Why do packets to 127.255.255.255 cause dial-up?

Nik Clayton nik at freebsd.org
Fri Mar 31 00:40:04 BST 2000


On Thu, Mar 30, 2000 at 03:23:15PM +0100, Richard Smith wrote:
> But...
> 
> It looks to me that the route to lo0 is 127.0.0.1/32 not 127/8, in which
> case the address 127.255.255.255 will use the default gateway, being
> tun0, QED (smug mode) :-)

Ah, indeed.  I hate it when I can't see the wood for the trees.  Nothing
a quick

    route add -net 127 lo0

can't solve.  This then gives the expected:

Destination        Gateway            Flags     Refs     Use     Netif Expire
default            158.152.1.222      UGSc       20      668     tun0
127                lo0                USc         0        0      lo0
127.0.0.1          127.0.0.1          UH          0        0      lo0
[...]

lines.  And no packets wending their way down tun0 by mistake.

The more interesting question is why is this happening?  The only other
IP enabled system I have to hand is Win98 (I know, I know), and a quick
"route print" shows that the 127.0.0.1 route has a /8 netmask rather than
a /32.

I've got the usual /etc/hosts,

    127.0.0.1   localhost.ngo.org.uk  localhost

entry, and a local nameserver, which resolves "localhost" to 127.0.0.1.

It's my understanding that 127/8 is the 'local net', and that packets to
any 127/8 address should go via the loopback interface.  That being the
default, shouldn't a 127/8 route automatically exist to lo0?

If it should, is it just my system that's misconfigured, and does 
everybody else have a 127/8 route on their systems?

N
-- 
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