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.. _kernel-interface:
****************
Kernel Interface
****************
There are several different methods for reading kernel routing table
information, updating kernel routing tables, and for looking up interfaces.
FRR relies heavily on the Netlink (``man 7 netlink``) interface to
communicate with the Kernel. However, other interfaces are still used
in some parts of the code.
- 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
Netlink first appeard in Linux kernel 2.0. It makes asynchronous
communication between the kernel and FRR possible, similar to a routing
socket on BSD systems. Netlink communication is done by reading/writing
over Netlink socket.
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