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Linux Networking. ITEC 370 George Vaughan Franklin University. TCP/IP and OSI Models. Origins of Unix. In the early 1960's, MIT experimented with Time sharing systems: CTSS and MAC. This developed into the Multics project.

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linux networking

Linux Networking

ITEC 370

George Vaughan

Franklin University

origins of unix
Origins of Unix
  • In the early 1960's, MIT experimented with Time sharing systems: CTSS and MAC.
  • This developed into the Multics project.
    • Funded by ARPA (ARPA also funded the creation of the Internet, originally known as ARPANET).
    • Project included MIT, GE and Bell Labs (Included Dennis Ritchie and Ken Thompson from Bell Labs).
    • Bell Labs withdrew in 1969.
origins of unix1
Origins of Unix
  • Ken Thompson wrote a game called 'Space Travel' but it was expensive to run on a Mainframe.
  • Thompson found an old PDP-7 with inadequate OS.
  • Thompson and Ritchie wrote the precursor to UNIX in assembly language.
origins of unix2
Origins of Unix
  • Thompson and Ritchie wanted a portable OS for programmers.
  • Ritchie developed the language C as mid-level language to implement UNIX. This was very important because:
    • It allowed UNIX to be portable.
    • It made it much easier to maintain/modify UNIX
  • Portability allowed UNIX to spread to other platforms very quickly.
origins of unix3
Origins of Unix
  • Bell Labs was part of AT&T. AT&T was a legal telecommunications monopoly and was not allowed to sell computers or software.
  • AT&T licensed UNIX source to Universities which made their own modifications and variations (specifically Berkley).
  • After divestiture of AT&T in 1984, AT&T commercializes UNIX.
  • Many commercial variants exist, including SUN Solaris, HP-UX, IBM AIX, Compaq TRUE64, etc. Source NOT available.
where did gnu and linux come from
Where did GNU and Linux Come From?
  • In the mid 80's, Richard Stallman, a researcher at MIT, felt it was morally wrong that companies would not share source code.
  • Stallman created the Free Software Foundation (FSF) with the goal a creating a free OS, called GNU (GNU stands for 'GNU is Not Unix').
    • see:
  • In the late 1980's, Stallman wins the McArthur Genious Award ($300K) and uses the money to support the GNU effort.
where did gnu and linux come from1
Where did GNU and Linux Come From?
  • GNU project decided to work on OS tools first (gcc, gdb, gnu make, gzip, etc) and the Kernel last.
  • In the early 1990's, Linus Torvalds wants source to Minix and is turned away. Torvalds creates a UNIX compliant Kernel and encourages other over the internet to help him.
  • In the mid 1990s complete, free, UNIX compliant OS's were created by merging GNU tools with Linux. This is what Redhat, Fedora, Knoppix, Debian, Mandrake, Suse, Knoppix, etc. have done.
  • GNU still working on its own Kernel (GNU HURD)
where did gnu and linux come from2
Where did GNU and Linux Come From?
  • In a strict sense, Linux refers to the kernel.
  • The kernel is a single executable file.
  • In Fedora Core 6.0, the kernel is:
    • located at /boot/vmlinuz
    • based on version 2.4
    • approximately 1.7 Mbytes in size (small)
what is the ip address of my machine
What is the IP Address of My Machine?
  • ifconfig command
  • ifconfig displays the status of the currently active network interfaces.
    • > ifconfig
    • eth0 Link encap:Ethernet HWaddr 00:A0:CC:3B:0E:53
    • inet addr: Bcast: Mask:
    • inet6 addr: fe80::2a0:ccff:fe3b:e53/64 Scope:Link
    • RX packets:136 errors:1 dropped:0 overruns:0 frame:0
    • TX packets:171 errors:0 dropped:0 overruns:0 carrier:0
    • collisions:0 txqueuelen:1000
    • RX bytes:14102 (13.7 KiB) TX bytes:18945 (18.5 KiB)
    • Interrupt:5 Base address:0xef00
what is the ip address of a remote machine
What is the IP Address of a Remote Machine?
  • nslookup command (also in Windows)
    • A program to query Internet domain name servers
  • Example:


    • Server:
    • Address:
    • Non-authoritative answer:
    • Name:
    • Address:
is the remote machine alive
Is the Remote Machine Alive?
  • Example of ping command: (also in Windows)
    • > ping
    • PING ( 56(84) bytes of data.
    • 64 bytes from icmp_seq=1 ttl=248 time=16.5 ms
    • 64 bytes from icmp_seq=2 ttl=248 time=13.0 ms
    • 64 bytes from icmp_seq=3 ttl=248 time=11.7 ms
    • 64 bytes from icmp_seq=4 ttl=248 time=14.9 ms
    • 64 bytes from icmp_seq=5 ttl=248 time=12.2 ms
    • --- ping statistics ---
    • 5 packets transmitted, 5 received, 0% packet loss, time 4000ms
    • rtt min/avg/max/mdev = 11.710/13.687/16.518/1.794 ms
etc hosts
  • /etc/host file:
    • Text file that associates IP addresses with hostnames (aliases).
  • /etc/hosts on Einstein
    • # Do not remove the following line, or various programs
    • # that require network functionality will fail.
    • localhost.localdomain localhost
    • qwprlx17
    • qwprwi04
etc protocols
  • A text file that maps protocol names with protocol numbers used as IDs in the IP layer.
  • Example from Einstein (very small subset):
    • ip 0 IP # internet protocol, pseudo protocol number
    • icmp 1 ICMP # internet control message protocol
    • tcp 6 TCP # transmission control protocol
    • udp 17 UDP # user datagram protocol
    • gre 47 GRE # Generic Routing Encapsulation
    • ipv6-crypt 50 IPv6-Crypt # Encryption Header for IPv6
    • ipv6-icmp 58 IPv6-ICMP # ICMP for IPv6
etc services
  • A text file providing that maps the following:
    • Human friendly textual names for internet services
    • Assigned port numbers
    • Protocol types
  • Port assignments made by IANA
    • See
  • Example from Einstein:
    • # service-name port/protocol [aliases ...] [# comment]
    • tcpmux 1/tcp # TCP port service multiplexer
    • rje 5/tcp # Remote Job Entry
    • echo 7/tcp
    • systat 11/tcp users
    • daytime 13/tcp
    • ftp 21/tcp
    • ssh 22/tcp # SSH Remote Login Protocol
    • telnet 23/tcp
  • Historically, each network service is supported by its own daemon process or processes.
  • A telnet daemon would support the telnet service, the ftp daemon would support the ftp process, etc.
  • Many daemons are running, often not being used.
  • Each service had to worry about security from the point of connection
  • inetd (precursor to xinetd) was created to address the issue of the abundance of network service daemons.
  • inetd was designed to listen on ports for network service requests.
  • when a request arrived at a port, inetd would fork the appropriate process (ftp, telnet, etc) to service the request.
  • Therefore services like ftp, telnet, etc were no longer daemons - they are now transient processes.
  • Inetd is sometimes referred to as a ‘Super Server’.
etc inetd conf inetd n d
/etc/inetd.conf(inetd, n.d.)
  • File format:


  • service
    • Gives the service name.
    • The service name has to be translated to a port number by looking it up in the /etc/services file.
  • type
    • Specifies a socket type
      • stream (for connection-oriented protocols)
      • dgram (for datagram protocols).
etc inetd conf inetd n d1
/etc/inetd.conf(inetd, n.d.)
  • protocol
    • Names the transport protocol used by the service.
    • Must be a valid protocol name found in the /etc/protocols
  • wait
    • This option applies only to dgram sockets.
    • It can be either wait or nowait.
    • If wait is specified, inetd executes only one server for the specified port at any time.
    • Otherwise, it immediately continues to listen on the port after executing the server.
etc inetd conf inetd n d2
/etc/inetd.conf(inetd, n.d.)
  • user
    • This is the login ID of the user who will own the process when it is executing.
  • server
    • Gives the full pathname of the server program to be executed.
  • cmdline
    • This is the command line to be passed to the server.
    • This field is empty for internal services.
etc inetd conf inetd n d3
/etc/inetd.conf(inetd, n.d.)
  • Sample File (Subset)
    • #
    • # inetd services
    • #
    • ftp stream tcp nowait root /usr/sbin/ftpd
    • login stream tcp nowait root /usr/sbin/rlogind
    • #
    • # inetd internal services
    • #
    • time stream tcp nowait root internal
    • time dgram udp nowait root internal
    • echo stream tcp nowait root internal
    • echo dgram udp nowait root internal
  • xinetd stands for eXtended InterNET services Daemon.
  • Created by Panos Tsirigotis at the University of Colorado.
  • More secure than inetd - designed to prevent Denial of Service attacks.
  • Can control access by:
    • address of remote host
    • time of access
    • name of remote host
    • domain of remote host
  • xinetd is sometimes referred to as the “super-server”.
xinetd conf
0001: #

0002: # Simple configuration file for xinetd

0003: #

0004: # Some defaults, and include /etc/xinetd.d/


0006: defaults

0007: {

0008: instances = 60

0009: log_type = SYSLOG authpriv

0010: log_on_success = HOST PID

0011: log_on_failure = HOST

0012: cps = 25 30

0013: }


0015: includedir /etc/xinetd.d


xinetd is the name of the daemon process.

xinetd config file: /etc/xinetd.conf

instances: max number of simultaneous servers for a given service


first number is max connections per second

second number is number of seconds to wait before re-enabling service after cps has been exceeded.

xinetd d
  • In addition to having a config file for the xinetd daemon itself, each supported service (ftp, telnet, etc) has its own config file in /etc/xinetd.d

[root@localhost xinetd.d]# ls

amanda cups-lpd eklogin ipop3 pop3s services time

amandaidx daytime finger klogin rexec sgi_fam time-udp

amidxtape daytime-udp gssftp krb5-telnet rlogin swat

chargen dbskkd-cdb imap kshell rsh talk

chargen-udp echo imaps ktalk rsync telnet

comsat echo-udp ipop2 ntalk servers tftp

xinetd d1
  • Example: What follows is the configuration file for telnet:

0001: # default: on

0002: # description: The telnet server serves telnet sessions; it uses \

0003: # unencrypted username/password pairs for authentication.

0004: service telnet

0005: {

0006: flags = REUSE

0007: socket_type = stream

0008: wait = no

0009: user = root

0010: server = /usr/sbin/in.telnetd

0011: log_on_failure += USERID

0012: disable = no

0013: }


  • SSHD - OpenSSH SSH daemon
  • replaces rsh and rlogin
  • forks a new sshd daemon for each new connection
  • communication is encrypted
  • used on einstein and can comes configured on RedHat 9.0
  • SSHD supports:
    • ssh
      • similar to telnet
      • client uses tool like putty (Windows), ssh (Linux/Unix)
    • secure ftp
      • similar to ftp
      • client uses tool like winscp2 (Windows), sftp (Linux, Unix)
  • Open Source application that runs on Linux, Unix, etc.
  • It allows Linux machine to appear like a Windows file server to Windows client machines
  • It also allows Windows clients to make use of printing services on a Linux machine.
  • Supports Server Message Block (SMB) Protocol.
  • Windows client machines can mount Unix directories to their PCs (as added drives)
  • Included in most Linux distributions.
nfs nfs n d
NFS(NFS, n.d.)
  • Network File System
  • A protocol developed by SUN in 1984
  • Allows a remote filesystem to be mounted on a local computer.
  • Originally implemented on top of UDP
  • Newer versions implement NFS protocol on top of TCP for better WAN support.
  • Inetd, n.d. The inetd Super Server. Retrieved on 02/20/2007
  • NFS, n.d. Network File System (Sun). Retrieved on 02/20/2007