By Paul Dunne
See that old 25MHz, 386-based PC? Yes, that's right, the
one gathering dust over in the corner of the office. It can't
run Windows 95, but it still boots -- surely it's not quite ripe
for use as that proverbial boat anchor? In this series of
articles, I will show how such a modest box can do sterling work
on your network -- providing Internet access, file and print
services, and even firewall protection -- simply by add
ing Linux.
So to start with, lets get Linux up and running and connected to
the Net. It will be presumed throughout that our attempt is very
much a budget one: a simple dialup account to the nearest ISP, a
no-name modem, and so forth.
Questions regarding this article should be directed to the
author at
paul@tiny1.demon.co.uk
Other installments of this Linux Internet server tutorial
series include:
A
previous
article
in UnixWorld Online has already covered Linux
background issues, and contain
s plenty of useful resources. If
you're not sure quite what Linux is, it might be helpful to take
a look there first.
Installing Linux is straightforward nowadays -- just pick a
distribution and run the install program. Before we go anywhere,
though, we must check that we CAN install Linux.
Hardware requirements
Linux is modest in its hardware requirements when compared to
those of most other modern OSes. Any Intel 386 or later CPU will
do. In a pinch we can get by with 40 megabytes or so of hard
disk space. Though it will run in only two megabytes of memory,
the minimum practical RAM required is four megabytes. Even that
can be a tight squeeze for some distribution's installation
programs, including Slackware's, so eight megabytes is the
practical base line. Linux supports a very wide range of PC
hardware, so most PCs will work without further ado. To find out
if a particular piece of hardware is supported, see
the
Linux Hardware HOWTO
.
Installation can be via floppy diskettes, from a CDROM, or by
NFS over the local network or the Internet.
My choice of Linux distribution (Slackware) explained
In this article, I will be using the Slackware distribution
for my examples. Why Slackware? It was for long the
``standard'' distribution. It remains the easiest to adjust to
one's needs, as long as one is not afraid to ``get one's hands
dirty,'' so to speak. It is also in my experience the quickest
to install.
Slackware installation routine
The Slackware setup program is simple and takes the user
through all the stages of installation and even does some basic
configuration. Rather than simply describe what the install
program does, step by step, which would be superfluous, I will
concentrate on explaining what is going on ``behind the
scenes.''
However, first let's back up a step and make boot and root
diskettes. You will most
probably be doing this from DOS, with
the
rawrite.exe
program provided on the CD. The
diskettes must be formatted first in DOS; then
rawrite.exe
is run to write the boot and root
images.
Secondly, reboot the computer with the boot diskette in the
``A:'' drive. If all goes well, Slackware will present you with
a prompt to enter additional parameters. Usually, you can just
ignore this and continue by pressing Enter. Sometimes, however,
you may need to pass parameters to the Linux kernel to tell it
what I/O address your CD-ROM drive uses, for example. A full
explanation of all the options is available from the
Linux
BootPrompt HOWTO
.
Your computer will then be booted into the Linux operating
system, and when the boot process is complete, you will be prompted
to replace the boot diskette with the root diskette.
The root diskette contains an image of a basic root file system,
a minimal system from which the rest of the Linux distribution can
be installed. Once this file system is mounted, you can login as
root, and type ``setup'' on the command line to start installing
the system.
To start with, the
fdisk
program will be run, and
you will be asked how many partitions you wish to set up, and
what types they should be. The simplest partition procedure is
simply to create two partitions, one to hold the root file
system, the other for swap space. This may be said to be
mandatory for a machine with only four megabytes of RAM, and
still highly recommended for those with more. As a crude rule of
thumb, let the swap space be twice the amount of physical RAM,
but always give yourself at least 16 megabytes total (physical
and virtual) memory.
A more sophisticated procedure, particularly if the PC has two
or more hard drives, is to create several more partitions. A good
basic split is five partitions:
/
,
/usr
,
/var
,
/tmp
, and the swap partition. This
makes no difference to performance when using a single IDE drive.
For multiple SCSI devices, however, it would make more sense to split
the file system appropriately among the various disks. More details
on optimising disk partitions for Linux may be found in the
Linux
Partition mini-HOWTO
.
Once the partitions have been set up, you select the
installation media and the destination media. Most commonly,
these will be respectively the CD-ROM drive and the hard disk
that you have just prepared. But, you do have the option of
installing from floppy disks, or over a network using NFS.
The installation process formats your chosen partition(s),
using the Ext2 file system, by default. You have no good reason
to chose any other file system. Formatting may be quick, or
complete (that is, including checking for bad blocks); it is
advisable to chose the latter. The time this takes will var
y
according to the size of hard disk, of course; it is somewhat
slower than the DOS format program, but then it is serving a
rather more sophisticated file system.
With the hard disk ready, we may choose which of the sets of
applications to install. Some are mandatory, others are
recommended, and others still are purely optional extras. For
our purposes, the ``A'' series should be installed in toto,
together with the ``N'' series. A minimal installation will fit
comfortably on a 40 megabyte partition; everything takes about
150 megabytes; however, you will want to allow considerably more
than that as working space.
Once Linux is installed, the Slackware setup program does
several of the simpler configuration tasks. Here's the low-down
on what it does.
Local keyboard map
For anyone save U.S. readers, a keymap other than the default
U.S. keymap is preferable. An alternative keyboard map file is
loaded by the
loadkeys(1)
pr
ogram. Please note that
some versions of Slackware have a broken installation routine
that offers to change your keyboard map, then happily continues
to use the U.S. keyboard! This is simple to fix: for example, to
use a U.K. keyboard, add the following line to the
/etc/rc.d/rc.local
system boot configuration file:
loadkeys /usr/lib/kbd/keytables/uk.map
By the way, the Linux keyboard is fully remappable. The
*.map
files are simple text files, easy to
understand and hack. As an example, on my standard 102-key
keyboard, I've swapped Caps Lock and Right Ctrl, so that the
Control key is where the Good Lord intended it to be!
Mouse
Linux supports most sorts of mouse.
Although we won't be running X on this box, a mouse is still
useful, as Linux supports mouse-assisted cut-and-paste between
virtual text consoles. The
selection(1)
program
supports this, by running as a daemon installing it is an option
during the p
ackage installation phase. A link is made from the
generic
/dev/mouse
file to the actual device file
controlling the serial port to which the mouse is attached.
Modem
Modem configuration is usually a matter of making an optional
link from the actual device file to
/dev/modem
. Any
standard internal or external modem should work, with the only
configuration work needed being to chose a free IRQ. Those few
modems that rely on a DOS program to download firmware at
runtime are obviously a problem; the solution is a garage sale!
Seriously, in a pinch, if we are really stuck with such a device,
booting DOS, setting up the modem, then warm-booting into Linux
will work around this problem.
Host Name
Give the machine a name. You'll probably have local naming
conventions to follow, so that the new machine fits in with the
existing network.
There is a wealth of information on Linux
networking, i
n a variety of sources, including The
Linux
Journal
, the
NET-3 HOWTO
, and
Linux Networking
News
.
Ethernet
Configuring the Ethernet link is simple, and is done for us by
the setup program. Here, I will run through what that program does
``"behind the scenes.''
There are two scripts,
rc.inet1
and
rc.inet2
. The first sets up basic networking, which
is what we will be concerned with here. The second deals with
NFS, which will be considered later.
HOSTNAME=`hostname`
This sets the host name by running
hostname(8)
.
But how does
hostname
know what he host name is (if
you see what I mean)? In Slackware, the file
/etc/HOSTNAME
should be manually edited so that it
holds the fully-qu
alified domain name. This file can then be
read at boot-up, and used to set the hostname using
hostname(8)
, like so:
/bin/hostname `cat /etc/HOSTNAME`
Next, we configure a special device called ``lo'', short for
``loopback''. The loopback device is like a dummy network, in
which the machine talks to itself. It has the standard address
of 127.0.0.1, and is always required. We use:
/sbin/ifconfig lo 127.0.0.1
/sbin/route add -net 127.0.0.0
Now an optional part, that can be useful if the internet
connection is a dial-up one, and is thus intermittent. We don't
want the Internet host name to be unusable when the link is down;
the special Linux ``dummy'' interface is designed specifically with
this in mind.
/sbin/ifconfig dummy ourhost
/sbin/route add ourhost
Rather then typing in these values over and over, we set them
once here.
IPADDR="192.168.1.1" # REPLACE with YOUR
IP address!
NETMASK="255.255.255.0" # REPLACE with YOUR netmask!
NETWORK="192.168.1.0" # REPLACE with YOUR network address!
BROADCAST="192.168.1.255" # REPLACE with YOUR broadcast address, if you
# have one. If not, leave blank and edit below.
Set up the Ethernet device:
/sbin/ifconfig eth0 ${IPADDR} broadcast ${BROADCAST} netmask ${NETMASK}
Add a route to the local network to the routing table:
/sbin/route add ${NETWORK}
Finished!
The other networking configuration file,
rc.inet2
, does not concern us here. However, note
that it starts various daemons, including the various NFS
servers,
rpc.portmapi
, and more. The install
program will take care of this for you.
Name Service
Several files in
/etc
determine how
host name to IP address translation is done.
The
/etc/host.conf
file
order hosts,bind
multi on
This file determines in what order name resolution shall be
attempted. Here, we have specified that first the resolver will
attempt to look up names in
/etc/hosts
, then, if
that fails, attempt to use the default nameserver (as specified
in
/etc/resolv.conf
). The ``multi on'' means the
more than one nameserver can be used.
The
/etc/hosts
file
This file holds a few hostname-to-address mappings that need
to be available at boot-time, when no name service is available.
Indeed, it can be used instead of a name server. A typical
minimum file looks like this:
# For loopbacking (this is mandatory)
127.0.0.1 local localhost
# this host
xxx.xxx.xx.xxx this.hosts.ip.address
# gateway
xxx.xxx.x.xxx the.gateway.to.the.internet
The
/etc/resolv.conf
file
nameserver 127.0.0.1
domain mycompany.com
The file
/etc/resolv.conf
file controls how
the
resolver library routines operate. Here, we specify localhost
for the address of the name server because I run a caching name
server. Alternatively, this could be the IP address of your
ISP's name server(s) -- there can be a list. If you are running
a local name server on another machine, then put its IP address
here. The domain parameter indicates the default domain for
unqualified host names.
Caching named
It can be useful to set up a minimal
named
. This
serves as a cache, so that name lookups only have to go out over
the Net once; thereafter, they are stored locally, decreasing
latency.
This is not the place for a tutorial in configuring
named
. For further details, there is a good
DNS
HOWTO
. For more general background, you may wish to examine
the UnixWorld Online
``DNS Database Files'' tutorial article
.
You may wish to use the Linux box to provide name service for
the local network. This is a subject for a later article; for now,
we will assume that this is being taken care of.
With installation complete, it is advisable to add a root
password, and set up one or more ordinary users. Do not succumb
to the temptation to use root for everyday work; this account is
far too powerful, and sooner or later you will do something you
will regret, such as the proverbial
rm -fr /
,
which zaps EVERYTHING! Slackware comes with a nice little
``adduser'' program, which will do everything necessary. In any
case, the manual steps are easy enough: edit
/etc/passwd
to add the user, the file being in the
format:
user name:password:user id:group id:real name:home directory:shell
Then, create the user's home directory; and copy any useful
files that may be in
/etc/skel
(common ones are a
basic
.profile
,
.less
and
.term
). Reboot and enjoy!
Having installed and configured Linux, our final step is to get
the Internet link up and running.
Dialup Link
The mechanics of setting up a dial-up link are so dependent on the
particular ISP chosen that there isn't much to say in a general
article like this one. I prefer SLIP, but many ISPs don't give you
the choice anymore. Again, I prefer a static IP address, but most
ISPs are now using dynamic addressing. It is worth shelling out
a little more to get either a dial-up link with a static address,
or a permanent line; but there are work arounds for the worst-case
scenario, PPP with dynamic addressing. Space is too limited
to consider to delve into making a Linux box happy with dynamic
addressing; see the
Linux
Dynamic IP mini-HOWTO
for details.
As an example, I will run through co
nfiguring a dial-up link
using my preferred configuration, SLIP with a static IP address.
The procedure here is simple. A script is written or, more
likely, copied from someone else and modified. Here's an example
of my local
dialup IP connection
support program
, which takes care of all the ``housekeeping''
of raising and dropping the link, which includes calling the
remote computer, adding routing table entries, and the rest.
This script is run as often as connections to the Internet are
desired, typically from
cron(8)
.
Using PPP is quite similar. The
pppd
program,
which generally has path name
/usr/sbin/pppd
, is
used. Here, for example, is the command line invocation (put in
a script so I don't have to type it out each time) that I would
use to connect to my ISP using PPP:
pppd connect 'chat "" ATDT01716640666 CONNECT "" \
ogin: tiny1 word: duh! ocol: PPP' \
/dev/cua3 115200 -detach debug cr
tscts modem defaultroute \
158.152.37.217:158.152.1.222
Where
chat
is another program called by
pppd
to actually dial up and log in to the remote
termianl server.
The arguments to the
chat
program are expect-send
string pairs:
""
- Expect nothing (don't wait for a prompt)
ATDT01716640666
- Send the dialing command to the modem
CONNECT
- Expect the answer ``CONNECT''
""
- Send a return (null text followed by usual return)
ogin: tiny1 word: duh! ocol: PPP
- This is the sequence of expect-send strings needed to log in
to my ISP. You can also think of these sequences as question-answer
pairs, the first being the ``question'' (sent from the ISP), the
second the ``answer'' (returned by
chat
). Note:
only the last
few characters need to be specified, as in
``word:'' instead of ``password:'', which has the
advantage that you don't need to worry if ``Password:'' is
capitalized or not.
The other options are as follows:
Here,
/dev/cua3
is the callout serial port that
my modem is on;
115200
is the baud rate on that
line;
-detach
tells
pppd
not
to put itself in the background;
crtscts
says to use
hardware flow control on the line;
modem
tells
pppd
that this is a modem device so that the program
will hang up the phone before terminating;
defaultroute
makes the PPP link the default route, which is usually what you
want; and finally,
158.152.37.217:158.152.1.222
specifies the local and the remote IP addresses, respectively.
Note that if you're using dynamic IP addressing, then the
noipdefault
option would be specified to request
this second IP address from the remote hos
t.
Permanent Line
A permanent line is simplicity itself, given what has gone
before. Use
slattach(8)
to attach the SLIP
interface to the device, like so:
slattach -p slip -s 19200 /dev/ttyS0
The
-p
option sets the protocol to use on the
line. The default is set to
cslip
, that is,
compressed SLIP. Other possible values are
slip
(normal SLIP),
ppp
(Point-to-Point Protocol) and
kiss
(AX.25 TNC protocol). The
-s
option sets a specific line speed, other than the default.
Then
ifconfig
and
route
are used to
configure the interface and add the routing table entries
respectively, in just the same way as the Ethernet device was
configured above.
/etc/ifconfig sl0 $IPADDR pointtopoint $REMADDR up
/etc/route add default gw $REMADDR
At the end of this process, we
have a fully-fledged Internet
host, capable of performing any of the tasks we might expect such
a box to undertake. In our next installment, we set up a
Firewall to protect your local network.
Paul Dunne is a writer and consultant who specialises in Linux.
He has a
home page
that
describes in more detail what he does.
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