SLIP (Serial Line Internet Protocol) and PPP instructions

This document contains instructions on how to go about getting a SLIP or PPP address and using SLIP or PPP on the SCS Cisco terminal servers (TS[5/6/7/8].SRV.CS.CMU.EDU). SLIP and PPP connections are supported from high speed modems. The "man dialup" command on a CS/RI/EDRC-supported workstation or time-sharing system contains information such as phone numbers to dial and how to use the Cisco terminal servers.

SLIP and PPP encapsulate IP datagrams using an escape byte. IP packets normally travel over 10 megabit/sec ethernet. With SLIP and PPP, they are traversing a link whose bandwidth might typically be less than 20 kilobits/sec. Thus, many IP-based applications might not work adequately over SLIP and PPP links. SLIP and PPP can support some TELNET sessions, and a modest FTP. While NFS and AFS might work, their use over SLIP and PPP is strongly discouraged.

If you need an IP address for SLIP and PPP

Using "dynamic" IP addresses:

If you don't need a dedicated IP address, you can use the default IP address assigned to the line you've dialed into. This means that you likely won't get the same IP addressed assigned each time you dial in, but most SLIP and PPP clients can parse and/or negotiate with the server to set the dynamic IP address on the client.

When you dial up the V.32bis or V.34 huntgroup, log in as you normally would with Kerberos ID and password. At the "TS?> prompt, type the command "slip default" to activate SLIP, or "ppp default" to activate PPP. The server will then print out information about the connection including the IP address assigned.

Requesting dedicated IP addresses:

If you need a hostname and/or IP address for a SLIP or PPP client, run the /usr/cs/bin/netregister program and use the 'add' command at the prompt. You will then be prompted for information about the host.

A response for the following fields is mandatory:

  • Name of host to add
  • CPU type
  • CPU model number
  • CPU asset number (if privately owned equipment, supply CPU serial number)
  • Primary user
  • Remarks: You MUST supply the following information:
  • the IP address will be used for SLIP and PPP
  • the method of access (e.g., modem)
  • the encrypted password string as generated by the gentacpass program (program use is documented below)
  • Example - Remarks: SLIP and PPP use, modem, Encrypted password string for hostname is "sGl79TRXp1Y3U"

    "netregister" will send mail to Within a couple of days, you will receive an e-mail response containing the IP address in addition to other information.

    If you don't have access to the netregister program, complete the form found in /afs/cs/help/www/03-Communication/03-Tcons_and_Dialups/netregister.form and send it to

    If you have an existing hostname/IP address

    If you presently have a machine with an existing hostname and IP address, it is preferable that you ask for a new hostname/IP address or request a new IP address for that hostname. The SLIP and PPP clients are assigned IP addresses on certain subnets only.

    To use SLIP and PPP with the SCS Cisco terminal servers

    The following instructions outline how to use SLIP and PPP with the SCS Cisco terminal servers. It is assumed the SLIP or PPP clients that were netregistered have suitable SLIP or PPP implementations and that you are familiar with its operation.

    1. Read about how to use the Cisco terminal server command interface.

    2. At the "TS>" prompt, type:
      slip hostname
      ppp hostname
      where hostname is the first component of the hostname. The server will prompt you with "Password:". Type the password (see below for instructions on getting a password) or just carriage return if a password has not been enabled. You should then see:
      Entering SLIP mode.
      Your IP address is 128.2.nnn.nnn, MTU is 1524 bytes

    3. If you want a password enabled for your SLIP or PPP connection, please follow this procedure (until something more formal is implemented):

      Run the following program:

      /afs/cs/project/systems-parker/slip/bin/@sys/gentacpass [-a | -g] hostname
      @sys supported are:
    4. -a will ask you for a password

    5. -g generates a password
    6. Whether you select your password or have one generated, the program will print the following line:
      Encrypted password string for hostname is "sGl79TRXp1Y3U"
      You should send this output in an e-mail message to to have your password updated/activated.

    Other Details:

  • Be careful if your SLIP or PPP IP address is being ARP-served for (either on a previous slipserver or if you happen to have another machine on the net using that address). If this occurs, when you try to enter slip mode, the terminal server prints: Bad slip command - invalid address instead of "Entering SLIP mode." as above. This probably means some machine answered an ARP for your SLIP or PPP address.

  • Another ARP-related problem is that the server will not proxy-ARP on behalf of your SLIP or PPP address such that the ARP entry on other hosts will be updated. This problem will manifest itself if you switch your connection between TS servers within a short period of time. Some hosts may still have the ARP entry for your SLIP or PPP address pointing to the ethernet hardware address of the server you were connected to previously. This may explain why you can't connect to certain systems in SCS and ALL systems outside of SCS (the gateway will have the "old" ARP entry). There is a fix for this being worked on. Until then, when a SLIP or PPP connection is dropped, and a quick reconnect is desired, attempt to connect to the same server.

  • For many SLIP or PPP implementations, you need to supply a "destination IP address", ie, the address on the other side of the point-to-point link. You can use either or One of these addresses should be your default gateway, too. Use the same IP address for both destination and default gateway. The terminal servers will route to any destination. Only telnet connections made normally (ie, as ascii terminal connections, not SLIP mode) are restricted to the 128.2 network.

  • Compressed SLIP (CSLIP) is available on all servers except TS1.SRV.CS.CMU.EDU.

  • The MTU for SLIP is currently set to 1524. This is not an issue if you run purely TCP over the SLIP link and your MTU doesn't match (The TCP MAXSEG option will take care of this. It might be an issue if you run UDP or other IP protocols and your MTU doesn't match. If your MTU is smaller (many SLIP implementations run with a default MTU of 1006), you will not correctly receive IP datagrams greater than your MTU.

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