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Linux Networking

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

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  1. Linux Networking ITEC 370 George Vaughan Franklin University

  2. TCP/IP and OSI Models

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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: http://www.gnu.org/ • In the late 1980's, Stallman wins the McArthur Genious Award ($300K) and uses the money to support the GNU effort.

  8. 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)

  9. 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)

  10. 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:192.168.1.20 Bcast:192.168.1.255 Mask:255.255.255.0 • inet6 addr: fe80::2a0:ccff:fe3b:e53/64 Scope:Link • UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 • 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

  11. What is the IP Address of a Remote Machine? • nslookup command (also in Windows) • A program to query Internet domain name servers • Example: > nslookupeinstein.franklin.edu • Server: 65.24.7.3 • Address: 65.24.7.3#53 • Non-authoritative answer: • Name: einstein.franklin.edu • Address: 66.77.177.143

  12. Is the Remote Machine Alive? • Example of ping command: (also in Windows) • > ping 65.24.7.3 • PING 65.24.7.3 (65.24.7.3) 56(84) bytes of data. • 64 bytes from 65.24.7.3: icmp_seq=1 ttl=248 time=16.5 ms • 64 bytes from 65.24.7.3: icmp_seq=2 ttl=248 time=13.0 ms • 64 bytes from 65.24.7.3: icmp_seq=3 ttl=248 time=11.7 ms • 64 bytes from 65.24.7.3: icmp_seq=4 ttl=248 time=14.9 ms • 64 bytes from 65.24.7.3: icmp_seq=5 ttl=248 time=12.2 ms • --- 65.24.7.3 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

  13. /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. • 127.0.0.1 localhost.localdomain localhost • 10.0.2.12 qwprlx17 qwprlx17.students.qw.franklin.edu • 10.0.2.118 qwprwi04

  14. /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

  15. /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 http://www.iana.org/assignments/port-numbers • 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

  16. inetd • 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

  17. inetd • 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’.

  18. /etc/inetd.conf(inetd, n.d.) • File format: servicetypeprotocolwaituserservercmdline • 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).

  19. /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.

  20. /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.

  21. /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

  22. xinetd • 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”.

  23. 0001: # 0002: # Simple configuration file for xinetd 0003: # 0004: # Some defaults, and include /etc/xinetd.d/ 0005: 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: } 0014: 0015: includedir /etc/xinetd.d 0016: xinetd is the name of the daemon process. xinetd config file: /etc/xinetd.conf instances: max number of simultaneous servers for a given service cps: 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.conf

  24. 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

  25. xinetd.d • 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: } 0014:

  26. SSHD • 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

  27. SSHD • 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)

  28. Samba • 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.

  29. 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.

  30. References • http://www.perldoc.com/perl5.6.1/lib/CGI.html • http://www.xinetd.org/faq.html • http://www.linuxfocus.org/English/November2000/article175.shtml • http://www.macsecurity.org/resources/xinetd/tutorial.shtml • http://www.bgw.org/tutorials/operating_systems/linux/inetd_tour.php3 • Inetd, n.d. The inetd Super Server. Retrieved on 02/20/2007 http://www.faqs.org/docs/linux_network/x-087-2-appl.inetd.html • NFS, n.d. Network File System (Sun). Retrieved on 02/20/2007 http://en.wikipedia.org/wiki/Network_File_System_%28Sun%29

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