Disabling IPv6 Support at Runtime

The FreeBSD stack starts up with IPv4 support in an essentially "disabled" state. It requires the application to take positive actions to assign IPv4 address and routes before the interfaces will respond to packets from the outside world.

This is not true of IPv6 support. If IPv6 support was enabled in the eCos build, the stack will start up with link-local addresses assigned and interfaces listening to certain multicast addresses. The stack will not only respond to IPv6 packets, it will spontaneously send IPv6 packets without actions from application code.

If an eCos application wishes to disable IPv6 support at runtime, the decision has to be made very early in the boot process and actions taken to prevent the invocation of IPv6 startup code in the network stack. This involves two rather obscure "tricks":

• Using a dummy device driver's "init" hook as a place to execute some code early in the startup process

• Monkey-patching the network stack's table of initializers by looping through it and checking the function pointers against certain globally visible symbols and replacing the two IPv6-specific function pointers with pointers to a "noop" function.

The code listing below shows an example of disabling IPv6 support at runtime if the global variable we_want_to_disable_ipv6 is non-zero. The assumption is that some code in the HAL initialization or somewhere else has made the decision whether or not to disable IPv6 support and set that variable accordingly.

// Sample code showing a method of disabling IPv6 support at runtime

// Do-nothing function we can patch into the network stack's init table
// when we want to disable one of the entries
static void
init_noop(void *dummy)
{
}

// Function that loops through the network stack's init table and
// disables the two IPv6 entries
static void
disable_ipv6(void)
{
    extern struct init_tab_entry __NET_INIT_TAB__[],
                    __NET_INIT_TAB_END__;
    struct init_tab_entry *init_entry;
    extern void     cyg_net_add_domain(void *);
    extern void     ip6_init2(void *);
    extern char     inet6domain[];

    for (init_entry = __NET_INIT_TAB__; init_entry != &__NET_INIT_TAB_END__;
         init_entry++)
        if ((init_entry->fun == ip6_init2)
            || (init_entry->fun == cyg_net_add_domain
                && init_entry->data == (void *)inet6domain))
            init_entry->fun = init_noop;
}

// Function called early in the inintialization process via a dummy
// device table entry
static bool
early_init(struct cyg_devtab_entry *tab)
{
    if (we_want_to_disable_ipv6)
        disable_ipv6();
    return 0;
}

// Dummy device driver table entry that will call the above early_init()
// function during the startup process
DEVTAB_ENTRY(device_master_early_init,
             "neveruse",
             NULL,
             NULL,
             early_init,
             NULL,
             NULL
    );