Re: ipv6 loopback

[David Wright <deblis@lionunicorn.co.uk>]

On Wed 10 Jun 2020 at 12:05:57 (-0400), Jim Popovitch wrote:
> Something that has always bugged me (and #debian hasn't yet answered)
> 
> What establishes the IPv6 loopback ::1 address, and why is there no need
> for "iface lo inet6 loopback" in /etc/network/interfaces?

AFAICT there's code in linux-source-4.19/net/ipv6/addrconf.c to make
sure that the loopback gets set up early enough. The comments are:

        /* The addrconf netdev notifier requires that loopback_dev
         * has it's ipv6 private information allocated and setup
         * before it can bring up and give link-local addresses
         * to other devices which are up.
         *
         * Unfortunately, loopback_dev is not necessarily the first
         * entry in the global dev_base list of net devices.  In fact,
         * it is likely to be the very last entry on that list.
         * So this causes the notifier registry below to try and
         * give link-local addresses to all devices besides loopback_dev
         * first, then loopback_dev, which cases all the non-loopback_dev
         * devices to fail to get a link-local address.
         *
         * So, as a temporary fix, allocate the ipv6 structure for
         * loopback_dev first by hand.
         * Longer term, all of the dependencies ipv6 has upon the loopback
         * device and it being up should be removed.

Cheers,
David.