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nat: add overview description about Network Address Translation

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@ -4,22 +4,267 @@
NAT
###
:abbr:`NAT (Network Address Translation)` is a common method of remapping one
IP address space into another by modifying network address information in the
IP header of packets while they are in transit across a traffic routing device.
The technique was originally used as a shortcut to avoid the need to readdress
every host when a network was moved. It has become a popular and essential tool
in conserving global address space in the face of IPv4 address exhaustion. One
Internet-routable IP address of a NAT gateway can be used for an entire private
network.
IP masquerading is a technique that hides an entire IP address space, usually
consisting of private IP addresses, behind a single IP address in another,
usually public address space. The hidden addresses are changed into a single
(public) IP address as the source address of the outgoing IP packets so they
appear as originating not from the hidden host but from the routing device
itself. Because of the popularity of this technique to conserve IPv4 address
space, the term NAT has become virtually synonymous with IP masquerading.
As network address translation modifies the IP address information in packets,
NAT implementations may vary in their specific behavior in various addressing
cases and their effect on network traffic. The specifics of NAT behavior are
not commonly documented by vendors of equipment containing NAT implementations.
The computers on an internal network can use any of the addresses set aside by
the :abbr:`IANA (Internet Assigned Numbers Authority)` for private addressing
(see :rfc:`1918`). These reserved IP addresses are not in use on the Internet,
so an external machine will not directly route to them. The following addresses
are reserved for private use:
* 10.0.0.0 to 10.255.255.255 (CIDR: 10.0.0.0/8)
* 172.16.0.0 to 172.31.255.255 (CIDR: 172.16.0.0/12)
* 192.168.0.0 to 192.168.255.255 (CIDR: 192.268.0.0/16)
If an ISP deploys a :abbr:`CGN (Carrier-grade NAT)`, and uses :rfc:`1918`
address space to number customer gateways, the risk of address collision, and
therefore routing failures, arises when the customer network already uses an
:rfc:`1918` address space.
This prompted some ISPs to develop a policy within the :abbr:`ARIN (American
Registry for Internet Numbers)` to allocate new private address space for CGNs,
but ARIN deferred to the IETF before implementing the policy indicating that
the matter was not a typical allocation issue but a reservation of addresses
for technical purposes (per :rfc:`2860`).
IETF published :rfc:`6598`, detailing a shared address space for use in ISP
CGN deployments that can handle the same network prefixes occurring both on
inbound and outbound interfaces. ARIN returned address space to the :abbr:`IANA
(Internet Assigned Numbers Authority)` for this allocation.
The allocated address block is 100.64.0.0/10.
Devices evaluating whether an IPv4 address is public must be updated to
recognize the new address space. Allocating more private IPv4 address space for
NAT devices might prolong the transition to IPv6.
Overview
========
Different NAT Types
-------------------
.. _source-nat:
Source NAT
==========
Source NAT (SNAT)
^^^^^^^^^^^^^^^^^
Source NAT is typically referred to simply as NAT. To be more correct, what
most people refer to as NAT is actually the process of **Port Address
Translation (PAT)**, or **NAT Overload**. The process of having many internal
host systems communicate to the Internet using a single or subset of IP
addresses.
Source NAT is the most common form of NAT and is typically referred to simply
as NAT. To be more correct, what most people refer to as NAT is actually the
process of :abbr:`PAT (Port Address Translation)`, or NAT Overload. SNAT is
typically used by internal users/private hosts to access the Internet - the
source address is translated and thus kept private.
.. _destination-nat:
Destination NAT (DNAT)
^^^^^^^^^^^^^^^^^^^^^^
While :ref:`source-nat` changes the source address of packets, DNAT changes
the destination address of packets passing through the router. DNAT is
typically used when an external (public) host needs to initiate a session with
an internal (private) host. A customer needs to access a private service
behind the routers public IP. A connection is established with the routers
public IP address on a well known port and thus all traffic for this port is
rewritten to address the internal (private) host.
.. _bidirectional-nat:
Bidirectional NAT
^^^^^^^^^^^^^^^^^
This is a common szenario where both :ref:`source-nat` and
:ref:`destination-nat` are configured at the same time. It's commonly used then
internal (private) hosts need to establish a connection with external resources
and external systems need to acces sinternal (private) resources.
NAT, Routing, Firewall Interaction
----------------------------------
There is a very nice picture/explanation in the Vyatta documentation which
should be rewritten here.
NAT Ruleset
-----------
:abbr:`NAT (Network Address Translation)` is configured entirely on a series
of so called `rules`. Rules are numbered and evaluated by the underlaying OS
in numerical order! The rule numbers can be changes by utilizing the
:cfgcmd:`rename` and :cfgcmd`copy` commands.
.. note:: Changes to the NAT system only affect newly established connections.
Already establiushed ocnnections are not affected.
.. hint:: When designing your NAT ruleset leave some space between consecutive
rules for later extension. Your ruleset could start with numbers 10, 20, 30.
You thus can later extend the ruleset and place new rules between existing
ones.
Rules will be created for both :ref:`source-nat` and :ref:`destination-nat`.
For :ref:`bidirectional-nat` a rule for both :ref:`source-nat` and
:ref:`destination-nat` needs to be created.
.. _traffic-filters:
Traffic Filters
---------------
Traffic Filters are used to control which packets will have the defined NAT
rules applied. Five different filters can be applied within a NAT rule
* **outbound-interface** - applicable only to :ref:`source-nat`. It configures
the interface which is used for the outside traffic that this translation rule
applies to.
Example:
.. code-block:: none
set nat source rule 20 outbound-interface eth0
* **inbound-interface** - applicable only to :ref:`destination-nat`. It
configures the interface which is used for the inside traffic the the
translation rule applies to.
Example:
.. code-block:: none
set nat destination rule 20 inbound-interface eth1
* **protocol** - specify which types of protocols this translation rule applies
to. Only packets matching the specified protocol are NATed. By default this
applies to `all` protocols.
Example:
* Set SNAT rule 20 to only NAT TCP and UDP packets
* Set DNAT rule 20 to only NAT UDP packets
.. code-block:: none
set nat source rule 20 protocol tcp_udp
set nat destination rule 20 protocol udp
* **source** - specifies which packets the NAT translation rule applies to
based on the packets source IP address and/or source port. Only matching
packets are considered for NAT.
Example:
* Set SNAT rule 20 to only NAT packets arriving from the 192.0.2.0/24 network
* Set SNAT rule 30 to only NAT packets arriving from the 192.0.3.0/24 network
with a source port of 80 and 443
.. code-block:: none
set nat source rule 20 source address 192.0.2.0/24
set nat source rule 30 source address 192.0.3.0/24
set nat source rule 30 source port 80,443
* **destination** - specify which packets the translation will be applied to,
only based on the destination address and/or port number configured.
.. note:: If no destination is specified the rule will match on any
destination address and port.
Example:
* Configure SNAT rule (40) to only NAT packets with a destination address of
192.0.2.1.
.. code-block:: none
set nat source rule 40 destination address 192.0.2.1
Address Conversion
------------------
Every NAT rule has a translation command defined. The address defined for the
translation is the addrass used when the address information in a packet is
replaced.
Source Address
^^^^^^^^^^^^^^
For :ref:`source-nat` rules the packets source address will be replaced with
the address specified in the translation command. A port translation can also
be specified and is part of the translation address.
.. note:: The translation address must be set to one of the available addresses
on the configured `outbound-interface` or it must be set to `masquerade`
which will use the primary IP address of the `outbound-interface` as its
translation address.
.. note:: When using NAT for a large number of host systems it recommended that
a minimum of 1 IP address is used to NAT every 256 private host systems.
This is due to the limit of 65,000 port numbers available for unique
translations and a reserving an average of 200-300 sessions per host system.
Example:
* Define a discrete source IP address of 100.64.0.1 for SNAT rule 20
* Use address `masquerade` (the interfaces primary address) on rule 30
* For a large amount of private machines behind the NAT your address pool might
to be bigger. Use any address in the range 100.64.0.10 - 100.64.0.20 on SNAT
rule 40 when doing the translation
.. code-block:: none
set nat source rule 20 translation address 100.64.0.1
set nat source rule 30 translation address 'masquerade'
set nat source rule 40 translation address 100.64.0.10-100.64.0.20
Destination Address
^^^^^^^^^^^^^^^^^^^
For :ref:`destination-nat` rules the packets destination address will be
replaced by the specified address in the `translation address` command.
Example:
* DNAT rule 10 replaces the destination address of an inbound packet with
192.0.2.10
.. code-block:: none
set nat destination rule 10 translation address 192.0.2.10
Configuration Examples
======================
To setup SNAT, we need to know:
* The internal IP addresses we want to translate;
* The outgoing interface to perform the translation on;
* The external IP address to translate to.
* The internal IP addresses we want to translate
* The outgoing interface to perform the translation on
* The external IP address to translate to
In the example used for the Quick Start configuration above, we demonstrate
the following configuration:
@ -138,7 +383,7 @@ Which results in a configuration of:
}
Destination NAT
===============
---------------
DNAT is typically referred to as a **Port Forward**. When using VyOS as a NAT
router and firewall, a common configuration task is to redirect incoming
@ -231,7 +476,7 @@ This would generate the following configuration:
additional rules to permit inbound NAT traffic.
1-to-1 NAT
==========
----------
Another term often used for DNAT is **1-to-1 NAT**. For a 1-to-1 NAT
configuration, both DNAT and SNAT are used to NAT all traffic from an external
@ -245,9 +490,6 @@ internal IP to a reserved external IP. This dedicates an external IP address
to an internal IP address and is useful for protocols which don't have the
notion of ports, such as GRE.
1-to-1 NAT example
------------------
Here's an extract of a simple 1-to-1 NAT configuration with one internal and
one external interface:
@ -270,15 +512,16 @@ Firewall rules are written as normal, using the internal IP address as the
source of outbound rules and the destination of inbound rules.
NPTv6
=====
-----
NPTv6 stands for Network Prefix Translation. It's a form of NAT for IPv6. It's
described in :rfc:`6296`. NPTv6 is supported in linux kernel since version 3.13.
**Usage**
NPTv6 is very useful for IPv6 multihoming. It is also commonly used when the external IPv6 prefix is dynamic,
as it prevents the need for renumbering of internal hosts when the extern prefix changes.
NPTv6 is very useful for IPv6 multihoming. It is also commonly used when the
external IPv6 prefix is dynamic, as it prevents the need for renumbering of
internal hosts when the extern prefix changes.
Let's assume the following network configuration:
@ -302,7 +545,7 @@ their address to the right subnet when going through your router.
* eth2 addr : 2001:db8:e2::1/48
VyOS Support
------------
^^^^^^^^^^^^
NPTv6 support has been added in VyOS 1.2 (Crux) and is available through
`nat nptv6` configuration nodes.
@ -333,16 +576,20 @@ Resulting in the following ip6tables rules:
NAT before VPN
==============
--------------
Some application service providers (ASPs) operate a VPN gateway to provide access to their internal resources,
and require that a connecting organisation translate all traffic to the service provider network to a source address provided by the ASP.
Some application service providers (ASPs) operate a VPN gateway to provide
access to their internal resources, and require that a connecting organisation
translate all traffic to the service provider network to a source address
provided by the ASP.
Example Network
---------------
^^^^^^^^^^^^^^^
Here's one example of a network environment for an ASP.
The ASP requests that all connections from this company should come from 172.29.41.89 - an address that is assigned by the ASP and not in use at the customer site.
The ASP requests that all connections from this company should come from
172.29.41.89 - an address that is assigned by the ASP and not in use at the
customer site.
.. figure:: _static/images/nat_before_vpn_topology.png
:scale: 100 %
@ -352,7 +599,7 @@ The ASP requests that all connections from this company should come from 172.29.
Configuration
-------------
^^^^^^^^^^^^^
The required configuration can be broken down into 4 major pieces:
@ -363,10 +610,11 @@ The required configuration can be broken down into 4 major pieces:
Dummy interface
^^^^^^^^^^^^^^^
"""""""""""""""
The dummy interface allows us to have an equivalent of the Cisco IOS Loopback interface - a router-internal interface we can use for IP addresses the router must know about,
but which are not actually assigned to a real network.
The dummy interface allows us to have an equivalent of the Cisco IOS Loopback
interface - a router-internal interface we can use for IP addresses the router
must know about, but which are not actually assigned to a real network.
We only need a single step for this interface:
@ -375,7 +623,7 @@ We only need a single step for this interface:
set interfaces dummy dum0 address '172.29.41.89/32'
NAT Configuration
^^^^^^^^^^^^^^^^^
"""""""""""""""""
.. code-block:: none
@ -391,8 +639,7 @@ NAT Configuration
set nat source rule 120 translation address '172.29.41.89'
IPSec IKE and ESP
^^^^^^^^^^^^^^^^^
"""""""""""""""""
The ASP has documented their IPSec requirements:
@ -408,7 +655,8 @@ The ASP has documented their IPSec requirements:
* DH Group 14
Additionally, we want to use VPNs only on our eth1 interface (the external interface in the image above)
Additionally, we want to use VPNs only on our eth1 interface (the external
interface in the image above)
.. code-block:: none
@ -429,11 +677,12 @@ Additionally, we want to use VPNs only on our eth1 interface (the external inter
set vpn ipsec ipsec-interfaces interface 'eth1'
IPSec VPN Tunnels
^^^^^^^^^^^^^^^^^
"""""""""""""""""
We'll use the IKE and ESP groups created above for this VPN.
Because we need access to 2 different subnets on the far side, we will need two different tunnels.
If you changed the names of the ESP group and IKE group in the previous step, make sure you use the correct names here too.
We'll use the IKE and ESP groups created above for this VPN. Because we need
access to 2 different subnets on the far side, we will need two different
tunnels. If you changed the names of the ESP group and IKE group in the previous
step, make sure you use the correct names here too.
.. code-block:: none
@ -452,7 +701,8 @@ If you changed the names of the ESP group and IKE group in the previous step, ma
Testing and Validation
""""""""""""""""""""""
If you've completed all the above steps you no doubt want to see if it's all working.
If you've completed all the above steps you no doubt want to see if it's all
working.
Start by checking for IPSec SAs (Security Associations) with: