ECE/CS 4984: Lecture 11

Lecture 10Mobile Networks: Nomadic

Services, DHCP, NAT, and VPNs

Wireless Networks and Mobile Systems

Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 2

Lecture Objectives

● Describe the role of nomadic services in mobile networking

● Describe the objectives and operation of IP virtual private networks (VPNs)

● Describe the objectives and operation of the Dynamic Host Configuration Protocol (DHCP)

● Describe the objectives and operation of network address translation (NAT)

● Describe firewall and packet filter functions, especially as related to NAT

● Provide some high-level background in web services, especially for a wireless “hot spot” service


Agenda

● Nomadic services● Virtual private networks (VPNs)● Dynamic Host Configuration Protocol (DHCP)● Network address translation (NAT)● Firewalls and packet filtering● HTML and web programming● Brief comments on a wireless “hot spot” service


Nomadic Services

● Nomadic services support hosts that attach to different networks, but where host reconfiguration is acceptable■ Compare to mobile services where hosts can move to a

different network without reconfiguring

● Functions ■ Changing the host’s IP address to that of the current

network to which it is attached DHCP■ Limited number of public Internet addresses available in the

current network (or any network) NAT■ Lack of trust of the current network (or any network) VPN

● A wireless “hot spot” usually combines DHCP, NAT, and firewall functions


Nomadic Services Functions

Addressvia DHCP

Secure Data,Private Address

Secure Data,Public Address

• VPN endpoint• VPN endpoint

• DHCP• NAT

Public NetworkPrivate Network Private Network

NomadicNode


Agenda

● Nomadic services

● Virtual private networks (VPNs)● Dynamic Host Configuration Protocol (DHCP)● Network address translation (NAT)● Firewalls and packet filtering● HTML and web programming● Brief comments on a wireless “hot spot” service


Virtual Private Networks (1)

● Virtual private networks (VPNs)■ Enable end-to-end security (authentication and, optionally,

privacy) for a single (mobile) host connecting to a private network over untrusted (public) intermediate networks

■ Enable security for private network-to-network communication over untrusted intermediate networks

■ Support quality-of-service and other attributes of a service level agreement over a shared network for network-to-network connectivity


Virtual Private Networks (2)

● Tunneling protocols■ Point-to-Point Tunneling Protocol (PPTP)■ Layer 2 Tuneling Protocol (L2TP)■ IP Security (IPSec)

VPNClient

VPNServer

GeneralHost

PublicNetwork

PrivateNetwork

Secure Tunnel


Point-to-Point Tunneling Protocol

● PPTP is an extension of the Point-to-Point Protocol (PPP) to support tunneling

● Can carry IP and non-IP packets

Layer 2Header

IPHeader

PPPPacket

GREHeader


Layer 2 Tunneling Protocol

● Resulted from the IETF’s merger of PPTP and the Layer 2 Forwarding Protocol (L2FP)

● Can carry IP and non-IP packets over IP and other networks

Packet Transport (UDP, FR, ATM, etc.)

L2TP Data Channel(unreliable)

L2TP Control Channel(unreliable)

L2TP Data Messages(unreliable)

PPPFrames

L2TP ControlMessages


IP Security

● IPSec has two main components■ Authentication Header (AH)■ Encapsulating Security Payload (ESP)

● Two modes■ Transport mode■ Tunnel mode

IPHeader

AH(or ESP)

IPPayload

Inner IPHeader

Original IP Datagram

TunnelMode


VPN References

K. Hamzeh, G. Pall, W. Verthein, J. Taarud, W. Little, G. Zorn, “Point-to-Point Tunneling Protocol,” RFC 2637, July 1999.

W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn, B. Palter, “Layer Two Tunneling Protocol ‘L2TP’,” RFC 2661, Aug. 1999.

S. Kent, R. Atkinson, “Security Architecture for the Internet Protocol,” RFC 2401, Nov. 1998.

D. Fowler, Virtual Private Networks, Morgan-Kaufmann Publishers, 1999.


Agenda

● Nomadic services● Virtual private networks (VPNs)

● Dynamic Host Configuration Protocol (DHCP)● Network address translation (NAT)● Firewalls and packet filtering● HTML and web programming● Brief comments on a wireless “hot spot” service


DHCP

● DHCP provides all necessary configuration information to allow a stationary node to become a viable Internet host

● Applications■ To simplify system administration in traditional networks■ To improve utilization of IP address space■ To allow mobile hosts to obtain collocated

care-of addresses on foreign networks

R. Droms, “Dynamic Host Configuration Protocol,” RFC 2131, March 1997.

C. E. Perkins, Mobile IP: Design Principles and Practices, Addison-Wesley, Reading, MA, 1998 (Chapter 9).


DHCP: Client-Server Model (1)

● DHCP adheres to a client-server model■ Client requests service■ Server provides response

● Request and reply must be sent without the benefit of the client being an Internet host

DHCPServer

DHCPClient 1

DHCPClient 2

request

reply


DHCP: Client-Server Model (2)

● Client broadcasts request to network■ Broadcast received by server or relay■ If a relay is used, it forwards request with other information

to the server

● Server responds with configuration information● Client acknowledges receipt● Server reserves IP address (for some lease time) and

notifies client that address is reserved● Client must renew the lease


DHCP Initialization (1)

● Client broadcasts a discover message (DHCPDISCOVER)■ Sent via UDP to port 67■ Received by one or more DHCP servers (or relays)

● Responding servers …■ Determine configuration■ Send an offer message (DHCPOFFER) to the client

● Client selects a configuration that it wants■ Sends a request message (DHCPREQUEST) to the selected

server■ Sends the same request message to servers not selected so

they can release reserved IP address



● Selected server …■ Commits configuration■ Replies with an acknowledge message (DHCPACK) to

complete initialization



Server 1(selected)

Client Server 2(not selected)

DHCPDISCOVER DHCPDISCOVER

DHCPOFFERDHCPOFFER

DHCPREQUEST DHCPREQUEST

DHCPACK


Lease and Renewals (1)

● Server grants use of the IP address for a limited time, the lease time

● Client should renew the lease about after about two-thirds of the lease time has expired

● Lease renewal …■ Client sends DHCPREQUEST message to the original

selected server via unicast■ Server responds with DHCPACK message■ If no response from the server, client must start again with

DHCP initialization


Lease and Renewals (2)

Server Client

DHCPREQUEST

DHCPACK


Graceful Shutdown

● Client can perform a graceful shutdown by sending a DHCP release message (DHCPRELEASE) to the server■ Allows server to release reserved IP address

● Often, clients just shutdown and IP address is released after the lease time expires

Server Client

DHCPRELEASE


DHCP Options

● DHCP servers can provide optional information beyond the assigned IP address■ Default router■ Subnet mask■ Network Time Protocol (NTP) servers■ Service Location Protocol (SLP) servers■ Domain Name System (DNS) servers■ Local domain name■ Host name

● Request in discover or request message● Response in offer or acknowledge message

■ Type, Length, Value (TLV) option


Agenda

● Nomadic services● Virtual private networks (VPNs)● Dynamic Host Configuration Protocol (DHCP)

● Network address translation (NAT)● Firewalls and packet filtering● HTML and web programming● Brief comments on a wireless “hot spot” service


Network Address Translation

● NAT “mangles” a packet’s addressing headers as it passes through a router to change either the source or destination address

● Most common form of NAT: Network and port address translation■ A.k.a. IP Masquerading – Linux■ A.k.a. Port Address Translation (PAT) – Cisco


What is Masquerading?

● One-to-many translation● The process of routing Internet-bound traffic from a

private network through a gateway router that modifies the traffic to look like its own

● On the return, the router, demultiplexes the traffic back to the appropriate hosts by source/destination port/address pairs (remembered from transmission)


Example Configuration

● Trace a packet from Host1 to google.com■ IP address: 216.239.39.101

Host1

Host2

Host3

Router

eth1 – 12.34.56.78

Internal Network – 192.168.1.xxx

External Network

.4

.3

.2

eth0 – 192.168.1.254


Packet Trace

● Packet sent to HTTP server at google.com

Interface Src IP Dest IP Src Prt Dest Prt

Host1:eth0 192.168.1.2 216.239.39.101 4356 80

Router:eth0 192.168.1.2 216.239.39.101 4356 80

NAT

Router:eth1 12.34.56.78 216.239.39.101 65013* 80

… routing

Google.com 12.34.56.78 216.239.39.101 65013* 80

*Note: Masquerading changes the source port as well as source address for assured demultiplexing. Value depends on implementation.


Packet Trace (2)

● Returning packet

Interface Src IP Dest IP Src Prt Dest Prt

Google.com 216.239.39.101 12.34.56.78 80 65013

…routing

Router:eth1 216.239.39.101 12.34.56.78 80 65013

NAT

Router:eth0 216.239.39.101 192.168.1.2 80 4356

Host1:eth0 216.239.39.101 192.168.1.2 80 4356


Implementation of Masquerading

● Linux – built into kernel firewall■ Resident for years■ ipfwadm, ipchains, iptables

● Windows – Internet Connection Sharing■ Partially with Microsoft Windows 98SE and Windows ME

(only share certain interfaces)■ Full implementation in Microsoft Windows 2000 and

Windows XP (share any interface)


Agenda

● Nomadic services● Virtual private networks (VPNs)● Dynamic Host Configuration Protocol (DHCP)● Network address translation (NAT)

● Firewalls and packet filtering● HTML and web programming● Brief comments on a wireless “hot spot” service


Firewalls

● Routers with “attitude”● Process packets based on rules● Rules based on any packet characteristics or

attributes■ Source and destination addresses and ports (e.g., source

port 1234 from host 10.0.3.23)■ Protocol flags (e.g., TCP SYN, TCP ACK)■ Protocol types (e.g., ICMP, UDP)■ Connection status (e.g., new or established)


Firewall Services

Data Link

Physical

Transport

Network

Presentation

Session

ApplicationApplication-specific proxy,

Application-specific filter

Gateway, User Filter

Port map, Port filter,Address map, Address filter

Address map, Address filter,Protocol filter

Address filter, Protocol filter


Types of Firewalls (1)

● Two types■ Stateful■ Stateless

● Stateless■ Simple, less secure than stateful■ Makes decisions based on individual packet information■ Does not maintain any connection status■ Example:

○ Allow all traffic inbound with destination port 80○ Deny all traffic from 192.168.1.0/24 on the external

interface


Types of Firewalls (2)

● Stateful■ All the attributes of a stateless firewall plus …■ Connection status (context for decisions)

○ Watches traffic for SYN, ACK, and FIN packets○ Knows connection status (established, initiating)

■ More complex, better security■ Example:

○ Deny all ICMP Echo Reply packets not associated with an Echo Request

○ Deny all TCP sessions not initiated from the inside network


Firewall Implementations

● Implementations■ Hardware and software

● Hardware (network devices)■ Cisco PIX, Sonicwall, Watchguard Firebox

● Software (applications)■ Windows – ZoneAlarm, Norton Personal Firewall, BlackICE■ Unix and variants – ipfw, ipchains, iptables, ipf


iptables (1)

● Linux firewall (and more)● Present with the 2.4 series kernel● Part of the netfilter project

■ http://www.netfilter.org/

● Consists of two parts■ Firewall code in the kernel■ User space “iptables” executable to manipulate kernel code

Oskar Andreasson, Iptables Tutorial 1.1.19,http://iptables-tutorial.frozentux.net/.


iptables (2)

● Three parts■ Rules■ Chains■ Tables


iptables (3)

● Rule■ Lowest-level (most basic) entity in firewalling■ A single tuple of what to do (action) and packets to which to

apply the action (filter)■ Filter – identifies packets to which the rule applies

○ Addresses, ports, status■ Action – what to do with the packet (stream)

○ Accept, reject (drop, but reply with ICMP error message), drop, redirect, masquerade, go to another chain, and more


iptables (4)

● Chains■ An ordered list of rules■ Traversed in order■ The first matching rule in the chain is selected■ Important predefined chains in FILTER table

○ INPUT – all incoming packets go here○ FORWARD – packets to be routed○ OUTPUT – all outgoing packets go here


iptables (5)

● Tables■ Separate different types of operations■ Three built-in tables

○ FILTER – general filtering ○ NAT – dealing with network address translation○ MANGLE – other packet changes

■ Each contain multiple chains


iptables (6)

● Incoming packet traversal

NetworkMangle

PREROUTINGNat

PREROUTING

RoutingDecision

MangleFORWARD

MangleINPUT

FilterINPUT

FilterFORWARD

Application *to output*

Local Non-Local

Example:Setting DSCP

Example:Redirecting

Example:Typical Firewall

FunctionsExample:Typical Firewall

Functions


iptables (7)

● Outgoing packet traversal

Application

MangleOUTPUT

RoutingDecision

NatOUTPUT

FilterOUTPUT

ManglePOSTROUTING

NatPOSTROUTING

Network

*from non-localinput*

Example:IP Masquerading

Example: Typical Firewall

Functions


iptables (8)

● Rule placement■ Rule type specifies table

○ Address translation and IP masquerading map to the NAT table

○ Simple packet filtering maps to the filter table■ Rule stage specifies chain

○ Prerouting versus postrouting○ Traffic from local application versus forwarded traffic


Firewall Comments

● “Good” firewall rules are difficult to write■ Must consider all possible traffic■ Only allow what should pass

● Stateful firewalls are more secure (and more complex) than stateless firewalls

● Stepping forward■ Intrusion Detection System (IDS) – “smarter” stateful firewall


Agenda

● Nomadic services● Virtual private networks (VPNs)● Dynamic Host Configuration Protocol (DHCP)● Network address translation (NAT)● Firewalls and packet filtering

● HTML and web programming● Brief comments on a wireless “hot spot” service


Web-Based Authentication

● Consider a wireless LAN “hot spot” service● This will require consideration and use of…

■ DHCP■ Firewalling■ Authentication■ IP masquerading (NAT)

● Authentication is commonly done using a web-based scheme − here is one approach…■ The first attempt to access any web page is redirected to an

authentication page for the service■ A script or program must perform authentication and

updates the configuration to allow access, if appropriate


HTML

● HyperText Markup Language (HTML)■ Web page “language” (content)■ Currently in version 4.01■ Maintained by the World Wide Web Consortium (W3C)

○ http://www.w3c.org■ Uses “tags”: <begin_tag>text</end_tag>■ Formatting language

○ Take data and add formatting, pictures, input, and/or links


HTML (2)

● Many extensions and add-ons■ Responsible for rich web content

● Tags interpreted by web browser; no server processing involved

● May be edited by hand or with a WYSWYG editor■ By hand: notepad, emacs, vi■ WYSWYG: MS Frontpage, Dreamweaver


Web Programming

● Common Gateway Interface (CGI)■ A way for web servers to interact with standard programs to

generate dynamic web content■ Input typically HTML form data■ Output dynamic content (web pages) ■ Can be written using C++, Perl, Fortran, or PHP■ Can do many functions with the appropriate library

(1) URL, param

(5) HTML, text, …

(2) CGI

(4) HTML, text, …

WebBrowser

HTTPServer

GatewayProgram

(3)

Pro

ce

ss


Web Programming (2)

● Model■ Client request■ Server reference■ Server processing (CGI, SSI, PHP)■ Request sent to client■ Browser processing (JavaScript, HTML, CSS)


No Experience?

● PHP suggested for those with no experience with web programming

● PHP code is embedded in HTML code■ No compilation■ Quick editing

● Familiar syntax■ Borrows syntax “look and feel” from Java, Perl, and C++


Agenda

● Nomadic services● Virtual private networks (VPNs)● Dynamic Host Configuration Protocol (DHCP)● Network address translation (NAT)● Firewalls and packet filtering● HTML and web programming

● Brief comments on a wireless “hot spot” service


A Test Network Configuration

● DHCP server● Firewall● IP masquerading● Web-based authentication

Private Network

“Public” Internet

PrivatePublic


Summary

● Nomadic services enable Internet access■ Security, addressing, filtering

● VPNs provide authentication and privacy for nomadic users and protect private networks

● DHCP allows nomadic users to obtain an IP address and other configuration information

● NAT conserves addresses in private networks, allowing support for nomadic hosts

● Firewalls and packet filtering provide security and enable access control

● HTML and web programming can be used to authenticate nomadic users for a hot spot service

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ECE/CS 4984: Lecture 11

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