GateD Configuration Guide - Kernel Statement

# The Kernel Statement

While the kernel interface isn't technically a routing protocol, it has many characteristics of one, and GateD handles it similarly to one. The routes GateD chooses to install in the kernel forwarding table are those that will actually be used by the kernel to forward packets.

The add, delete and change operations GateD must use to update the typical kernel forwarding table take a non-trivial amount of time. This does not present a problem for older routing protocols (RIP, EGP), which are not particularly time critical and do not easily handle very large numbers of routes anyway. The newer routing protocols (OSPF, BGP) have stricter timing requirements and are often used to process many more routes. The speed of the kernel interface becomes critical when these protocols are used.

To prevent GateD from locking up for significant periods of time installing large numbers of routes (up to a minute or more has been observed on real networks), the processing of these routes is now done in batches. The size of these batches may be controlled by the tuning parameters described below, but normally the default parameters will provide the proper functionality.

During normal shutdown processing, GateD normally deletes all the routes it has installed in the kernel forwarding table, except for those marked with retain. Optionally, GateD can leave all routes in the kernel forwarding table by not deleting any routes. In this case changes will be made to insure that routes with a retain indication are installed in the table. This is useful on systems with large numbers of routes as it prevents the need to re-install the routes when GateD restarts. This can greatly reduce the time it takes to recover from a restart.

## Forwarding tables and Routing tables

The table in the kernel that controls the forwarding of packets is a forwarding table, also know in ISO speak as a forwarding information base, or FIB. The table that GateD uses internally to store routing information it learns from routing protocols is a routing table, known in ISO speak as a routing information base, or RIB. The routing table is used to collect and store routes from various protocols. For each unique combination of network and mask an active route is chosen, this route will be the one with the best (numerically smallest) preference. All the active routes are installed in the kernel forwarding table. The entries in this table are what the kernel actually uses to forward packets.

## Updating the Forwarding Table

There are two main methods of updating the kernel FIB, the `ioctl()` interface and the routing socket interface. Their various characteristics are described here.

### Updating the Forwarding Table with the ioctl interface

The `ioctl` interface to the forwarding table was introduced in BSD 4.3 and widely distributed in BSD 4.3. This is a one-way interface, it only allows GateD to update the kernel forwarding table. It has several other limitations:

Fixed subnet masks
  The BSD 4.3 networking code assumed that all subnets of a given network had the same subnet mask. This limitation is enforced by the kernel. The network mask is not stored in the kernel forwarding table, but determined when a packet is forwarded by searching for interfaces on the same network.
One way interface
  GateD is able to update the kernel forwarding table, but it is not aware of other modifications of the forwarding table. GateD is able to listen to ICMP messages and guess how the kernel has updated the forwarding table with response to ICMP redirects.
Blind updates
  GateD is not able to detect changes to the forwarding table resulting from the use of the the route command by the system administrator. Use of the route command on systems that use the `ioctl()` interface is strongly discouraged while GateD is running.
Changes not supported
  In all known implementations, there is no change operation supported, to change a route that exists in the kernel, the route must be deleted and a new one added.

### Updating the Forwarding Table with the routing socket interface

The routing socket interface to the kernel forwarding table was introduced in BSD 4.3 Reno, widely distributed in BSD 4.3 Net/2 and improved in BSD 4.4.