gated route filtering

Routes are filtered by specifying configuration language that will match a certain set of routes by destination or by destination and mask. Among other places, route filters are used on martians, import, and export statements.

The action taken when no match is found is dependent on the context. For instance, import and export route filters assume an ``all reject ;'' at the end of a list.

A route will match the most specific filter that applies. Specifying more than one filter with the same destination, mask, and modifiers will generate an error. The filtering syntax is as follows:

   network
   network mask mask
   network mask-length number
   all
   host host

In most cases, it is possible to specify additional parameters relevant to the context of the filter. For example, on a martian statement, it is possible to specify the allow keyword; on an import statement, you can specify a preference; and on an export statement you can specify a metric.

network
network mask mask
network mask-length number

Matching usually requires both an address and a mask, although the mask is implied in the shorthand forms listed below. These three forms vary in how the mask is specified. In the first form, the mask is implied to be the natural mask of the network. In the second, the mask is explicitly specified. In the third, the mask is specified by the number of contiguous one bits.

---

NOTE: The mask-length keyword may also be masklength, masklen, or mask-len.

If no additional parameters are specified, any destination that falls in the range given by the network and mask is matched; the mask of the destination is ignored. If a natural network is specified, the network, any subnets, and any hosts will match.

---

all

This entry matches anything. It is equivalent to:

   0.0.0.0 mask 0.0.0.0

host host

Matches the specific host. To match, the address must exactly match the specified host, and the network mask must be a host mask, that is, all ones. This is equivalent to:

   host mask 255.255.255

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