netdev - Re: ipv6 ::1 and lo dev

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Message-ID: <20210818201755.GA4899@1wt.eu>
Date:   Wed, 18 Aug 2021 22:17:55 +0200
From:   Willy Tarreau <w@....eu>
To:     Thorsten Glaser <t.glaser@...ent.de>
Cc:     Oleg <lego12239@...dex.ru>, netdev@...r.kernel.org
Subject: Re: ipv6 ::1 and lo dev

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