scis.regis.edu ● scis@regis.edu

CS 468: Advanced UNIXClass 7

Dr. Jesús BorregoRegis University

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Topics

•IA Lab visit was scheduled for last week; rescheduled for next week

•Review of Homework 5•Networking•Homework 6•Review for final•Q&A

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Networking

•Computers connecting to other computers•Computers connecting to the Internet•Network Topologies•Network Devices•Internetworking•Communicating with Users•Data distribution•NFS

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Communication model

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CommunicationsInfrastructure

Source

Transmitter

Transmission System

Receiver

Destination

OSI Model

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Physical

Data Link

Network

Transport

Session

Presentation

Application

Internet Protocol Stack

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Transport

Physical

Data Link Control

Network/ Internet

Transport

Application

OSI vs. IP

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Physical

Data Link

Network

Transport

Session

Presentation

Application

Physical

Data Link Control

Network/ Internet

Transport

Application

Ethernet

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100 Mbps

100 Mbps

100 Mbps

1 Gbps

server

Ethernetswitch

institutionalrouter

to institution’sISP

typically used in companies, universities, etc▫ 10 Mbps, 100Mbps, 1Gbps, 10Gbps Ethernet▫ today, end systems typically connect into

Ethernet switch

Bridges

•Connects separate networks▫One Ethernet network to another one▫“Bridges” two network segments together▫Makes it appear as if the two segments

were a single one•Wire length is limited due to degradation

of signal•Bridges allow extension of physical

limitation of wire

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Routers

•Bridges cannot accommodate large networks

•Routers connect two or more networks▫“Routes” incoming messages to

appropriate network•Can be used to connect a LAN to an ISP

(Internet Service Provider)•Can be used to link the different networks

in the global Internet

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Gateway

•Used to connect remote LANs to a WAN (Wide Area Network)

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Topologies

Typical LAN topologies include:•Bus

▫Single link for all computers•Ring (Token)

▫Each computer connected to at least 2 other computers

•Star▫Central server

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Internetworking – Packet Switching

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sequence of A & B packets has no fixed timing pattern

Time Division Multiplexing (TDM)

A

B

C100 Mb/sEthernet

1.5 Mb/s

D E

statistical multiplexing

queue of packetswaiting for output

link

Internet Address

IPv4 – 4 Octets•Class A: 0*.*.*.* (two reserved, all 0’s and all 1’s)•Class B: 10*.*.*.*•Class C: 110*.*.*.*•Class D: 111*.*.*.*•Class E: 1111*.*.*.*Many subnet calculators available online

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Ports and common services

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Users in your system

•users – local host users•rusers - users on local network•who – more info than users•rwho – more info than rusers•w – more information than who•whois – information about major internet

sites•hostname – displays local host name•finger – lists information about a user

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Finger utility

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User communication on a network•write – send individual lines to user•talk – interactive split screen two-way chat

•wall – send messages to all users on local host

•mail – send email messages•mesg – disables incoming messages to your terminal

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File transfer on network

•Rcp (remote copy) – copy files from one host to another

•uucp (unix-to-unix copy) – like rcp, copies files between two hosts

•ftp (file transfer protocol/program) – copy files between local host and other hosts

•Commands for ftp: UPU page 338

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Distributed access commands

•rlogin – provides login to remote servers

•rsh – execute shell commands on remote Unix hosts

•telnet – executes commands on remote telnet hosts

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Network File System (NFS)

•Public domain specification developed by Sun Microsystems

•Allows you to mount several local file systems into a single network file hierarchy

•Provides remote mount capability•Uses RPC to mount a file system on

remote machine

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Internet control

•ICANN – Internet Corporation for Assigned Names and Numbers – allocates names and domains

•ISOC – Internet Society – represents Internet users, technical advisory society

•IGF – Internet Governance Forum, global forum established by the United Nations in 2005

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Network Standards

RFC: Request for Comments•RFC 114/959: A File Transfer Protocol •RFC 791: Internet Protocol•RFC 793: Transmission Control Protocol•RFC 1945: Hypertext Transfer Protocol HTTP 1.0•RFC 2251: Lightweight Directory Access Protocol•RFC 2460: Internet Protocol v6 (IPv6)•RFC 4251: Secure Shell (SSH) Protocol Architecture

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Internet Protocols

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Internet Protocols

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BGP - Border Gateway Protocol

FTP - File Transfer Protocol

HTTP - Hypertext Transfer Protocol

ICMP - Internet Control Message Protocol

IGMP - Internet Group Management Protocol

IP - Internet Protocol

MIME - Multipurpose Internet Mail Extension

Source: Stallings, W. (2007). Data and computer communications (8th ed.). Upper Saddle River, NJ: Pearson Prentice Hall.

Internet Protocols (Cont’d)

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OSPF – Open Shortest Path FirstRSVP – Resource ReSerVation

ProtocolSMTP – Simple Mail Transfer

ProtocolSNMP – Simple Network

Management ProtocolTCP – Transmission Control

ProtocolUDP – User Datagram Protocol

Source: Stallings, W. (2007). Data and computer communications (8th ed.). Upper Saddle River, NJ: Pearson Prentice Hall.

Sample Flow

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Server

Application

Presentation

Session

Transport

Network

Data Link

Physical

Data

Data

Data

Data

Data

Data

Data

Server

Application

Presentation

Session

Transport

Network

Data Link

Physical Data

Data

Data

Data

Data

Data

Data

IPv4

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IPv6

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Internet Addressing•Media Access Control (MAC): used by

hardware•IPv4 and IPv6 used by software to

determine source, destination, and component location (NIC, not computers)

•Hostnames used by people•Data link layer maps IPs to hardware•Hostnames can map names to IPs

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Sample subnet calculator

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Source:http://www.subnetonline.com/pages/subnet-calculators/ip-subnet-calculator.php

CIDRClassless Inter-Domain Routing•Netmasks that do not end in a byte boundary•Each byte has 8 bits•To subnet 128.138.243.0 with 26 bits – not a byte boundary (8, 16, 24), we use the convention 128.138.243.0/26

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26 bits 6 bits

CIDR Calculator

39Source: http://www.subnet-calculator.com/cidr.php

NAT•Private addresses can be used

internally by an organization•NAT captures internal addresses and

prevents them from exiting the corporate environment

•NAT maintains a table of internal versus external addresses to ensure that no internal addresses escape to the global Internet

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Routing•The process of determining the

output path for an incoming packet•Routing tables are maintained in the

kernel and also in routers throughout the Internet

•If the server does not know where to send it next, it uses the Address Resolution Protocol to determine next action

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Routing

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1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding table

header value output link

0100010101111001

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ARP•ARP discovers the hardware address

associated with an IP address•If the destination address is not in the

same network, ARP determines the next hop router

•If address is not known, it send a broadcast message “Does anybody know where X is?”

•Response is received and then the protocol uses the response

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DHCP•Dynamic Host Configuration Protocol

(RFC 2131)•When a host connects to a network, it

obtains a ‘lease’ on an IP address, gateways, DNS name servers, Syslog hosts, and others.

•If the lease is not renewed, it expires

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DHCP interaction

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DHCP server: 223.1.2.5 arriving client

time

DHCP discover

src : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

DHCP offer

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

Security Issues•Default IP forwarding on a server

should be disabled to prevent the server to act as a router

•ICMP redirect (you should not send packet to me, send to XYZ) can compromise system

•Source routing can slip through firewalls▫Do not want to accept or forward

source-routed packets

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Security Issues (Cont’d)•IP spoofing means changing source or

destination in packet header▫Receiver may believe source and reply

to a malicious server (man-in-the-middle attack)

•Host-based firewalls are preferred to client-based firewalls

•VPN – allow remote uses to create ‘tunnels’ to the private network▫Requires encryption

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Virtual Private Network (VPN)•IP spoofing means changing source or

destination in packet header▫Receiver may believe source and reply

to a malicious server (man-in-the-middle attack)

•Host-based firewalls are preferred to client-based firewalls

•VPN – allow remote uses to create ‘tunnels’ to the private network▫Requires encryption

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VPN

Routing•Routing has different meanings:

▫Actual forwarding packets▫Management of routing tables

•Routing consists of determining the ‘next hop’ in the route towards the destination

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Routing Daemons•Routing daemons collect information

from three sources:▫Configuration files▫Existing routing tables▫Routing daemons on other systems

•Daemons collect this information to determine optimal route and new routes are added to routing tables

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Homework 6 (last)1. What is the difference between a bridge, a

router, and a gateway?2. Describe 3 Internet Protocols and provide

examples of where they can be used.3. What are the differences between ftp and rcp?

Which one is better and why?4. Explain how ICMP redirection can cause

vulnerabilities in a network.5. Using an IP subnet calculator of your choice,

answer the questions in E14.3 found in the USAH book. Capture the calculator screen.

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Review for Final

•Same format as Midterm▫2 hour, take home▫8 questions▫Email to jborrego@regis.edu by midnight

4/26▫All material from week 4-7

•Week 8: 2 hour class, 2 hour take home final

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Questions?

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