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diff --git a/docs/nat.rst b/docs/nat.rst index 916f6aba..8aafe300 100644 --- a/docs/nat.rst +++ b/docs/nat.rst @@ -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 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 -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. + +* **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: |