HP OpenVMS Systems Documentation

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Compaq TCP/IP Services for OpenVMS

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3.2.2 Removing the PPP Configuration

To remove the PPP configuration, follow these steps:

  1. If you created a PPP interface, return the associated terminal port to general use. Enter:


    In this example, n is the number of the interface. If you omit the interface number, PP0 is assumed.
  2. If you added special route and proxy entries with the PPP line, remove them.
  3. If you changed any terminal settings in preparation for PPP, restore them. Enter the DCL command SET TERMINAL, and wait for the modem to reset and free the port and phone line.

3.3 Setting Up a SLIP Interface

Configuring the network interface for SLIP is the same as configuring the interface for Ethernet connections. In this case, the network interface is the modem connection. Remember that before you can configure a SLIP line, you must choose an IP address for the interface at each end of the line and establish a physical connection.

Use the following commands to set up the SLIP interface:

  • SET INTERFACE SLn, where n is the number of the interface. If you omit the interface number, SL0 is assumed. This command takes effect immediately and stays in effect until the next TCP/IP Services shutdown.
  • SET CONFIGURATION INTERFACE SLn, where n is the number of the interface. If you omit the interface number, SL0 is assumed. This command makes the change part of the permanent configuration. The change takes effect at the next product startup.

Table 3-3 describes the command qualifiers used for configuring SLIP interfaces.

Table 3-3 Command Qualifiers Used for Configuring SLIP
Qualifier Description
/[NO]AUTO_START Optional. The default is /AUTO_START. Automatically creates the interface on startup.
/COMPRESS=[ON|OFF|AUTOMATIC] Optional. The default is no compression. Enables or disables TCP header compression (CSLIP). With /COMPRESS=AUTOMATIC, compression remains off unless the remote host begins to use it.
/[NO]FLOWCONTROL Optional. The default is No flow control. Enables the special handling of XON and XOFF characters to work properly with modems that are configured to interpret these characters locally.

Specify /FLOWCONTROL only if the host at the other end of the line is another host running TCP/IP Services. If you cannot use /FLOWCONTROL, configure your modem to pass all the XON and XOFF characters through transparently.

/HOST=( host_name, IP_address) Required. Host name or IP address of the local host. If your host is multihomed, you must specify an address in dotted-decimal notation.
/NETWORK_MASK= subnet_address Required. The subnet mask of the local SLIP interface in dotted-decimal notation.
/SERIAL_DEVICE= device Required for hard-wired or dedicated modem connections. Optional for dynamic connections.

Identifies the OpenVMS device name assigned to the SLIP interface, for example, TTA1.

For example, the following command configures SLIP interface SL5, using the local IP address assigned to host CROW, with a subnetwork mask of The interface uses the terminal device TTA3:. The /COMPRESS qualifier enables TCP header compression (CSLIP). The /FLOWCONTROL qualifier enables special handling of XON and XOFF characters.


3.3.1 Setting Up Hard-Wired SLIP Lines

To configure SLIP with hard-wired lines, follow these steps:

  1. Establish a physical connection. Plug in a serial cable between the two host systems or ensure that they are both cabled to opposite ends of a leased line.
  2. Obtain an IP address if necessary.
  3. Configure the SLIP interface. Enter the SET INTERFACE command with the /HOST and /SERIAL_DEVICE qualifiers, which are required.

3.3.2 Setting Up SLIP Dialup Lines

You can configure either a terminal server port or an OpenVMS system to answer dialin calls.

Follow these steps:

  1. Configure the appropriate settings for the terminal port to which you will connect. Begin a dialog of dialing (or answering) commands with your modem. The specific required commands depend on the type of modem you are using.
    For example, to prevent the modem from hanging up when you exit the DTE session to bring up the SLIP line, enter the following command:


    To disable interactive logins on the line, enter the following command:


    Any SLIP data that arrives before you enter the SET INTERFACE command is ignored. Otherwise, this command triggers the creation of a new interactive login process.
    To enable interactive logins after a user sends a Break, enter the following command:

  2. Configure the modem. Enter the appropriate commands to dial the telephone and establish communication.
  3. Unless you are setting up a SLIP line between two hosts running TCP/IP Services and plan to use the /FLOWCONTROL qualifier at both ends, disable modem recognition of XON and XOFF characters. (If SLIP packets have Ctrl/S and Ctrl/Q characters embedded in them as data, you must prevent the modem from trying to interpret these characters.)
    Either use hardware flow control or disable flow control entirely. The following examples disable all flow control.
    • With a DECmodem V32 in AT command mode, set the following values:
      • AT%F0 --- No speed buffering flow control
      • AT%M0 --- Disable speed buffering (optional)
    • With a DECmodem V32 in DMCL mode, set the following values:
      • SET P2/SBU
      • SET P1/SBU
      • prompts appropriate_answers
    • With a U.S. Robotics Sportster modem, set the following values:
      • AT&B0 --- Variable, follows connection rate (optional)
      • AT&H0 --- Flow control disabled
      • AT&I0 --- Software flow control disabled
  4. Obtain IP addresses if necessary.
  5. To dial in, follow these steps:
    1. Enter the SET HOST /DTE command:

      $ SET HOST /DTE nx
    2. Type the telephone number. For example:

      atdt telephone_number
    3. The connected system displays its interactive (command mode) prompt. You are talking to the terminal server and can now make the connection.

The following example shows a user named SLIP-USER at a PC named ROBIN with a 9600-baud modem, using terminal device TTA2 and connecting it to the port of a terminal server. In this example:

  • The terminal server is a DECserver 700 terminal server.
  • The user directs the modem to dial the telephone number 222-2222.
  • The password prompt of the terminal server is # .
  • The terminal server's current login password is hootowl .
  • The terminal server's prompt is Local> .
  • The user types Ctrl/\ (Ctrl key plus backslash) to escape from the terminal server to the SLIP host.
  • The user defines interface SL2 and identifies it as SLIP device TTA1: with IP address Communication on this line will use CSLIP.


%REM-I-TOQUIT, connection established
Press Ctrl/\ to quit, Ctrl/@ for command mode

atdt 2222222


# hootowl (not echoed)

Network Access SW V1.5 for DS700-16
(c) Copyright 1994, Digital Equipment Corporation - All Rights Reserved
Please type HELP if you need assistance

Enter username>SLIP-USER



3.3.3 Setting Up Your Host as a SLIP Dialup Provider

You can configure your host to answer calls and establish connections initiated by users on remote hosts.

To set up your host as a SLIP provider:

  1. Over the line you will define as a SLIP line, dial in to the host.
  2. Log in to the remote host.
  3. Enter an appropriate SET INTERFACE command with the /SERIAL_DEVICE qualifier to turn the line into a SLIP line.
    For example, the following command creates a SLIP interface named SL5, using the terminal device associated with the session where the command is entered.

  4. Log out.

As soon as you log out, your terminal port becomes a SLIP interface. Without causing the modem to hang up, start SLIP on the remote system.

To facilitate connection setup for end users, create a dedicated user name for each remote host that dials in. These users need to have a LOGIN.COM procedure that invokes appropriate SET TERMINAL commands and TCP/IP management SET INTERFACE commands, terminating with a LOGOUT command. Every user should specify a different SLIP interface name and host name (or IP address). These users require the OPER privilege to create interfaces.

You can enable IP forwarding on the SLIP provider host and start dynamic routing. For example, enter the following commands:



To send notifications automatically on all connected LANs when new hosts or networks become reachable, use dynamic routing with the /SUPPLY option. For example, every time a SLIP connection is set up to a new remote subnetwork, RIP (Routing Information Protocol) advertises a corresponding route. For example, enter the following commands:



3.3.4 Connecting a Host to the LAN

You can make your SLIP-connected host appear as if it were directly connected to the LAN. This is possible using a proxy ARP server (usually the same host that is acting as a SLIP gateway into the LAN).

To use proxy ARP (Address Resolution Protocol), assign to the remote host an IP address in the same subnetwork as the LAN. As other hosts on the LAN attempt to communicate with the remote host, the SLIP gateway answers ARP queries for the remote host by giving its own LAN address. The gateway then forwards packets across the SLIP line.

Many DECserver terminal server products support SLIP connections and implement proxy ARP. If you dial in from an OpenVMS host to a terminal server, the terminal server automatically detects your IP address and begins responding to ARP queries, forwarding packets as necessary.

To use proxy ARP with a DECserver terminal server, assign an IP address in the same subnetwork as the terminal server.

At the terminal server, enter the TCP/IP management command SHOW PORT SLIP. Verify that:

  • An IP address has not already been associated with your port.
  • Header compression is available, if you plan to use it.

3.3.5 Setting Up a SLIP Gateway with Proxy ARP

It is also possible to set up your host as a SLIP gateway with proxy ARP. You might prefer this approach if your dialin modems are attached directly to an OpenVMS system rather than to a terminal server.

Follow these steps on the host to become a SLIP gateway:

  1. Create a SLIP interface in another network or subnetwork, for example:

  2. Add a host route for the remote system. For example:

  3. Configure an ARP entry for the remote host, listing your own Ethernet address (as shown in TCPIP SHOW INTERFACE /FULL). For example:

    $ TCPIP SET ARP 08-00-2B-2C-4F-46 FINCH /PUBLIC
  4. Enable IP packet forwarding, if not already done. Enter:


When your host is set up as a SLIP gateway, create an interface on the remote host at the other end of the serial line. Specify an address in the same subnetwork as the LAN.

Although the two ends of the SLIP line are in different subnetworks, traffic can flow properly due to the interface route you added with the SET ROUTE command.

3.3.6 Shutting Down SLIP

To terminate a SLIP connection, follow these steps:

  1. Return the associated terminal port to general use. Enter:

  2. If you added special route and proxy entries in conjunction with the SLIP line, remove them.
  3. If you changed any terminal settings in preparation for SLIP, restore them using the SET TERMINAL command.

3.4 Solving Serial Line Problems

If you have problems dialing in to an OpenVMS system using SLIP or PPP after following the instructions in this chapter, perform the following steps to isolate the cause of the problem:

  1. Check the equipment used by both the client and the dialin provider:
    • Do the cables work?
    • Are the modems configured properly?
    • Are the DIP switches on the modems set correctly?
    • Are the modem software settings correct? Make sure that flow control is disabled.
    • Are all clients and dialup providers using unique addresses?

    After a software upgrade, be sure to reboot and restart TCP/IP Services.
  2. Make sure the SET HOST attempts have not exceeded the OpenVMS security level. To check and then delete, if necessary, any information about these attempts, enter the following commands. Note that SECURITY privilege must be enabled to use these commands.

  3. Make sure that IP forwarding is enabled using the following command:

  4. Make sure the terminal characteristics for the terminal device associated with the interface are set up as follows:


    Make sure you specify the /TYPE_AHEAD qualifier when you enter the SET TERMINAL command to set up an asynchronous port.
  5. Enter the SET HOST/DTE command to make sure you can log in to the system:


    If you cannot log in to or communicate with the system, you may be using the wrong terminal device name (TTnx).
  6. Set up OPCOM to receive messages using the DCL command REPLY/ENABLE. You need OPER privileges to use OPCOM.
  7. You need NETMBX and OPER privileges to establish a successful connection. If these privileges are not enabled when you enter the CONNECT command, you will see messages similar to the following:

    $ PPPD
    \}`}"}(}"6~ <CTRL/@>
    %PPPD-I-CONNECTTERM, converting connection on device _TTA0: to a
    Point-to-Point connection
    %PPPD-E-CALLBACKERR, error calling network callback
    %SYSTEM-F-NOPRIV, insufficient privilege or object protection violation
    %PPPD-F-ABORT, fatal error encountered; operation terminated

    Note that the extraneous data in this sample is an ASCII representation of IP packets transmitted over the open line.
    PPP sets up a default route on the client if one did not exist. Typically, a default route exists if another interface exists on the client.
  8. Attempt to ping the remote system:

    TCPIP> PING host-name

    Watch the modem's LED display as you attempt to communicate using the PING command.
    You might not be able to ping the system if the serial line is tied up with a large FTP operation.
  9. Use the TCPTRACE command to see packets going in and out of the local system. For information about using TCPTRACE, enter:

  10. Display a count of the packets being sent and received on the problem interface, in full screen format, updated every second. For a SLIP problem, enter:


    To display the packet counts for PPP problem, enter:


    In these commands, n is the interface number.

3.4.1 Solving PPP Problems

Keep the following in mind for PPP-specific problems:

  • If the virtual terminal software has not been loaded, the following error will be displayed when you try to connect:

    %PPPD-E-NEEDVIRTTERM, point-to-point connection on device _TTB0: must
    be done on a virtual terminal

    Correct this problem by entering the following commands before you dial out:


    To make this permanent, add the following commands to the SYS$MANAGER:SYSTARTUP_VMS.COM file:


    Be sure to terminate any old virtual terminal sessions.
  • If you are trying to use OpenVMS as a PPP client to your ISP (Internet service provider), check where the ISP uses an authentication protocol, such as CHAP, PAP, or RADIUS. These protocols are not supported and will prevent a connection to OpenVMS.

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