[glugot] Minutes of the meeting #2
- From: Joe Steeve <joe_steeve@xxxxxxx>
- To: glugot@xxxxxxxxxxxxx
- Date: Tue, 13 Jan 2004 20:06:54 +0530 (IST)
Hello,
It took me sometime to finish up with this minutes. I've enclosed it
here. I wrote it in a hurry. So please bear with typing errors
:S. I've created the aligning to 70 columns which should look good
with most the email-service providers, if it dosent look good er.,
dont blame me blame your email service providerr :(
Please post errors in the minutes to me or to the list so that they
can be discussed.
Cheers,
Joe
--
"Software is like sex; Its better when it is free"
-- Linus Torvalds
visit : http://www.joesteeve.tk/
-- Start of MOM --
GLUGOT
(GNU/Linux User Group Of TCE)
Minutes of meeting #2
Date : 10-01-2004
Time : 16:45 to 17:45
Venue : Z0 Hall,
CSE Department,
TCE, Madurai - 624015
Speaker : I. Amalan Joe Steeve ( I ME CSE )
joe_steeve@xxxxxxx
Topic : `Networking and GNU/Linux -- Administration & Configuring
(Basics)'
-------------------------------------------------------------------------------
Introduction:
-------------
A `computer network' is a collection of computers connected together
by some physical medium. These computers communicate with each other
by emitting signals on the medium. They follow some agreed upon
protocol in doing this. There are different levels of these
protocols. TCP/IP is one such protocol which is very popular
today. The Internet is driven by TCP/IP. TCP/IP is the child of the
`DARPA' (Defence Advanced Research Projects Agency). Novell promoted
SPX/IPX. IBM promoted SNA. Xerox promoted a protocol suite of their
own. Today., TCP/IP has become a standard among computer networks. The
Linux kernel has a very flexible support for networking. A major part
of the kernel code is for networking. Linux supports TCP/IP,SPX/IPX,
etc.. The Linux kernel can perform the best on TCP/IP based networks.
IP Addressing:
--------------
In a TCP/IP network, each computer is known by an IP number. An IP
number is a 32-bit number denoted in a form called the `dotted
notation'. When two program on different machines want to communicate
via the network, they do so by sending messages to each other. They
address each other using a ip-address and port-no pair. The ip-address
identifies a machine on the netork. A port-no identifies a program
running on the machine. Server programs usually listen on some
predefined port. (Eg. SMTP servers listen on port number 25. POP
servers listen on port number 110. HTTP servers (web server) listen on
port number 80.) The port number range 0 to 1000 are reserved for
standard protocols and services. On any UNIX, for a program to listen
on one of these ports, it should be run as `super-user'.
The `Internet Engineering Task Force' (IETF) moniters and maintains
the technical details of the Internet and TCP/IP standards. When
TCP/IP was introduced, the IETF identified the need to classify the
IP-Address space and introduced a addressing scheme called `classful'
addressing. In this scheme, the IP-Address space was divided into
four classes. The class in which an ip-number belongs can be easily
identified by examining its first few bits. The classifying scheme is
explained below:
0xxxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx --> Class A
10xxxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx --> Class B
110xxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx --> Class C
1110xxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx --> Class D
x --> Either a `0' or a `1'
The Internet is comprised of many small networks. Hence its called the
`INTER NETwork'. In order to make the addressing more simpler a need
arised to consider a part of the ip-address as a network id and the
rest as the host id. Along with this conventions were chosen to
address a full network and to address the current network (explained
later in this text). Now a problem arose as to how many bits should
comprise of the `network-id' and how many bits should be as the
`host-id'. Different organisations needed different lengths of
`host-id' and `network-id's depending upon the size of their
networks. In order to cater the needs of various establishments, the
IETF proposed the `classful' addressing scheme. Where the initial bits
of the ip-address was used to determine the bit-length of the host-id
and the network-id part of the ip-address.
In a class A network, the first bit is `0'. The rest of the 7 bits of
the first byte comprises of the `network-id' and the rest 3 bytes
comprises of the `host-id'. In class B, the first two bits were `10'
and the next 14 bits was used as the network-id. The last 2 bytes was
used as the `host-id'. In class C, the first three bits is `110'. The
next 21 bits are the `network-id' and the last 1 byte is used as the
host-id. Class D is a special case. It is used for multicasting. The
first four bits is `1110' and rest of the 28 bits correspond to a
group of machines. This is summarised in the following diagram.
class A := 0xxxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
|net-id| host-id |
class B := 10xxxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
| net-id | host-id |
class C := 110xxxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
| net-id |host-id |
class D := 1110xxx.xxxxxxxx.xxxxxxxx.xxxxxxxx
| multicast address |
When the Internet was commercialised, a lot of Internet Service
Providers sprouted up and a new scheme came up called the `classless'
addressing. This scheme removed the hassles of `classful'
addressing. The ip-number always carried along with it a integer which
specified the number of bits which are the `network-id' and the number
of bits that are the `host-id'. A new routing called `Classless Inter
Domain Routing' (CIDR) came up. This is the currently used
scheme. Organisations use `classful' addressing in their private
networks, but the public networks are addressed using the `classless'
addressing scheme. In classless addressing, an ip-address is always
accompanied by a net-mask or an integer. It is denoted as follows.
a.b.c.d/8
OR
ip-number= a.b.c.d
net-mask = 255.0.0.0
This means that the first 8 bits of the ip-number is used as a network
id and the rest of them are used as the host-id. Netmasks are also
used in the `classful' addressing scheme to specify subnets.
Networking in Linux:
--------------------
The hardware component which connects a computer to the network is
called the interface. There are a variety of interfaces
available. The most commonly used device in LANs are `Ethernet
controllers'. In the case of connection to other networks modems are
used. These modems can either be a regular telephone modem or a ISDN
modem or leased line modem or anything else. These interfaces are
exported as a device file in the `/dev' directory by any conventional
UNIX. But Linux dosent export this as a file. Instead these devices
are initialised and maintained internally. They can be configured by
userland software. This adds flexibility to use the kernel for any
networking purpose.
Internally the interfaces are named depending on their type. Ethernet
controllers are named as eth[0...n]. PPP interfaces (modems) are named
as ppp[0...n]. When the kernel finds a Ethernet device, the kernel
initializes the corresponding device. In the case of ppp devices, user
software should create the link and then inform the kernel about
it. This way the user gets to makeup a ppp link to an ISP or any other
network at his/her own wish.
Each of these interfaces can hold more than on ip-address. This
feature is helpful in cases where a machine acts as the gateway
between logical subnets on the same physical network. In this case the
devices are named as follows.
eth0 --> first interface
eth0:1 --> second interface
eth0:2 --> third interface ( and so on )
The command to assign an ip-address to an interface is;
#ifconfig eth0 192.169.2.1 netmask 255.255.255.0 up
The above command assigns `192.168.2.1' to eth0 and sets the netmask
as `255.255.255.0'. Now the kernel assumes the broadcast address and
the network address as `192.168.2.255' and `192.168.2.0'
respectively. To shutdown an interface, the following command is
used.
#ifconfig eth0 down
The ifconfig is a tool used to control various parameters of the
interface. `ifconfig' stands for `interface config'.
Name resolver:
--------------
On the Internet, it would be difficult to remember the ip-addresses of
the needed web-sites. Hence, the individual computers on a network are
given a name and are referred using this name. In this case., a method
is needed to convert these human readable names to ip-addresses. This
is the work of the resolver code of the Linux kernel. The resolver
code can either look into a locally available configuration file
(/etc/hosts.conf) for a `hostname' to `ip-address' mapping, or it can
request another server called the `nameserver' or `DNS server' to give
the ip-address for the required hostname.
The `/etc/host.conf' file controls how the resolver should go about
resolving a given hostname. It specifies where the resolver should
look first, either the local file or the server. The following is an
example `/etc/host.conf'.
# These are comment lines
# /etc/host.conf
#
order hosts,bind
multi on
In the above case, the resolver code first looks into the
`/etc/host.conf' file to find a match for a particular hostname. If it
dosent find one, then it requests an external server to provide the
ip-address for the required hostname.
The `/etc/resolve.conf' defines the addresses of the nameservers. When
the resolver code decides to query a external server about a hostname
it consults this file to find the list of `nameservers' it can query.
TCP/IP standard defines `127.0.0.1' as the loopback interface. It
means the current machine. This address is used for checking and
debugging. This is often called the loopback device. This can be
configured like anyother interface. The kernel handles these
interfaces specially. (i.e.) packets to this interface are not
broadcasted on the network but are subjected to all the protocol
layers like any other packet. The loopback interface can be configured
using the following commands.
#ifconfig lo 127.0.0.1
#route add -host 127.0.0.1 lo
The second command `route' adds a route in the kernel's routing table
about how to reach this interface. Routes are needed in the case where
multiple networks are interconnected (like in the Internet). When
setting up a ppp link with a ISP or some other computer, the `pppd'
(PPP software) assigns the route of all the packets as the other end
of the ppp-link. When configuring a `network-router' the administrator
should set up the routing tables accordingly. However discussion of
routing is out of the scope of this text.
Other config files:
-------------------
A few other configuration files are worth discussion. These are mostly
defined automatically by the installer scripts.
`/etc/services' -> This file describes the different services and
their respective port numbers.
`/etc/protocols' -> This file describes the various protocols and
their respective numeric ids.
`/etc/hosts.allow' -> A listing of hosts and the respective services
they are allowed to access.
`/etc/hosts.deny' -> A listing of hosts and the respective services
they are denied from accessing.
It is highly recomended that the user check these files to make sure
what he/she allows and denies on his box.
Conclusion:
-----------
I've tried my best to keep this text as simple as possible. Please
notify errors in this text directly to the author. If it is a subject
of discussion, you are free to start a thread of discussion on
`glugot@xxxxxxxxxxxxx'
-- EoF --
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