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IRIX Advanced Site and Server Administration Guide

Chapter 22
SLIP and PPP

This chapter introduces the Silicon Graphics implementation of the Serial Line Internet Protocol (SLIP) and Point to Point Protocol. SLIP/PPP offers:

SLIP allows the simultaneous operation of multiple processes on a serial cable or telephone line. Processes such as ftp and rlogin can share a link that an IRIS station is using for electronic mail or UUCP. Networks can be connected with the Internet Protocol features that network users expect.

The Silicon Graphics implementation provides both RFC 1144 data compression and its own proprietary data compression, which compresses header framing, checksum, and TCP/IP information to three bytes. SLIP is part of the eoe1 software subsystem, and is installed via the eoe1.sw.slip package. Use the versions(1M) command to check to see if you have SLIP installed on your system.

PPP offers substantially the same features as SLIP, but is generally considered to be faster. PPP is part of the eoe1 software subsystem, and is installed via the eoe1.sw.ppp package. Use the versions(1M) command to check to see if you have PPP installed on your system. Information for configuring SLIP is also generally true for configuring PPP. The types of changes and specifics added to the files in /etc/uucp are the same. The differences in usage and configuration are detailed in "Configuring PPP". Once installed and working, a PPP link works in the same way as a SLIP link. Thus, the sections on file transfer and NFS are true for both protocols.

The following sections are included in this chapter:

SLIP Configuration Information

SLIP can be used to connect stations or networks. SLIP configuration allows you to choose the modem and port you wish to use. If you have installed uucp or used a modem on your system, you are probably already familiar with most of the configuration files used by SLIP.

SLIP can be used over a telephone line with a modem at each end of the connection, or over a serial cable. This section assumes that you have read the information in "Turning On Dial-In/Dial-Out Modem Software" or in the Personal System Administration Guide, or are already familiar with modem installation.

If you have never used IRIX serial devices or if you need information on serial cable pin requirements for modem use, you may want to consult "Cabling the Serial Ports" or the Personal System Administration Guide for information on using the ports. You may also wish to review the reference pages for slip(1M), uugetty(1M), getty(1M), and inittab(1M).

Modem Specifications

A modem capable of at least 9600 bits per second (bps, sometimes referred to as ``baud'') is required for use with SLIP. However, at this speed, your SLIP link will be quite slow. A good modem choice is any modem that supports the v.32bis standard and a speed of 14400 bps or greater. A half-duplex modem can be used but is less desirable for interactive tasks.

Silicon Graphics does not manufacture these modems and cannot be responsible for changes to the modems. Silicon Graphics cannot guarantee modem compatibility or the quality of the telephone line used with SLIP.

If your modem supports both v.32 and PEPTM modes (or another batch-oriented mode), you can take advantage of these capabilities while using SLIP. The v.32 mode is preferred for most uses, but switching to PEP before using a file-transfer utility such as ftp or rcp will yield higher performance.

If you are using a high-speed modem, be sure it supports hardware flow control using RTS/CTS. With such a modem, use the ttyf* device name.

Note: Silicon Graphics supports DSI, Intel, Telebit, Zyxel, US Robotics, Hayes, and most Hayes-compatible modems. While many others work, configuring them may be much more complicated and their operation is not guaranteed. There are "fix" scripts in your software distribution that are placed in /etc/uucp when you install your system. Most commercially available modems that support the AT command set will work under IRIX.

Cable Specifications

The pin definitions for a cable connecting a suitable modem are described in "Attaching a Modem" and in the Personal System Administration Guide.

Configuring PPP

The main configuration file for PPP is /etc/ppp.conf. This file is described in detail in the ppp(1M) reference page. Here is an example of a working ppp.conf file:

ppp-dialup out remotehost=pppserv.dialup.com 
          lochost=mymachine.dialul.com
          uucp_name=pppserv netmask=255.255.252.0
          active_timeout=300 inactive_timeout=300 add_route

The entry ppp-dialup should match an entry in the /etc/uucp/Systems file, such as the following:

ppp-dialup Any ACUSLIP 38400 555-1212 "" @\r\c \ 
           ogin: mymachine ssword: TbZ4QlMA PPP

There should also be a corresponding entry in the /etc/uucp/Dialers file:

ppp-dialup  =W-, "" \d\r\pATs2=128s0=1&C0\r\c \
           OK\r-ATs2=128s0=1&C0\r\c-OK\r \
           ATdtw\T\r\c CONNECT

And an entry in the /etc/uucp/Devices file:

ACUslip ttyf1 null 38400 212 x ppp-dialup

You should also have your /etc/inittab file set up to expect the modem port speed you are using (in this case, 38400 bps) and to have getty or uugetty turned off. Specific details on editing the /etc/inittab file and restarting telinit are found in the Personal System Administration Guide or in "Adding a Terminal or Modem". The following entry works with the above listed file entries for PPP:

t1:23:off:/etc/uucp/uugetty ttyd1 dx_38400  # ppp modem

You may use dial-out PPP with the above listed entry, but for dial-in PPP, you will need to configure uugetty to answer the line, as described in "Adding a Terminal or Modem".

It is important to note that the above listed entries are simply examples of one configuration that works for one example site. The same entries may not yield satisfactory results in every case, due to other differences in site configuration and modem manufacturer and model. For example, the PPP site you are dialing into may require different settings in the /etc/ppp.conf file and the entry in the Dialers file assumes a specific brand and model of modem. Also, the example assumes you have issued the correct fix script command (found in the /etc/uucp directories and described in "Modem ''fix'' Scripts".)

When you have configured the files, you must issue the ppp command as root. The following command works for the above listed example file entries:

ppp -r ppp-dialup

For complete information on the ppp command and its options, see the ppp(1M) reference page.

Connecting Two Systems with SLIP

A SLIP link between two workstations or servers offers a TCP/IP work environment similar to that of Ethernet. SLIP provides the ability to use familiar TCP/IP utilities such as ftp and rlogin from any IRIS station accessible by telephone.

A SLIP connection can be initiated by either or both of the stations you wish to link. The simplest, most common SLIP connection is initiated by one of the two stations. The first part of this chapter describes the configuration of a simple SLIP link between two stations with distinct roles; one station always calls in and the other station always receives the call. The changes necessary to convert a simple SLIP link to a SLIP link that can be initiated by either station are also described in this chapter. See "Demand Dialing SLIP". Demand-Dialing SLIP is the most economical method of using SLIP for an ``as-needed'' network connection.

In this chapter, the term local station refers to the station that initiates the SLIP connection. The remote station receives a call from the local station. When the local station successfully connects, a slip process is started on the remote station. Once the connection is established, the slip processes sleep on both stations until one or both are terminated.

Overview of Configuration

Before beginning configuration of SLIP, verify that the following conditions exist:

To link two systems with SLIP, make these changes:

    Edit the file /etc/uucp/Devices on the local station to reflect the modem and port used by SLIP.

    Change the attributes for the port selected on the local station in its /etc/inittab, and the attributes for the port selected on the remote station in its /etc/inittab.

    Create a line in /etc/uucp/Systems on the local station that contains dialing and login information.

    Create an entry in the remote station's /etc/passwd file for the SLIP link.

    Add a line to /usr/etc/remoteslip on the remote station that contains the slip command you want to execute on the remote station.

    Install and configure a modem or connect a cable at each end of the SLIP connection.

The Local Station

This section explains how to configure the local station. The examples that follow use the local station name of wenders.

/etc/uucp/Devices

The file /etc/uucp/Devices is used to configure the desired device, modem speed, and dialer program for SLIP on your IRIS station. The correct format for a line appropriate for SLIP in /etc/uucp/Devices is: 

ACUSLIP device  null speed 212 x dialer

device can be any flow control device associated with a port not currently in use. speed can be any speed supported by your modem. dialer can be any dial program listed in /etc/uucp/Dialers.

If you want to configure more than one modem speed, use a different port, or use a modem that supports a different command set, you must create a new line in /etc/uucp/Devices that reflects the change.

For example, the following command configures SLIP to use a TelebitTM T2500 modem at 38,400 bps on the serial port 2 hardware flow control device in /etc/uucp/Devices: 

ACUSLIP ttyf2 null 38400 212 x t25slip

/etc/inittab

To use the port you specified in /etc/uucp/Devices for dialing out, change its attributes in /etc/inittab. Ensure that either getty is turned off or uugetty is in use instead.

Edit the file /etc/inittab and find the line that corresponds to the name of the port selected. For example, on the local station, SLIP uses ttyf2, so the line is changed to read: 

t2:23:off:/etc/getty ttyf2 co_38400         # port 2

This line turns off the getty program on port number 2.

If the SLIP link can be initiated by either station, you must turn on uugetty. For example, to configure a symmetric link using Telebit T2500 modems, change the line to:

t2:23:respawn:/usr/lib/uucp/uugetty -Nt 60 -it25in,conn ttyf2 \ dx_38400

Changes made to /etc/inittab are acted upon when init reexamines the /etc/inittab file. To make init reexamine /etc/inittab immediately, use this command:

/etc/telinit q

For more information, see inittab(4).

/etc/uucp/Systems

SLIP uses the information in /etc/uucp/Systems to call the remote station. The remote station's node name and telephone number, as well as the local modem's speed and a password, are all kept here and are used to log in to the remote station. In this example, the local station wenders uses this line in its /etc/uucp/Systems file to call station lynch. The connection is made at 38,400 bps with SLIP options recorded on lynch in /usr/etc/remoteslip. The password is "hopper". SLIP logs in to the remote station by responding with "slip-wenders'' to the login prompt, and ``hopper'' to the password prompt. 

lynch Any ACUSLIP 38400 5551212 "" @\r\c ogin:--ogin: \ @slip-wenders asswd: hopper SLIP

The information must be in one continuous line. The last string, "SLIP", forces the local station to wait for the remote station to announce that it is starting the SLIP protocol.

For more information on the /etc/uucp/Devices, /etc/uucp/Systems, and /etc/uucp/Dialers files, see Chapter 21, "UUCP." The file /usr/etc/remoteslip is discussed in the next section.

To call lynch from wenders, invoke slip:

/usr/etc/slip -o -p comp -r lynch

The Remote Station

SLIP requires an entry in /etc/passwd in order to log in. The user ID and group ID must both be zero (0). Instead of the shell specified at the end of a normal entry in /etc/passwd, SLIP uses the file /usr/etc/remoteslip. To log in as "slip-wenders," the station lynch should have this line in /etc/passwd:

slip-wenders:3RsB768WRAN2.:0:0:slip from wenders: \ /usr/people/slip-wenders:/usr/etc/remoteslip

An entry like this one is necessary for each station that calls in by using SLIP. For maximum system security, it is strongly suggested that you create a home directory for each SLIP user. Using open directories such as /tmp opens your system up to a variety of threats.

Note: The encrypted password in the example does not represent a real password. You must use passwd(1M) to set the password for the SLIP login.

/usr/etc/remoteslip

In the /etc/passwd entry for slip-wenders, the login shell is specified as the file /usr/etc/remoteslip. This file is used to invoke SLIP on a remote station. In /usr/etc/remoteslip, the slip command can specify the remote station's name and any other options appropriate to that connection.

/usr/etc/remoteslip is a Bourne shell script. You can add to the case statement as you would to any Bourne shell script case statement. The sh(1) online reference page contains detailed information about shell script programming. Each SLIP connection should have an entry in the following format:

slip-nodename )
    exec /usr/etc/slip options
    ;;

nodename is the name of the remote station. Options for slip are detailed in the slip(1M) reference page.

The /usr/etc/remoteslip file might contain this entry on the station lynch:

# Edit the case statement as required.  
case $USER in
       slip-wenders)
              exec /usr/etc/slip -p comp -i -r wenders 
              ;;
       *)
              exec /usr/etc/slip -i -r $USER
              ;;
esac

The connection between lynch and wenders uses the Silicon Graphics proprietary header prediction/compression for faster data transfer because the -p comp option is specified. To use RFC 1144 compression, use -p cslip instead. The option tells SLIP that the session is input from another station. The slip option, -r wenders, specifies the remote station's name.

/etc/inittab

There must be a getty or uugetty running on the incoming port on the remote station. In other words, the incoming port must be enabled. For example, if the remote station uses serial port 2 with Telebit T2500 modems, change the line in its /etc/inittab to:

t2:23:respawn:/usr/lib/uucp/uugetty -Nt 60 -it25in,conn ttyf2 \ dx_38400

Configuring the Modem

Modem configuration utilities are provided to facilitate the configuration of two popular brands of modems, Hayes and Telebit, and true work-alikes.

To send information across a telephone line, SLIP requires a modem at each end. To configure a modem at each end of the connection between the stations wenders and lynch, you must connect the modems and configure the software switches in the modems. Each modem must be physically connected to the desired port on one of the stations and a telephone line.

Modem ''fix'' Scripts

To configure your modem you must use the Superuser account (root). Go to the directory /etc/uucp. There you will find a number of modem ''fix'' scripts. They are named with the form

fix-modemtype

These scripts contain useful information and sample lines for /etc/inittab.

The scripts fix-hayes and fix-telebit can be used to configure modems that fully conform to two of the most popular modem configuration standards. The options to fix-hayes and fix-telebit are:

-i

Configures modem to accept incoming calls

-o

Configures modem to initiate outgoing calls

-io

Configures modem for both incoming and outgoing calls

The last argument must be followed by a serial port number, in the form d#. That is, to configure the modem to use the device ttyf2, use d2 in your command line.

Once you have connected a modem to the station, you can use the appropriate utility to configure your modem. For example, if you want to install a Telebit T2500 modem on ttyf2 of the station wenders so that station lynch can dial in, type: 

/etc/uucp/fix-telebit -io d2

In addition to Hayes and Telebit fix scripts, there are also scripts provided for DSI, Intel, and Zyxel modems. Most modems that support the AT command set can be correctly configured by one of these scripts.

Configuring a Bidirectional SLIP Link

A simple SLIP link, in which one station must always initiate the connection, can be changed to allow either station to initiate the connection.

Follow the procedure described in "Connecting Two Systems with SLIP" for configuring the files Devices, Systems, inittabpasswd, and remoteslip on the local and remote stations. Each station can then connect to the other. In addition, the modem fix script should be used to configure the modem on each station for incoming and outgoing calls.

The getty utility normally used to facilitate the login process interferes with attempts to call out. To call in and out using the same port, configure the port to use uugetty, which is part of the UUCP software subsystem.

Edit the /etc/inittab file on both stations to use uugetty as described above. Edit /usr/etc/remoteslip on the local station and /etc/uucp/Systems on the remote station. In other words, each station will now be both "local" and "remote." Therefore, both stations need both sets of changes described in "The Local Station".

Connecting Networks with SLIP

SLIP can be used to connect Ethernet or FDDI networks that use the Internet Protocol. Network SLIP connections are set up like other SLIP connections.

To have a SLIP connection between networks start automatically, create a file in /etc/init.d for that purpose. For example, the file /etc/init.d/slipstart can be used to automate the connection between the stations wenders and lynch. The slipstream file resides on the station where you want to initiate the SLIP link and should be linked to an appropriate startup file in the /etc/rc2.d directory. For example, to have lynch automatically generate a SLIP connection between networks at station startup, on lynch you would:

    Create the /etc/init.d/slipstart file and add the following lines:

    #!/bin/sh
# Bourne shell script for starting SLIP connection.
/usr/etc/slip -c -p comp -r wenders &

    The -c option automates maintenance by redialing the remote station (in this case wenders) should the telephone line fail.

    Create the necessary /etc/rc2.d link on lynch: 

    ln -s /etc/init.d/slipstart /etc/rc2.d/S32slipstart

Demand Dialing SLIP

If you have a SLIP link that is used irregularly but frequently, it is likely to be economical for you to make that link a demand-dialing link. With demand-dialing, the SLIP program will make the telephone connection on an as-needed basis, when there is network traffic to be transmitted, and it will drop the connection when there is no traffic. By default, much of the non-essential network traffic will not cause a link to be established. For example, the traffic generated by the time daemon (timed) would not cause a call to be placed, but a request for file transfer would cause the connection to be made.

To use demand-dialing, add the -q option to your slip command on system that initiates the connection. Demand-dialing mode is also known as ``quiet'' mode. For complete information on this and other options, see the slip(1M) reference page.

Debugging a SLIP Link

If you are having trouble bringing up your SLIP link, test the line with another utility. If you are familiar with uucp, you may want to establish a uucp link between the stations as a means of debugging the connection. Most SLIP users will find cu an easier way to debug a SLIP link.

Note: cu requires a direct entry in the /etc/uucp/Devices file. Refer to "The Devices File" for more details.

When debugging a SLIP connection, check each station separately. First check the port and modem on the local station by using a cu(1C) command like this: 

cu -d -s speed -l port

For example, to test the port and modem installed on the station wenders, you would use this cu command: 

cu -d -s 38400 -l ttyf2

(It may be necessary to turn off the (uu)getty first by changing respawn to off on the line for the port in the file /etc/inittab.)

The modem should respond. Many modems respond by printing AT. If the modem does not respond as expected, review the SLIP configuration procedure for the local station and review the modem configuration and documentation. If the modem does respond as expected, disconnect from cu by typing a tilde followed by a dot:

~.

Connect the stations you want to link as you would for the SLIP link. On the local station, use cu to call the remote station through the port and connection you have already verified with cu.

Check the connection to the remote station with a cu command like this: 

cu -d -sspeed telno

For example, to test the connection between wenders and lynch, you would call lynch from wenders with this cu command: 

cu -d -s38400 5552002

You should see the local station tell the modem to call the remote station. Eventually, you should see the login prompt. Type the send strings from the /etc/uucp/Systems file in response to the expect strings.

NFS Under SLIP

You can run NFS over a SLIP link. NFS will be very slow because of the amount of information transferred in NFS transactions. You may be able to improve performance with these measures:

File Transfer Under SLIP

File transfer may be slow if other demanding utilities share the SLIP link. For faster file transfer, try these solutions:

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