Network Address
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Transcript of 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
Source = 128.143.71.21Destination = 213.168.112.3
public address: 213.168.112.3NATdevice
Source = 213.168.112.3Destination = 128.143.71.21
Source = 213.168.112.3Destination = 10.0.1.2
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
Source = 10.0.1.2Destination = 213.168.112.3
Source = 128.143.71.21Destination = 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 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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Supporting migration between network service providers
H1
private address: 10.0.1.2public address: 128.143.71.21
128.195.4.120
Source = 10.0.1.2Destination = 213.168.112.3
NATdevice
PrivateAddress
PublicAddress
10.0.1.2128.143.71.21128.195.4.120
128.143.71.21
128.195.4.120
Source = 128.143.71.21Destination = 213.168.112.3
Source = 128.195.4.120Destination = 213.168.112.3
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
Source = 128.143.71.21Source port = 2100
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
Source = 10.0.1.3Source port = 3020
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
Source = 213.168.12.3Destination = 128.143.71.21
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
Source = 128.195.4.120Destination = 128.143.71.21
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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
public address:128.195.4.120
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
private address: 10.0.1.3public address: 128.143.72.21
Internet
FTP client FTP server
PORT 10.0.1.3/1027 PORT 128.143.72.21/1027
200 PORT command successful200 PORT command successful
RETR myfile
establish data connection
RETR myfile
150 Opening data connection150 Opening data connection
establish data connection
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NAT and FTP
FTP in passive mode and NAT.
H1
Private network
NATdevice
H2
private address: 10.0.1.3public address: 128.143.72.21
Internet
FTP client FTP server
PASV PASV
Entering Passive Mode128.195.4.120/10001
Entering Passive Mode128.195.4.120/10001
public address:128.195.4.120
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