Routing messages'.

     Today, the "/dev/route" special device file is obsolete.  Netlink
     communication is done by reading/writing over Netlink socket.

     After the kernel configuration, please reconfigure and rebuild
     FRR.  You can use Netlink as a dynamic routing update channel
     between FRR and the kernel.