7.5. Running in Server Mode

Setting up your SLIP client was the hard part. Configuring your host to act as a SLIP server is much easier.

There are two ways of configuring a SLIP server. Both ways require that you set up one login account per SLIP client. Assume you provide SLIP service to Arthur Dent at dent.beta.com. You might create an account named dent by adding the following line to your passwd file:

dent:*:501:60:Arthur Dent's SLIP account:/tmp:/usr/sbin/diplogin

Afterwards, you would set dent's password using the passwd utility.

The dip command can be used in server mode by invoking it as diplogin. Usually diplogin is a link to dip. Its main configuration file is /etc/diphosts, which is where you specify what IP address a SLIP user will be assigned when he or she dials in. Alternatively, you can also use the sliplogin command, a BSD-derived tool featuring a more flexible configuration scheme that lets you execute shell scripts whenever a host connects and disconnects.

When our SLIP user dent logs in, dip starts up as a server. To find out if he is indeed permitted to use SLIP, it looks up the username in /etc/diphosts. This file details the access rights and connection parameter for each SLIP user. The general format for an /etc/diphosts entry looks like:

# /etc/diphosts
Each of the fields is described in Table 7-2.

Table 7-2. /etc/diphosts Field Description

Field Description

The username of the user invoking dip that this entry will apply to.


Field 2 of the /etc/diphosts file is used to add an extra layer of password-based security on the connection. You can place a password in encrypted form here (just as in /etc/passwd ) and diplogin will prompt for the user to enter the password before allowing SLIP access. Note that this password is used in addition to the normal login-based password the user will enter.


The address that will be assigned to the remote machine. This address may be specified either as a hostname that will be resolved or an IP address in dotted quad notation.


The IP address that will be used for this end of the SLIP link. This may also be specified as a resolvable hostname or in dotted quad format.


The netmask that will be used for routing purposes. Many people are confused by this entry. The netmask doesn't apply to the SLIP link itself, but is used in combination with the rem-addr field to produce a route to the remote site. The netmask should be that used by the network supported by that of the remote host.


This field is free-form text that you may use to help document the /etc/diphosts file. It serves no other purpose.


This field is where you specify what protocol or line discipline you want applied to this connection. Valid entries here are the same as those valid for the –p argument to the slattach command.


The maximum transmission unit that this link will carry. This field describes the largest datagram that will be transmitted across the link. Any datagram routed to the SLIP device that is larger than the MTU will be fragmented into datagrams no larger than this value. Usually, the MTU is configured identically at both ends of the link.

A sample entry for dent could look like this:

dent::dent.beta.com:vbrew.com: Dent:CSLIP,296

Our example gives our user dent access to SLIP with no additional password required. He will be assigned the IP address associated with dent.beta.com with a netmask of His default route should be directed to the IP address of vbrew.com, and he will use the CSLIP protocol with an MTU of 296 bytes.

When dent logs in, diplogin extracts the information on him from the diphosts file. If the second field contains a value, diplogin will prompt for an “external security password.” The string entered by the user is encrypted and compared to the password from diphosts. If they do not match, the login attempt is rejected. If the password field contains the string s/key, and dip was compiled with S/Key support, S/Key authentication will take place. S/Key authentication is described in the documentation that comes in the dip source package.

After a successful login, diplogin proceeds by flipping the serial line to CSLIP or SLIP mode, and sets up the interface and route. This connection remains established until the user disconnects and the modem drops the line. diplogin then returns the line to normal line discipline and exits.

diplogin requires superuser privilege. If you don't have dip running setuid root, you should make diplogin a separate copy of dip instead of a simple link. diplogin can then safely be made setuid without affecting the status of dip itself.