1 network address translation (nat) and dynamic host configuration protocol (dhcp) relates to lab 7....

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Network Address Translation (NAT) and Dynamic Host Configuration Protocol (DHCP)

Relates to Lab 7.

Module about private networks and NAT.

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Network address translation: why?

• A fix to 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

• We’ve learned another solution. What is it?

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Basic operation of NAT

• NAT device has address translation table

•H1

•private address: 10.0.1.2•public address: 128.143.71.21

•H5

•Private•network

•Internet

•Source •= 10.0.1.2•Destination •= 213.168.112.3


•public address: • 213.168.112.3•NAT•device



•Private•Address

•Public•Address

•10.0.1.2 •128.143.71.21

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Private Network

• Private IP network is an IP network that is not directly connected to the Internet

• IP addresses in a private network can be assigned arbitrarily. – Not registered and not guaranteed to be globally unique– Question: how is public IP address assigned?

• 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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Main uses of NAT

• Pooling of IP addresses

• Supporting migration between network service providers

• IP masquerading

• 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

H1

private address: 10.0.1.2public address:

H5

Privatenetwork

Internet

Source = 10.0.1.2Destination = 213.168.112.3


public address: 213.168.112.3NATdevice

PrivateAddress

PublicAddress

10.0.1.2

Pool of addresses: 128.143.71.0-128.143.71.30

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Supporting migration between network service providers

• Scenario: In 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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Supporting migration between network service providers

H1

private address: 10.0.1.2public address: 128.143.71.21

128.195.4.120


NATdevice

PrivateAddress

PublicAddress

10.0.1.2128.143.71.21128.195.4.120

128.143.71.21

128.195.4.120



ISP 2allocates address block

128.195.4.0/24 to privatenetwork:

Privatenetwork

ISP 1allocates address block

128.143.71.0/24 to privatenetwork:

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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

H1

private address: 10.0.1.2

Private network

Source = 10.0.1.2Source port = 2001


NATdevice

PrivateAddress

PublicAddress

10.0.1.2/2001 128.143.71.21/2100

10.0.1.3/3020 128.143.71.21/4444

H2

private address: 10.0.1.3


Internet

Source = 128.143.71.21Destination = 4444

128.143.71.21

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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


NATdevice

PrivateAddress

PublicAddress

10.0.1.2 128.143.71.21

Inside network

10.0.1.4 128.143.71.21

Internet128.143.71.21

S1

S2

S3

10.0.1.4

10.0.1.3

10.0.1.2

PublicAddress

128.195.4.120

Outside network

213.168.12.3


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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.

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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


RETR myfile

150 Opening data connection

establish data connection

• Normal FTP operation

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NAT and FTP

• NAT device with FTP support

H1

Private network

NATdevice

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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NAT and FTP

• FTP in passive mode and NAT.

H1

Private network

NATdevice

H2


Internet


PASV PASV

Entering Passive Mode128.195.4.120/10001

Entering Passive Mode128.195.4.120/10001


Establish data connection Establish data connection

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Configuring NAT in Linux

• Linux uses the Netfilter/iptable package to add filtering rules to the IP module

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 iptable

• First example:iptables –t nat –A POSTROUTING –s 10.0.1.2 –j SNAT --to-source 128.143.71.21

• Pooling of IP addresses:iptables –t nat –A POSTROUTING –s 10.0.1.0/24 –j SNAT --to-source 128.128.71.0–128.143.71.30

• 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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Dynamic Host Configuration Protocol (DHCP)

Relates to Lab 7.

Module about dynamic assignment of IP addresses with 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

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DHCP

• Dynamic Host Configuration Protocol (DHCP) – Designed in 1993– Supports temporary allocation (“leases”) of IP addresses– DHCP client can acquire all IP configuration parameters

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DHCP Interaction (simplified)

Argon00: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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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 AddressLength

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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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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DHCP operations

Src: 0.0.0.0, 68Dest: 255.255.255.255, 67DHCPDISCOVERYYiaddr: 0.0.0.0Transaction ID: 654

Src:128.195.31.1, 67

DHCPOFFERYiaddr: 128.195.31.147Transaction ID: 654

Dest: 255.255.255.255, 68

Lifetime: 3600 secsServer ID: 128.195.31.1

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DHCP operations

Src: 0.0.0.0, 68Dest: 255.255.255.255, 67DHCPREQUESTYiaddr: 128.195.31.147Transaction ID: 655server ID: 128.195.31.1Lifetime: 3600 secs

Src:128.195.31.1, 67

DHCPACKYiaddr: 128.195.31.147Transaction ID: 655

Dest: 255.255.255.255, 68

Lifetime: 3600 secsServer ID: 128.195.31.1

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More on DHCP operations

• A client may receive DCHP offers from multiple servers• The DHCPREQUEST message accepts offers from one

server.• Other servers who receive this message considers it as a

decline• A client can use its address after receiving DHCPACK• DHCP replies can be unicast, depending on implmentation

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DHCP relay agent

DHCPDISCOVERGiaddr: 0

Src: 0.0.0.0., 68Dest: 255.255.255.255, 67

128.195.31.1 128.195.41.1

DHCPDISCOVERGiaddr: 128.195.41.1

Src: 0.0.0.0., 68Dest: 255.255.255.255, 67

DHCPOFFER

……

Giaddr: 128.195.41.1

Src: 128.195.31.10, 67Dest: 128.195.41.1, 67

DHCPOFFER

……

Giaddr: 128.195.41.1

Src: 128.195.41.1, 67Dest: 255.255.255.255, 68

128.195.31.10

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Lab question:

• Why does DHCP choose well-known client port?

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History of DHCP

• Three Protocols:– RARP (until 1985, no longer used)– BOOTP (1985-1993)– DHCP (since 1993)

• Only DHCP is widely used today.

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Solutions for dynamic assignment of IP addresses

• Reverse Address Resolution Protocol (RARP)– RARP is no longer used– 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

subnetmask)

RARP

Ethernet MACaddress(48 bit)

ARPIP address(32 bit)

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BOOTP

• BOOTstrap Protocol (BOOTP) • Host can configure its IP parameters at boot time. • 3 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 assigns 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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BOOTP Interaction

• BOOTP can be used for downloading memory image for diskless workstations

• Assignment of IP addresses to hosts is static

Argon00:a0:24:71:e4:44 BOOTP Server

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

Argon128.143.137.144

00:a0:24:71:e4:44 DHCP ServerBOOTP Response:IP address: 128.143.137.144Server IP address: 128.143.137.100Boot file name: filename

(a) (b)

Argon128.143.137.14400:a0:24:71:e4:44 DHCP Server

128.143.137.100

TFTP“filename”

(c)

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Lab errata

• In Figure 7.1, the private network interface of Router2 should be labeled with IP address "10.0.1.1/24" (instead of 10.0.0.1/24).

1 network address translation (nat) and dynamic host configuration protocol (dhcp) relates to lab 7....

Documents

public address

ip address depletion

single public ip address

address pool

ethernetcomputerbridg

private addresses

port address translation

pool of public ip addresses