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.
- 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.
- 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.