Add NAT chapter

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diff --git a/docs/ch08-nat.rst b/docs/ch08-nat.rst
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+NAT
+===
+
+Source NAT
+----------
+
+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.
+
+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:
+
+  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:: sh
+
+  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:: sh
+
+  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.
+
+**Note:** Avoiding NAT breakage in the absence of split-DNS
+
+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 reffered to as **NAT Reflection**, or **Hairpin NAT**.
+
+In this example, we will be using the example Quick Start configuration above
+as a starting point.
+
+To setup a NAT reflection rule, we need to create a rule to NAT connections
+from the internal network to the same internal network to use the source
+address of the internal interface.
+
+.. code-block:: sh
+
+  set nat source rule 110 description 'NAT Reflection: INSIDE'
+  set nat source rule 110 destination address '192.168.0.0/24'
+  set nat source rule 110 outbound-interface 'eth1'
+  set nat source rule 110 source address '192.168.0.0/24'
+  set nat source rule 110 translation address 'masquerade'
+
+Which results in a configuration of:
+
+.. code-block:: sh
+
+  rule 110 {
+      description "NAT Reflection: INSIDE"
+      destination {
+          address 192.168.0.0/24
+      }
+      outbound-interface eth1
+      source {
+          address 192.168.0.0/24
+      }
+      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: http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers
+
+Our configuration commands would be:
+
+.. code-block:: sh
+
+  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:: sh
+
+  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 that 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:: sh
+
+  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:: sh
+
+  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.
+
+1-to-1 NAT example
+------------------
+
+Here's an extract of a simple 1-to-1 NAT configuration with one internal and
+one external interface:
+
+.. code-block:: sh
+
+  set interfaces ethernet eth0 address '192.168.1.1/24'
+  set interfaces ethernet eth0 description 'Inside interface'
+  set interfaces ethernet eth1 address '1.2.3.4/24'
+  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 '1.2.3.4'
+  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 '1.2.3.4'
+
+Firewall rules are written as normal, using the internal IP address as the
+source of outbound rules and the destination of inbound rules.
+
+NPTv6 (RFC6296)
+---------------
+See here : [[How_to_do_NPTv6]]
+
diff --git a/docs/index.rst b/docs/index.rst
index 9619b1fc..78e938e3 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -33,6 +33,8 @@ as a router and firewall platform for cloud deployments.
 
 .. include:: ch07-firewall.rst
 
+.. include:: ch08-nat.rst
+
 Indices and tables
 ==================