path: root/handbook/slips.sgml

<!-- This is an SGML version in the linuxdoc DTD of the SLIP Server
     FAQ by Guy Helmer.

     This guide provides instruction in configuring and preparing
     a FreeBSD system to be a dialup SLIP server.

<title>
Setting up FreeBSD as a SLIP Server
<author>Guy Helmer, <tt/ghelmer@alpha.dsu.edu/
<date>v1.0, 15 May 1995

-->

<sect><heading>Setting up a SLIP server<label id="slips"></heading>
<p><em>Contributed by &a.ghelmer;.<newline>
v1.0, 15 May 1995.</em>

This document provides suggestions for setting up SLIP Server services
on a FreeBSD system, which typically means configuring your system to
automatically startup connections upon login for remote SLIP clients.
The author has written this document based on his experience;
however, as your system and needs may be different, this document may
not answer all of your questions, and the author cannot be responsible
if you damage your system or lose data due to attempting to follow the
suggestions here.

This guide was originally written for SLIP Server services on a
FreeBSD 1.x system.  It has been modified to reflect changes in the
pathnames and the removal of the SLIP interface compression flags in
early versions of FreeBSD 2.X, which appear to be the only major
changes between FreeBSD versions.  If you do encounter mistakes in
this document, please email the author with enough information to
help correct the problem.

<sect1><heading>Prerequisites<label id="slips:prereqs"></>

<p>
This document is very technical in nature, so background knowledge is
required.  It is assumed that you are familiar with the TCP/IP network
protocol, and in particular, network and node addressing, network
address masks, subnetting, routing, and routing protocols, such as
RIP.  Configuring SLIP services on a dial-up server requires a
knowledge of these concepts, and if you are not familiar with them,
please read a copy of either Craig Hunt's <em>TCP/IP Network
Administration</em> published by O'Reilly &amp; Associates, Inc. (ISBN
Number 0-937175-82-X), or Douglas Comer's books on the TCP/IP
protocol.

It is further assumed that you have already setup your modem(s) and
configured the appropriate system files to allow logins through your
modems.  If you have not prepared your system for this yet, please see
the tutorial for configuring dialup services; if you have a World-Wide
Web browser available, browse the list of tutorials at
<tt>http://www.freebsd.org/</tt>; otherwise, check the place
where you found this document for a document named <tt/dialup.txt/ or
something similar.  You may also want to check the manual pages for
<tt/sio(4)/ for information on the serial port device driver and
<tt/ttys(5)/, <tt/gettytab(5)/, <tt/getty(8)/, &amp; <tt/init(8)/ for
information relevant to configuring the system to accept logins on
modems, and perhaps <tt/stty(1)/ for information on setting serial
port parameters &lsqb;such as <tt/clocal/ for directly-connected
serial interfaces&rsqb;.

<sect1><heading>Quick Overview</heading>
<p>

In its typical configuration, using FreeBSD as a SLIP server works as
follows: a SLIP user dials up your FreeBSD SLIP Server system and logs
in with a special SLIP login ID that uses <tt>/usr/sbin/sliplogin</tt>
as the special user's shell.  The <tt/sliplogin/ program browses the
file <tt>/etc/sliphome/slip.hosts</tt> to find a matching line for
the special user, and if it finds a match, connects the serial line to
an available SLIP interface and then runs the shell script
<tt>/etc/sliphome/slip.login</tt> to configure the SLIP interface.

<sect2><heading>An Example of a SLIP Server Login</heading>
<p>

For example, if a SLIP user ID were <tt>Shelmerg</tt>, <tt/Shelmerg/'s
entry in <tt>/etc/master.passwd</tt> would look something like this
(except it would be all on one line):

<tscreen><verb>
Shelmerg:password:1964:89::0:0:Guy Helmer - SLIP:
        /usr/users/Shelmerg:/usr/sbin/sliplogin
</verb></tscreen>

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

<tscreen><verb>
Shelmerg        dc-slip sl-helmer       0xfffffc00      autocomp
</verb></tscreen>

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

<tscreen><verb>
/etc/sliphome/slip.login 0 19200 Shelmerg dc-slip sl-helmer 0xfffffc00 autocomp
</verb></tscreen>

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

OK, enough of the examples -- let us dive into setting up the system.

<sect1><heading>Kernel Configuration</heading>
<p>
FreeBSD's default kernels usually come with two SLIP interfaces
defined (<tt>sl0</tt> and <tt>sl1</tt>); you can use <tt>netstat
-i</tt> to see whether these interfaces are defined in your kernel.

Sample output from <tt>netstat -i</tt>:

<tscreen><verb>
Name  Mtu   Network     Address            Ipkts Ierrs    Opkts Oerrs  Coll
ed0   1500  <Link>0.0.c0.2c.5f.4a         291311     0   174209     0   133
ed0   1500  138.247.224 ivory             291311     0   174209     0   133
lo0   65535 <Link>                            79     0       79     0     0
lo0   65535 loop        localhost             79     0       79     0     0
sl0*  296   <Link>                             0     0        0     0     0
sl1*  296   <Link>                             0     0        0     0     0
</verb></tscreen>

The <tt>sl0</tt> and <tt>sl1</tt> interfaces shown in <tt>netstat
-i</tt>'s output indicate that there are two SLIP interfaces built
into the kernel.  (The asterisks after the <tt>sl0</tt> and
<tt>sl1</tt> indicate that the interfaces are ``down''.)

However, FreeBSD's default kernels do not come configured to forward
packets (ie, your FreeBSD machine will not act as a router) due to
Internet RFC requirements for Internet hosts (see RFC's 1009
&lsqb;Requirements for Internet Gateways&rsqb;, 1122
&lsqb;Requirements for Internet Hosts -- Communication Layers&rsqb;,
and perhaps 1127 &lsqb;A Perspective on the Host Requirements
RFCs&rsqb;), so if you want your FreeBSD SLIP Server to act as a
router, you will have to edit the <tt>/etc/sysconfig</tt> file and change
the setting of the <bf>gateway</bf> variable to <tt>YES</tt>.  If you
have an older system which does not have the <tt>/etc/sysconfig</tt>
file, then add the following command:
<verb>
sysctl -w net.inet.ip.forwarding = 1
</verb>
to your <tt>/etc/rc.local</tt> file.

<p>You will then need to reboot for the new settings to take effect.

<p>You will notice that near the end of the default kernel configuration
file (<tt>/sys/i386/conf/GENERIC</tt>) is a line that reads:

<tscreen><verb>
pseudo-device sl 2
</verb></tscreen>

which is the line that defines the number of SLIP devices available in
the kernel; the number at the end of the line is the maximum number of
SLIP connections that may be operating simultaneously.

Please refer to <ref id="kernelconfig" name="Configuring the FreeBSD Kernel">
for help in reconfiguring your kernel.

<sect1><heading>Sliplogin Configuration</heading>

<p>

As mentioned earlier, there are three files in the
<tt>/etc/sliphome</tt> directory that are part of the configuration
for <tt>/usr/sbin/sliplogin</tt> (see <tt>sliplogin(8)</tt> for the
actual manual page for <tt>sliplogin</tt>): <tt>slip.hosts</tt>, which
defines the SLIP users &amp; their associated IP addresses;
<tt>slip.login</tt>, which usually just configures the SLIP interface;
and (optionally) <tt>slip.logout</tt>, which undoes
<tt>slip.login</tt>'s effects when the serial connection is
terminated.

<sect2><heading>slip.hosts Configuration</heading>

<p>

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

<itemize>
<item> SLIP user's login ID
<item> Local address (local to the SLIP server) of the SLIP link
<item> Remote address of the SLIP link
<item> Network mask
</itemize>

The local and remote addresses may be host names (resolved to IP
addresses by <tt>/etc/hosts</tt> or by the domain name service,
depending on your specifications in <tt>/etc/host.conf</tt>), and I
believe the network mask may be a name that can be resolved by a
lookup into <tt>/etc/networks</tt>.  On a sample system,
<tt>/etc/sliphome/slip.hosts</tt> looks like this:

<tscreen><verb>
----- begin /etc/sliphome/slip.hosts -----
#
# login local-addr      remote-addr     mask            opt1    opt2 
#                                               (normal,compress,noicmp)
#
Shelmerg  dc-slip       sl-helmerg      0xfffffc00      autocomp
----- end /etc/sliphome/slip.hosts ------
</verb></tscreen>

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

<itemize>
<item> <tt>normal</tt> - no header compression
<item> <tt>compress</tt> - compress headers
<item> <tt>autocomp</tt> - compress headers if the remote end allows it
<item> <tt>noicmp</tt> - disable ICMP packets (so any ``ping'' packets will be
	dropped instead of using up your bandwidth)
</itemize>

Note that <tt/sliplogin/ under early releases of FreeBSD 2 ignored
the options that FreeBSD 1.x recognized, so the options
<tt/normal/, <tt/compress/, <tt/autocomp/, and <tt/noicmp/ had no effect
until support was added in FreeBSD 2.2 (unless your <tt/slip.login/ script
included code to make use of the flags).

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 document 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 <ref id="slips:prereqs"> section 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 either need to configure a
static route to the SLIP subnet via your SLIP server on your nearest
IP router, or install <tt>gated</tt> on your FreeBSD SLIP server and
configure it to talk the appropriate routing protocols to your other
routers to inform them about your SLIP server's route to the SLIP
subnet.

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
<tt>/etc/sliphome/slip.login</tt> and
<tt>/etc/sliphome/slip.logout</tt> scripts to use <tt>arp(8)</tt> to
manage the proxy-ARP entries in the SLIP server's ARP table.

<sect2><heading>slip.login Configuration</heading>

<p>
The typical <tt>/etc/sliphome/slip.login</tt> file looks like this:

<tscreen><verb>
----- begin /etc/sliphome/slip.login -----
#!/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
----- end /etc/sliphome/slip.login -----
</verb></tscreen>

This <tt>slip.login</tt> file merely ifconfig's 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
<tt>/etc/sliphome/slip.login</tt> file will need to look something
like this:

<tscreen><verb>
----- begin /etc/sliphome/slip.login for "proxy ARP" -----
#!/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
----- end /etc/sliphome/slip.login for "proxy ARP" -----
</verb></tscreen>

The additional line in this <tt>slip.login</tt>, <tt>arp -s &dollar;5
00:11:22:33:44:55 pub</tt>, 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 a 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 (<tt>00:11:22:33:44:55</tt>) 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 <tt>netstat -i</tt>; the second line of the output
should look something like:

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

which indicates that this particular system's Ethernet MAC address is
<tt>00:02:c1:28:5f:4a</tt> -- the periods in the Ethernet MAC address
given by <tt>netstat -i</tt> 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 <tt>arp(8)</tt> desires; see the manual page on
<tt>arp(8)</tt> for complete information on usage.

Note that when you create <tt>/etc/sliphome/slip.login</tt> and
<tt>/etc/sliphome/slip.logout</tt>, the ``execute'' bit (ie,
<tt>chmod 755 /etc/sliphome/slip.login
/etc/sliphome/slip.logout</tt>) must be set, or <tt>sliplogin</tt>
will be unable to execute it.

<sect2><heading>slip.logout Configuration</heading>

<p>

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

<tscreen><verb>
----- begin /etc/sliphome/slip.logout -----
#!/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
----- end /etc/sliphome/slip.logout -----
</verb></tscreen>

If you are using ``proxy ARP'', you will want to have
<tt>/etc/sliphome/slip.logout</tt> remove the ARP entry for the SLIP
client:

<tscreen><verb>
----- begin /etc/sliphome/slip.logout for "proxy ARP" -----
#!/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
----- end /etc/sliphome/slip.logout for "proxy ARP" -----
</verb></tscreen>

The <tt>arp -d &dollar;5</tt> removes the ARP entry that the ``proxy ARP''
<tt>slip.login</tt> added when the SLIP client logged in.

It bears repeating: make sure <tt>/etc/sliphome/slip.logout</tt> has
the execute bit set for after you create it (ie, <tt>chmod 755
/etc/sliphome/slip.logout</tt>).

<sect1><heading>Routing Considerations</heading>

<p>
If you are not using the ``proxy ARP'' method for routing packets
between your SLIP clients and the rest of your network (and perhaps
the Internet), you will probably either have to add static routes to
your closest default router(s) to route your SLIP client subnet via
your SLIP server, or you will probably need to install and configure
<tt>gated</tt> on your FreeBSD SLIP server so that it will tell your
routers via appropriate routing protocols about your SLIP subnet.

<sect2><heading>Static Routes</heading>

<p>
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 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.

<sect2><heading>Running gated</heading>

<p>
An alternative to the headaches of static routes is to install
<tt>gated</tt> 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.  <tt/gated/ is available via anonymous ftp
from <tt>ftp.gated.cornell.edu</tt> in the directory
<tt>/pub/gated</tt>; I believe the current version as of this writing
is <tt>gated-R3_5Alpha_8.tar.Z</tt>, which includes support for
FreeBSD ``out-of-the-box''.  Complete information and documentation on
<tt>gated</tt> is available on the Web starting at
<tt>http://www.gated.cornell.edu/</tt>.  Compile and install it, and
then write a <tt>/etc/gated.conf</tt> file to configure your gated;
here is a sample, similar to what the author used on a FreeBSD SLIP
server:

<tscreen><verb>
----- begin sample /etc/gated.conf for gated version 3.5Alpha5 -----
#
# 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 ;
} ;

----- end sample /etc/gated.conf -----
</verb></tscreen>

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

When you get <tt>gated</tt> built and installed and create a
configuration file for it, you will need to run <tt>gated</tt> in place
of <tt>routed</tt> on your FreeBSD system; change the
<tt>routed/gated</tt> startup parameters in <tt>/etc/netstart</tt> as
appropriate for your system.  Please see the manual page for
<tt>gated</tt> for information on <tt>gated</tt>'s command-line
parameters.

<sect1><heading>Acknowledgments</heading>

<p>
Thanks to these people for comments and advice regarding this tutorial:

<descrip>
<tag/&a.wilko;/

<tag/Piero Serini/ &lt;Piero@Strider.Inet.IT&gt;
</descrip>

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