My solution to the 'link via null modem(laplink) cable' problem

My solution to the 'link via null modem(laplink) cable' problem

Post by Graeme Wo » Thu, 13 May 1999 04:00:00

 Hi list,
          I just thought I'd post my solution to the above statement as the
 ppp-howto poved to be useless in my case.

 The following is to set up a machine to talk to machine

On the machine of ip= type in the following set of    
 commands :

  1. pppd -crtscts local passive silent /dev/ttyS0 38400 &

      This command brings the ppp0 interface into being. The local option
      tells the link not to use the modem control lines. The passive option
      tells the link to not answer any ad-hoc packets coming in and the silent
      option  tells the link not to send any ad-hoc packets out.
      When doing this the silent and passive options were vital as the link
       seemed to crash after a short time period.
      Note also /dev/ttyS0 is com1 and ttyS1 is com2. You can use either..

 2 Type in             ifconfig -a

      This queries your interfaces eth0, eth1 ppp0 etc.
      It essentially finds out what interfaces are there on your system.
      At this point do not worry if your ppp0 interface does not show up
      here..It doesn't mean it hasn't been created.It just ain't shown yet !!
      I haven't a clue why this happens but proceed to the next step...  

  3 Type in

 ifconfig ppp0 pointopoint netmask

      If you're feeling adventurous at this point type in   ifconfig -a  and
      if  your ppp0 interface didn't show up in step 2 ,it should now...

  4. Type in     ifconfig ppp0 up

      This should bring the interface up at this point. This is vital.
       At step 2 after typing in   ifconfig -a   the interface would not have
       been up,,, now it is. Not it will say UP somewhere on the ppp0
       interface output garble from ifconfig -a. From what I can remember it
       doesn't say DOWN when  it is down,,,most confusing..      

  5. Type in   route add -host

  We will now go to the other machine and repeat steps 1 to 5.
  for the other end of the connection........

 6 type in    pppd -crtscts local passive silent /dev/ttyS0 38400 &

 7. type in   ifconfig -a

                  Note that ppp0 may not appear here....yet.

 8. type in  

 ifconfig ppp0 pointopoint netmask

 Note here how the ip no's are different for the opposite end of the link..

 9. type in    ifconfig ppp0 up

   Again type in ifconfig -a at this point to see what is what.

 10 type in

     route add -host

     Note here how the ip no is different from the route command made on the
     other end of the link....

 11 And finally ping away  ......ecstasy....

     For newbies  - on machine type in


     It does form quite a stable link although ftp bombs out on fairly
     heavy loads.. I don't know why this is. You can plug and unplug
     the laplink cable while  a ping is going and it will just recover
     and keep on pinging...I suppose tcp/ip really is that robust...

 I hope this helps a few people out.It took me ages to do. I note that you
 have to deal with 3 seperate things - ppp itself,  the ppp0 interface which
 you really treat like an eth0 interface and finally the route to the other
 Would appreciate any feedback on successes, failure etc




1. Linking linux boxes via null-modem cable

I would like to connect two linux boxes together via a null-modem
cable.  I'm trying to run agetty on one machine (helium) and minicom
on the other (dialup).  The problem is that I can login to helium from
dialup when helium has been booted, but after I exit minicom and go
back into it I don't get any response from helium.  In /etc/inittab
I'm using /sbin/agetty -L ttyS3 38400.  The -L is to ignore the
carrier detect status.

I've also tried using kermit on dialup to access helium after minicom
has been exited and the same thing happens.

I'm not sure whether the problem is with agetty on helium or
minicom/kermit on dialup.  Would be grateful for any help.

Pat Cain

2. Kermit

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10. connecting laptop to desktop via null-modem cable

11. rsh machine -n 'command >&/dev/null </dev/null &'

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