Modems

Topics

Modem Configuration
Serial Line Networking Solutions
Connecting with Chat
Configuring PPP
Dialers
Security
Configuring a PPP Server

Assignment

Modem Configuration

A modem uses a serial port to connect to the network. |-----------------------| | I/O Buss | |-----------------------| | | |-----------------------| | Serial Port | |-----------------------| | | |-----------------------| | Modem | |-----------------------|

Serial ports are bidirectional, but Linux treat them as two devices, ttySn for input and cuan for output. However, this is changing, and in the future, both in and out will be a ttyS device.
PC's have four standard ports, ttyS0-ttyS3 and cua0-cua3. These are device numbers (major:minor) 4:64,65,66,67 and 5:64,65,66,67.

If you don't have these, create them. e.g. MAKEDEV ttyS0 or mknod -m 666 /dev/ttyS0 c 4 64 chown root.root /dev/ttyS0

An internal modem requires one serial device.
Normally, serial ports and modems are configured to some default values.
If these aren't satisfactory, they need to be changed in the config files, primarily /etc/ttytab and /etc/inittab
/etc/ttytab is the historical file, but Linux typically uses /etc/gettydefs

Or use setserial, which is universally available and much easier to use.

setserial

setserial [options] device [parameters] - set port parameters setserial -g [options] device [parameters] - display port settings

Options
- -a = print all data
- -b = print summary of device configuration
- -G = print the information if a format to be used for setserial
- -q = be quiet
- -v = be noisy
parameters (limited list)
- port portnum -- port 0x3f8
- irq irqnum -- irq 5
- uart uarttype -- uart 16650
- autoconfig
- auto_irq
- baud_base baudbase -- baud_base 115200
- spd_hi - use 57.6kb when request is for 38.4kb
- spd_vhi - use 115kb when request is for 38.4kb
- spd_normal - use 38.4kb when request is for 38.4kb
- session_lockout - one process at a time can open a port
- pgrp_lockout - one process group at a time can open a port
- hup_notify - notify a waiting process when a port clears
Examples
- setserial /dev/ttyS0 skip_test autoconfig auto_irq
- setserial /dev/cua1 irq 5 port 0x3f8 uart 16650 baud_base 57600
- Use autoconfig if at all possible.
Making it automatic
- Put the setserial commands in /etc/rc.d/rc.serial and execute it from /etc/rc.d/rc.local

Turn on hardware flow control (handshake)

stty crtscts < /dev/cua1 - for example stty -a < /dev/cua1 - displays current flags (-crtscts = not set)

Checking it out

minicom is a nice utility that lets you communicate directly with your modem. Start it up and you can issue commands directly to control the modem and set up communications.

ATDT 555-1212

Serial Line Networking Solutions

SLIP (Serial Line IP) - Older and it has some deficiencies
PPP (Point-to-Point Protocol) - it allows the negotiation of some of the protocol parameters like the IP address and the maximum datagram size. It also allows client authentication.
- Uses High-Level Data Link Control Protocol (HDLC).
- Can encapsulate protocols other than IP.
- Link Control Protocol (LCP) is used to negotiate options
- Can use Password Authentication Protocol (PAP) or Challenge Handshake Authentication Protocol (CHAP) for authentication.
- Internet Protocol Control Protocol (IPCP) is used to negotiate IP parameters.
- Uses Van Jacobsen header compression (VJ Header Compression).
Configuring PPP Clients
Establishing A Connection with chat
- Options
  - -f give the file name of the script
  - -e turn echo on
  - -t set the default timeout
  - -T pass in a phone number to substitute for \T
  - -U pass in a string to substitute for \U
  - -v log all activities to syslog local2
- script - if there is no -f option, enter the script here.
- scripts consist of "expect-send" pairs, separated by spaces and subexpect-subsend pairs separated by dashes
  Examples
  Script Files
  Often, a chat script is more likely to be used.
  Examples
Configuring PPP
- wvdial and rc3 are two utilities that will help you set up PPP .
- pppd [tty_name] [speed] [options]
- tty_name is the tty device
- speed is the desired connection flow rate in bps
- Options
  - auth - require authentication (default).
  - call name - read options from /etc/ppp/peers/name.
  - connect scriptname - use scriptname to set up the serial line (usually a chat script).
  - crtscts - use hardware flow control.
  - nocrtscts - do not use hardware flow control.
  - defaultroute - add a default route to the routing table using the peer as the gateway.
  - detach - when connected, detach as a background process and leave the controlling terminal available.
  - disconnect scriptname - use this script after the connection is terminated.
  - kdebug n - pppd will log contents of all packets through syslog facility daemon, level debug (see below).
  - idle n - disconnect if idle for n seconds.
  - modem - use the modem control lines, CD and DTR (default)
  - dozens of others
  Example
  - pppd /dev/cua0 38400 -detatch crtscts modem defaultroute
  - pppd /dev/cua0 call campuscwix connect chat.script
  PPP configuration
  - /etc/ppp
  - ppp-on - example of a script to bring up a ppp connection
  - ppp-down - example of a script to shut down a ppp connection
  - options - contains the default options for pppd that cannot be overridden (in most cases).
    auth -- authentication on
    lock -- exclusive use via lock file in /var/spool/uucp
    domain cs.montana.edu -- for authentication purposes
  - ip-up and ip-down - scripts to establish routing for ppp links. These are executed when ppp starts and when it shuts down to setup and tear-down routing. Don't modify these - use ip-up.local and ip-down which are called.
  Example Setup
  - The PPP HOWTO has good examples of these.
    ppp.script = ppp.on
    #! /bin/csh exec /usr/sbin/pppd connect \ 'chat -v -s -f /etc/ppp/chatscript' \ -detach crtscts modem defaultroute \ /dev/modem 57600
    chatscript
    TIMEOUT 5 "" ATZ OK ATDT5824120 ABORT "NO CARRIER" ABORT BUSY ABORT "NO DIALTONE" ABORT WAITING TIMEOUT 45 CONNECT "" TIMEOUT 5 "ogin:" harkin "assword:" Its$A#Secret # "ption:" P
    Command Output While Connected
    Script started on Wed Nov 10 19:19:16 1999 [root@rainbow ppp]# ifconfig -a lo Link encap:Local Loopback inet addr:127.0.0.1 Bcast:127.255.255.255 Mask:255.0.0.0 UP BROADCAST LOOPBACK RUNNING MTU:3584 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 TX packets:0 errors:0 dropped:0 overruns:0 ppp0 Link encap:Point-to-Point Protocol inet addr:207.0.103.63 P-t-P:207.0.103.13 Mask:255.255.255.0 UP POINTOPOINT RUNNING MTU:1500 Metric:1 RX packets:6 errors:0 dropped:0 overruns:0 TX packets:6 errors:0 dropped:0 overruns:0 [root@rainbow ppp]# netstat -r Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface pm02-montana.ca * 255.255.255.255 UH 1500 0 0 ppp0 127.0.0.0 * 255.0.0.0 U 3584 0 0 lo default pm02-montana.ca 0.0.0.0 UG 1500 0 0 ppp0 [root@rainbow ppp]# traceroute esus.cs.montana.edu traceroute to esus.cs.montana.edu (153.90.192.1), 30 hops max, 40 byte packets 1 pm02-montana.campuscwix.net (207.0.103.13) 132.578 ms 139.826 ms 138.877 ms 2 gw01-montana.campuscwix.net (207.0.103.1) 129.574 ms 139.465 ms 139.725 ms 3 3gateway.msu.montana.edu (153.90.3.254) 129.618 ms 129.474 ms 129.732 ms 4 esus.cs.montana.edu (153.90.192.1) 129.641 ms 129.186 ms 129.752 ms [root@rainbow ppp]#
  Running PPP
  - Manually - execute ppp.script as root. Or set the suid bit and allow other users access. chmod o+srx /etc/ppp chmod 4755 /etc/ppp chmod o+sx /etc/ppp/ppp.script chmod 4755 /etc/ppp/ppp.script chmod o+r /etc/ppp/chatscript
  - Use dial-on-demand. This is available in the 2.3.4 version of pppd. In /etc/ppp/options enter:
  - Set up your pap-secrets and chap-secrets files if you are using one or the other.
  - It's probably better to put your chatscript into an executable directory.
  - To test it out, enter pppd : where is the address of your provider. Actually, it could be anything, because the ipcp-accept-remote option tells pppd to accept the remote servers suggestion for its IP. For example, you could enter pppd :1.2.3.4 and it would probably work.
  Debugging
  - set up a daemon entry on /etc/syslog.conf and possibly a log for chat. daemon.* /var/log/ppplog local2.* /var/log/chatlog
  - Turn on kdebug, kdebug n, where n is the sum of:
    - 1 = general debug
    - 2 = contents of received packets
    - 4 = contents of transmitted packets
  - Look at the log files to see what is going on between the client and server.
  - Dial in like this
```
      tail -f /var/log/messages
      ppp-on
      
      
```
  you will get to see the system messages as they happen.
Dialers

There are a number of products that will help you configure your dial-up access. These typically have a user-friendly interface to get the data and a command-line or GUI to do the actual dial-in. Four of these are rp3 (Red Hat PPP), wvdial, kppp (installed with KDE) and linuxconf.
Virtually all these work the same. They provide an interface that allows you to select the modem device (if there is more than one), specify the telephone number, user name and password. They also allow you to specify such things as the IP address (or that it is provided by the ISP when you dial-in), gateway IP and DNS server IP's. The best documentation can be found at the Linux Doc PPP HOWTO

rp3

rp3 consists of two parts; rp3-config and rp3. When you run rp3-config it configures the PPP connection. When you run rp3, it runs a gui that allows you to connect and disconnect easily as needed.

wvdial

wvdial also has two parts; wvdialconf and wvdial. As with rp3, wvdialconf elicits the necessary information and builds a configuration file (/etc/wvdial.conf). You run wvdial to connect and halt to program to disconnect.

kppp

When you run kppp, you are presented with a tab style interface that allows you to enter all of the perinent data. The Accounts tab also has a Connect button that is used to connect when you need to do that, and to disconnect when ready.

linuxconf

If you are familiar with linuxconf, you will have no problem with the configuration. You will find it under
Config > Networking > Client Tasks > PPP/SLIP/PLIP
As with the others, fill in the fields and save the changes. The command to use to manage the connection is netconf.
Auto-dial
One feature you would probably like to have is auto-dial, so that when you do something that requires network access, your system will automatically dial your ISP. This can be accomplished with diald. diald sets up a fake network interface and changes your default gateway to point to this interface. It then monitors the interface and if any traffic comes in, it dials your ISP and forwards all traffic to your PPP connection. If the interface is idle for a length of time you set, it will disconnect.
The diald configuration is /etc/diald/diald.options. This file controls how diald will behave on the system. diald must be started and that is normally done at startup. When diald is installed, it should be configured to start in the /etc/rc.d/rcX.d files. If not, you will have to do that manually. If you change /etc/diald/diald.options you will have to restart diald in order for it to process the new configuration.
diald Example

Security

PAP versus CHAP - CHAP is more robust. PAP uses passwords sent in the clear. Also, CHAP challenges the client at intervals so that it can't be replaced.
/etc/ppp/chap-secrets can be used to provide the necessary secret words.
Make sure that the /etc/ppp/chap-secrets or /etc/ppp/pap-secrets files are readable only by root!!!
Format: "client name" "server name" "secret" "IP addresses"
chap-secrets

# CHAP secrets example # # client server secret addrs #----------------------------------------------------------- batman.comic.org ppp.isp.net "Whos There?" batman.comic.org ppp.isp.net batman.comic.org "The Shadow Knows" ppp.isp.net * batman.comic.org "Ive Got A Secret" *.comic.org

ppp.isp.net serves batman.comic.org and requires the given secret string.
The addrs field can be used to give alternative domain names or IP addresses (good for mobile hosts).
batman.comic.net will challenge ppp.isp.net to insure that it is who it says it is.
batman.comic.org will let any other host connect to it if its domain name is in the comic.org zone.

Configuring a PPP Server

Put the pppd startup in /etc/rc.d/rc.local, or .../rc3.d.
mesg n stty -echo exec pppd -detach silent modem crtscts

Connecting a Serial Device

cat /proc/devices to see what devices are there
cat /proc/ioports to see what ports are used
cat /proc/interrupts to see the IRQ's - if needed
cat /proc/tty/driver/serial for config (RedHat)

The init process runs a copy of getty for each port that can have a connected device.
Devices are configured in

/etc/ttys - line definitions - old and going out of fashion
/etc/gettydefs - speed definitions for getty
Identifier Iflags Fflags login_message Next_identifier
Identifier = definition name
Iflags = initial flags prior to sending login message (see stty)
Fflags = final flags after login runs (see stty)
login_message
Next_identifier = config to use if this one fails 57600# B57600 HUPCL CS8 CLOCAL # B57600 SANE IXANY TAB3 CLOCAL #login: #38400
57600 is the name
B57600 is the bit rate (57600 bps)
Initial flags are HUPCL (no modem control), CS8 (8 bit word), CLOCAL (assume no modem control on line)
Final flags are SANE (reset all values to something reasonable), IXANY (any character can restart), TAB3 (style of delay for horizontal tabs), CLOCAL
The login prompt is "login:"
If this fails, try 38400

/etc/inittab - initialization file

/etc/issue - will use the contents of this file as the login banner
/etc/conf.getty[.tty] is default configuration /sbin/getty -c gettydefs_file /sbin/getty [options] line

/sbin/getty ttyS0 /sbin/getty -c /etc/gettydefs.ttyS1 /sbin/getty -t 10 ttyS2 57600 # hold line during init, drop after # 10 seconds if nothing typed

/securetty - which terminals should permit logins as root
/dev/console - has to be here local:0 - Xwindows console :0 - any Xterminal pty - any pseudo terminal

Line configuration in /etc/rc.d/rc.local -> rc.serial using setserial.

Assignment
Due:

If you have a modem, set up communications using the modem to establish a PPP connection and implement network services.
If you don't have a modem, look into what you would have to do to set up a PC or dumb terminal to connect to your system via a serial port.

Modems

Contents

Topics

Assignment

Modem Configuration

ttytab

setserial

Turn on hardware flow control (handshake)

Checking it out

Serial Line Networking Solutions

Configuring PPP Clients

Establishing A Connection with chat

Examples

Script Files

Examples

Configuring PPP

Example

PPP configuration

Example Setup

ppp.script = ppp.on

chatscript

Command Output While Connected

Running PPP

Debugging

Dialers

rp3

wvdial

kppp

linuxconf

Auto-dial

Security

chap-secrets

Configuring a PPP Server

Connecting a Serial Device

Assignment