Userland PPP/PPTP tunneling problem

[Wes Peters]

On Friday 18 April 2003 15:41, Brett Glass wrote:
> At 04:22 PM 4/18/2003, Chris Luke wrote:
> >Tunnels are point-to-point connections. Each end of the link
> >has an address, even if inherited from another interface,
> >and these addresses are either known in advance, or exchanged or
> >negotiated by a higher-level protocol, such as the negotiation
> >stuff in PPP. Thus the address of the far end is known, and is
> >entered as a route into the forwarding table.
>
> Even assuming that you don't need ARP (and SOMEONE has to do
> ARP if you're going to get to other addresses on the LAN you're
> tunneling into),

The "other end" does the ARP.  Your packets reach the "other end" because 
that's where your route pointed them.  The same effect works on your 
local LAN gatewayed to the internet every day.  For instance, when I send 
mail to you, none of my hosts know the MAC address for mail.lariat.org 
becuase that address is not on the local network.  My workstation looks 
up the address 63.229.157.2 in the routing table and matches on the 
default route:

default            204.68.178.1       UGSc        2        0    dc0

My workstation *does* know how to ARP for 204.68.178.1:

frankenrouter.softweyr.com (204.68.178.1) at 00:09:5b:37:a1:e2 on dc0 
[ethernet]

So it sends the packet there, and frankenrouter sends it through a point 
to point interface (the cable modem) to a router at san.rr.com, and so 
on.  Eventually it gets to the router upstream of mail.lariat.org, which 
presumably does ARP for the address of your mailserver.

> there are many applications that do need
> to send out a broadcast. HP JetDirect and LapLink are two which
> I know these folks to be using. The broadcast address should
> be the correct one for the LAN into which you're tunneling, or
> these products won't work.

Since by definition your PPTP client is on the same network as the 
JetDirect, the PPTP server at the other end had better forward the 
broadcast (and multicast) packets through the tunnel, right?

-- 

        Where am I, and what am I doing in this handbasket?

Wes Peters                                               wes at softweyr.com