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Leased line Mini HOWTO

Rob van der Putten


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Revision 2.3b7           2005-09-05           Revised by: RvdP               
Additional PPPD options and routing                                          
Revision 2.3b6           2005-01-19           Revised by: RvdP               
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 Configuring your modem and pppd to use a 2 wire twisted pair leased line.

Table of Contents
1. Introduction
    1.1. Copyright and License
    1.2. What is a leased line
    1.3. Assumptions
2. Modem
    2.1. Modem Configuration
    2.2. Test
    2.3. Examples
    3.1. Configuration
    3.2. Scripts
    3.3. Test

 The most recent (beta) version of this HOWTO can be found at: [http://]

1. Introduction

1.1. Copyright and License

 This document is distributed under the terms of the GNU Free Documentation
License. You should have received a copy along with it. If not, it is
available from []

1.2. What is a leased line

 Any fixed, that is permanent, point to point data communications link, which
is leased from a telco or similar organisation. The leased line involves
cables, such as twisted pair, coax or fiber optic, and may involve all sorts
of other hardware such as (pupin) coils, transformers, amplifiers and

This document deals with:
     Configuring your modem and pppd to use a 2 wire twisted pair leased
This document does NOT deal with:
     SLIP, getting or installing pppd, synchronous data communication,
    baseband modems, xDSL.

1.3. Assumptions

 You should already have a working pppd on your system. You also need Minicom
or a similar program to configure your modems.

2. Modem

 A leased line is not connected to a telephone exchange and does not provide
DC power, dial tone, busy tone or ring signal. This means that your modems
are on their own and have to be able to deal with this situation.

 You should have 2 identical (including firmware version) external modems
supporting both leased line and dumb mode. Make sure your modems can actually
do this! Also make sure your modem is properly documented. You also need:

��*� 2 fully wired shielded RS232 cables. The shield should be connected to
    the connector shell (not pin 1) at both ends (not at one end).
��*� A RS232 test plug may be handy for test purposes.
��*� 2 RJ11 cords, one for each end of the leased line.
��*� A basic understanding of `AT' commands.

2.1. Modem Configuration

 A note on modem configuration and init strings in general: Configure your
modem software such as minicom or (m)getty to use the highest possible speed;
57600 bps for 14k4 and 115200 bps for 28k8 or faster modems. Lots of people
use very long and complicated init strings, often starting with AT&F and
containing lots of modem brand and -type specific commands. This however is
needlessly complicated. Most programs feel happy with the same modem
settings, so why not write these settings in the non volatile memory of all
your modems, and only use `ATZ' as an init string in all your programs. This
way you can swap or upgrade your modems without ever having to reconfigure
any of your software.

 Most programs require you to use the following settings;

��*� Fixed baud rate (no auto baud)
��*� Hardware bidirectional RTS-CTS flow control (no x-on/x-off)
��*� 8 Bits, no parity, 1 stopbit
��*� The modem should produce the TRUE DCD status (&C1)
��*� The modem should NOT ignore the DTR status (&D2 or &D3)

Check this with AT&V or AT&Ix (consult your modem documentation)

 These settings are not necessarily the same as the default factory profile
(&F), so starting an init string with AT&F is probably not a good idea in the
first place. The smart thing to do is probably to use AT&F only when you have
reason to believe that the modem setup stored in the non volatile memory is
really screwed up. If you think you have found the right setup for your
modems, write it to non volatile memory with AT&W and test it thoroughly with
Z-modem file transfers of both ASCII text and binary files. Only if all of
this works perfectly should you configure your modems for leased line.

 Find out how to put your modem into dumb mode and, more importantly, how to
get it out of dumb mode; The modem can only be reconfigured when it is not in
dumb mode. Make sure you actually configure your modems at the highest
possible speed. Once in dumb mode it will ignore all `AT' commands and
consequently will not adjust its speed to that of the COM port, but will use
the speed at which it was configured instead (this speed is stored in a
S-register by the AT&W command).

 Now configure your modem as follows;

��*� Reset on DTR toggle (&D3, this is sometimes a S register). This setting
    is required by some ISP's!
��*� Leased line mode (&L1 or &L2, consult your modem documentation)
��*� The remote modem auto answer (S0=1), the local originate (S0=0)
��*� Disable result codes (Q1, sometimes the dumb mode does this for you)
��*� Dumb mode (\D1 or %D1, this is sometimes a jumper) In dumb mode the
    modem will ignore all AT commands (sometimes you need to disable the ESC
    char as well).

Write the configuration to non-volatile memory (&W).

2.2. Test

 Now connect the modems to 2 computers using the RS232 cables and connect the
modems to each other using a RJ11 lead. Use a modem program such as Minicom
(Linux), procom or telix (DOS) on both computers to test the modems. You
should be able to type text from one computer to the other and vice versa. If
the screen produces garbage check your COM port speed and other settings. Now
disconnect and reconnect the RJ11 cord. Wait for the connection to
reestablish itself. Disconnect and reconnect the RS232 cables, switch the
modems on and off, stop and restart Minicom. The modems should always
reconnect at the highest possible speed (some modems have speed indicator
leds). Check whether the modems actually ignores the ESC (+++) character. If
necessary disable the ESC character.

  If all of this works you may want to reconfigure your modems; Switch off
the sound at the remote modem (M0) and put the local modem at low volume

2.3. Examples

2.3.1. Hi-Tech

 This is a rather vague `no name clone modem'. Its config string is however
typical and should work on most modems.

Originate (local):
Answer (remote):

2.3.2. Tornado FM 228 E

 This is what should work;

Originate (local):
Answer (remote):

Move the dumb jumper from position 2-3 to 1-2.

  Due to a firmware bug, the modems will only connect after being hard reset
(power off and on) while DTR is high. I designed a [
hardware/modem-reset.html#l2h] circuit which hard resets the modem on the low
to high transition of DTR. The FreeBSD pppd however, isn't very happy about
this. By combining the setting &D0 with a [
modem-reset.html#h2l] circuit which resets on the high to low transition
instead, this problem can be avoided.

2.3.3. Tron DF

 The ESC char should be disabled by setting S2 > 127;


2.3.4. US Robotics Courier V-Everything

 The USR Sportster and USR Courier-I do not support leased line. You need the
Courier V-everything version for this job. There is a webpage on the USR site
`explaining' how to set-up your Courier for leased line. However, if you
follow these instructions you will end up with a completely brain dead modem,
which can not be controlled or monitored by your pppd.

 The USR Courier can be configured with dip switches, however you need to
feed it the config string first. First make sure it uses the right factory
profile. Unlike most other modems it has three; &F0, &F1 and &F2. The
default, which is also the one you should use, is &F1. If you send it an AT&
F, however it will load the factory profile &F0! For the reset on DTR toggle
you set bit 0 of S register 13. This means you have to set S13 to 1.
Furthermore you need set it to leased line mode with &L1; ATS13=1&L1&W The
dip switches are all default except for the following:

     OFF Disable result codes
     ON Disable offline commands
     ON For originate, OFF For answer
     OFF Dumb mode


 You need a pppd (Point to Point Protocol Daemon) and a reasonable knowledge
of how it works. Consult the relevant RFC's or the [
/PPP-HOWTO/index.html] Linux PPP HOWTO if necessary. Since you are not going
to use a login procedure, you don't use (m)getty and you do not need a (fake)
user associated with the pppd controlling your link. You are not going to
dial so you don't need any chat scripts either. In fact, the modem circuit
and configuration you have just build, are rather like a fully wired null
modem cable. This means you have to configure your pppd the same way as you
would with a null modem cable.

  For a reliable link, your setup should meet the following criteria;

��*� Shortly after booting your system, pppd should raise the DTR signal in
    your RS232 port, wait for DCD to go up, and negotiate the link.
��*� If the remote system is down, pppd should wait until it is up again.
��*� If the link is up and then goes down, pppd should reset the modem (it
    does this by dropping and then raising DTR), and then try to reconnect.
��*� If the quality of the link deteriorates too much, pppd should reset the
    modem and then reestablish the link.
��*� If the process controlling the link, that is the pppd, dies, a watchdog
    should restart the pppd.

3.1. Configuration

 Suppose the modem is connected to COM2, the local IP address is `Loc_Ip' and
the remote IP address is `Rem_Ip'. We want to use 576 as our MTU. The /etc/
ppp/options.ttyS1 would now be:
mru 576                                                                      
mtu 576                                                                      
#maxfail 0                                                                   
#holdoff 10                                                                  
Stuff like `asyncmap 0', `lock', `modem' and `-detach' are probably already
in /etc/ppp/options. If not, add them to your /etc/ppp/options.ttyS1. So, if
the local system is and the remote system is, then /etc/
ppp/options.ttyS1 on the local system would be:
mru 576                                                                      
mtu 576                                                                      
#maxfail 0                                                                   
#holdoff 10                                                                  
The options.ttyS1 on the remote system would be:
mru 576                                                                      
mtu 576                                                                      
#maxfail 0                                                                   
#holdoff 10                                                                  
The passive option limits the number of (re)connection attempts. The persist
option will keep pppd alive in case of a disconnect or when it can't connect
in the first place. If you telnet a lot while doing filetransfers (FTP or
webbrowsing) at the same time, you might want to use a smaller MTU and MRU
such as 296. This will make the remote system more responsive. If you don't
care much about telnetting during FTP, you could set the MTU and MRU to 1500.
Keep in mind though, that UDP cannot be fragmented. [
netfone/] Speakfreely for instance uses 512 byte UDP packets. So the minimum
MTU for speakfreely is 552 bytes. The noauth option may be necessary with
some newer distributions. `maxfail 0' may be necessary with newer PPPDs.
After the connection is lost, PPPD will wait for a while before reconnecting.
This time can be set with the holdoff option. The default holdoff used to be
30 seconds, but is now zero. A holdoff of 10 is often recommended.

3.2. Scripts

3.2.1. Starting the pppd and keeping it alive

 You could start the pppd form a boot (rc) script. However, if you do this,
and the pppd dies, you are without a link. A more stable solution, is to
start the pppd from /etc/inittab;
s1:23:respawn:/usr/sbin/pppd /dev/ttyS1 115200                               
This way, the pppd will be restarted if it dies. Make sure you have a
`-detach' option (nodetach on newer systems) though, otherwise inittab will
start numerous instances of pppd, while complaining about `respawning too

 Note: Some older systems will not accept the speed `115200'. In this case
you will have to set the speed to 38400 and set the `spd_vhi' flag with
setserial. Some systems expect you to use a `cua' instead of `ttyS' device.

3.2.2. Setting the routes

 The default route can be set with the defaultroute option or with the /etc/
ppp/ip-up script;
case $2 in                                                                   
          /sbin/route add -net gw Rem_Ip netmask             
Ip-up can also be used to sync your clock using netdate.

 Of course the route set in ip-up is not necessarily the default route. Your
ip-up sets the route to the remote network while the ip-up script on the
remote system sets the route to your network. If your network is
and your ppp interface, the ip-up script on the remote machine
looks like this;
case $2 in                                                                   
      /sbin/route add -net gw netmask  
The `case $2' and `/dev/ttyS1)' bits are there in case you use more than one
ppp link. Ip-up will run each time a link comes up, but only the part between
`/dev/ttySx)' and `;;' will be executed, setting the right route for the
right ttyS. You can find more about routing in the Linux Networking HOWTOs
section on routing.

 Some systems use dynamic ttys, in which case you can't route on a tty basis.
In this case it might be handy to translate the ip address to a ppp interface
and then do the routing (and firewalling) on a ppp interface basis. For this
purpose I edited /etc/ppp/ip-up;
# These variables are for the use of the scripts run by run-parts            
# translate ip to ppp                                                        
echo $PPP_IFACE > "/var/run/ppp/if-$PPP_LOCAL"                               
sleep 1                                                                      
# Rerun firewall.                                                            
# Take care of the (default) route(s)                                        
case $PPP_LOCAL in                                                           
                /sbin/route add -net gw $PPP_REMOTE netmask  
# Fix things missed at boot                                                  
if ! ( netstat -an | grep 'My_Ip_Address:53' > /dev/null 2>&1 )              
        # Just booted                                                        
        # Sync clock                                                         
        /usr/local/sbin/ &                                         
        # Set the null routes                                                
        /usr/local/sbin/ &                                      
        # Bind 9 needs this;                                                 
        sleep 1                                                              
        /etc/init.d/bind9 restart                                            
# An audiable notification                                                   
/bin/echo -ne "\007" >> /dev/tty1                                            
Replace 'My_Ip_Address' with your Ip address. /usr/local/sbin/
synchronises the clock. It stops the NTPD, syncs using ntpdate and then
starts the NTPD again. /usr/local/sbin/ is a script which sets
null routes;
route add -net    netmask   reject                        
route add -net  netmask reject                        
route add -net netmask reject                        
If you have RFC 1918 addresses in use, the above null routes won't interfere
provided you use a smaller netmask. A network won't be
bothered by the null route;
Kernel IP routing table                                                       
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface UH    0      0        0 eth1 UH    0      0        0 ppp0   U     0      0        0 eth0   U     0      0        0 eth1     -          !     0      -        0 -      -          !     0      -        0 -        -            !     0      -        0 -         UG    0      0        0 ppp0  

3.3. Test

 Test the whole thing just like the modem test. If it works, get on your bike
and bring the remote modem to the remote side of your link. If it doesn't
work, one of the things you should check is the COM port speed; Apparently, a
common mistake is to configure the modems with Minicom using one speed and
then configure the pppd to use an other. This will NOT work! You have to use
the same speed all of the time!

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