Transcript Introduction

Network Address Translation (NAT) and
Dynamic Host Configuration Protocol (DHCP)
Relates to Lab 7.
Module about private networks and NAT.
1
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?
2
Basic operation of NAT
•Private
•network
•Source
•Destination
•= 10.0.1.2
•= 213.168.112.3
•Source
•Destination
•NAT
•device
•private address: 10.0.1.2
•public address: 128.143.71.21
•H1
•Internet
•Source
•Destination
•= 213.168.112.3
•= 10.0.1.2
•= 128.143.71.21
•= 213.168.112.3
•public address:
•Source
•Destination
•Private
•Address
•Public
•Address
•10.0.1.2
•128.143.71.21
• 213.168.112.3
•= 213.168.112.3
•= 128.143.71.21
•H5
• NAT device has address translation table
3
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
4
Main uses of NAT
• Pooling of IP addresses
• Supporting migration between network service providers
• IP masquerading
• Load balancing of servers
5
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
Private
network
Internet
Source
= 10.0.1.2
Destination = 213.168.112.3
Source
= 128.143.71.21
Destination = 213.168.112.3
NAT
device
private address: 10.0.1.2
public address:
H1
public address:
213.168.112.3
H5
Private
Address
Public
Address
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.
8
Supporting migration between network service
providers
9
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
10
IP masquerading
11
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
13
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.
14
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.
15
NAT and FTP
• Normal FTP operation
16
NAT and FTP
• NAT device with FTP support
17
NAT and FTP
• FTP in passive mode and NAT.
18
Configuring NAT in Linux
• Linux uses the Netfilter/iptable package to add filtering rules
to the IP module
To application
From application
filter
INPUT
nat
OUTPUT
filter
OUTPUT
Yes
Destination
is local?
nat
PREROUTING
(DNAT)
Incoming
datagram
No
filter
FORWARD
nat
POSTROUTING
(SNAT)
Outgoing
datagram
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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.
21
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
22
DHCP
• Dynamic Host Configuration Protocol (DHCP)
– Designed in 1993
– Supports temporary allocation (“leases”) of IP addresses
– DHCP client can acquire all IP configuration parameters
23
DHCP Interaction (simplified)
Argon
128.143.137.144
00:a0:24:71:e4:44
DHCP Server
DHCP Response:
IP address: 128.143.137.144
Default gateway: 128.143.137.1
Netmask: 255.255.0.0
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DHCP Message Format
OpCode
Hardware Type
Number of Seconds
Hardware Address
Hop Count
Length
Unused (in BOOTP)
Flags (in DHCP)
Transaction ID
Client IP address
Your IP address
Server IP address
Gateway IP address
Client hardware address (16 bytes)
Server host name (64 bytes)
Boot file name (128 bytes)
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, 68
Dest: 255.255.255.255, 67
DHCPDISCOVERY
Yiaddr: 0.0.0.0
Transaction ID: 654
Src:128.195.31.1, 67
Dest: 255.255.255.255, 68
DHCPOFFER
Yiaddr: 128.195.31.147
Transaction ID: 654
Server ID: 128.195.31.1
Lifetime: 3600 secs
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DHCP operations
Src: 0.0.0.0, 68
Dest: 255.255.255.255, 67
DHCPREQUEST
Yiaddr: 128.195.31.147
Transaction ID: 655
server ID: 128.195.31.1
Lifetime: 3600 secs
Src:128.195.31.1, 67
Dest: 255.255.255.255, 68
DHCPACK
Yiaddr: 128.195.31.147
Transaction ID: 655
Server ID: 128.195.31.1
Lifetime: 3600 secs
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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
Src: 0.0.0.0., 68
Dest: 255.255.255.255, 67
Giaddr: 128.195.41.1
DHCPDISCOVER
Src: 0.0.0.0., 68
Dest: 255.255.255.255, 67
Giaddr: 0
DHCPDISCOVER
128.195.31.10
128.195.31.1 128.195.41.1
Src: 128.195.41.1, 67
Src: 128.195.31.10, 67
Dest: 255.255.255.255, 68
Dest: 128.195.41.1, 67
Giaddr: 128.195.41.1
Giaddr: 128.195.41.1
DHCPOFFER
DHCPOFFER
……
……
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Lab question:
• Why does DHCP choose well-known client port?
32
History of DHCP
• Three Protocols:
– RARP (until 1985, no longer used)
– BOOTP (1985-1993)
– DHCP (since 1993)
• Only DHCP is widely used today.
33
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)
IP address
(32 bit)
ARP
RARP
Ethernet MAC
address
(48 bit)
34
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
(a)
Argon
00:a0:24:71:e4:44
BOOTP Server
Argon
128.143.137.144
00:a0:24:71:e4:44
(b)
DHCP Server
BOOTP Response:
IP address: 128.143.137.144
Server IP address: 128.143.137.100
Boot file name: filename
BOOTP Request
00:a0:24:71:e4:44
Sent to 255.255.255.255
(c)
• BOOTP can be used for
downloading memory
image for diskless
workstations
• Assignment of IP addresses
to hosts is static
36
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).
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