Understanding Linux Network Internals

Helper Routines

Here are a few routines used by some of the functions we will cover in this chapter:

fib_validate_source

Validates the source IP address of a packet received on a given device, to detect spoofing attempts. Among other things, this function makes sure that unless asymmetric routing is enabled, the source IP address of the packet is reachable through the same interface the packet was received from (see the section "Reverse Path Filtering" in Chapter 31). It also returns the preferred source address spec_dst to use for the reverse direction, as described in the upcoming section "Preferred Source Address Selection," and initializes the routing tag, as described in the section "Routing Table Based Classifier" in Chapter 31.

inet_select_addr

Given a device dev, an IP address dst, and a scope scope, returns the first primary address with scope scope, to use in sending a packet to the address dst out of device dev.

This routine is needed because a device can be configured with multiple addresses, and each can have its own scope.

The reason for the dst argument is that if dev is configured with different IP addresses on different subnets, dst allows this function to return an IP address configured on the same subnet as dst.

In the section "Scope" in Chapter 30, we saw that there are primary and secondary addresses; inet_select_addr returns only primary addresses.

If no address configured on dev meets the conditions specified by scope and dst, the function tries the rest of the ...