Re: Solaris and lack of loopback routes

From: Crist J. Clark (
Date: 08/09/02

Date: Thu, 8 Aug 2002 16:13:21 -0700
From: "Crist J. Clark" <>
To: "Chris A. Mattingly" <>

On Tue, Aug 06, 2002 at 12:28:39PM -0400, Chris A. Mattingly wrote:
> Jon wrote:
> >
> > (reposted)
> >
> > On Tue, Jul 30, 2002 at 05:12:45PM -0400, Chris Mattingly wrote:
> > > On FreeBSD, there is no route for the 127/8 network, but the stack
> > > actually pays attention to the fact that lo0 has a netmask for the
> > > entire /8 network, and traffic to any 127 address stays within the lo0
> > > interface (never shows up on any of the other three physical
> > > interfaces).
> >
> > Just to satisfy my curiousity, I dropped the loopback network route and
> > changed the netmask of my loopback device to be on my linux
> > box (I don't have a Solaris box available at the moment). Still, all
> > traffic bound for the loopback network was sent over the loopback device.
> > This may be a linux specific feature that I'm not familiar with. Does a
> > similar test on a Solaris box yield different results?
> On Solaris, here's the breakdown:
> o With a netmask of and no routes for the 127/8 network,
> Solaris sends 127/8 traffic (except for which there
> is a host route) out the "default" interface. It should not do
> this if it paid attention to the netmask on lo0.

The whole issue is that,

  # ifconfig lo0 inet


  lo0: flags=849<UP,LOOPBACK,RUNNING,MULTICAST> mtu 8232
          inet netmask ff000000
Even though it doesn't actually add a route corresponding to that
network. Doing an ifconfig(1M) on other types of interfaces does put
the entry in the routing table.

> So what it's boiling down to is that Solaris doesn't care what
> the netmask is of the lo0 interface. Only when there is a
> specific route in place will traffic stay on lo0 when it is
> supposed to stay on lo0.

Huh. Strange. I thought this type of behavior was only a BSDism. I'd
be curious to find what other Unix variants have this "feature."

Crist J. Clark                     |
                                   |    |