Network Address

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Network Address Translation (NAT)

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Overview

Motivation End-to-end principle Role of IP addresses Basic NAT types and their behaviors NAT traversal: STUN

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History Early 1990s

IPv4 Address consumption concern Two approaches

• IPv6 and NAT

NATs were initially intended to allow devices to share an address pool dynamically First RFC about NAT in 1994 NAT vs. DHCP?

NAT goes against Internet end-to-end principle IETF hates NATs No standardization -> backfire

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Motivation

DSL and cable modem business model Not simultaneous access, no servers

ISP wants to save money In PSTNs, there is extension

Changing next higher ISP becomes easier Even multi-homing

Security: Inbound traffic filtering stateful firewall

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End-to-end principle RFC 1958: “An end-to-end protocol design

should not rely on the maintenance of state (i.e., information about the state of the end-to-end communication) inside the network. Such state should be maintained only in the endpoints, in such a way that the state can only be destroyed when the endpoint itself breaks (known as fate-sharing). An immediate consequence of this is that datagrams are better than classical virtual circuits. The network's job is to transmit datagrams as efficiently and flexibly as possible. Everything else should be done at the fringes.”

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Middle boxes

Middle box = “any intermediary device performing functions other than the normal, standard functions of an IP router on the datagram path between a source host and destination host”

L2, L3, L4, L7, … explicit vs. implicit functional vs. optimizing routing vs. processing soft vs. hard state fail-over vs. restart

Examples: NATs SOCKS gateway IP tunnel endpoint Transport relay Load balancers Application firewalls Transcoders (RFC 3234) Proxies Caches Modified DNS servers Anonymizers

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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 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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Network Address Translation (NAT)

NAT is a router function where IP addresses (and possibly port numbers) of IP datagrams are replaced at the boundary of a private network

NAT is a method that enables hosts on private networks to communicate with hosts on the Internet

NAT is run on routers that connect private networks to the public Internet, to replace the IP address-port pair of an IP packet with another IP address-port pair. Topology sensitive

• inside (private) vs. outside (public)

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Basic operation of NAT

NAT device has address translation table

H1

private address: 10.0.1.2public address: 128.143.71.21

H5

Privatenetwork

Internet

Source = 10.0.1.2Destination = 213.168.112.3


public address: 213.168.112.3NATdevice



PrivateAddress

PublicAddress

10.0.1.2 128.143.71.21

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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



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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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 static 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.

Note: The difference to the use of NAT with IP address pooling is that the

mapping of public and private IP addresses is static.

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H1


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)

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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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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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• Hamper peer-to-peer applications

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

Typically, the address-port mapping is maintained soft-state (in minutes)

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Concerns about NAT

IP address in application data: Applications that carry IP addresses in the

payload of the application data generally do not work across a private-public network boundary.

Some NAT devices inspect the payload of widely used application layer protocols and, if an IP address is detected in the application-layer header or the application payload, translate the address according to the address translation table.

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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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NAT Traversal

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NAPT Traversal

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NAT types

Symmetric Port restricted cone Address restricted cone Full cone Hairpin

Different not only on a vendor-by-vendor basis but also on a model-by-model basis

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Symmetric NAT

•NAT mapping btw src_addr/src_port and dest_addr/dest_port is fixed•The most restrictive form•It has been observed to be rare

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Full-cone NAT

The least restrictive form Binding of a local address/port can be used by any remote host

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(Address) Restricted-cone NAT

NAT binding is accessible only by the destination host different port can be used

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Port-restricted-cone NAT

NAT binding is accessible by any remote host But only same port should be used

Typically, the internal host had previously sent a packet the remote host

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Hairpin NAT

A local host can direct a packet to the public address/port of an already mapped local host

D

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Nondeterministic NATs

NATs change their types of behavior when a binding conflict occurs

ExampleSome NATs attempt to preserve the port

number in the bindingIf the port number is available, the NAT

behaves like a full-cone NATIf that port is already occupied by other

host, the NAT may change the type, e.g. a symmetric NAT

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What is STUN? Who uses it?

STUN – Simple Traversal of User Datagram Protocol through Network Address Translators.

The protocol is defined in RFC 3489. Protocols like SIP and applications

like Google Talk use STUN to gather important information about the network configuration.

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What does it do?

STUN is a client-server protocol that “allows entities behind a NAT to first discover the presence of a NAT and the type of NAT, and then to learn the addresses bindings allocated by the NAT.”

In other words, it’s a means of discovering the public IP and port numbers that a NAT assigns to a node on a private LAN.

In addition, STUN does not require any special network configuration and works with a variety of existing networks, but not all.

*http://tools.ietf.org/html/rfc3489

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STUN and NAT terminology

A STUN Client is a node that generates the STUN requests.

A STUN Server is a node that receives the STUN requests and generates the STUN responses.

NATusually part of a firewall or router

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A picture i worth 1000 words

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How STUN generally works

A STUN Client sends a STUN request to a STUN Server. The Client then waits for the Server to send a STUN response STUN client is typically embedded in application STUN server has two IP addresses

The trick is to analyze the response from the server to determine the type of NAT router and the associated bindings the router has given to internal nodes.

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The STUN Message

The following STUN attributes in the payload are especially important: MAPPED-ADDRESS : Found in STUN responses. It

contains the IP address and port number of the STUN request. I.e., the public IP and port of the STUN client.

CHANGE-REQUEST : Found in STUN requests. It contains flags for the IP address and port number of the server. If set, the client is asking the server to send the response from a different IP and port. (We will see why later)

CHANGED-ADDRESS – Found in STUN responses. It contains the alternate IP address and port number of the server due to CHANGE-REQUEST

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NAT discovery (test 1)

To determine if a NAT router/firewall is present, send a STUN request to the server. Wait for a response and analyze it.

If the IP address and port number in the MAPPED-ADDRESS attribute of the payload in the STUN response equal the local IP address and port number that it bound to when sending the request, then the client is NOT behind a NAT router. Otherwise, it is behind a NAT router.

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NAT discovery – Full Cone (test 2)

Full Cone NAT router – The client sets the IP address and port number flags in the CHANGE-REQUEST of the STUN request. This causes the server to send the response from the alternate IP and port number. If the client receives the STUN response,

then the client is behind a full cone router. Otherwise, it is behind one of the other three

NAT routers.

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NAT discovery – Symmetric (test 3)

Symmetric NAT – The client sends two STUN requests. One request is sent to a server at IP address X and port P, and another to a server at IP address Y and port P. If the IP addresses and ports from the

MAPPED-ADDRESS attributes in the two responses do not match, then it is behind a Symmetric NAT router.

If they do match, then it is behind one of the remaining two NAT routers.

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NAT discovery – Restricted (test 4)

Restricted NAT – The port flag in the CHANGE-REQUEST attribute of the request is set. This instructs the server to send a response from a different port. If the response is received, it is behind a

restricted NAT router. If no response is received, it is behind a port restricted NAT router.

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Limitations of STUN Does not address incoming TCP connections.

STUNT and other proposals Does not allow incoming UDP connections through a symmetric

NAT STUN “is not a cure-all for the problems associated with NAT.” “The problems in STUN have to do with the lack of standardized

behaviors and controls in NATs, which results in a proliferation of devices whose behavior is highly unpredictable, extremely variable, and uncontrollable. Ultimately, the solution is to make the environment less hostile, and to introduce controls and standardized behaviors into NAT. However, until such time as that happens, STUN provides a good short term solution given the terrible conditions under which it is forced to operate.”

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NAT traversal

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Other issues

Symmetric NATsThe first packet for hole punching will

be droppedAnd the port number (for the other

peer) is changed from the one for the server

Port prediction technique• May not work

Nested NATs

Network Address

Technology

Transcript of Network Address