COMS W4995-1 COMS W4995-1 Lecture 8. NAT, DHCP & Firewalls.

COMS W4995-1

Lecture 8

NAT, DHCP & Firewalls

Outline

Network Address Translation (NAT)

Dynamic Host Configuration Protocol (DHCP)

Firewalls

Typical Application Domains where we use them

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. But it became a standard: RFC 1631 - The IP Network Address

Translator (NAT)

Provides a form security by acting as a firewall home users. Small companies.

Network Address Translation: a hack

Is there any other solution to the IP address problem?

Basic operation of NAT

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

•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

Private Network

Private IP network is an IP network with Private IP Addresses (Can it be connected directly to the Internet?)

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 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

Main uses of NAT

Pooling of IP addresses

Supporting migration between network service providers

IP masquerading and internal firewall

Load balancing of servers


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.


•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



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.


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:


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:

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

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


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.


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


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.

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 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

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


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

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)

Configuring NAT with iptable

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

Dynamic Host Configuration Protocol

(DHCP)

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 Wireless networking and Home NATs

No static IP means that we have to depend on DNS for the packet routing Use of a DDNS (Dynamic DNS entry) Free sites for that service in the internet

Dynamic Host Configuration Protocol (DHCP)

Designed in 1993

Requires a server and free IP address space

Supports temporary allocation (“leases”) of IP addresses

DHCP client can acquire all IP configuration parameters

Any potential security risks?

Can we use something that can prevent unauthorized users?

DHCP Interaction (simplified)

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

DHCP Request 00:a0:24:71:e4:44Sent to 255.255.255.255

Argon 128.16.23.144

00:a0:24:71:e4:44 DHCP ServerDHCP Response: IP address: 128.16.23.144Default gateway: 128.16.23.1Netmask: 255.255.0.0

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)

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

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

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: 128.59.20.147Transaction ID: 654

Dest: 255.255.255.255, 68

Lifetime: 3600 secsServer ID: 128.59.18.1

Src: 0.0.0.0, 68Dest: 255.255.255.255, 67DHCPREQUESTYiaddr: 128.59.20.147Transaction ID: 655server ID: 128.195.31.1Lifetime: 3600 secs

Src:128.59.18.1, 67

DHCPACKYiaddr: 128.59.20.147Transaction ID: 655

Dest: 255.255.255.255, 68

Lifetime: 3600 secsServer ID: 128.59.18.1

DHCP operations

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

DHCP relay agent

DHCPDISCOVERGiaddr: 0

Src: 0.0.0.0., 68Dest: 255.255.255.255, 67

128.16.31.1 128.16.41.1

DHCPDISCOVERGiaddr: 128.16.41.1

Src: 0.0.0.0., 68Dest: 255.255.255.255, 67

DHCPOFFER

……

Giaddr: 128.16.41.1

Src: 128.16.31.10, 67Dest: 128.16.41.1, 67

DHCPOFFER

……

Giaddr: 128.16.41.1

Src: 128.16.41.1, 67Dest: 255.255.255.255, 68

128.16.31.10

History of DHCP

Three Protocols: RARP (until 1985, no longer used) BOOTP (1985-1993) DHCP (since 1993) Secure DHCP – not a standard yet…

Only DHCP is widely used today.

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)

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

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)

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).

Firewalls (Slides to be added)

COMS W4995-1 COMS W4995-1 Lecture 8. NAT, DHCP & Firewalls.

Documents

private ip addresses

ip address problem

private address space

address pool

pool of public ip addresses

ip address depletion

corporate network

private networks