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Linux Networking

ITEC 370

George Vaughan

Franklin University

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TCP/IP and OSI ModelsTCP/IP and OSI Models (OSI-Model, n.d.) and (Tomsho, 2007)

TCP/IP Layers

PDU OSI Layers Function Devices - Apps Standards

7 Application Network process to application, Initiates or accepts a request to transfer data

Browsers, servers, Gateways

HTTP, SNMP, FTP, Telnet

6 Presentation Adds formatting, display, and encryption of information

Gateways ASCII, MPEG, SSH, SSL

Application Data

5 Session Adds communication session control information, Login/Logout

DNS, Gateways

NetBIOS

Transport Segments 4 Transport Adds End-to-end connections and reliability, re-sequencing, flow control

Gateways TCP, UDP

Network Packets 3 Network Path determination and logical addressing (IP), translates MAC address to logical address

Routers IP, ICMP, ARP, NetBEUI, IPSec

LLC Frames 2 Data Link

MAC

Adds error checking and physical addressing (MAC & LLC)

Switches, Bridges, NICs

802.3, 802.11, FDDI

Link

Bits 1 Physical Media, signal and binary transmission, sends data as a bit stream

Hubs, Repeaters

10Base-T, T1, E1

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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.

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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.

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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.

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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.

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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.

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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)

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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)

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What is the IP Address of My Machine?

• ifconfig command• ifconfig displays the status of the currently active network

interfaces.1. > ifconfig2. eth0 Link encap:Ethernet HWaddr 00:A0:CC:3B:0E:533. inet addr:192.168.1.20 Bcast:192.168.1.255 Mask:255.255.255.04. inet6 addr: fe80::2a0:ccff:fe3b:e53/64 Scope:Link5. UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:16. RX packets:136 errors:1 dropped:0 overruns:0 frame:07. TX packets:171 errors:0 dropped:0 overruns:0 carrier:08. collisions:0 txqueuelen:10009. RX bytes:14102 (13.7 KiB) TX bytes:18945 (18.5 KiB)10. Interrupt:5 Base address:0xef00

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What is the IP Address of a Remote Machine?

• nslookup command (also in Windows)– A program to query Internet domain name

servers

• Example:> nslookup einstein.franklin.edu1. Server: 65.24.7.32. Address: 65.24.7.3#53

3. Non-authoritative answer:4. Name: einstein.franklin.edu5. Address: 66.77.177.143

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Is the Remote Machine Alive?

• Example of ping command: (also in Windows)1. > ping 65.24.7.32. PING 65.24.7.3 (65.24.7.3) 56(84) bytes of data.3. 64 bytes from 65.24.7.3: icmp_seq=1 ttl=248 time=16.5 ms4. 64 bytes from 65.24.7.3: icmp_seq=2 ttl=248 time=13.0 ms5. 64 bytes from 65.24.7.3: icmp_seq=3 ttl=248 time=11.7 ms6. 64 bytes from 65.24.7.3: icmp_seq=4 ttl=248 time=14.9 ms7. 64 bytes from 65.24.7.3: icmp_seq=5 ttl=248 time=12.2 ms

8. --- 65.24.7.3 ping statistics ---9. 5 packets transmitted, 5 received, 0% packet loss, time

4000ms10. rtt min/avg/max/mdev = 11.710/13.687/16.518/1.794 ms

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/etc/hosts

• /etc/host file:– Text file that associates IP addresses with

hostnames (aliases).

• /etc/hosts on Einstein1. # Do not remove the following line, or various programs

2. # that require network functionality will fail.

3. 127.0.0.1 localhost.localdomain localhost

4. 10.0.2.12 qwprlx17 qwprlx17.students.qw.franklin.edu

5. 10.0.2.118 qwprwi04

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/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):1. ip 0 IP # internet protocol, pseudo protocol number2. icmp 1 ICMP # internet control message protocol3. tcp 6 TCP # transmission control protocol4. udp 17 UDP # user datagram protocol5. gre 47 GRE # Generic Routing Encapsulation6. ipv6-crypt 50 IPv6-Crypt # Encryption Header for IPv67. ipv6-icmp 58 IPv6-ICMP # ICMP for IPv6

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/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:1. # service-name port/protocol [aliases ...] [# comment]2. tcpmux 1/tcp # TCP port service multiplexer3. rje 5/tcp # Remote Job Entry4. echo 7/tcp5. systat 11/tcp users6. daytime 13/tcp7. ftp 21/tcp8. ssh 22/tcp # SSH Remote Login Protocol9. telnet 23/tcp

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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

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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’.

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/etc/inetd.conf(inetd, n.d.)

• File format:service type protocol wait user server cmdline

• 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).

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/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.

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/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.

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/etc/inetd.conf(inetd, n.d.)

• Sample File (Subset)1. # 2. # inetd services3. #4. ftp stream tcp nowait root /usr/sbin/ftpd 5. login stream tcp nowait root /usr/sbin/rlogind6. #7. # inetd internal services8. #9. time stream tcp nowait root internal10. time dgram udp nowait root internal11. echo stream tcp nowait root internal12. echo dgram udp nowait root internal

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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”.

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xinetd.conf0001: #0002: # Simple configuration file for xinetd0003: #0004: # Some defaults, and include

/etc/xinetd.d/0005:0006: defaults0007: {0008: instances = 600009: log_type = SYSLOG authpriv0010: log_on_success = HOST PID0011: log_on_failure = HOST0012: cps = 25 300013: }0014:0015: includedir /etc/xinetd.d0016:

• 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.

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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]# lsamanda cups-lpd eklogin ipop3 pop3s services timeamandaidx daytime finger klogin rexec sgi_fam time-udpamidxtape daytime-udp gssftp krb5-telnet rlogin swatchargen dbskkd-cdb imap kshell rsh talkchargen-udp echo imaps ktalk rsync telnetcomsat echo-udp ipop2 ntalk servers tftp

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xinetd.d

• Example: What follows is the configuration file for telnet:0001: # default: on0002: # description: The telnet server serves telnet sessions; it uses \0003: # unencrypted username/password pairs for authentication.0004: service telnet0005: {0006: flags = REUSE0007: socket_type = stream0008: wait = no0009: user = root0010: server = /usr/sbin/in.telnetd0011: log_on_failure += USERID0012: disable = no0013: }0014:

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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

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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)

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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.

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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.

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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.php

3• 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