Convert to GutenPalm or to PalmDoc by Guido Socher About the author: Guido is a long time Linux fan. All his computers in the house are integrated into a network. Content: 10Base2 10baseT 100baseT and 100baseT4 RJ-45 Connector Cable types Cross connect cable Hub LAN Cards Network interface Routing PLIP IP addresses Files IP-Masquerading (NAT) IRQ Example network References Talkback form for this article

Home Networking, glossary and overview

Abstract:

Networking becomes more and more popular. Your old 486 can perfectly work as file-server or run IP-Masquerading to give the whole family Internet access from different computers at the same time. This article will provide you with the basic overview to start a home network. It will explain the vocabulary which you need to understand literature and other articles on networking. The focus of the article is a small scale home network.

10Base2

10baseT

100baseT and 100baseT4

RJ-45 Connector

• 10baseT uses pairs 2 and 3 only
• 100baseT uses pairs 2 and 3 only
• 100baseT4 uses pairs 1,2,3 and 4

Note: you need special pliers to connect the RJ-45 connector to the cable.

Cable types

• 50 Ohm coaxial cable. Used for 10base2 networks, max 10 Mbit/s
• Twisted pair:
  • UTP (Unshielded Twisted Pair) cable, the cheapest type of Twisted Pair cable, thin and flexible as there is not shielding. The chances for radio interferences are high. Radio interferences can go in both directions. You may disturb a close by short-wave radio receiver and a longer cable can also pick up strong electric fields (e.g from an electric heating going on and off). This cable is however totally sufficient for home networking where the distances are usually only a few meters.
  • STP (Shielded Twisted Pair) cable with one single shielding going around all the wires. This cable is very common. It is still a bit flexible and also not very expensive. Possibilities for radio interferences are reduced.
  • STP (Shielded Twisted Pair) cable will separate shielding around each wire pair. Expensive and not recommended for home networking.

• Cat 3 cable: max 16 MHz bandwidth, used for 10BaseT
• Cat 5 cable: max 100 MHz bandwidth, used for 10BaseT, 100BaseT and 100BaseT4

The mapping between the pins on the RJ-45 connector and the Twisted Pair wires is standardized. There are several standards and they use slightly different colors. Valid pin assignments are:

Wire pair number	Connector pins	standard 1	standard 2	standard 3	standard 4
1	4/5	blue/white	white/blue	white/blue	white/brown
2	3/6	white/green	red/orange	cyan/purple	green/yellow
3	1/2	white/orange	black/green	white/orange	grey/pink
4	7/8	white/brown	yellow/brown	cyan/purple	blue/red

Cabling is really only relevant if you want to run 20 or more meters of cable through the whole house. If you want to connect just a few computers in the same room then you can get cheap ready made cables with connectors. These are typically available with a length between 1 and 10 meters.

Cross connect cable

Hub

LAN

Cards

My personal experience is that the cheapest cards are the best. The more expensive ones do usually not give you any additional features. ISA PNP cards may cause trouble. Most cards which are sold as ISA PNP cards come with a DOS floppy disk and software where you can change the flash ROM settings of that card and configure it to not use PNP. You can set a fixed interrupt and IO address. (Note: if you have a PNP bios then you must also tell it to disable PNP for this interrupt). The standard IO address is normally io=0x300. You can see which address and interrupts are already used by typing:
cat /proc/interrupts /proc/ioports | more
(Note: These are the interrupts and addresses which are really used by a driver at the moment.)

If you bought such an ISA PNP card then you will probably find that the DOS floppy disk is not of much use for a Linux only user. You or a friend might have an old boot-able dos floppy somewhere. Copy the program that came with the network card to that floppy. Boot from it and change the setting of the network card. The settings are stored in flash Rom so, you just do this once after you installed the hardware and then never again.

The cheapest cards are usually NE2000 cards and cost today about $15 (or 15 Euro). These are well supported under Linux. NE2000 cards are available as ISA or PCI bus cards.
The ISA bus cards need the kernel modules "ne" and "8390". You can either let the driver probe for it or provide IRQ and IO at the command line. Loading of kernel module without auto-probing can be done as follows:
insmod 8390
insmod ne irq=10 io=0x300

With a PCI card you don't usually have any trouble with interrupts and IO addresses but they are a bit more expensive. The kernel modules for the NE2000 PCI cards are called ne2k-pci and 8390.

Network interface

> /sbin/ifconfig -a
lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Bcast:127.255.255.255  Mask:255.0.0.0
          UP BROADCAST LOOPBACK RUNNING  MTU:3584  Metric:1
          RX packets:1664 errors:0 dropped:0 overruns:0
          TX packets:1664 errors:0 dropped:0 overruns:0

eth0      Link encap:10Mbps Ethernet  HWaddr 00:80:AD:71:52:19
          inet addr:10.0.0.1  Bcast:10.255.255.255  Mask:255.0.0.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:5993 errors:0 dropped:0 overruns:0
          TX packets:4073 errors:0 dropped:0 overruns:0
          Interrupt:5 Base address:0x300

dummy0    Link encap:10Mbps Ethernet  HWaddr 00:00:00:00:00:00
          inet addr:10.0.0.1  Bcast:10.255.255.255  Mask:255.0.0.0
          UP BROADCAST RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0
          TX packets:0 errors:0 dropped:0 overruns:0

ppp0      Link encap:Point-Point Protocol
          inet addr:199.94.253.30  P-t-P:199.94.253.4  Mask:255.255.255.0
          UP POINTOPOINT RUNNING  MTU:552  Metric:1
          RX packets:7 errors:0 dropped:0 overruns:0
          TX packets:8 errors:0 dropped:0 overruns:0

Routing

PLIP

Parallel port IP is a possibility to use the parallel port as network interface. The kernel configuration parameter is called CONFIG_PLIP and you should preferably configure it as module. PLIP is a cheap possibility to connect your Laptap to another computer and via this computer to your home network. You need to buy a parallel port cable with 2 male connectors also known as LapLink cable. If you want to build the cable your self then the wiring is:

IP addresses

You need to decide which addresses to use for your network. Normally you will get one IP addresses assigned form your Internet service provider when you "go online". This IP addresses is more or less automatically assigned to the ppp interface. You do normally not have to worry about it. What you need to decide now is what addresses to assign to the other interfaces of your private network (e.g eth0 and plip1 interface). A number of addresses are reserved for private use. This addresses are not routed in the Internet and can not cause you any trouble when you re-use them. Their ranges are:

Netmask	Network Addresses
255.0.0.0	10.0.0.0 - 10.255.255.255
255.255.0.0	172.16.0.0 - 172.31.255.255
255.255.255.0	192.168.0.0 - 192.168.255.255

The first block is a single Class A network number, the second block is a set of 16 contiguous Class B network numbers, and the third block is a set of 255 contiguous Class C network numbers.

Which address should you pick? Well, it does not really matter but I recommend using consecutive numbers. E.g you have 2 computers connected via Ethernet and you need now 2 addresses to assign to the 2 network cards (you assign it to the software representation of the cards. This software representation is called interface). Just use 192.168.0.1 and 192.168.0.2

Files

Most distributions come these days with pretty graphical user interfaces (GUIs) to configure networking (e.g yast or netcfg). You can use them if you want. Refer to the user manual of your Linux distribution on how to use them. This article shall not focus on a specific distribution. It explains therefore the underlying configuration files that will somehow magically be modified if you use the GUIs from your distribution. This gives you also the possibility to verify what the configuration GUIs did.

/etc/hosts and /etc/host.conf

For a small home network it does not make much sense to have DNS server running. Instead you can just enter the names of the machines in a file called /etc/hosts Example:

# ipaddr. fullname alias
127.0.0.1 localhost
192.168.0.1 obelix.mynet obelix
192.168.0.2 asterix.mynet asterix
192.168.0.3 idefix.mynet idefix

# /etc/host.conf
order hosts bind
# Allow multiple addrs
multi on

/etc/resolv.conf

The file /etc/resolv.conf says where to resolve anything that can not be found in /etc/hosts. You need to ask your ISP for the ip address of his DNS server. You should enter more than one DNS server if possible. If the first DNS server is slow or down then the next will be asked. The syntax is:

#/etc/resolv.conf
#nameserver ipaddr.
nameserver 123.456.789.1
# alternative DNS server:
nameserver 123.456.111.9

setting the hostname

/etc/HOSTNAME or (/etc/sysconfig/network for redhat) is a file where many distributions configure the name of the local machine. But in reality the host name of a machine is set via the command hostname. What happens in these distributions is that during startup (e.g /etc/rc.d/rc.sysinit) the file /etc/HOSTNAME is read and then its content is feed to the command hostname. Which file is exactly used depends really on the peculiarities of your specific installation. It should however now be easy for you to investigate this since you know that you should grep for hostname.
Example how to set the hostname:

hostname obelix.mynet

tcp wrapper

/etc/hosts.allow this is a configuration file for a security system called tcp wrapper. All you need to know, is that you should add the ip-addresses of all the machines in your local network, excluding the machine where this file is located, behind the keyword ALL:.
Example: obelix has 2 network interfaces that go to other hosts in your local net. The interface with the ip address 192.168.0.2 goes to asterix and the interface with the ip address 192.168.0.3 goes to idefix. In this case you write:

# hosts.allow
# See tcpd(8) and hosts_access(5) for a description
ALL: 192.168.0.2   192.168.0.3

portmap: 192.168.0.2   192.168.0.3

/etc/hosts.deny is also for the tcp wrapper it should never be changed and should always look as follows:

portmap: ALL
ALL: ALL

/etc/hosts.equiv

/etc/hosts.equiv allows local users to log into other machines (e.g with rsh) without password. You may decide if you want this. The syntax is: On full name (as in /etc/hosts) of a trusted host per line. E.g

asterix.mynet
idefix.mynet

/etc/hosts.lpd

The /etc/hosts.lpd file is important if you would like to enable network printing services via lpd. How to configure network printing is another story but this file must be present on the machine where you run the lp-daemon and it must list the names of all the machines that are allowed to print. The syntax is the same as hosts.equiv. The network printer configuration it self is best done with the system admin tools from your distribution (e.g yast in suse and printtool in redhat). The Printing-HOWTO from the Linux Documenation project describes how to set it up if you do not want to use the system admin tools from your distribution.

sendmail

The file /etc/service.switch is used by sendmail. On the hosts inside your network (the ones that do not have a direct Internet connection) you write:

hosts   files
aliases files

Sendmail reads at startup a configuration file called /etc/sendmail.cf and this file is quite difficult to read. The sendmail sources use therefore a macro language called m4 to generate the sendmail.cf out of a macro file. To do this you need the m4 macro processor (probably already installed on your system) and a part of the sendmail sources. Redhat Linux installs also these sources to /usr/lib/sendmail-cf other distributions may not come with it but you can always unpack the sendmail sources. Documentation on how the sendmail configuration works can be obtained from www.sendmail.org.

A central mail host is called mail-hub. The idea is that all the different machines in your network forward just their mail to this hub and the hub stores the mail. You then access it via NFS or pop/imap. A m4 file to do exactly that looks as follows:

#divert(-1)
include(`../m4/cf.m4')
VERSIONID(`mc file for asterix, A HOST INSIDE THE PRIVATE NETWORK')
undefine(`UUCP_RELAY')
undefine(`BITNET_RELAY')
OSTYPE(`linux')
# put here your outside mail domain. E.g if your user id is
# joe and your mail address joe@linuxnet.somewhere.org then you write:
MASQUERADE_AS(linuxnet.somewhere.org)
# Do not show these internal hostnames to the outside world:
MASQUERADE_DOMAIN(asterix.mynet)
MASQUERADE_DOMAIN(asterix)
MASQUERADE_DOMAIN(localdomain)
MASQUERADE_DOMAIN(localhost)
FEATURE(masquerade_envelope)
#the /etc/passwd entry: mail:*:8:12:mail:/var/spool/mail:
define(`confDEF_USER_ID',``8:12'')
# the centail mail hub is obelix, Note dot at the end
define(`MAIL_HUB', `obelix.mynet.')dnl
define(`confHOSTS_FILE',/etc/hosts)dnl
MAILER(smtp)dnl
MAILER(local)dnl

Please note: to just get networking running you will not need sendmail at all and you can send and receive mail with netscape without bothering about sendmail. Sendmail is probably something for the more advances Linux users.

A bit more difficult is the configuration of your mailhost. It must deliver local mail and forward external mail. External mail can e.g be forwarded via smtp (Simple Mail Transfer Protocol) in which case you should read the sendmail documentation ( www.sendmail.org) about "define(`SMART_HOST', smtp:provider.mail.host)dnl". For outgoing mail via smtp you need also on your mailhost computer a /etc/service.switch that enables DNS lookup:

hosts   files dns
aliases files

IP-Masquerading (NAT)

IP Masquerading is the Linux term and most other people call it Network Address Translation. This is a very valuable thing for a small home network. It gives you the possibility to access the Internet from any of your local machines although only one machine is known in the Internet. This one machine is probably connected via modem/PPP and has a valid Internet wide routed ip-address assigned dynamically (or statically) to its ppp0 interface. With IP-Masquerading several people on different computers in your home network can access the Internet at the same time. LinuxFocus will have a separate article on how to setup IP-Masquerading. I recommend also reading the howto on IP-Masquerade. For the moment I will just explain how it works in principle.

To understand how IP-Masquerade works you must understand how a TCP connection is set up. When you connect to a host on the Internet with your web-browser then you connect to the ip address of that machine and the web server application. The application (e.g a web server) is selected by something called port number. The port numbers are pre-defined. Port 80 is the port number for the web server application. Thus if you type www.linuxfocus.org in your netscape then it knows that it must connect to the IP address 195.53.25.18 (= www.linuxfocus.org) and port 80.
For a full tcp connection where data can be send and received, pairs of IP address and port numbers are needed. That is 195.53.25.18 and 80 plus your own ip address and a local port number to which the web server can reply. The local port number is usually allocated at random. E.g:

www at www.linuxfocus.org:    your.machine.at.your.isp:
IP=195.53.25.18 Port=80 ----- IP=123.123.456.1 Port=4123

www at www.linuxfocus.org:    your.machine.at.your.isp:
IP=195.53.25.18 Port=80 ----- IP=123.123.456.1 Port=4123
IP=195.53.25.18 Port=80 ----- IP=123.123.456.1 Port=4541
Note: twice port 80 is not correct but
we just say so to make the example simpler.

www at www.linuxfocus.org --- your.machine.at.your.isp --- privatehost1.mynet
                                                        |
                                                        +- privatehost2.mynet

IRQ

Problems with interrupts are not specific to networking. They can occur anytime you add new hardware to your computer. Unfortunately the PC hardware does only allow for 16 interrupts and most of them are already in use. It is possible to share interrupts between two PCI cards at the expense of some CPU power. It is not possible to share interrupts between ISA bus cards. If you have a PNP Bios and insert a ISA bus card with fixed IRQ into your computer then you must first go into the BIOS setup and reserve that IRQ otherwise the BIOS may try to assign the IRQ to other hardware and that will not work.

Under Linux you can see how often certain hardware has already generated an interrupt by using the command cat /proc/interrupts. Hardware which is not used will not generate any interrupts.

The following is a list of the pre-defined IRQs. You can only use them for other purposes if you do not use the hardware component assigned to it. E.g if you use SCSI drives instead of IDE then you can use IRQ 14/15 for other things.

IRQ	Assigned to
0	timer
1	keyboard
2	cascade (programmable IRQ controller)
3	serial 2
4	serial 1
5	free, usually sound card
6	floppy
7	parallel port, but under linux printers will not generate any iterrrupt
8	real time clock
9	free
10	free
11	free
12	ps2 mouse (/dev/psaux)
13	math co-processor
14	IDE 1
15	IDE 2

Example network

After so much theory I would like to discuss a little example network with you. Why do people build home networks? Here some ideas:

1. Networks allow us to share resources. One computer my have a modem and Internet connection, all share it and use it. One computer may have a printer, all computers can share it. One computer may have a special drive (e.g zip-dirve, cd-rom, dvd) all computers can use it and mount it via NFS. One computer may have a large disk with lots of images and sound files. All computers can access it.
2. It is a technically interesting thing.

>ping 192.168.0.2
PING 192.168.0.2 (192.168.0.2): 56 data bytes
64 bytes from 192.168.0.2: icmp_seq=0 ttl=64 time=0.9 ms
64 bytes from 192.168.0.2: icmp_seq=1 ttl=64 time=0.8 ms
64 bytes from 192.168.0.2: icmp_seq=2 ttl=64 time=0.9 ms

--- ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.8/0.8/0.9 ms

Basically your network is there but nothing really cool works. Now you must edit the different configuration files (use also the nice graphical tools that are available with the various distributions). Finally you setup IP-Maskerade and NFS. Now your network is fully functional.

I hope that this gave you a good overview. Don't be disappointed if I did not tell you exactly how to really configure the software part. There will be separate articles covering ifconfig/route/netstat, IP-Maskerade, PPP setup, network printing with lpd and NFS. This article was supposed to give a good overview to make it easier to understand other articles on networking.


Happy networking

ps: LinuxFocus is always happy to receive articles on networking. Not all articles are written yet and this is just the start of a series. Let us know if you are an experienced user and would like to write an article.

References

• The Linux Network Andministators's Guide www.linuxdoc.org/LDP/nag/nag.html
• The PPP-HOWTO
• The Networking-Overview-HOWTO
• The NET-3-HOWTO
• The IP-Masquerade HOWTO
• The ISP-Connectivity mini HOWTO
• The PLIP mini HOWTO
• The PLIP installation
• The Printing-HOWTO

Talkback form for this article

2001-01-27, generated by lfparser version 2.8