path: root/docs/nat.rst

.. _nat:

###
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:

SNAT
^^^^

:abbr:`SNAT (Source Network Address Translation)` is the most common form of
:abbr:`NAT (Network Address Translation)` and is typically referred to simply
as NAT. To be more correct, what most people refer to as :abbr:`NAT (Network
Address Translation)` 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:

DNAT
^^^^

:abbr:`DNAT (Destination Network Address Translation)` changes the destination
address of packets passing through the router, while :ref:`source-nat` changes
the source address of packets. 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 scenario 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 access internal (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 underlying 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
  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 address 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

In the example used for the Quick Start configuration above, we demonstrate
the following configuration:

.. code-block:: none

  set nat source rule 100 outbound-interface 'eth0'
  set nat source rule 100 source address '192.168.0.0/24'
  set nat source rule 100 translation address 'masquerade'

Which generates the following configuration:

.. code-block:: none

  rule 100 {
      outbound-interface eth0
      source {
          address 192.168.0.0/24
      }
      translation {
          address masquerade
      }
  }

In this example, we use **masquerade** as the translation address instead of
an IP address. The **masquerade** target is effectively an alias to say "use
whatever IP address is on the outgoing interface", rather than a statically
configured IP address. This is useful if you use DHCP for your outgoing
interface and do not know what the external address will be.

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 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: For an ~8,000 host network a source NAT pool of 32 IP addresses is
recommended.

A pool of addresses can be defined by using a **-** in the
`set nat source rule [n] translation address` statement.

.. code-block:: none

  set nat source rule 100 translation address '203.0.113.32-203.0.113.63'

.. note:: Avoiding "leaky" NAT

Linux netfilter will not NAT traffic marked as INVALID. This often confuses
people into thinking that Linux (or specifically VyOS) has a broken NAT
implementation because non-NATed traffic is seen leaving an external interface.
This is actually working as intended, and a packet capture of the "leaky"
traffic should reveal that the traffic is either an additional TCP "RST",
"FIN,ACK", or "RST,ACK" sent by client systems after Linux netfilter considers
the connection closed. The most common is the additional TCP RST some host
implementations send after terminating a connection (which is implementation-
specific).

In other words, connection tracking has already observed the connection be
closed and has transition the flow to INVALID to prevent attacks from
attempting to reuse the connection.

You can avoid the "leaky" behavior by using a firewall policy that drops
"invalid" state packets.

Having control over the matching of INVALID state traffic, e.g. the ability to
selectively log, is an important troubleshooting tool for observing broken
protocol behavior. For this reason, VyOS does not globally drop invalid state
traffic, instead allowing the operator to make the determination on how the
traffic is handled.

.. _hairpin_nat_reflection:

Hairpin NAT/NAT Reflection
--------------------------

A typical problem with using NAT and hosting public servers is the ability for
internal systems to reach an internal server using it's external IP address.
The solution to this is usually the use of split-DNS to correctly point host
systems to the internal address when requests are made internally. Because
many smaller networks lack DNS infrastructure, a work-around is commonly
deployed to facilitate the traffic by NATing the request from internal hosts
to the source address of the internal interface on the firewall.

This technique is commonly referred to as NAT Reflection or Hairpin NAT.

Example:

* Redirect Microsoft RDP traffic from the outside (WAN, external) world via
  :ref:`destination-nat` in rule 100 to the internal, private host 192.0.2.40.

* Redirect Microsoft RDP traffic from the internal (LAN, private) network via
  :ref:`destination-nat` in rule 110 to the internal, private host 192.0.2.40.
  We also need a :ref:`source-nat` rule 110 for the reverse path of the traffic.
  The internal network 192.0.2.0/24 is reachable via interface `eth0.10`.

.. code-block:: none

  set nat destination rule 100 description 'Regular destination NAT from external'
  set nat destination rule 100 destination port '3389'
  set nat destination rule 100 inbound-interface 'pppoe0'
  set nat destination rule 100 protocol 'tcp'
  set nat destination rule 100 translation address '192.0.2.40'

  set nat destination rule 110 description 'NAT Reflection: INSIDE'
  set nat destination rule 110 destination port '3389'
  set nat destination rule 110 inbound-interface 'eth0.10'
  set nat destination rule 110 protocol 'tcp'
  set nat destination rule 110 translation address '192.0.2.40'

  set nat source rule 110 description 'NAT Reflection: INSIDE'
  set nat source rule 110 destination address '192.0.2.0/24'
  set nat source rule 110 outbound-interface 'eth0.10'
  set nat source rule 110 protocol 'tcp'
  set nat source rule 110 source address '192.0.2.0/24'
  set nat source rule 110 translation address 'masquerade'

Which results in a configuration of:

.. code-block:: none

  vyos@vyos# show nat
   destination {
       rule 100 {
           description "Regular destination NAT from external"
           destination {
               port 3389
           }
           inbound-interface pppoe0
           protocol tcp
           translation {
               address 192.0.2.40
           }
       }
       rule 110 {
           description "NAT Reflection: INSIDE"
           destination {
               port 3389
           }
           inbound-interface eth0.10
           protocol tcp
           translation {
               address 192.0.2.40
           }
       }
   }
   source {
       rule 110 {
           description "NAT Reflection: INSIDE"
           destination {
               address 192.0.2.0/24
           }
           outbound-interface eth0.10
           protocol tcp
           source {
               address 192.0.2.0/24
           }
           translation {
               address masquerade
           }
       }
   }


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
traffic to a system behind the firewall.

In this example, we will be using the example Quick Start configuration above
as a starting point.

To setup a destination NAT rule we need to gather:

* The interface traffic will be coming in on;
* The protocol and port we wish to forward;
* The IP address of the internal system we wish to forward traffic to.

In our example, we will be forwarding web server traffic to an internal web
server on 192.168.0.100. HTTP traffic makes use of the TCP protocol on port 80.
For other common port numbers, see: https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers

Our configuration commands would be:

.. code-block:: none

  set nat destination rule 10 description 'Port Forward: HTTP to 192.168.0.100'
  set nat destination rule 10 destination port '80'
  set nat destination rule 10 inbound-interface 'eth0'
  set nat destination rule 10 protocol 'tcp'
  set nat destination rule 10 translation address '192.168.0.100'

Which would generate the following NAT destination configuration:

.. code-block:: none

  nat {
      destination {
          rule 10 {
              description "Port Forward: HTTP to 192.168.0.100"
              destination {
                  port 80
              }
              inbound-interface eth0
              protocol tcp
              translation {
                  address 192.168.0.100
              }
          }
      }
  }

.. note:: If forwarding traffic to a different port than it is arriving on,
   you may also configure the translation port using
   `set nat destination rule [n] translation port`.

This establishes our Port Forward rule, but if we created a firewall policy it
will likely block the traffic.

It is important to note that when creating firewall rules that the DNAT
translation occurs **before** traffic traverses the firewall. In other words,
the destination address has already been translated to 192.168.0.100.

So in our firewall policy, we want to allow traffic coming in on the outside
interface, destined for TCP port 80 and the IP address of 192.168.0.100.

.. code-block:: none

  set firewall name OUTSIDE-IN rule 20 action 'accept'
  set firewall name OUTSIDE-IN rule 20 destination address '192.168.0.100'
  set firewall name OUTSIDE-IN rule 20 destination port '80'
  set firewall name OUTSIDE-IN rule 20 protocol 'tcp'
  set firewall name OUTSIDE-IN rule 20 state new 'enable'

This would generate the following configuration:

.. code-block:: none

  rule 20 {
      action accept
      destination {
          address 192.168.0.100
          port 80
      }
      protocol tcp
      state {
          new enable
      }
  }

.. note::

  If you have configured the `INSIDE-OUT` policy, you will need to add
  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
IP address to an internal IP address and vice-versa.

Typically, a 1-to-1 NAT rule omits the destination port (all ports) and
replaces the protocol with either **all** or **ip**.

Then a corresponding SNAT rule is created to NAT outgoing traffic for the
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.

Here's an extract of a simple 1-to-1 NAT configuration with one internal and
one external interface:

.. code-block:: none

  set interfaces ethernet eth0 address '192.168.1.1/24'
  set interfaces ethernet eth0 description 'Inside interface'
  set interfaces ethernet eth1 address '192.0.2.30/24'
  set interfaces ethernet eth1 description 'Outside interface'
  set nat destination rule 2000 description '1-to-1 NAT example'
  set nat destination rule 2000 destination address '192.0.2.30'
  set nat destination rule 2000 inbound-interface 'eth1'
  set nat destination rule 2000 translation address '192.168.1.10'
  set nat source rule 2000 description '1-to-1 NAT example'
  set nat source rule 2000 outbound-interface 'eth1'
  set nat source rule 2000 source address '192.168.1.10'
  set nat source rule 2000 translation address '192.0.2.30'

Firewall rules are written as normal, using the internal IP address as the
source of outbound rules and the destination of inbound 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.

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.

.. figure:: _static/images/nat_before_vpn_topology.png
   :scale: 100 %
   :alt: NAT before VPN Topology

   NAT before VPN Topology


Configuration
^^^^^^^^^^^^^

The required configuration can be broken down into 4 major pieces:

* A dummy interface for the provider-assigned IP;
* NAT (specifically, Source NAT);
* IPSec IKE and ESP Groups;
* IPSec VPN tunnels.


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.

We only need a single step for this interface:

.. code-block:: none

  set interfaces dummy dum0 address '172.29.41.89/32'

NAT Configuration
"""""""""""""""""

.. code-block:: none

  set nat source rule 110 description 'Internal to ASP'
  set nat source rule 110 destination address '172.27.1.0/24'
  set nat source rule 110 outbound-interface 'any'
  set nat source rule 110 source address '192.168.43.0/24'
  set nat source rule 110 translation address '172.29.41.89'
  set nat source rule 120 description 'Internal to ASP'
  set nat source rule 120 destination address '10.125.0.0/16'
  set nat source rule 120 outbound-interface 'any'
  set nat source rule 120 source address '192.168.43.0/24'
  set nat source rule 120 translation address '172.29.41.89'

IPSec IKE and ESP
"""""""""""""""""

The ASP has documented their IPSec requirements:

* IKE Phase:

  * aes256 Encryption
  * sha256 Hashes

* ESP Phase:

  * aes256 Encryption
  * sha256 Hashes
  * DH Group 14


Additionally, we want to use VPNs only on our eth1 interface (the external
interface in the image above)

.. code-block:: none

  set vpn ipsec ike-group my-ike ikev2-reauth 'no'
  set vpn ipsec ike-group my-ike key-exchange 'ikev1'
  set vpn ipsec ike-group my-ike lifetime '7800'
  set vpn ipsec ike-group my-ike proposal 1 dh-group '14'
  set vpn ipsec ike-group my-ike proposal 1 encryption 'aes256'
  set vpn ipsec ike-group my-ike proposal 1 hash 'sha256'

  set vpn ipsec esp-group my-esp compression 'disable'
  set vpn ipsec esp-group my-esp lifetime '3600'
  set vpn ipsec esp-group my-esp mode 'tunnel'
  set vpn ipsec esp-group my-esp pfs 'disable'
  set vpn ipsec esp-group my-esp proposal 1 encryption 'aes256'
  set vpn ipsec esp-group my-esp proposal 1 hash 'sha256'

  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.

.. code-block:: none

  set vpn ipsec site-to-site peer 198.51.100.243 authentication mode 'pre-shared-secret'
  set vpn ipsec site-to-site peer 198.51.100.243 authentication pre-shared-secret 'PASSWORD IS HERE'
  set vpn ipsec site-to-site peer 198.51.100.243 connection-type 'initiate'
  set vpn ipsec site-to-site peer 198.51.100.243 default-esp-group 'my-esp'
  set vpn ipsec site-to-site peer 198.51.100.243 ike-group 'my-ike'
  set vpn ipsec site-to-site peer 198.51.100.243 ikev2-reauth 'inherit'
  set vpn ipsec site-to-site peer 198.51.100.243 local-address '203.0.113.46'
  set vpn ipsec site-to-site peer 198.51.100.243 tunnel 0 local prefix '172.29.41.89/32'
  set vpn ipsec site-to-site peer 198.51.100.243 tunnel 0 remote prefix '172.27.1.0/24'
  set vpn ipsec site-to-site peer 198.51.100.243 tunnel 1 local prefix '172.29.41.89/32'
  set vpn ipsec site-to-site peer 198.51.100.243 tunnel 1 remote prefix '10.125.0.0/16'

Testing and Validation
""""""""""""""""""""""

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:

.. code-block:: none

  $ show vpn ipsec sa

  Peer ID / IP                            Local ID / IP
  ------------                            -------------
  198.51.100.243                          203.0.113.46

      Tunnel  State  Bytes Out/In   Encrypt  Hash    NAT-T  A-Time  L-Time  Proto
      ------  -----  -------------  -------  ----    -----  ------  ------  -----
      0       up     0.0/0.0        aes256   sha256  no     1647    3600    all
      1       up     0.0/0.0        aes256   sha256  no     865     3600    all

That looks good - we defined 2 tunnels and they're both up and running.

.. _ULAs: https://en.wikipedia.org/wiki/Unique_local_address