Understanding Linux Network Internals

Primary and Secondary IP Addresses

We saw in the section "Primary and Secondary Addresses" in Chapter 30 that an IP address can be configured as primary or secondary on a device. The kernel often needs to browse all the addresses configured on a device to find one that matches a given condition. Let's see how the two types of addresses are distinguished and how addresses are browsed.

Secondary IPv4 addresses are tagged with the IFA_F_SECONDARY flag in their in_ifaddr data structures (see Chapter 19). Because there are only two configurations—primary and secondary—there is no need for an IFA_F_PRIMARY flag: if an address is not secondary, it is considered primary.

The kernel provides macros in include/linux/inetdevice.h that make it easier to browse interfaces meeting specific criteria. For each criterion selected, there are usually two macros that are used to bracket a loop: the programmer places the code to process a single address in a block that is started by one macro and terminated by another. The effect is to run a loop applying the code to each address meeting selected criteria.

Here is an example of the macros in use:

for_ifa {
 
        do something with ifa
 
} endfor_ifa

The for_ifa macro starts a loop with the variable ifa to represent each address selected. The code between the macros does not need to be placed in brackets, but it usually is, to make variables such as ifa local and usable only within the loop.

A few such macros include:

for_ifa

endfor_ifa

Given a device, these two ...