COMS W49951 Lecture 8

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COMS W4995-1Lecture 8
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NAT, DHCP Firewalls
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Outline

• Network Address Translation (NAT)
• Dynamic Host Configuration Protocol (DHCP)
• Firewalls
• Typical Application Domains where we use them

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Network Address Translation a hack

• 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.

Is there any other solution to the IP address
problem?
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Basic operation of NAT

• Private Network

• Internet

• private address 10.0.1.2

NATDevice

• public address 128.143.71.21

• Host

Public Host
64.236.24.4

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

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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

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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

• Private Network

• Internet

• private address 10.0.1.2

NATDevice

• public address 128.143.71.21

• Host

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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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Supporting migration between network service
providers
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Supporting migration between network service
providers
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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
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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
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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

• Normal FTP operation

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NAT and FTP

• NAT device with FTP support

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NAT and FTP

• FTP in passive mode and NAT.

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Configuring NAT in Linux

• Linux uses the Netfilter/iptable Kernel package

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Configuring NAT with iptable

• First exampleiptables t nat A POSTROUTING s
  10.0.1.2 j SNAT --to-source
  128.16.71.21
  
• Pooling of IP addressesiptables t nat A
  POSTROUTING s 10.0.1.0/24 j SNAT
  --to-source 128.16.71.0128.16.71.30
  
• 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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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
• 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

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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?

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DHCP Interaction (simplified)
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DHCP Message Format
(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.

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DHCP operations
Src128.195.31.1, 67
Dest 255.255.255.255, 68
DHCPOFFER
Yiaddr 128.59.20.147
Transaction ID 654
Server ID 128.59.18.1
Lifetime 3600 secs
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DHCP operations
Src 0.0.0.0, 68
Dest 255.255.255.255, 67
DHCPREQUEST
Yiaddr 128.59.20.147
Transaction ID 655
server ID 128.195.31.1
Lifetime 3600 secs
Src128.59.18.1, 67
Dest 255.255.255.255, 68
DHCPACK
Yiaddr 128.59.20.147
Transaction ID 655
Server ID 128.59.18.1
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
128.16.31.10
128.16.41.1
128.16.31.1
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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.

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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)
  

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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
(b)
(a)

• BOOTP can be used for downloading memory image
  for diskless workstations
  
• Assignment of IP addresses to hosts is static

(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).

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Firewalls (Slides to be added)