Internet

1
Internet

• An Introduction

2
Packet

• Information sent in packets
• Header
• sender
• receiver
• type of service
• checksum
• Body
• data
• size 1500 - 65000 bytes

3
IP layers

• Physical layer
• unreliable connection between two computers
• Link layer
• reliable connection between two computers
• IP layer
• reliable connection between two computers across
  the LAN
• TCP/UDP layer
• virtual connections
• ports
• Application layer
• HTTP, mail, ...
• ISO/OSI Reference model session and
  presentation layer

4
Physical and link layer

• Physical layer
• ensures data sending/receiving
• Ethernet, RS 232, WiFi, ATM, Novell,
• Link layer - Ethernet
• corrects errors of physical layer
• each net card has its own (unique) MAC (Machine
  Address Code) address
• 00-11-D8-C8-D6-6A

5
IP protocol

• sends data across heterogeneous networks
• router
• controls a subnet of the Internet
• source target address
• IP address - 32-bit number

6
IP address

• IP
• 10011110.11000011.10101010.10xxxxxx
• 158.195.85.?
• mask
• 11111111.11111111.11111111.11000000
• 255.255.255.192
• special addresses
• network 158.195.85.128
• broadcast 158.195.85.191

7
ICMP and other IP protocols

• Internet Control Message Protocol
• ping
• echo request
• echo reply
• destination unreachable
• TTL exceeded
• Other IP protocols
• IGMP, ARP, RARP, BOOTP, DHCP, TCP, UDP

8
Static vs. dynamic IP

• Static address
• manual IP address, mask, DNS server setting
• DHCP protocol (based on BOOTP)
• Dynamic Host Control Protocol
• given a MAC address IP address of some IP address
  range
• also gives information about correct netmask and
  DNS information for a particular LAN network

9
TCP/UDP

• TCP (Transmission Control Protocol)
• the most common connection-oriented protocol
• most of Internet services use this protocol POP3,
  ...
• UDP (User Datagram Protocol)
• no check, whether the data was received correctly
  
• video, SMB, P2P

10
Port

• Every (virtual) connection between two computer
  uses some port
• Well-known ports lt 1024
• used for some well-known services FTP, HTTP, SSH,
  ...
• Dynamic ports gt 1024
• secondary communication and data transfer

11
Packet example
12
Communication example

• POP3 service port 110
• telnet sccg.sk 110
• Trying 158.195.85.131...
• Connected to sccg.sk (158.195.85.131).
• Escape character is ''.
• OK POP3 fractal.dam.fmph.uniba.sk 2004.89mdk
  server ready
• user kubini
• OK User name accepted, password please
• pass tralala
• OK Mailbox open, 3 messages
• retr 2
• OK 386 octets
• Return-Path Received (from user.123_at_localhost)
  by pop.foo.bar (8.8.8/8.8.8) id SAA29614 for
  user.123 Wed, 3 Dec 1997 185543 -0500 (EST)
  Date Wed, 3 Dec 1997 185543 -0500 (EST) From
  Pat Gunn
• dele 1
• OK Message 2 has been deleted.
• quit
• OK Sayonara

13
Summary

• Internet
• Packets
• ICMP (Internet Control Message Protocol)
• IP (Internet Protocol)
• TCP/UDP (Transmission Control Protocol, User
  Datagram Protocol)
• Ports
• Application specific information

14
Server configuration
15
Linux basics

• bash (shell)
• man bash
• cd, cp, mv, rm, w, ps,
• path
• absolute /home/kubini/public_html
• relative public_html
• home directory
• prompt kubini_at_fractal
• mount
• /proc
• /dev
• /etc
• /mnt
• /bin, /sbin, /usr/bin, /usr/sbin
• user, group - chmod, chown, chgrp

16
MAIL(SMTP, POP3)

• mail sending
• postfix /etc/postfix/main.cf,
  /etc/init.d/postfix/restart
• mail receiving
• pop3(Post Office Protocol) /etc/xinetd.d/pop3(s)
  
• unsecured protocol
• mails are stored on the client (usually)
• imap(Internet Mail Access Protocol)
  /etc/xinetd.d/imap(s)
• newer safer protocol
• mails are stored on the server
• /etc/init.d/xinetd restart
• client mail, pine, kmail

17
DNS

• 158.195.85.151 (ns.dam.fmph.uniba.sk)
• www.zoznam.sk lt-gt 62.65.179.31
• MX, PTR, A, CNAME
• hierarchical DNS name structure
• .uniba.sk
• .fmph.uniba.sk
• .dai.fmph.uniba.sk
• /etc/named.conf
• /var/named/
• /etc/init.d/named restart
• client dig, host nslookup, ping -a

18
Webserver (httpd)

• Apache(Pyramid)
• server webpages (sccg.sk, )
• user webpages
• Configuration
• vim /etc/httpd/conf/httpd2.conf, /etc/php.ini
• /etc/init.d/httpd restart
• Client lynx, links, mozilla, IE

19
Database

• MySQL server(fractal)
• port 3306
• configuration
• vim /etc/my.cnf
• /etc/init.d/mysqld restart
• relational database
• SQL (Structured Query Language)
• Client
• http//www.sccg.sk/phpMyAdmin
• mysql u user p pass

20
SMB(Server Message Block)

• Samba server
• ports TCP 139, 440, UDP 137, 138,
• /etc/samba/smb.conf
• WorkGroup(KG, KPGSO) browstat
• Domain (M-115)
• Client smbmount, smbclient, smbumount,
  explorer(\\fractal\pub), Windows shared disks
• net use Q \\fractal\pub /userAdministrator
• print, ipc,

21
FTP(File Transfer Protocol)

• Server (not running)
• proftpd
• /etc/proftpd.conf
• /etc/init.d/proftpd restart
• anonymous ftp
• Client
• ftp, mc, explorer (ftp//...), Total Commander
• passive vs. active ftp

22
Telnet

• Server (not running7)
• Port 23
• remote keyboard
• Client
• telnet (Linux and Windows)
• Ctrl if the connection gets broken

23
SSH(Secure shell)

• Secure shell server
• Safe telnet replacement
• /etc/ssh/sshd.conf
• /etc/init.d/sshd restart
• Client
• SecureCRT, putty, ssh(Secure Shell)
• scp(sftp), WinScp(Secure coPy ftp replacement)

24
Firewall/Router

• router
• computer responsible for a subnet
• routes packets to/from the correct computers
• firewall
• ensures safety of the subnet
• iptables
• firewall(offon)

25
Spam filter

• SpamAssassin
• /etc/mail/spamassassin/local.cf
• spam, whitelist, blacklist, ...
• Client configuration
• /.spamassassin/local.cf
• similar to server configuration

26
Proxy server

• Proxy server
• content based filtering
• from
• to
• type of service
• SQUID application proxy
• application layer content filtering
• M-115 has restricted access to KPGSO LAN
• /etc/squid.conf
• /etc/init.d/squid restart

27
X-Windows

• Server
• X-Window
• physical console Alt-F7
• local server WinAxe
• Client
• applications
• xterm, ...
• kclock, kmail

28
DHCP

• Server
• dhcpd
• /etc/init.d/dhcpd restart
• /etc/dhcpd/dhcpd.conf
• dynamically assign IP address from some range
• MAC filtering
• DHCP assigns IP address to computers with known
  MAC address
• Client
• ipconfig /renew,

29
Our LAN
30
References

• www.sccg.sk/kubini/Internet
• Internet
• IBM TCP/IP reference
• RFC (Request for comments)
• Google
• Linux
• man
• standard documentation and Internet (google)
• Interesting software
• Ethereal (packet analyzer)