netdev - Re: ipv6 ::1 and lo dev

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Message-ID: <20210818141544.75b15e93@hermes.local>
Date:   Wed, 18 Aug 2021 14:15:44 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Willy Tarreau <w@....eu>
Cc:     Thorsten Glaser <t.glaser@...ent.de>, Oleg <lego12239@...dex.ru>,
        netdev@...r.kernel.org
Subject: Re: ipv6 ::1 and lo dev

On Wed, 18 Aug 2021 22:17:55 +0200
Willy Tarreau <w@....eu> wrote:

> On Wed, Aug 18, 2021 at 07:47:37PM +0200, Thorsten Glaser wrote:
> > On Wed, 18 Aug 2021, Oleg wrote:
> >   
> > > I try to replace ::1/128 ipv6 address on lo dev with ::1/112 to
> > > access more than 1 address(like with ipv4 127.0.0.1/8). But i get  
> > 
> > AIUI this is not possible in IPv6, only :: and ::1 are reserved,
> > the rest of ::/96 is IPv4-compatible IPv6 addresses.
> > 
> > I never understood why you'd want more than one address for loopback
> > anyway (in my experience, the more addresses a host has, the more
> > confused it'll get about which ones to use for what).  
> 
> It's because you've probably never dealt with massive hosting :-)
> Sometimes binding a full /24 (or smaller) on the loopback, be it
> for listening addresses or to be used as sources to connect to
> next hops, is extremely useful.
> 
> Willy

In the large scale routing world addresses are assigned to loopback
interface because it keeps routing protocols happy. If interface goes
down loopback stays around.