HELP: DIP, SLIP Servers, and IP Forwarding

HELP: DIP, SLIP Servers, and IP Forwarding

Post by Christopher O'Mall » Tue, 01 Aug 1995 04:00:00

        Hi everyone.  I know this is a pretty common problem and there are FAQ
concerning it, but I'm still stuck.

        I'm running a Linux 1.1.59 box as a SLIP server which is connected to our
Ethernet subnet.  Its got a standard 14.4k modem on one of the COM ports with a
SLIP client (regular old PC w/ WFW311 and Chameleon's TCP/IP stack) on the
other end.
        The client connects to the server fine, and you can ping/telnet/ftp/etc to
and from
the client/server.  You cannot, however, do the same from the client to any other
machine on
our Ethernet or from any other machine on the Ethernet to the client.  I have
compiled in IP
Forwarding on the server (but I don't know how to check to make sure its working).  
And I am
using ProxyARP (I think).  Still, no dice.  I can't get past the Linux box in either
So, things look like this:

   Ethernet                           eth0  SLIP Server  sl0                                             SLIP
   |---------------------------------------------|                   |--------------------------------------------|      

   So, the Ethernet is subnet
        Slip Server's eth0 is
        Slip Server's sl0   is
        Slip Client's           is
        Subnet Router      is

  Server's routing table:
    Destination      GW         Genmask         Flags   Iface         *    UH            sl0         *         UH            sl0            *                          *               U     eth0
    loopback           *                       U      lo
    default                      *               UG    eth0

  arp -a  00:00:C0:49:9A:73  00:00:C0:49:9A:73  00:00:A2:00:1A:FD

    sl0 Link encap: VJ Serial Line IP
    inet addr:  P-t-P:  Mask:
    UP POINTOPOINT RUNNING      MTU:1006        Metric: 1

    eth0        Link encap: 10MBs Ethernet      HW Addr: 00:00:C0:49:9A:73
    inet addr:  Bcast:   Mask:

        I have the Client pointing to as its default Gateway.  
Anyone see
any outright mistakes?  Please respond to this group or send email to:


HELP: DIP, SLIP Servers, and IP Forwarding

Post by Terry Dawso » Tue, 08 Aug 1995 04:00:00

>    I have the Client pointing to as its default Gateway.  
> Anyone see
> any outright mistakes?  Please respond to this group or send email to:

heh, the first one is not restricting your posting to 80 columns
but that is a nitpick.

I'm basing my response on what I understood of your diagram, it
got lost in screen overlap.

In this situation you probably shouldn't use proxy arp. You should
either run a routing daemon like gated/routed to advertise the
subnet to the hosts on the big ethernet, or have the machines on
the other side manually route to the subnet using your linux server
as the gateway. How you do this is going to be dependent on the
nature of the machines on the other side.



1. SLIP, DIP and Ip forwarding

I'm trying to use my desktop machine as a SLIP server for my laptop.  I've
built cslip and slip in to both kernels as well as ipforwarding on the desktop
machine.  I can successfully get a cslip connection from the laptop to the
desktop but if I try to contact machine beyond the server, my packets don't
appear to be forwarded.  

I'm using diplogin which should handle proxy arps.  My understanding is that
I shouldn't need to use gated or whatever.  What am I missing?  Can I probe a
kernel to check what's built in, say Ip forwarding?  I'm wondering if my
config file might be lying to me about what is in the kernel Alternatively,
where abouts in /proc would the userful information be.

Any useful information gratefully received.  Wife, daughter and several hundred
computers to support.  Insufficient time, insufficient money.

                Many thanks,

                        Chris Cook - Postmaster

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