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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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 3
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 4
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 5
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 6
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 7
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 8
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 9
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 10
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 11
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 12
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.
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 13
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 14
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).
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 15
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 16
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 17
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 18
DHCP Initialization (2)
● Selected server …■ Commits configuration■ Replies with an acknowledge message (DHCPACK) to
complete initialization
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 19
DHCP Initialization (3)
Server 1(selected)
Client Server 2(not selected)
DHCPDISCOVER DHCPDISCOVER
DHCPOFFERDHCPOFFER
DHCPREQUEST DHCPREQUEST
DHCPACK
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 20
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 21
Lease and Renewals (2)
Server Client
DHCPREQUEST
DHCPACK
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 22
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 23
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 24
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 25
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 26
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)
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 27
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 28
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.
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 29
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 30
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)
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 31
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 32
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)
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 33
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 34
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 35
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 36
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 37
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/.
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 38
iptables (2)
● Three parts■ Rules■ Chains■ Tables
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 39
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 40
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 41
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 42
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 43
iptables (7)
● Outgoing packet traversal
Application
MangleOUTPUT
RoutingDecision
NatOUTPUT
FilterOUTPUT
ManglePOSTROUTING
NatPOSTROUTING
Network
*from non-localinput*
Example:IP Masquerading
Example: Typical Firewall
Functions
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 44
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 45
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 46
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 47
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 48
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 49
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 50
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 51
Web Programming (2)
● Model■ Client request■ Server reference■ Server processing (CGI, SSI, PHP)■ Request sent to client■ Browser processing (JavaScript, HTML, CSS)
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 52
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++
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 53
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
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 54
A Test Network Configuration
● DHCP server● Firewall● IP masquerading● Web-based authentication
Private Network
“Public” Internet
PrivatePublic
Mobile Networks: Nomadic Services, DHCP, NAT, and VPNs 55
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