Chapter 12: Configuring the Serial Line Internet Protocol (SLIP)

Chapter 12: Configuring the Serial Line Internet Protocol (SLIP)

Chapter 12
Configuring the Serial Line Internet Protocol (SLIP)

SCO SLIP is a STREAMS-based computer networking facility that provides for the transmission and reception of IP packets over serial lines. As such, SCO SLIP provides for the use of TCP/IP networking applications such as rlogin and telnet over serial lines. SCO SLIP is an implementation of the Serial Line Internet Protocol (SLIP) as defined in the following Internet documents: RFC 1055, RFC 1144.

Use SLIP when an Ethernet or Token-Ring network connection between the local host and another location is not possible but a serial line connection is. SLIP can be used to connect the local host to another host via a single, physical serial line connection between serial ports or over longer distances using telephone lines and modems. 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 the network and the computers at the far ends of the serial lines.

This chapter describes:

Configuring SLIP with the Network Configuration Manager. This section assumes you are familiar with SLIP concepts.
SLIP concepts. Read this section if you are unfamiliar with serial communications in general and SLIP in particular.
Troubleshooting SLIP.
Other sources of SLIP information.

Also available for serial line communication, for use instead of SLIP, is PPP. See Chapter 11, ``Configuring the Point-to-Point Protocol (PPP)'', for more information on PPP, and ``Serial line communications'' for a comparison of the two protocols.

SCO SLIP supports only asynchronous communications. SCO provides SLIP as a part of the discrete PPP package within the SCO TCP/IP runtime system. To use SLIP, you must either install the entire TCP/IP runtime system or ensure that PPP is one of the packages selectively installed.

Terminology used in this chapter:

line: as in serial line -- the physical medium over which data flows
link: as in SLIP link -- the virtual connection between two machines over which data flows
interface: as in SLIP interface -- the abstract entity to which the IP driver routes datagrams

SLIP link configurations

SCO SLIP supports the following link types:

The link type that should be configured at each end of a link depends on the way that the endpoint will view the connection. The possible combinations of link types that can be used to connect systems using SCO SLIP are illustrated in Figure 12-1, ``Possible connections between systems using SCO SLIP''. The arrows show the direction in which a SLIP connection can be initiated.

Figure 12-1 Possible connections between systems using SCO SLIP

Dynamic incoming link characteristics

An incoming SLIP link permits the local system to accept an incoming SLIP connection request from a remote host using a getty(M) process running on the local system. 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 a dynamic outgoing type of link, and must specify this account name and its associated password in the UUCP Systems 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. See ``Adding a SLIP link'' for more details.

You must configure the remote side to use a dynamic outgoing link type for its end of the link. See ``Dynamic outgoing link characteristics'' for details about this type of link.

Dynamic outgoing link characteristics

An outgoing SLIP link allows the physical equipment, such as a modem and telephone line, to be shared with other SLIP connections or other networking services, such as UUCP. In fact, UUCP facilities are used to implement this type of connection. The remote end of the connection must be configured as a dynamic incoming link. It must accept the login and password information forwarded by UUCP to establish the connection. See ``Dynamic incoming link characteristics'' for details about this type of link.

See ``Adding a SLIP link'' for more details. Also see ``Configuring UUCP for dynamic outgoing links'' for additional UUCP requirements for this type of link.

Dedicated manual link characteristics

A manual link must only be used with a serial line that is always up and dedicated to (that is, reserved exclusively for) a single connection. Examples of such lines include a leased line between buildings or a serial cable connecting two machines. The remote host must also configure its end of the connection as a dedicated link. Either host may initiate a connection to the other.

See ``Adding a SLIP link'' for more details.

Configuring SLIP

Use the Network Configuration Manager and its counterpart, slconf, to perform most SLIP configuration tasks. You can add or remove the SLIP stack and add, modify, or remove one or more SLIP interfaces.

You can also configure advanced options for each interface as described in ``Advanced SLIP configuration''.

Adding the SLIP protocol stack

You must add the SLIP protocol stack before configuring any SLIP links. To do so:

Log in as root on the machine on which you want to configure the SLIP stack, and run the Network Configuration Manager.
Select Add new WAN connection from the Hardware menu.
Select SCO TCP/IP SLIP driver from the list of WAN connection types.
Select SCO TCP/IP from the list of networking protocols that can be used with SLIP, and click on Add.
To add a link endpoint, choose a link configuration type and follow the instructions in ``Adding a SLIP link'', beginning with step 5.
To defer configuring links, click on Cancel. A message appears indicating that the SLIP driver has been successfully configured.
Select Exit from the Hardware menu.
Select Yes when prompted to relink the kernel, and enter y at the two prompts that appear shortly after.
Shut the system down and reboot it by using the System Shutdown Manager as described in the SCO OpenServer Handbook. Remember to select Reboot after shutdown when using the manager.

Adding a SLIP link

Use the Network Configuration Manager or slconf to configure SLIP links after you configure the SLIP stack.

If you are using slconf to configure SLIP links, enter /usr/bin/slconf at the shell prompt and begin with step 4.

To add a SLIP link:

Run the Network Configuration Manager as root.
Highlight the SCO TCP/IP SLIP driver from the list.
Select Modify hardware configuration from the Hardware menu.
Select Add and press OK.
Choose the type of desired link configuration:
- Dedicated manual link -- choose manual
- Dynamic incoming link -- choose incoming
- Dynamic outgoing link -- choose outgoing
If you want to add the SLIP drivers to the kernel without performing any link configuration, select the manual link option, and then skip to step 8.
Enter link configuration data in the form.
To enter advanced configuration options for either the dynamic outgoing or dedicated links such as compression, authentication, and filtering options, select the Advanced Options button.
Click on OK to confirm the link configuration.
If you configured a dynamic incoming link, enter the SLIP login account's user ID and password at the prompts.
Choose No if you are done configuring links, or Yes to configure an additional link. If you choose Yes, return to step 5 and choose a configuration type.
After your choose No, a message appears indicating that the link was configured successfully.
Select Exit from the Add hardware menu. slconf users skip this step.
Make any needed changes to other SLIP and UUCP files. See ``Configuring UUCP for dynamic outgoing links'' for more information.

The links you configured are available the next time you reboot your system or restart TCP/IP.

Configuring a dynamic incoming link

When you use the Network Configuration Manager or slconf to configure a dynamic incoming link, you must provide the following information:

You must create the login shell script you enter in the Network Configuration Manager. It should contain an slattach line similar to the following:

   /etc/slattach /dev/tty1a 10.0.118.6 10.0.246.12 255.255.0.0 38400&

In this example, SLIP accepts incoming requests from the system 10.0.246.12 via the interface 10.0.118.6 on a modem connected to /dev/tty1a. The baud rate in this example is 38400. For more information on slattach command line syntax, see slattach(ADMN).

Configuring a dynamic outgoing link

When you use the Network Configuration Manager or slconf to configure a dynamic outgoing link, you must provide the following information:

Tty line for SLIP. You must set this to none. If you set this to any other value, you configure a dedicated link.
Baud rate for tty. Do not set a baud rate for this type of link.
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.
Netmask. The netmask to use over this link. The netmask must match that used by the remote system. For more information on netmasks, see ``Netmask setting'' in Configuring Network Connections.
UUCP system name. Enter the name of the entry found in /usr/lib/uucp/Systems that this link uses to establish UUCP connectivity. For more information on basing your SLIP connection on UUCP, see ``Configuring UUCP for dynamic outgoing links''.
Gateway. If this network interface is not the first on your system, and you want to be able to route packets between the different interfaces, select Yes. This causes the kernel parameters ipforwarding and ipsendredirects to be enabled. For more information, see ``TCP/IP parameters'' in the Performance Guide.

NOTE: After you configure the link with the Network Configuration Manager, configure UUCP to allow your system to connect to the remote site. See ``Configuring UUCP for dynamic outgoing links'' for more information.

Link configuration information is stored in a line in /etc/tcp which calls the slattach program. Here is a sample slattach entry, broken over two lines for ease of reading:

   /etc/slattach -d slipsite 10.0.118.55 10.0.246.77\
   255.255.255.0&

This link connects to a UUCP host named slipsite. The remote system's IP address is 10.0.246.77, the local address is 10.0.118.55. The netmask is specified as 255.255.255.0.

See ``Advanced SLIP configuration'' for more information about using the Advanced Options or directly editing the slattach entry in /etc/tcp to configure SLIP.

Configuring UUCP for dynamic outgoing links

Dynamic outgoing SLIP links use the UUCP facilities as the physical media over which the link is made. These media usually consist of modems at the remote and local hosts, and the telephone lines connecting the modems.

After you configure a dynamic outgoing SLIP link with the Network Configuration Manager, you must edit the UUCP Systems and Devices files to define information about the remote site. You can also use uuinstall(ADM) to perform this task.

A dynamic outgoing SLIP link, defined in /etc/tcp, must specify the UUCP name for the remote site (uucp_sitename) as the argument to the -d option. (slattach will fail if you specify -d but not uucp_sitename. If you do not specify -d, slattach erroneously tries to configure a dedicated link instead of a dynamic outgoing link.) uucp_sitename must exist as a ``sitename'', listed in the UUCP file /usr/lib/uucp/Systems, which identifies the remote host. This forms the link between SLIP and UUCP.

An entry for uucp_sitename may be entered in the /usr/lib/uucp/Systems file (see Systems(F)) by manually editing this file or by running uuinstall(ADM), which prompts you for the necessary information. The login name and password associated with the account must match those defined by the SLIP configuration at the remote end.

NOTE: The entry in /usr/lib/uucp/Systems requires shared information with other UUCP files. For complete information on configuring a UUCP link, see Chapter 7, ``Connecting to other computers with UUCP'' in the System Administration Guide.

SLIP's use of UUCP in establishing a connection requires that UUCP can be used alone to connect the local and remote hosts without using SLIP. When a dynamic outgoing SLIP link is brought up, SLIP requests a UUCP link to the remote host. UUCP must be able to establish a UUCP link using only UUCP information, such as that found in the files /usr/lib/uucp/Systems, /usr/lib/uucp/Devices, and /usr/lib/uucp/Dialers. During UUCP link establishment, characteristics such as baud rate and flow control are determined by the UUCP information and not by the SLIP link configuration. Once UUCP has established a link, it informs SLIP of the tty to use for that link. At this point SLIP takes over the link. Therefore, it is best to test a UUCP link between the local and remote hosts before trying to run SLIP over the link.

For complete information on configuring a UUCP link, see Chapter 7, ``Connecting to other computers with UUCP'' in the System Administration Guide.

Configuring a dedicated manual link

When you use the Network Configuration Manager or slconf to configure a dedicated link, you must provide the following information:

Tty line for SLIP. The serial port used by this link. Choose a line from the point-and-pick list. You should choose a line which does not support modem control (those lines ending in lower case letters). The default line is /dev/tty1a.
Baud rate for tty. The line speed for the designated tty, in bits per second. Choose a speed from the point-and-pick list. The default value is 38400.
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.
Netmask. The netmask to use over this link. The netmask must match that used by the remote system. For more information on netmasks, see ``Netmask setting'' in Configuring Network Connections.
UUCP system name. This field applies to dynamic outgoing links only.
Gateway. If this network interface is not the first on your system, and you want to be able to route packets between the different interfaces, select Yes. This causes the kernel parameters ipforwarding and ipsendredirects to be enabled. For more information, see ``TCP/IP parameters'' in the Performance Guide.

Link configuration information is stored in a line in /etc/tcp which calls the slattach program. Here is a sample slattach entry, broken over two lines for ease of reading:

   /etc/slattach /dev/tty1a 10.0.118.55 10.0.246.77\
   255.255.255.0 9600&

This link uses tty1a at 9600 baud. It communicates with a remote system, 10.0.246.77, via the interface 10.0.118.55. The netmask is specified as 255.255.255.0.

See ``Advanced SLIP configuration'' for more information about using the Advanced Options or directly editing the slattach entry in /etc/tcp to configure SLIP.

Modifying a SLIP link

To modify a SLIP link:

Log into the system as root.
Bring the link down with ifconfig(ADMN). For example, if the link you are reconfiguring is the first SLIP link on your system (sl0), enter ifconfig sl0 down at the command line.
Enter /usr/bin/slconf.
Highlight a link to modify from the list.
Select Modify, then click on OK.
Enter the desired information.
Enter any desired advanced configuration options, such as for compression, authentication, and timeout, by selecting the Advanced options button. Click on OK when done.
Click on OK to confirm the link configuration.
You are prompted to configure more links. Select No. If you select Yes, refer to ``Adding a SLIP link''.
Click on OK to leave slconf.
Make any needed changes to other SLIP and UUCP files.
Restart the link with ifconfig. For example, you could enter ifconfig sl0 up.

NOTE: You can also use the Network Configuration Manager to modify a link. To do so, highlight the SCO TCP/IP for UNIX entry directly beneath the SCO TCP/IP SLIP driver, then select Modify protocol from the Protocol menu and join the previous procedure at step 3.

Removing a SLIP link

To remove a SLIP link:

Log into the system as root.
Bring the link down with ifconfig(ADMN). For example, if the link you are reconfiguring is the first SLIP link on your system (sl0), enter ifconfig sl0 down at the command line.
Enter /usr/bin/slconf.
Highlight the link to delete.
Select Remove, then click on OK.
Click on OK to exit slconf.

Removing the SLIP stack

To remove the SLIP stack and its associated links:

Run the Network Configuration Manager as root.
Select SCO TCP/IP SLIP driver from the list.
Select Remove network device from the Hardware menu.
Select Yes when prompted to remove the link. You see several messages as the SLIP drivers are removed from the kernel.
Select Exit from the Add hardware menu.
Select Yes when prompted to relink the kernel, and enter y at the two prompts that appear shortly thereafter.
Shut the system down and reboot it by using the System Shutdown Manager as described in the SCO OpenServer Handbook. Remember to select Reboot after shutdown when using the manager.

Administering SLIP

The slattach 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.
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.

NOTE: Before slattach can create a SLIP link, the SLIP protocol stack (a linked chain of software modules and drivers) must have been configured the kernel, and the system must have been rebooted. See ``Configuring SLIP'' for details.

Multiple applications (or multiple instances of the same application) on a local host can communicate with the same remote host using the same SLIP link. Use of the same SLIP link requires only that the applications use the same IP address to specify the remote host. By using different IP addresses to specify the same remote host, two applications can also use individual SLIP links to the same host, provided a separate SLIP link exists for each remote IP address.

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

root kills the associated slattach process to remove the SLIP interface.
root marks the associated SLIP interface down using ifconfig(ADMN), for example: ifconfig slip_interface down.

CAUTION: Marking an interface down using ifconfig does not remove the link. Therefore, marking down the interface for a dynamic 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.

Once the link has been removed, root should remove the route associated with the SLIP link from the routing table using the route(ADMN) command:

route delete destination_address gateway_address

Running slattach for users other than root

The slattach command allows you to specify the local and remote IP addresses on the command line and to turn on proxy-ARP. As an unscrupulous user could use these features to intercept network traffic, ordinary users are prevented from using this command by the permissions and ownership set on slattach. Only root and users in the group network (with group ID 10) can run slattach. In this way, root can set up accounts for dial-in access by SLIP provided that they are in the network group. For instructions on how to add a user to a group, see ``Changing a user's group membership'' in the System Administration Guide.

NOTE: You cannot use the scoadmin Account Manager to add a dial-in SLIP user to the network group. You must edit /etc/group instead. For example, the following entry from /etc/group defines the users network and nslip as members of the network group:

   network::10:network,nslip

NOTE: Because slattach is a setuid program, a user must have the execsuid kernel privilege to be able to run it. If the system is operating with a High or Improved security profile, users do not have the execsuid kernel privilege by default. root can assign this privilege using the scoadmin(ADM) Account Manager. For instructions on how to change a user's kernel privileges, see ``Changing system privileges'' in the System Administration Guide.

NOTE: Only root can kill the slattach process to remove a SLIP interface. Additionally, only root can remove the associated route from the routing table.

How SCO SLIP works

SCO SLIP provides for the use of TCP/IP network applications (for example, telnet) over serial lines by providing for the establishment, use, and removal of SLIP links. A SLIP link is a serial communication path between two computers over which data is transferred in the form of serialized IP packets and over which both machines use SLIP to govern the transmission and receipt of these packets. The serial communication path can be a physical cable connecting a serial port on one computer with a serial port on the other or can be telephone lines between a pair of modems, a modem at each computer.

SCO SLIP features

This release of SCO SLIP provides:

Support for up to 64 simultaneous SLIP links if no PPP links are active. For each active PPP link, remove one from the maximum number of SLIP links that can be supported.
Support for SLIP communications initiated from the local host and SLIP communications initiated from a remote host.
Use of serial lines dedicated to a single SLIP link, or dynamic acquisition of serial lines using UUCP on an as-needed basis.
Dynamic addition and removal of SLIP links. This makes it unnecessary to relink the kernel or to stop and restart TCP/IP when adding or removing links.
Packet header compression.
Packet filtering. SCO SLIP provides a mechanism for specifying what types of IP packet (for example, those carrying FTP data versus those carrying RIP data) will pass through a SLIP network interface. For more information, see ``SLIP packet filtering''.
SLIP gateway services. See ``SLIP gateway''.
In this release, users other than root can run slattach, see ``Running slattach for users other than root'' for details.

SLIP configuration files

SCO SLIP uses the following configuration files:

/etc/pppfilter (optional): This file is used for SLIP packet filter specifications and is shared with PPP. It is created when the SLIP STREAMS stack is installed, unless PPP has already created it. When created, it contains only comments; actual filter entries must be added by manually editing this file. This file and filter entries within it are required only if packet filtering is needed for one or more SLIP links. See ``PPP packet filtering'' and the packetfilter(SFF) manual page for more information on packet filtering and the /etc/pppfilter file.

SCO SLIP uses the following configuration files indirectly:

/etc/hosts: Any remote IP address to which a SLIP link will be established must be listed here with any host names by which a user may specify the remote host. If the Domain Name Service (DNS) is being used on the local host, then hostname-to-IP address mapping is done by that system instead of through this file.
/usr/lib/uucp/Systems, /usr/lib/uucp/Devices: These two files are UUCP systems files. Dynamic outgoing SLIP links require entries in these files for SLIP to be able to use UUCP facilities for these links. This dependency is described in ``Configuring UUCP for dynamic outgoing links''.

SLIP packet filtering

SLIP permits the transfer of IP packets over a serial line via a SLIP network interface. Packet filtering permits control of the traffic through a SLIP network interface based on the contents of the packets passed to the interface. Unlike PPP, which allows ``keep up'' and ``bring up'' filtering as well, SLIP only provides ``pass'' filtering.

By default, any packet passed to an interface is allowed to pass through that interface. Packet filtering provides for specifying that only some types of packet may pass through a SLIP interface.

The following packet content can be used to qualify or disqualify a packet:

transport level protocol type (that is, tcp, udp, icmp)
IP destination or source fields
destination or source port field in IP/TCP or IP/UDP packets
packet length
IP packet type (that is, broadcast or multicast)

Packet filtering can be specified separately for each link. For information on configuring packet filtering for each link, see ``Configuring packet filtering''.

SLIP gateway

A SLIP gateway is a computer that has two or more link interfaces, of which at least one is a SLIP link.

For example, a SLIP gateway might have one or more SLIP links (direct or dynamic) and also be connected to an Ethernet or Token-Ring network. The gateway machine provides Ethernet/Token-Ring network access to the computers connected to the gateway's SLIP links. When the gateway machine receives a SLIP packet addressed to a machine other than itself, it encapsulates the data in an Ethernet/Token-Ring packet and places it on the Ethernet/Token-Ring network. In addition to having interfaces configured for SLIP links and Ethernet/Token-Ring access, the machine must be configured as a SLIP gateway.

Proxy ARP

SLIP and PPP each use the same proxy ARP scheme to allow packet routing between machines directly connected to an Ethernet network and those connected indirectly via SLIP or PPP. For a complete discussion of proxy ARP in PPP and SLIP, see ``Proxy ARP''.

Complex SLIP connection scenarios

Many networking scenarios use SLIP to connect two hosts over a dedicated line or via modems. While these scenarios may require some effort to achieve a successful connection, they are simple in that they involve configuration of just one connection. This section provides a description of two more complex scenarios that require multiple connections, including one or more SLIP links, to resolve the scenario's networking needs.

SLIP gateway scenario

The first complex connection scenario is illustrated by Figure 12-2, ``SLIP gateway scenario''.

Figure 12-2 SLIP gateway scenario

This scenario assumes that host A needs to access the Internet but has no direct access. To gain Internet access, host A must use SLIP to connect to host B, which does have Internet access. Host B must act as a gateway to the Internet for host A. The special configuration issues for this scenario are:

A successful SLIP link must be established between host A and host B.
Host B must be configured as a SLIP gateway.
Host B may need to have proxy ARP entries for other hosts to be able to send packets to host A. See ``Proxy ARP''.
Host A must have a default route in its IP routing table that points to the SLIP interface associated with the SLIP link to host B. To add such a default route, execute:

route add default remote_IP_address 1
remote_IP_address must be the same address used to identify the remote host when configuring the SLIP link. To find this address (if necessary) for an existing SLIP link, execute:

ps -ef
and find the entry for the slattach process for the link.

SLIP server scenario

The second complex connection scenario is illustrated by Figure 12-3, ``SLIP server scenario''.

Figure 12-3 SLIP server scenario

The requirements of this scenario are for a single host to act as a SLIP server. Multiple hosts connect to this server using SLIP, and the server provides services (such as Internet access) to these hosts over the SLIP connections. Following are the special configuration issues for the SLIP server:

The SLIP server must be configured as a SLIP gateway.
The server must have configured a unique login account for each client that wishes to log in. Each login account must call a unique SLIP shell script. The shell script must execute slattach with a unique pair of IP addresses.

Advanced SLIP configuration

Advanced configuration consists of using the Advanced Options or directly editing the slattach entry in /etc/tcp to configure:

Configuring proxy address resolution

To configure proxy address resolution, perform one of these actions:

In the Network Configuration Manager's Advanced Options window for SLIP link configuration, set Proxy ARP to On or Off.
Edit the slattach line in /etc/tcp or the incoming login script to contain the +a flag.

Configuring TCP/IP header compression

To configure TCP/IP header compression, perform one of these actions:

In the Network Configuration Manager's Advanced Options window for SLIP link configuration, set TCP/IP header compression and Auto detect header compression to On or Off.
Edit the slattach line in /etc/tcp or the incoming login script to contain the c or e flags.
With the flag +c set, the compression mode is turned on. The interface sends compressed packets.
With the flag +e set, the interface attempts to detect compressed packets and, if it detects such packets, to respond with compressed packets.
The +c flag overrides any setting of the e flag.

Configuring flow control

To configure hardware flow control, perform one of these actions:

In the Network Configuration Manager's Advanced Options window for SLIP link configuration, set Hardware flow control On or Off.
Edit the slattach line in /etc/tcp or the incoming login script to contain the f flag.
Setting the flag to +f enables hardware flow control. -f disables flow control.

Configuring ICMP packet suppression

To configure ICMP packet suppression, perform one of these actions:

In the Network Configuration Manager's Advanced Options window for SLIP link configuration, set ICMP packet suppression On or Off.
Edit the slattach line in /etc/tcp or the incoming login script to contain the i flag.
Setting the flag to +i suppresses ICMP packets. -i allows them through the interface.

Configuring the maximum transmission unit

To configure the maximum transmission unit (MTU) for each link, perform one of these actions:

In the Network Configuration Manager's Advanced Options window for SLIP link configuration, set the Maximum transmission unit (MTU) to the desired value in bytes.
Edit the slattach line in /etc/tcp or the incoming login script to contain the -m flag. This flag is in the form -m value, where value is a number 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 transmittal. You can experiment with this value to obtain the most desirable results.

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.

Configuring SLIP debugging messages

To configure debugging mode, perform one of these actions:

In the Network Configuration Manager's Advanced Options window for SLIP link configuration, set Debugging to On.
Edit the slattach line in /etc/tcp or the incoming login script to contain the v flag.
Setting the flag to +v enables verbose mode. Each time a SLIP link is initiated, various messages about the status of the link appear on the console. Setting the flag to -v removes this behavior.

Configuring packet filtering

To specify a packet filter for a SLIP link:

Create a filter entry in the file /etc/pppfilter. See the packetfilter(SFF) manual page for a description of the format for the filter file.
Perform one of the following actions:
- In the Network Configuration Manager's Advanced Options window for SLIP link configuration, set the Filter file tag name to match that found in /etc/pppfilter.
- Edit the slattach line in /etc/tcp or the incoming login script to contain the -p filter_tag entry, where filter_tag matches the name of the entry you created in pppfilter.

NOTE: Two or more SLIP link configurations may specify the same filter file entry. Also, a SLIP link may share a filter file entry with a PPP link. If the entry contains the bringup or keepup keywords for use in filtering PPP packets, SLIP ignores these parts of the entry.

If /etc/pppfilter does not exist or no filter file entry is specified for a link, then all packets are passed.

Following is an example filter entry:

   # tag      keyword     filter
   sample     pass        !port ntp and !port who and !port timed

The above specification does not allow ntp, who, and timed packets to pass through the network interface. For this packet filter to apply to a SLIP link, the tag ``sample'' must be specified on the slattach command line with the -p option.

Troubleshooting SLIP

This section contains the following procedures:

Troubleshooting dedicated manual SLIP links

To troubleshoot a dedicated manual SLIP link:

Verify the soundness of the serial line connections. For SLIP to run over a dedicated serial line, the line itself and its connections at each end must be sound.
Verify that the local and remote systems on the connection are set to use the same data transfer speed. For the local SCO system, check the value of the baud_rate parameter for this SLIP link. For the remote system, check the data transfer speed setting in its configuration for this link.
Test the serial line as follows if the line connections appear sound but the problem still appears to be in the line.
NOTE: For this procedure, you must have access to the remote system, and the remote system must be running the UNIX operating system.
a.
Log into both machines as root.
b.
Disable SLIP for this serial line at both machines by killing the associated slattach process.
c.
Enable getty at the remote machine by executing the following command there:

enable tty_number

d.
At the local machine, edit the file /usr/lib/uucp/Devices to be sure that the tty port for the serial line being used is configured for direct access and both systems specify the same speed. For example:
```
   Direct tty1a - 9600 direct
```
e.
Enter the following command on the source machine: cu -l tty dir, where tty is the name of the tty line you use.
Your serial line is sound if the screen displays connected and the login prompt appears. Type ~. at the beginning of a line to exit the cu program.
If there is no response, the problem probably lies with the cable. Check the cable's connections or try another cable. If an error message appears, see Chapter 7, ``Connecting to other computers with UUCP'' in the System Administration Guide.
Retry the slattach command with the +v flag in addition to your other parameters. This turns on verbose mode, allowing you to see the ongoing progress of slattach at the command line and identify potential problems.
If you have still not located the source of the problem, see ``Using ping to troubleshoot SLIP link problems''.

Troubleshooting dynamic outgoing SLIP links

To troubleshoot a dynamic outgoing SLIP link:

Verify that the local host can establish a UUCP connection to the remote host without SLIP. Dynamic outgoing SLIP links use UUCP to establish a physical connection with the remote host. (For more information, see ``Configuring UUCP for dynamic outgoing links''.)

a.
Enable getty at the remote host by executing the following command there:

enable tty_number

b.
Use the cu(C) command to try to connect to the remote system. For example, for a SLIP connection on tty1A running at 1200 baud and calling a modem at extension 5555, enter the following command on the local machine:

cu -ltty1A -s1200 5555
The modem should dial, connect, and present a login prompt. If you do not see a login prompt after many seconds, type ~%B to make sure that the remote modem did not accidentally cycle past the correct baud rate. Try ~%B three times, waiting several seconds in between each try.
c.
If you still do not get a login prompt, make sure that the entries in your Devices file are correct, and that the remote tty setup is correct.
d.
Enter the following command on the local machine:

cu remote_hostname
You should get the same results as when you specify the extension number. If not, make sure that your /usr/lib/uucp/Systems file includes an entry for the remote_hostname.
e.
If you are still unable to establish a UUCP link, see Chapter 7, ``Connecting to other computers with UUCP'' in the System Administration Guide.
Retry the slattach command with the +v flag in addition to your other parameters. This turns on verbose mode, which causes slattach to display at the console the UUCP chat script execution. Using this, you will be able to detect if problems are occurring with the UUCP connection.
If you have still not located the source of the problem, see ``Using ping to troubleshoot SLIP link problems''.

Troubleshooting dynamic incoming SLIP links

To troubleshoot dynamic incoming SLIP links:

Run the w command to check for a successful login to the local host from the remote host.
Run the ps -ef command and look for slattach to verify the startup of a SLIP shell on the local host.
If you have still not located the source of the problem, see ``Using ping to troubleshoot SLIP link problems''.

Using ping to troubleshoot SLIP link problems

Many common problems with all types of SLIP links can be diagnosed with the ping command. ping the remote host:

ping remote_host_IP_address

If ping fails, check the systems at both ends of the SLIP connection to make sure the following are true:

slattach is running at both ends of the connection.
The source address of the first machine matches the destination address of the second machine, and the destination address of the first machine matches the source address of the second machine.
Both ends have the same header compression setting (that is, either both ends expect to send and receive packets with compressed headers, or neither end does).
Both ends are using the same baud rate.
The modems on both ends are using the same protocols (does not apply to dedicated links).

If ping is successful when using the IP address of the remote host, you may wish to ping the remote host using the host names by which it should be reachable. Execute the following command:

ping remote_hostname

If ping fails, it may fail with the following message:

   ping: unknown host remote_hostname

This error message indicates that the remote_hostname used on the ping command line does not have an entry in /etc/hosts or is not resolvable with DNS. This can be resolved by:

editing the /etc/hosts file on the local host to include an entry for the remote host.
ensuring that DNS can resolve the remote host name (if DNS is being used).
ensuring that users use the correct name for the remote host.

If ping is successful, but you are still unable to rlogin or telnet, continue to the next section.

Troubleshooting rlogin and telnet problems

If ping works correctly but rlogin(TC) or telnet(TC) fails, consult the appropriate manual page to make sure that your command syntax is correct. If the syntax is correct, verify that your /etc/hosts file has an entry for the target host.

The following telnet or rlogin error message indicates either that there are no more pseudo-ttys available to make additional connections or that the terminal database does not contain entries for the /dev/ttyp## that the system is attempting to use:

   Cannot obtain database information on this terminal.
   Connection closed.

For information on gaining access to additional pseudo-ttys, see ``Adding or removing pseudo-ttys''.

For more about SLIP

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

 ------------------------------------------------------------------------
 Manual page         Information provided
 ------------------------------------------------------------------------
 ipf(ADMP)           IP packet filter module
 mbcl(ADMP)          message to clist conversion driver
 packetfilter(SFF)   SLIP and PPP packet filter specification file format
 slattach(ADMN)      the command for assigning a tty line to a network
                     interface
 slip(ADMP)          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