<!-- 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>Contribudted 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 FreeBSD 2.x, which appear to be the only major changes between FreeBSD versions. If you do run encounter mistakes in this document, please email the author with enough information to help correct the problem. For FreeBSD 1.x users, all of the files referenced in the directory <tt>/etc/sliphome</tt> are actually in the <tt>/etc</tt> directory. <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 & Associates, Inc. (ISBN Number 0-937175-82-X), or Douglas Comer's books on the TCP/IP protocol. It's further assumed that you have already setup your modem(s) and configured the appropriate system files to allow logins through your modems. If you haven't 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/How</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)/, & <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 [such as <tt/clocal/ for directly-connected serial interfaces]. <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's 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 [Requirements for Internet Gateways], 1122 [Requirements for Internet Hosts -- Communication Layers], and perhaps 1127 [A Perspective on the Host Requirements RFCs]), so if you want your FreeBSD SLIP Server to act as a router, you'll have to add the line <tt>options GATEWAY </tt> to your machine's kernel configuration file and re-compile the kernel anyway. (Trivia: ``Gateways'' are the Internet's old name for what are now usually called ``routers''.) Please see the BSD System Manager's Manual chapter on ``Building Berkeley Kernels with Config'' [the source for which is in <tt>/usr/src/share/doc/smm</tt>] and ``FreeBSD Configuration Options'' [in <tt>/sys/doc/options.doc</tt>] for more information on configuring and building kernels. You may have to unpack the kernel source distribution if haven't installed the system sources already (<tt>srcdist/srcsys.??</tt> in FreeBSD 1.1, <tt>srcdist/sys.??</tt> in FreeBSD 1.1.5.1, or the entire source distribution in FreeBSD 2.0) to be able to configure and build kernels. You'll notice that near the end of the default kernel configuration file (<tt>/sys/i386/conf/GENERICAH</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. See the document ``Building Berkeley Kernels with Config'' and the manual page for <tt>config(8)</tt> to see how to configure and build kernels. <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 & 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> It appears that <tt/sliplogin/ under FreeBSD 2.x ignores the options that FreeBSD 1.x recognized, so the options <tt/normal/, <tt/compress/, <tt/autocomp/, and <tt/noicmp/ will have no effect under FreeBSD 2.x unless your <tt/slip.login/ script includes 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's not ``true'' proxy ARP, but that is the terminology used in this document to describe it). If you're 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'll 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 $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> isn't 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'll 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 $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 don't 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's 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'll 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'll 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'll 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>Acknowledgements</heading> <p> Thanks to these people for comments and advice regarding this tutorial: <descrip> <tag/Wilko Bulte/ <wilko@yedi.iaf.nl> <tag/Piero Serini/ <Piero@Strider.Inet.IT> </descrip> <!-- </article> -->