slip.html

FreeBSD操作系统的详细使用手册

you might get:</p>

<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">ifconfig sl0</kbd>
sl0: flags=10&lt;POINTOPOINT&gt;
        inet 136.152.64.181 --&gt; 136.152.64.1 netmask ffffff00
</pre>
</li>

<li>
<p>If you get ``<tt class="ERRORNAME">no route to host</tt>'' messages from ping, there
may be a problem with your routing table. You can use the <tt class="COMMAND">netstat
-r</tt> command to display the current routes :</p>

<pre class="SCREEN">
<samp class="PROMPT">#</samp> <kbd class="USERINPUT">netstat -r</kbd>
Routing tables
Destination      Gateway            Flags     Refs     Use  IfaceMTU    Rtt    Netmasks:

(root node)
(root node)

Route Tree for Protocol Family inet:
(root node) =&gt;
default          inr-3.Example.EDU  UG          8   224515  sl0 -      -
localhost.Exampl localhost.Example. UH          5    42127  lo0 -       0.438
inr-3.Example.ED water.CS.Example.E UH          1        0  sl0 -      -
water.CS.Example localhost.Example. UGH        34 47641234  lo0 -       0.438
(root node)
</pre>

<p>The preceding examples are from a relatively busy system. The numbers on your system
will vary depending on network activity.</p>
</li>
</ul>
</div>
</div>

<div class="SECT2">
<h2 class="SECT2"><a id="SLIPS" name="SLIPS">21.7.2 Setting Up a SLIP Server</a></h2>

<p>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.</p>

<div class="SECT3">
<h3 class="SECT3"><a id="SLIPS-PREREQS" name="SLIPS-PREREQS">21.7.2.1
Prerequisites</a></h3>

<p>This section 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 <span class="emphasis"><i class="EMPHASIS">TCP/IP Network
Administration</i></span> published by O'Reilly &amp; Associates, Inc. (ISBN Number
0-937175-82-X), or Douglas Comer's books on the TCP/IP protocol.</p>

<p>It is further assumed that you have already set up 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 <a
href="../../../../docs.html" target="_top">http://www.FreeBSD.org/docs.html</a>. You may
also want to check the manual pages for <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=sio&sektion=4"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">sio</span>(4)</span></a> for information
on the serial port device driver and <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=ttys&sektion=5"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">ttys</span>(5)</span></a>, <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=gettytab&sektion=5"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">gettytab</span>(5)</span></a>, <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=getty&sektion=8"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">getty</span>(8)</span></a>, &amp; <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=init&sektion=8"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">init</span>(8)</span></a> for
information relevant to configuring the system to accept logins on modems, and perhaps <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=stty&sektion=1"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">stty</span>(1)</span></a> for
information on setting serial port parameters (such as <var class="LITERAL">clocal</var>
for directly-connected serial interfaces).</p>
</div>

<div class="SECT3">
<h3 class="SECT3"><a id="AEN31476" name="AEN31476">21.7.2.2 Quick Overview</a></h3>

<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 class="FILENAME">/usr/sbin/sliplogin</tt> as the special user's shell. The
<tt class="COMMAND">sliplogin</tt> program browses the file <tt
class="FILENAME">/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 class="FILENAME">/etc/sliphome/slip.login</tt> to
configure the SLIP interface.</p>

<div class="SECT4">
<h4 class="SECT4"><a id="AEN31483" name="AEN31483">21.7.2.2.1 An Example of a SLIP Server
Login</a></h4>

<p>For example, if a SLIP user ID were <tt class="USERNAME">Shelmerg</tt>, <tt
class="USERNAME">Shelmerg</tt>'s entry in <tt class="FILENAME">/etc/master.passwd</tt>
would look something like this:</p>

<pre class="PROGRAMLISTING">
Shelmerg:password:1964:89::0:0:Guy Helmer - SLIP:/usr/users/Shelmerg:/usr/sbin/sliplogin
</pre>

<p>When <tt class="USERNAME">Shelmerg</tt> logs in, <tt class="COMMAND">sliplogin</tt>
will search <tt class="FILENAME">/etc/sliphome/slip.hosts</tt> for a line that had a
matching user ID; for example, there may be a line in <tt
class="FILENAME">/etc/sliphome/slip.hosts</tt> that reads:</p>

<pre class="PROGRAMLISTING">
Shelmerg        dc-slip sl-helmer       0xfffffc00       autocomp
</pre>

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

<pre class="PROGRAMLISTING">
/etc/sliphome/slip.login 0 19200 Shelmerg dc-slip sl-helmer 0xfffffc00 autocomp
</pre>

<p>If all goes well, <tt class="FILENAME">/etc/sliphome/slip.login</tt> will issue an <tt
class="COMMAND">ifconfig</tt> for the SLIP interface to which <tt
class="COMMAND">sliplogin</tt> attached itself (slip interface 0, in the above example,
which was the first parameter in the list given to <tt class="FILENAME">slip.login</tt>)
to set the local IP address (<tt class="HOSTID">dc-slip</tt>), remote IP address (<tt
class="HOSTID">sl-helmer</tt>), network mask for the SLIP interface (<tt
class="HOSTID">0xfffffc00</tt>), and any additional flags (<var
class="LITERAL">autocomp</var>). If something goes wrong, <tt
class="COMMAND">sliplogin</tt> usually logs good informational messages via the <var
class="LITERAL">daemon</var> syslog facility, which usually logs to <tt
class="FILENAME">/var/log/messages</tt> (see the manual pages for <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=syslogd&sektion=8"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">syslogd</span>(8)</span></a> and <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=syslog.conf&sektion=5"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">syslog.conf</span>(5)</span></a> and
perhaps check <tt class="FILENAME">/etc/syslog.conf</tt> to see to what <tt
class="COMMAND">syslogd</tt> is logging and where it is logging to).</p>

<p>OK, enough of the examples -- let us dive into setting up the system.</p>
</div>
</div>

<div class="SECT3">
<h3 class="SECT3"><a id="AEN31521" name="AEN31521">21.7.2.3 Kernel Configuration</a></h3>

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

<p>Sample output from <tt class="COMMAND">netstat -i</tt>:</p>

<pre class="SCREEN">
Name  Mtu   Network     Address            Ipkts Ierrs    Opkts Oerrs  Coll
ed0   1500  &lt;Link&gt;0.0.c0.2c.5f.4a         291311     0   174209     0   133
ed0   1500  138.247.224 ivory             291311     0   174209     0   133
lo0   65535 &lt;Link&gt;                            79     0       79     0     0
lo0   65535 loop        localhost             79     0       79     0     0
sl0*  296   &lt;Link&gt;                             0     0        0     0     0
sl1*  296   &lt;Link&gt;                             0     0        0     0     0
</pre>

<p>The <tt class="DEVICENAME">sl0</tt> and <tt class="DEVICENAME">sl1</tt> interfaces
shown from <tt class="COMMAND">netstat -i</tt> indicate that there are two SLIP
interfaces built into the kernel. (The asterisks after the <var class="LITERAL">sl0</var>
and <var class="LITERAL">sl1</var> indicate that the interfaces are ``down''.)</p>

<p>However, FreeBSD's default kernel does not come configured to forward packets (by
default, your FreeBSD machine will not act as a router) due to Internet RFC requirements
for Internet hosts (see RFCs 1009 [Requirements for Internet Gateways], 1122
[Requirements for Internet Hosts -- Communication Layers], and perhaps 1127 [A
Perspective on the Host Requirements RFCs]). If you want your FreeBSD SLIP Server to act
as a router, you will have to edit the <tt class="FILENAME">/etc/rc.conf</tt> file and
change the setting of the <var class="LITERAL">gateway_enable</var> variable to <var
class="OPTION">YES</var>.</p>

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

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

<pre class="PROGRAMLISTING">
pseudo-device sl 2
</pre>

<p>This 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.</p>

<p>Please refer to <a href="kernelconfig.html">Chapter 8</a> on Configuring the FreeBSD
Kernel for help in reconfiguring your kernel.</p>
</div>

<div class="SECT3">
<h3 class="SECT3"><a id="AEN31553" name="AEN31553">21.7.2.4 Sliplogin
Configuration</a></h3>

<p>As mentioned earlier, there are three files in the <tt
class="FILENAME">/etc/sliphome</tt> directory that are part of the configuration for <tt
class="FILENAME">/usr/sbin/sliplogin</tt> (see <a
href="http://www.FreeBSD.org/cgi/man.cgi?query=sliplogin&sektion=8"><span
class="CITEREFENTRY"><span class="REFENTRYTITLE">sliplogin</span>(8)</span></a> for the
actual manual page for <tt class="COMMAND">sliplogin</tt>): <tt
class="FILENAME">slip.hosts</tt>, which defines the SLIP users and their associated IP
addresses; <tt class="FILENAME">slip.login</tt>, which usually just configures the SLIP
interface; and (optionally) <tt class="FILENAME">slip.logout</tt>, which undoes <tt
class="FILENAME">slip.login</tt>'s effects when the serial connection is terminated.</p>

<div class="SECT4">
<h4 class="SECT4"><a id="AEN31566" name="AEN31566">21.7.2.4.1 <tt
class="FILENAME">slip.hosts</tt> Configuration</a></h4>

<p><tt class="FILENAME">/etc/sliphome/slip.hosts</tt> contains lines which have at least
four items separated by whitespace:</p>

<ul>
<li>
<p>SLIP user's login ID</p>
</li>

<li>
<p>Local address (local to the SLIP server) of the SLIP link</p>
</li>

<li>
<p>Remote address of the SLIP link</p>
</li>

<li>
<p>Network mask</p>
</li>
</ul>

<p>The local and remote addresses may be host names (resolved to IP addresses by <tt
class="FILENAME">/etc/hosts</tt> or by the domain name service, depending on your
specifications in the file <tt class="FILENAME">/etc/nsswitch.conf</tt> on
FreeBSD&nbsp;5.X, in <tt class="FILENAME">/etc/host.conf</tt> if you use
FreeBSD&nbsp;4.X), and the network mask may be a name that can be resolved by a lookup
into <tt class="FILENAME">/etc/networks</tt>. On a sample system, <tt
class="FILENAME">/etc/sliphome/slip.hosts</tt> looks like this:</p>

<pre class="PROGRAMLISTING">
#
# login local-addr      remote-addr     mask            opt1    opt2 
#                                               (normal,compress,noicmp)
#
Shelmerg  dc-slip       sl-helmerg      0xfffffc00      autocomp
</pre>

<p>At the end of the line is one or more of the options.</p>

<ul>
<li>
<p><var class="OPTION">normal</var> -- no header compression</p>
</li>

<li>
<p><var class="OPTION">compress</var> -- compress headers</p>
</li>

<li>
<p><var class="OPTION">autocomp</var> -- compress headers if the remote end allows it</p>
</li>

<li>
<p><var class="OPTION">noicmp</var> -- disable ICMP packets (so any ``ping'' packets will
be dropped instead of using up your bandwidth)</p>
</li>
</ul>

<p>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 (<a
href="slip.html#SLIPS-PREREQS">Section 21.7.2.1</a>) and/or consult your IP network
manager.</p>

<p>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.</p>