Configuring the Serial Line Internet Protocol (SLIP)

SLIP can be used to connect one host to another via a single, physical serial line connection between serial ports or over longer distances using a modem at each end of a telephone line. A computer that is running SLIP over one or more serial lines and that is also connected to a computer network (such as an Ethernet) can serve as a communication gateway between computers on a network and computers at the far end of each serial line.

SLIP is an old protocol with a limited number of features. If you need any of the following features, you should use PPP rather than SLIP:

• support for multiple physical conections within a link (multilink)

• CHAP or PAP authentication

• packet filtering

• error detection

• negotiation of IP addresses

• bringup on demand of links

• automatic reacquisition of links

• timing out of inactive links

• extensibility to network protocols other than IP

Configuring SLIP links

To configure a SLIP link using the Network Configuration Manager:

1. Select View WAN and then select Software Configure SLIP.

2. When presented with a list of currently configured SLIP link definitions, click on Add to define a new link and choose the desired type of link configuration:

   • outgoing

   • incoming
   When you have completed entering details for the link, click on OK. If you configure an incoming link, you will then also be prompted to enter the SLIP login account's user ID and password.

   To modify the configuration of a link, highlight it in the list and click on Modify. Click on OK when you have finished changing the details of the link's configuration.

   To delete a link configuration, highlight it in the list and click on Delete. Click on OK when asked to confirm the deletion.

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   NOTE: If you modify or delete a link configuration and the link is currently established, the changes you have made will only take effect when you next reboot your system.

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3. Click on Advanced Options to configure advanced features such as header compression.

4. If you need to configure a Systems(4bnu) file entry for an outgoing link, select Call Services Outgoing. See ``The Dialup Systems Manager'' for more information.

5. If you need to configure a modem to be used for an outgoing or an incoming link, select Hardware Add WAN device. For incoming links, configure the serial port to accept incoming connections and turn on logging in and auto-answer.

Configuring an outgoing link

An outgoing link can be configured to be connected over a dedicated serial line, or to use the facilities offered by UUCP to dial up a remote system over a modem and telephone line.

If configured to connect to a system in the UUCP Systems(4bnu) file, an outgoing SLIP link allows the physical equipment, such as a modem and telephone line, to be shared with other SLIP connections or other WAN networking services, such as PPP and UUCP. The remote end of the connection must configure an incoming link. It must accept the login and password information forwarded by UUCP to establish the connection.

Provide the following information to configure an outgoing link:

UUCP system name

Enter the name of the entry for the remote system in the Systems file if the link will use this to establish UUCP connectivity.

Do not enter a name in this field if you want the system to establish a SLIP link over a dedicated serial line.

Tty line for SLIP

Baud rate for tty

Enter values in these fields if you want the system to establish an outgoing SLIP link over a dedicated serial line.

Do not enter values in these fields if the SLIP link will be connected using entries in the UUCP Systems file.

Local host name

The name by which the local SLIP interface is known.

Host IP address

The address by which the local SLIP interface is known. The remote system must agree to use the specified address or the local system will refuse to establish the link.

If both a local name and an IP address are entered, the IP address takes precedence.

Destination name

The name by which the host at the remote end of the SLIP interface is known.

Destination IP address

The IP address for the host at the remote end of the SLIP interface. The remote system must agree to use the specified address or the local system will refuse to establish the link.

If both a destination name and an IP address are entered, the IP address takes precedence.

Gateway

If this network interface is not the first on your system, and you want to be able to forward packets between the different interfaces, select Yes.

Configuring an incoming link

An incoming SLIP link permits the local system to accept an incoming SLIP connection request from a remote host. This type of link requires a login account in the /etc/passwd file on the local host. A remote host wishing to initiate a SLIP link with the local host must use an outgoing type of link, and must specify this account name and its associated password in the chat script in the entry for the local system in the remote system's UUCP Systems(4bnu) file.

When you use the Network Configuration Manager to configure this type of link, it creates a SLIP login account which uses a special shell script instead of a standard shell. The shell script must execute an slattach command to establish the SLIP link. You must configure the remote side to use an outgoing link type for its end of the link.

Provide the following information to configure an incoming link:

Login name for SLIP

The name of the account used to service SLIP sessions.

Login shell script

The name of the script containing the slattach command that is used to establish the link.

   /usr/sbin/slattach /dev/term/00 132.147.118.6 132.147.246.12 255.255.0.0 38400 &

Advanced SLIP configuration

Advanced configuration of SLIP may be performed using the Advanced Options available from the Network Configuration Manager.

Proxy ARP

If set to On, add a proxy entry for the IP address of the remote end of the SLIP link in the ARP table. This will allow other hosts on the local network to contact the remote system if its IP address belongs to the local subnet.

TCP/IP header compression

If set to On, TCP/IP header compression is enabled and the interface sends compressed packets. The remote end of the link must be willing to accept such packets.

Auto detect header compression

If set to On, the interface will attempt to detect compressed packets. If it does, it will respond with compressed packets.

Auto-detection has no effect if TCP/IP header compression is already configured on.

Hardware flow control

If set to On, use hardware flow control on the serial line.

ICMP packet suppression

If set to On, stop ICMP packets (such as those used for redirects, ping, traceroute, and router discovery) from passing through the interface.

Maximum transmission unit

Enter the size of the maximum transmission unit (MTU) in bytes.

The default MTU value is 296. Increasing this value may increase throughput as packets can contain more data. However, it may also decrease throughput if packets are routed through systems with lower MTU values, as each packet is split (fragmented) into smaller packets for transmission. You can experiment with this value to obtain the most desirable results.

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NOTE: The suggested value for the MTU is 40 plus some power of 2. For example, the default value, 296, is 40 plus 2^8. Other possible values include 552, 1064, and 2088.

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Debugging

If set to On, enable logging of SLIP debug messages about the state of the link to the system console.

Deleting all SLIP links

To delete all SLIP link definitions using the Network Configuration Manager:

1. Select View WAN and then select Software Delete SLIP.

2. When asked if you want to delete all SLIP link definitions, click on OK.

Administering SLIP

The slattach(1Mtcp) command establishes a connection over a pre-defined SLIP link. Each time slattach is executed, a new slattach process is created to control the additional SLIP link in use. slattach creates a network interface based on the local and remote IP addresses that are passed to it as parameters.

Parameters to the slattach command define:

• the serial communication path for a link by specifying either a tty device or a UUCP site name (the latter requires the -d option)

• an IP address for the local side of the communication path and an IP address for the remote side of the communication path

• other IP and serial line attributes for the link

Once a SLIP link is established, it remains active until:

• you mark the associated SLIP interface down using ifconfig(1Mtcp). For example, the command to mark the first SLIP interface down would be:
  
  ifconfig sl0 down

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  CAUTION: Marking an interface down using ifconfig does not remove the link. Therefore, marking down the interface for an outgoing link that uses a modem and telephone line does not hang up the line. The costs of the line usage continue until the link is removed.

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• you kill the associated slattach process to remove the SLIP interface.

This should only be necessary if the system is not running a routing daemon such as routed(1Mtcp).

For more about SLIP

To obtain more information about SLIP files and commands, consult the following manual pages:

Manual page	Information provided
slattach(1Mtcp)	Command for assigning a tty line to a network interface
slip(7tcp)	Serial Line Internet Protocol (SLIP) and driver

To get more information about SLIP, see the following Request for Comments (RFCs). For information on how to obtain RFCs, see ``Obtaining RFCs from the Internet''.

RFC	Title
1055	Nonstandard for transmission of IP datagrams over serial lines: SLIP
1144	Compressing TCP/IP headers for low-speed serial links