LEASED LINE MINI HOWTO Rob van der Putten, rob@sput.dsl.nl v1.1, 22 December 1997 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 FDDI, and may involve all sorts of other hardware such as (pupin) coils, transformers, amplifiers and regenerators. This document deals with: Configuring your modem and pppd to use a 2 wire twisted pair leased line. This document does _NOT_ deal with: SLIP, getting or installing pppd, synchronous data communication, baseband modems. Assumptions You should already have a working pppd on your system. You also need Minicom or a similar program to configure your modems. 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 _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. Modem Configuration Configure your modem at the highest possible speed; 57600 bps for 14k4 and 115200 bps for 28k8 or faster modems. Once the modem is in dumb mode it will use the speed at witch it was configured. Configure both your modem software (Minicom) and your modem 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 DCD status (&D2 or &D3) Check this with AT&V or AT&Ix (consult your modem documentation) 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. Now configure your modem as follows; * Reset on DTR toggle (&D3) * Leased line mode (&Lx, consult documentation) * The remote modem auto answer (S0=1), the local originate (S0=0) * Disable result codes (Q1) * Dumb mode (this is sometimes a jumper) In dumb mode the modem will ignore all AT commands. Write the configuration to non-volatile memory (&W). Test Now connect the modems to 2 computers using the 232 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 one of the screens 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 (L1). PPPD You need a pppd (Point to Point Protocol Daemon) and a reasonable knowledge of how it works. Consult the relevant RFC's or the 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 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. Configuration Suppose the modem is connected to COM2, the local IP address is $LOCAL and the remote IP address is $REMOTE. We want to use 576 as our MTU. The /etc/ppp/options.ttyS1 would now be: crtscts mru 576 mtu 576 netmask 255.255.255.255 passive $LOCAL:$REMOTE -chap modem -pap persist 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. Scripts From /etc/init.d/network the script /usr/local/sbin/test-remote-ppp is called. This script checks whether the remote interface exists and will try to start the pppd if it doesn't. It starts with a sleep, making sure the boot process configures the COM ports first. #!/bin/bash /usr/bin/sleep 30 while true do if ! ( /sbin/ifconfig | grep $REMOTE > /dev/null ) then # PPP gone logger "Remote PPP gone ; restarted" /usr/local/sbin/Premote.sh & fi sleep 300 done You could of course, remove the 'sleep 300' and the do-done loop and start the whole thing from cron instead. 'Remote' is the remote host name. The default route can be set with the defaultroute option or with a /etc/ppp/ip-up script; #!/bin/bash case $2 in /dev/ttyS1) /sbin/route add -net 0.0.0.0 gw $REMOTE netmask 0.0.0.0 ;; esac 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 192.168.1.0 and your ppp interface 192.168.1.1, the ip-up script on the remote machine looks like this; #!/bin/bash case $2 in /dev/ttyS1) /sbin/route add -net 192.168.1.0 gw 192.168.1.1 netmask 255.255.255.0 ;; esac 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 NET-3-HOWTO section on routing Though the persist option should make this superfluous, the pppd can also be restarted using ip-down; #!/bin/bash case $s in /dev/ttyS1) /usr/bin/sleep 30 /usr/local/sbin/Premote.sh & ;; esac The pppd is started using the /usr/local/sbin/Premote.sh script #!/bin/bash ( /usr/sbin/pppd /dev/ttyS1 115200 crtscts $LOCAL:$REMOTE persist ) & I suppose some of the command line options make some of the options statements superfluous. Well, better safe then sorry. 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.