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

ioctl
     The ioctl 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
     sysctl 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
     proc filesystem 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     Today, the /dev/route special device file is obsolete.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.