25.7. Using SLIP

25.7.2.2.1. An Example of a SLIP Server Login

For example, if a SLIP user ID were Shelmerg, Shelmerg's entry in /etc/master.passwd would look something like this:

Shelmerg:password:1964:89::0:0:Guy Helmer - SLIP:/usr/users/Shelmerg:/usr/sbin/sliplogin

When Shelmerg logs in, sliplogin will search /etc/sliphome/slip.hosts for a line that had a matching user ID; for example, there may be a line in /etc/sliphome/slip.hosts that reads:

Shelmerg        dc-slip sl-helmer       0xfffffc00		  autocomp

sliplogin will find that matching line, hook the serial line into the next available SLIP interface, and then execute /etc/sliphome/slip.login like this:

/etc/sliphome/slip.login 0 19200 Shelmerg dc-slip sl-helmer 0xfffffc00 autocomp

If all goes well, /etc/sliphome/slip.login will issue an ifconfig for the SLIP interface to which sliplogin attached itself (SLIP interface 0, in the above example, which was the first parameter in the list given to slip.login) to set the local IP address (dc-slip), remote IP address (sl-helmer), network mask for the SLIP interface (0xfffffc00), and any additional flags (autocomp). If something goes wrong, sliplogin usually logs good informational messages via the syslogd daemon facility, which usually logs to /var/log/messages (see the manual pages for syslogd(8) and syslog.conf(5) and perhaps check /etc/syslog.conf to see to what syslogd is logging and where it is logging to).

25.7.2.4.1. slip.hosts Configuration

/etc/sliphome/slip.hosts contains lines which have at least four items separated by whitespace:

• SLIP user's login ID

• Local address (local to the SLIP server) of the SLIP link

• Remote address of the SLIP link

• Network mask

The local and remote addresses may be host names (resolved to IP addresses by /etc/hosts or by the domain name service, depending on your specifications in the file /etc/nsswitch.conf, or in /etc/host.conf if you use FreeBSD 4.X), and the network mask may be a name that can be resolved by a lookup into /etc/networks. On a sample system, /etc/sliphome/slip.hosts looks like this:

#
# login local-addr      remote-addr     mask            opt1    opt2
#                                               (normal,compress,noicmp)
#
Shelmerg  dc-slip       sl-helmerg      0xfffffc00      autocomp

At the end of the line is one or more of the options:

• normal —— no header compression

• compress —— compress headers

• autocomp —— compress headers if the remote end allows it

• noicmp —— disable ICMP packets (so any “ping” packets will be dropped instead of using up your bandwidth)

Your choice of local and remote addresses for your SLIP links depends on whether you are going to dedicate a TCP/IP subnet or if you are going to use “proxy ARP” on your SLIP server (it is not “true” proxy ARP, but that is the terminology used in this section to describe it). If you are not sure which method to select or how to assign IP addresses, please refer to the TCP/IP books referenced in the SLIP Prerequisites (µÚ 25.7.2.1 節) and/or consult your IP network manager.

If you are going to use a separate subnet for your SLIP clients, you will need to allocate the subnet number out of your assigned IP network number and assign each of your SLIP client's IP numbers out of that subnet. Then, you will probably need to configure a static route to the SLIP subnet via your SLIP server on your nearest IP router.

Otherwise, if you will use the “proxy ARP” method, you will need to assign your SLIP client's IP addresses out of your SLIP server's Ethernet subnet, and you will also need to adjust your /etc/sliphome/slip.login and /etc/sliphome/slip.logout scripts to use arp(8) to manage the proxy-ARP entries in the SLIP server's ARP table.

25.7.2.4.2. slip.login Configuration

The typical /etc/sliphome/slip.login file looks like this:

#!/bin/sh -
#
#       @(#)slip.login  5.1 (Berkeley) 7/1/90

#
# generic login file for a slip line.  sliplogin invokes this with
# the parameters:
#      1        2         3        4          5         6     7-n
#   slipunit ttyspeed loginname local-addr remote-addr mask opt-args
#
/sbin/ifconfig sl$1 inet $4 $5 netmask $6

This slip.login file merely runs ifconfig for the appropriate SLIP interface with the local and remote addresses and network mask of the SLIP interface.

If you have decided to use the “proxy ARP” method (instead of using a separate subnet for your SLIP clients), your /etc/sliphome/slip.login file will need to look something like this:

#!/bin/sh -
#
#       @(#)slip.login  5.1 (Berkeley) 7/1/90

#
# generic login file for a slip line.  sliplogin invokes this with
# the parameters:
#      1        2         3        4          5         6     7-n
#   slipunit ttyspeed loginname local-addr remote-addr mask opt-args
#
/sbin/ifconfig sl$1 inet $4 $5 netmask $6
# Answer ARP requests for the SLIP client with our Ethernet addr
/usr/sbin/arp -s $5 00:11:22:33:44:55 pub

The additional line in this slip.login, arp -s $5 00:11:22:33:44:55 pub, creates an ARP entry in the SLIP server's ARP table. This ARP entry causes the SLIP server to respond with the SLIP server's Ethernet MAC address whenever another IP node on the Ethernet asks to speak to the SLIP client's IP address.

When using the example above, be sure to replace the Ethernet MAC address (00:11:22:33:44:55) with the MAC address of your system's Ethernet card, or your “proxy ARP” will definitely not work! You can discover your SLIP server's Ethernet MAC address by looking at the results of running netstat -i; the second line of the output should look something like:

ed0   1500  <Link>0.2.c1.28.5f.4a         191923	0   129457     0   116

This indicates that this particular system's Ethernet MAC address is 00:02:c1:28:5f:4a —— the periods in the Ethernet MAC address given by netstat -i must be changed to colons and leading zeros should be added to each single-digit hexadecimal number to convert the address into the form that arp(8) desires; see the manual page on arp(8) for complete information on usage.

25.7.2.4.3. slip.logout Configuration

/etc/sliphome/slip.logout is not strictly needed (unless you are implementing “proxy ARP”), but if you decide to create it, this is an example of a basic slip.logout script:

#!/bin/sh -
#
#       slip.logout

#
# logout file for a slip line.  sliplogin invokes this with
# the parameters:
#      1        2         3        4          5         6     7-n
#   slipunit ttyspeed loginname local-addr remote-addr mask opt-args
#
/sbin/ifconfig sl$1 down

If you are using “proxy ARP”, you will want to have /etc/sliphome/slip.logout remove the ARP entry for the SLIP client:

#!/bin/sh -
#
#       @(#)slip.logout

#
# logout file for a slip line.  sliplogin invokes this with
# the parameters:
#      1        2         3        4          5         6     7-n
#   slipunit ttyspeed loginname local-addr remote-addr mask opt-args
#
/sbin/ifconfig sl$1 down
# Quit answering ARP requests for the SLIP client
/usr/sbin/arp -d $5

The arp -d $5 removes the ARP entry that the “proxy ARP” slip.login added when the SLIP client logged in.

It bears repeating: make sure /etc/sliphome/slip.logout has the execute bit set after you create it (i.e., chmod 755 /etc/sliphome/slip.logout).

25.7.2.5.1. Static Routes

Adding static routes to your nearest default routers can be troublesome (or impossible if you do not have authority to do so...). If you have a multiple-router network in your organization, some routers, such as those made by Cisco and Proteon, may not only need to be configured with the static route to the SLIP subnet, but also need to be told which static routes to tell other routers about, so some expertise and troubleshooting/tweaking may be necessary to get static-route-based routing to work.

25.7.2.5.2. Running GateD®

An alternative to the headaches of static routes is to install GateD on your FreeBSD SLIP server and configure it to use the appropriate routing protocols (RIP/OSPF/BGP/EGP) to tell other routers about your SLIP subnet. You will need to write a /etc/gated.conf file to configure your GateD; here is a sample, similar to what the author used on a FreeBSD SLIP server:

#
# gated configuration file for dc.dsu.edu; for gated version 3.5alpha5
# Only broadcast RIP information for xxx.xxx.yy out the ed Ethernet interface
#
#
# tracing options
#
traceoptions "/var/tmp/gated.output" replace size 100k files 2 general ;

rip yes {
  interface sl noripout noripin ;
  interface ed ripin ripout version 1 ;
  traceoptions route ;
} ;

#
# Turn on a bunch of tracing info for the interface to the kernel:
kernel {
  traceoptions remnants request routes info interface ;
} ;

#
# Propagate the route to xxx.xxx.yy out the Ethernet interface via RIP
#

export proto rip interface ed {
  proto direct {
      xxx.xxx.yy mask 255.255.252.0 metric 1; # SLIP connections
  } ;
} ;

#
# Accept routes from RIP via ed Ethernet interfaces

import proto rip interface ed {
  all ;
} ;

The above sample gated.conf file broadcasts routing information regarding the SLIP subnet xxx.xxx.yy via RIP onto the Ethernet; if you are using a different Ethernet driver than the ed driver, you will need to change the references to the ed interface appropriately. This sample file also sets up tracing to /var/tmp/gated.output for debugging GateD's activity; you can certainly turn off the tracing options if GateD works correctly for you. You will need to change the xxx.xxx.yy's into the network address of your own SLIP subnet (be sure to change the net mask in the proto direct clause as well).

Once you have installed and configured GateD on your system, you will need to tell the FreeBSD startup scripts to run GateD in place of routed. The easiest way to accomplish this is to set the router and router_flags variables in /etc/rc.conf. Please see the manual page for GateD for information on command-line parameters.