COMS/CSEE 4140 Networking Laboratory Lecture 08 Salman Abdul Baset Spring 2008.

COMS/CSEE 4140 Networking Laboratory

Lecture 08

Salman Abdul BasetSpring 2008

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Announcements Prelab 7 and Lab report 6 due next week

before your lab slot Assignment 3 due next week Monday Project groups

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Last time… Interconnection devices (hub,

bridge/switch, router) Bridges/LAN switches vs. routers Bridge concepts, PDU Spanning tree algorithm Linux packet reception

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Agenda Private network and addresses NAT (Network Address Translator)

Basic operation Issues (binding, filtering, state maintenance) Main uses of NAT

Dynamic Host Configuration Protocol (DHCP)

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Private Network Private IP network is an IP network with private IP

addresses

IP addresses in a private network can be assigned arbitrarily but they are usually picked from the reserved pool (can we use any?) Not registered and not guaranteed to be globally unique

Generally, private networks use addresses from the following experimental address ranges (non-routable addresses): 10.0.0.0 – 10.255.255.255 172.16.0.0 – 172.31.255.255 192.168.0.0 – 192.168.255.255

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Private Addresses

H1

R1

H2

10.0.1.3

10.0.1.1

10.0.1.2

H3

R2

H4

10.0.1.310.0.1.2

Private network 1

Internet

H5

10.0.1.1Private network 1

213.168.112.3

128.195.4.119 128.143.71.21

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A hack to fix the IP address depletion problem. NAT is a router function where IP addresses (and possibly

port numbers) of IP datagrams are replaced at the boundary of a private network.

Breaks the End-to-End argument. RFC 1631 - The IP Network Address Translator (NAT) Not an Internet standard (RFC 3700) but…

Provides a form of security by acting as a firewall Home users Small companies

Network Address Translator

Other solutions to the IP address problem are?

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Basic Operation of NAT

NAT device stores the address and port translation tables (Binding) In the this example we mapped only addresses.

NAT devices filters incoming traffic (Filtering)

•Host

•private address: 10.0.1.2•public address: 128.143.71.21

Public Host

•Private Network •Internet

64.236.24.4

NATDevice

PrivateAddress

PublicAddress

10.0.1.1 128.59.16.21

•Source = 10.0.1.2•Destination = 64.236.24.4

•Source = 10.0.1.2•Destination = 64.236.24.4•Source = 128.143.71.21•Destination = 64.236.24.4




•Source = 64.236.24.4•Destination = 128.59.16.21•Source = 64.236.24.4•Destination = 128.59.16.21•Source = 64.236.24.4•Destination = 128.59.16.21•Source = 64.236.24.4•Destination = 10.0.0.2

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NAT Issues Private-to-public address mapping

Static NAT Dynamic NAT Overloading (NAPT or PAT)

State maintenance Linux: /proc/net/ip_conntrack

Binding and Filtering Behavior Binding: endpoint-independent, address

dependent, address and port-dependent Filtering: endpoint-independent filtering,

address dependent filtering, address and port-dependent filtering.

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Static mapping

Dynamic mapping

NAPT/PAT

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Binding: Endpoint-independent

NATN1:n1Y1:y1N1:n1 Y1:y2N1:n1 Z1:z1N1:n1 Z1:z2

IPaddr: X1

IPaddr: Y1

IPaddr: Z1

Endpoint-independent mapping

X1:x1 Y1:y1

X1:x1 Z1:z1X1:x1 Y1:y2

X1:x1 Z1:z2

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Binding: Address-dependent

NAT

X1:x1 Y1:y1

X1:x1 Z1:z1

N1:n1 Y1:y1N1:n1 Y1:y2N1:n2 Z1:z1N1:n2 Z1:z2

IPaddr: X1

IPaddr: Y1

IPaddr: Z1

Address-dependent mapping

X1:x1 Y1:y2

X1:x1 Z1:z2

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Binding: Address and port dependent

NAT

N1:n1 Y1:y1N1:n2 Y1:y2N1:n3 Z1:z1N1:n4 Z1:z2

IPaddr: X1

IPaddr: Y1

IPaddr: Z1

Address and port dependent

X1:x1 Y1:y1

X1:x1 Z1:z1X1:x1 Y1:y2

X1:x1 Z1:z2

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Filtering: Endpoint-independent

NAT

IPaddr: X1

IPaddr: Y1

IPaddr: Z1

X1:x1 N1:n1 Y1:y1

Y1:y2 N1:n1

Z1:z1 N1:n1

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Filtering: Address-dependent

NAT

IPaddr: X1

IPaddr: Y1

IPaddr: Z1

(1) X1:x1 N1:n1 Y1:y1

Y1:y2 N1:n1

Z1:z1 N1:n1

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Filtering: Address and port dependent

NAT

IPaddr: X1

IPaddr: Y1

IPaddr: Z1

(1) X1:x1 N1:n1 Y1:y1

Z1:z1 N1:n1

Y1:y1 N1:n1

Y1:y2 N1:n1

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NAT Issues Port preserving Hair pinning

Discovering binding lifetime

NAT

IPaddr: X1

IPaddr: X2

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Main uses of NAT Pooling of IP addresses

Supporting migration between network service providers

IP masquerading and internal firewall

Load balancing of servers

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Pooling of IP addresses Scenario: Corporate network has many hosts

but only a small number of public IP addresses.

NAT solution: Corporate network is managed with a private address

space.

NAT device, located at the boundary between the corporate network and the public Internet, manages a pool of public IP addresses.

When a host from the corporate network sends an IP datagram to a host in the public Internet, the NAT device picks a public IP address from the address pool, and binds this address to the private address of the host.

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Pooling of IP addresses

•Host

•private address: 10.0.1.2•public address: 128.143.71.21

•Private Network •Internet

Public Host64.236.24.4

NATDevice

PrivateAddress

PublicAddress

10.0.1.2 128.59.16.21


•Source = 10.0.1.2•Destination = 64.236.24.4•Source = 128.143.71.21•Destination = 64.236.24.4


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Supporting migration between network service providers Scenario: In practice (using CIDR), the IP

addresses in a corporate network are obtained from the service provider. Changing the service provider requires changing all IP addresses in the network.

NAT solution: Assign private addresses to the hosts of the corporate

network NAT device has address translation entries which bind

the private address of a host to the public address. Migration to a new network service provider merely

requires an update of the NAT device. The migration is not noticeable to the hosts on the network.

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Host

private address: 10.0.1.2public address: 128.14.71.21

Source = 10.0.1.2Destination = 213.168.112.3

NAT device

Private Address

PublicAddress

10.0.1.2 128.14.71.21

128.14.71.21


Private network

ISP 1allocates address

block 128.14.71.0/24 to private network:

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Host


150.140.4.120


NAT device

Private Address

PublicAddress

10.0.1.2128.14.71.21150.140.4.120

128.14.71.21150.140.4.120


ISP 2allocates address block

150.140.4.0/24 to private network:

Private network

ISP 1allocates address

block 128.14.71.0/24 to private network:

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IP masquerading Also called: Network address and port

translation (NAPT), port address translation (PAT).

Scenario: Single public IP address is mapped to multiple hosts in a private network.

NAT solution: Assign private addresses to the hosts of the

corporate network NAT device modifies the port numbers for outgoing

traffic

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IP masquerading

NAT device

Host 2

private address: 10.0.1.2

Private network

Source = 10.0.1.2Source port = 2001


Private Address

PublicAddress

10.0.1.2/2001 128.143.71.21/80

10.0.1.3/3020 128.143.71.21/4444

Host 1

private address: 10.0.1.3


Internet

Source = 128.59.71.21Destination = 4444

128.16.71.2110.0.0.1

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Load balancing of servers Scenario: Balance the load on a set of

identical servers, which are accessible from a single IP address

NAT solution: Here, the servers are assigned private addresses NAT device acts as a proxy for requests to the

server from the public network The NAT device changes the destination IP address

of arriving packets to one of the private addresses for a server

A sensible strategy for balancing the load of the servers is to assign the addresses of the servers in a round-robin fashion.

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Load balancing of servers

Private network


NAT device

Private Address

PublicAddress

10.0.1.2 128.59.71.21

Inside network

10.0.1.4 128.59.71.21

Internet128.59.71.21

S1

S2

S3

10.0.1.4

10.0.1.3

10.0.1.2

PublicAddress

64.30.4.120

Outside network

101.248.22.3


When does this work?When does this fail?

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Concerns about NAT Performance

Modifying the IP header by changing the IP address requires that NAT boxes recalculate the IP header checksum.

Modifying port number requires that NAT boxes recalculate TCP checksum.

Fragmentation Care must be taken that a datagram that is

fragmented before it reaches the NAT device, is not assigned a different IP address or different port numbers for each of the fragments.

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Concerns about NAT End-to-end connectivity

NAT destroys universal end-to-end reachability of hosts on the Internet.

A host in the public Internet often cannot initiate communication to a host in a private network.

The problem is worse, when two hosts that are in a private network need to communicate with each other.

NAT and applications NAT break applications such as file transfer, VoIP

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NAT and FTP

H1 H2

public address:128.143.72.21

FTP client FTP server

PORT 128.143.72.21/1027

200 PORT command successful

public address:128.195.4.120

RETR myfile

150 Opening data connection

establish data connection

Normal FTP operation

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NAT and FTP

H1

Private network

NAT device

H2


Internet


PORT 10.0.1.3/1027 PORT 128.143.72.21/1027

200 PORT command successful200 PORT command successful

RETR myfile RETR myfile

150 Opening data connection

NAT device without FTP support

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NAT and FTP

NAT device with FTP support

H1

Private network

NAT device

H2


Internet


PORT 10.0.1.3/1027 PORT 128.143.72.21/1027

200 PORT command successful200 PORT command successful

RETR myfile


RETR myfile

150 Opening data connection150 Opening data connection


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Configuring NAT/firewall in Linux iptables Table (queue)

Filter, NAT, Mangle Chain

Place within the table where firewall/NAT rules are placed.

Packets pass through chains where tables are looked up and a decision per packet is made.

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Configuring NAT/firewall in LinuxQueue Type

Queue Function

Packet transformation chain

Chain Function

Filter Packet filtering

FORWARD Packets being forwarded

INPUT Packets destined for firewall

OUTPUT Packets originating from firewall

NAT Network address translation

PREROUTING Address translation occurs before routing (DNAT)

POSTROUTING Address translation occurs after routing (SNAT)

OUTPUT Address translation for packets generated by firewall

Mangle TCP header modification

PREROUTING POSTROUTING OUTPUT INPUT FORWARD

Modification of TCP quality of service bits before routing

35Source: http://www.linuxhomenetworking.com/wiki/index.php/Quick_HOWTO_:_Ch14_:_Linux_Firewalls_Using_iptables#What_Is_iptables.3F

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Configuring NAT in Linux Linux uses the Netfilter/iptable Kernel

package

Incomingdatagram

filterINPUT

Destinationis local?

filterFORW ARD

natOUTPUT

To application From application

Outgoingdatagram

natPOSTROUTING

(SNAT)

No

Yes filterOUTPUT

natPREROUTING

(DNAT)

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Configuring NAT with iptables First example:

iptables –t nat –A POSTROUTING –s 10.0.1.2 –j SNAT --to-source 128.16.71.21

Pooling of IP addresses:iptables –t nat –A POSTROUTING –s 10.0.1.0/24 –j SNAT --to-source 128.16.71.0–128.16.71.30

IP masquerading: iptables –t nat –A POSTROUTING –s 10.0.1.0/24 –o eth1 –j MASQUERADE

Load balancing:iptables -t nat -A PREROUTING -i eth1 -j DNAT --to-destination 10.0.1.2-10.0.1.4

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Agenda Private network and addresses NAT (Network Address Translator)

Basic operation Issues (binding, filtering, state maintenance) Main uses of NAT

Dynamic Host Configuration Protocol (DHCP)

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Dynamic Assignment of IP addresses Dynamic assignment of IP addresses is

desirable for several reasons: IP addresses are assigned on-demand Avoid manual IP configuration Support mobility of laptops / handheld WiFi

devices

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Solutions for dynamic assignment of IP addresses Reverse Address Resolution

Protocol (RARP) Works similar to ARP Broadcast a request for the IP address

associated with a given MAC address RARP server responds with an IP address Only assigns IP address (not the default

router and subnet mask)

RARP

Ethernet MACaddress(48 bit)

ARPIP address(32 bit)

Why not a good solution?

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BOOTP (RFC 951) BOOTstrap Protocol (BOOTP)

Predecessor of DHCP Host can configure its IP parameters at boot time. Three services

IP address assignment. Detection of the IP address for a serving machine. The name of a file to be loaded and executed by the client

machine (boot file name) Not only assign IP address, but also default router,

network mask, etc. Sent as UDP messages (UDP Port 67 (server) and 68

(host)) Use limited broadcast address (255.255.255.255):

These addresses are never forwarded

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DHCP Dynamic Host Configuration Protocol

(DHCP) From 1993 An extension of BOOTP, very similar to DHCP Same port numbers as BOOTP Extensions:

Supports temporary allocation (“leases”) of IP addresses DHCP client can acquire all IP configuration parameters

needed to operate DHCP is the preferred mechanism for dynamic

assignment of IP addresses DHCP can interoperate with BOOTP clients.

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DHCP Interaction (simplified)Argon

00:a0:24:71:e4:44 DHCP Server

DHCP Request00:a0:24:71:e4:44Sent to 255.255.255.255

Argon128.143.137.144

00:a0:24:71:e4:44 DHCP ServerDHCP Response:IP address: 128.143.137.144Default gateway: 128.143.137.1Netmask: 255.255.0.0

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BOOTP/DHCP Message Format

Number of Seconds

OpCode Hardware Type

Your IP address

Unused (in BOOTP)Flags (in DHCP)

Gateway IP address

Client IP address

Server IP address

Hardware Address Length

Hop Count

Server host name (64 bytes)

Client hardware address (16 bytes)

Boot file name (128 bytes)

Transaction ID

Options

(There are >100 different options !!!)

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BOOTP/DHCP OpCode: 1 (Request), 2(Reply)

Note: DHCP message type is sent in an option

Hardware Type: 1 (for Ethernet) Hardware address length: 6 (for Ethernet) Hop count: set to 0 by client Transaction ID: Integer (used to match reply to response) Seconds: number of seconds since the client started to

boot Client IP address, Your IP address, server IP address,

Gateway IP address, client hardware address, server host name, boot file name: client fills in the information that it has, leaves rest blank

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DHCP Message Type Message type is sent

as an option. Value Message Type

1 DHCPDISCOVER

2 DHCPOFFER

3 DHCPREQUEST

4 DHCPDECLINE

5 DHCPACK

6 DHCPNAK

7 DHCPRELEASE

8 DHCPINFORM

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Other options (selection) Other DHCP information that is sent as an

option:

Subnet Mask, Name Server, Hostname, Domain Name, Forward On/Off, Default IP TTL, Broadcast Address, Static Route, Ethernet Encapsulation, X Window Manager, X Window Font, DHCP Msg Type, DHCP Renewal Time, DHCP Rebinding, Time SMTP-Server, SMTP-Server, Client FQDN, Printer Name, …

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DHCP Operation

DHCP DISCOVER

DHCP Client00:a0:24:71:e4:44 DHCP Server

DHCPDISCOVERSent to 255.255.255.255

DHCP Server


DHCP Server

DHCPOFFER

DHCPOFFER DHCP OFFER

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DHCP OperationDHCP Client


DHCP Server

DHCPREQUEST

DHCPACK DHCP DISCOVER

At this time, the DHCP client can start to use the IP address


DHCP Server

DHCPREQUEST

DHCPACK

Renewing a Lease(sent when 50% of lease has expired)

If DHCP server sends DHCPNACK, then address is released.

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DHCP OperationDHCP Client


DHCP Server

DHCPRELEASE

DCHP RELEASE

At this time, the DHCP client has released the IP address

COMS/CSEE 4140 Networking Laboratory Lecture 08 Salman Abdul Baset Spring 2008.

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