Subject: Re: nat configuration
To: Andrew Brown <>
From: Brian Somers <>
List: tech-net
Date: 01/22/2001 01:36:25
> >> >Hmmm.... I just tried it, and now it works! I thought it didn't used to.
> >> >Either I misremembered, or it's been fixed.
> >> 
> >> um...what works?  a more current nat can mux pings?
> >
> >Yes. My 1.5 NAT box seems to be multiplexing pings. I had one box ping
> >, and another ping, and they both worked. My
> >nat config looks like yours, except that I have my hard IP in there
> >instead of, and I am using the outgoing ethernet card. :-)
> make it more interesting, just to amuse me?  ping the same outside
> address ( i usually use from two machines inside the nat
> and lemme know if it works.

Yes, it works.  libalias (used by user-ppp) recognises icmp traffic, 
and nat's the sequence number and IP.

> >> >All my machines are running 1.5.
> >> 
> >> all my machines are running current with less than a two month lag
> >> behind today.
> >
> >I hope it didn't get fixed then broken.
> me too.

It still works (and will continue to).

> >> >If it really works with userland ppp (which I thought was a downgrade from
> >> >1.5's ppp) but not kernel ppp, then there's a ppp bug.
> >> 
> >> the userspace ppp is, afaik, a *completely* separate and distinct
> >> implementation of ppp.  all it requires of the kernel is a serial
> >> interface (with a modem) and a tunnel interface (for packets to go
> >> through).  it's not a downgrade...perhaps a "sidegrade".
> >
> >I thought they were based on the same ppp project. The reason for the ppp
> >package was that it's set to version 2.3.11, which is newer than the ppp
> >in 1.4. But 1.5 and current are using ppp 2.4, which is newer, thus an
> >overall downgrade.
> they might have some common roots, if you dig far enough back (like
> netbsd and freebsd), but the userspace one and the kernel space one
> are very different.

user-ppp was originally written by IIJ and was picked up by me and 
almost entirely re-written (multi-link support made this necessary) 
since then.

>                      some examples: the userspace one doesn't use
> does it all itself;

Although it can use an external chat program too (see the man page).

>                               the userspace one does nat all by
> itself, it doesn't rely on the kernel;

Using libalias.  libalias does a whole bunch of things, including 
non-passive ftp, irc, cuseeme, transparent proxying, pptp, NetBIOS 
and realaudio (smedia).

>                                        the userspace one also claims
> to support mppp, which i've not tried,

Yep, works nicely with i4b :-)

>                                        whereas pppd says that only
> works under linux.  that might sounds a bit slanted, but those were
> the first things i thought of.

The only downside with user-ppp is the overhead of passing everything 
out then back into the kernel.  This is being addressed (albeit 
slowly) by making user-ppp use netgraph(4) if it's available.

> >> the nat (called aliasing) in the userspace ppp is what actually
> >> handles the multiple outbound pings.  i imagine it's fiddling with the
> >> icmp echo request identifier and using it as it uses the local port
> >> number rewriting for udp and tcp.
> >
> >Does it work with that aliasing off?
> not for me, no, since none of my inside machines addresses are routed
> back to me properly.  that's something i've been meaning to deal with,
> but haven't yet.

It'll work with ipnat turned off, but not with user-ppp's -nat (or 
``nat enable yes'') disabled.

> on a side note, i just thougfht about it a tiny bit more and
> remembered that my nat rules rewrite traffic over ppp0, not tun0,
> which is what the userspace ppp uses.  so, no conflict.

They shouldn't get in eachother's way.

I've been intending to make a ppp package for NetBSD, and now see 
that there already is one :-)  Now that exports 
pkgsrc, I'll look into keeping things more ``available'' :-)

> -- 
> |-----< "CODE WARRIOR" >-----|
>             * "ah!  i see you have the internet
> (Andrew Brown)                that goes *ping*!"
>       * "information is power -- share the wealth."

Brian <>                        <brian@[uk.]>
      <>                   <brian@[uk.]>
Don't _EVER_ lose your sense of humour !