Re: [gentoo-user] Bind to 127.0.0.N for any N

[Grant Taylor]

On 1/28/21 5:38 PM, Grant Edwards wrote:
> I've just recently realized something about the "lo" interface.

I don't think this is as much about the interface as it is the routes 
that are created.  (More below.)

> You can bind a socket to any 127.0.0.N address, even though only
> 127.0.0.1/8 is configured in /etc/config/net, and "ip addr" only shows
> 127.0.0.1/8 for that interface.

Yes.  But for specific reasons. (...)

> In the past, when I wanted to use other 127.0.0.N address, I
> always added them to /etc/config/net. The last time, I forget to do
> that. Later, I realized it was working anyway. I've since removed
> all of the extra "lo" addresses from /etc/config/net, and everything
> still works.

Because of a very special route.

> Apparently "lo" is special.
>
> Perhaps I don't even need to have 127.0.0.1/8 listed in
> /etc/config/net...

I think that you still want 127.0.0.1 in /etc/config/net even if only to 
bring the interface up (a la 'ip link set dev lo up', sans IP).

I believe the ""magic that is allowing this to work is one of the four 
following routes:

# ip route show table local | grep 127.0.0 | nl
      1	broadcast 127.0.0.0 dev lo proto kernel scope link src 127.0.0.1
      2	local 127.0.0.0/8 dev lo proto kernel scope host src 127.0.0.1
      3	local 127.0.0.1 dev lo proto kernel scope host src 127.0.0.1
      4	broadcast 127.255.255.255 dev lo proto kernel scope link src 
127.0.0.1

Lines 1, 3, and 4, are typical routes.  You should have something 
similar for other IPs and devices.

But line 2 is very special.  Notice how it's assigning the entire 127/8 
to the lo device.

Reformatting the route with some white space makes it somewhat more obvious.

      2	local 127.0.0.0/8 dev lo proto kernel scope host src 127.0.0.1
      3	local 127.0.0.1   dev lo proto kernel scope host src 127.0.0.1

#3 is a more typical /host/ route.
#2 is a less typical /net/ route.

#2 actually tells the kernel that anything and everything in the 127/8 
destination network can be reached directly via the lo adapter.

This network route is more efficient than having multiple host routes to 
cover some portion of the same IP space.

My understanding -- which may be wrong, and please correct me if you 
think it is -- is that this special route (#2) is how the kernel sends 
the entire 127/8 network to the lo adapter, even if the IP addresses 
aren't bound to the adapter.

Now, as for things receiving the connections, I think it is highly 
dependent on if the thing is listening to 0.0.0.0 or specific IP 
addresses.  Because if it's listening to 0.0.0.0, I think it will 
happily serve connections to other addresses in 127/8.  If it's 
listening to explicitly 127.0.0.1, then it likely will not serve 
connections to other addresses in 127/8.

I believe the same technique can be applied to other addresses outside 
of the 127/8 network.  Though it's much less often done.  You'd most 
likely see this with a service that wants to serve for an entire /24; 
e.g. 192.0.2.0/24 while listening to 0.0.0.0.

Admittely it's been a while since I last delt with this, so I could be 
mis-remembering.  But I think the special route, #2, is at the root of 
what you're asking about.

Again, I believe you do want the 127.0.0.1 in /etc/config/net to 
actually bring the interface up.  You probably don't even need to bind 
an IP to it.  I think the kernel does the 127/8 automatically /if/ the 
interface is simply up, a la 'ip link set dev lo up'.



-- 
Grant. . . .
unix || die