Disables network packet translation on the outside host device. Traffic that does not match any existing dynamic translations or static port translations are redirected, and packets are not dropped.
Dynamic mapping and interface overload can be configured for gaming devices. For online games, outside traffic comes on a different UDP port. To avoid unwanted traffic or DoS attacks, use access lists. For traffic going from the PC to the outside, it is better to use a route map so that extended entries are created.
When the RTSP protocol passes through a NAT router, the embedded address and port must be translated for the connection to be successful. RTSP is enabled by default. Configuring support for users with static IP addresses enables those users to establish an IP session in a public wireless LAN environment. Optional Displays active NAT translations and additional information for each translation table entry, including how long ago the entry was created and used.
The following is sample output from the show ip nat translations verbose command:. A specific host, access control list, or VRF instance generating an unexpectedly high number of NAT requests may be the source of a malicious virus or worm attack. Configures the maximum number of NAT entries that are allowed from the specified source. The maximum number of allowed NAT entries is , although a typical range for a NAT rate limit is to entries. The following example shows how inside hosts addressed from the Further, packets from outside hosts that are addressed from the NAT is configured as inside source static one-to-one translation.
The following example shows how inside hosts addressed from either the The following example shows how only traffic local to the provider edge PE device running NAT is translated:. The following example shows how to create a pool of addresses that is named net The pool contains addresses from Access list 1 allows packets with SA from If no translation exists, packets matching access list 1 is translated to an address from the pool.
The router allows multiple local addresses The router retains port numbers to differentiate the connections.
In the following example, the goal is to define a virtual address, connections to which are distributed among a set of real hosts. The pool defines addresses of real hosts. The access list defines the virtual address. If a translation does not exist, TCP packets from serial interface 0 the outside interface , whose destination matches the access list, are translated to an address from the pool.
The following example shows how to configure a route map A and route map B to allow outside-to-inside translation for a destination-based Network Address Translation NAT :. The following example shows how to enable static IP address support for the device at The following example shows how to limit the maximum number of allowed NAT entries to The following example shows how to limit the host at IP address NAT commands: complete command syntax, command mode command history, defaults, usage guidelines, and examples.
Network Address Translation on a Stick technology note. Internet Assigned Numbers Authority. Address Allocation for Private Internets. The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies.
Access to most tools on the Cisco Support and Documentation website requires a Cisco. The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train.
Unless noted otherwise, subsequent releases of that software release train also support that feature. The NAT Ability to Use Route Maps with Static Translation feature provides a dynamic translation command that can specify a route map to be processed instead of an access list.
A route map allows you to match any combination of the access list, next-hop IP address, and output interface to determine which pool to use. The ability to use route maps with static translations enables NAT multihoming capability with static address translations. In addition to giving users more control over how NAT addresses are used, the Rate Limiting NAT Translation feature can be used to limit the effects of viruses, worms, and denial-of-service attacks.
Skip to content Skip to search Skip to footer. Book Contents Book Contents. Find Matches in This Book. PDF - Complete Book 4.
Updated: November 11, The following requirements help you decide how to configure and use NAT: Define the NAT inside and outside interfaces if: Users exist off multiple interfaces.
Multiple interfaces connect to the internet. Define what you need NAT to accomplish: Allow internal users to access the internet. Allow the internet to access internal devices such as a mail server. Allow overlapping networks to communicate. Allow networks with different address schemes to communicate. Use NAT during a network transition. Some applications use embedded IP addresses in such a way that translation by a NAT device is impractical.
These applications may not work transparently or not work at all through a NAT device. NAT hides the identity of hosts, which may be an advantage or a disadvantage, depending on the desired result.
A device configured with NAT must not advertise the local networks to the outside. However, routing information that NAT receives from the outside can be advertised in the stub domain as usual. If you specify an access list with a NAT command, NAT will not support the permit ip any any command that is commonly used in the access list.
On Cisco Catalyst Series Switches, if you have a NAT overload configuration, we recommend that you limit the number of NAT translations to less than , by using the ip nat translation max-entries command. If the number of NAT translations is or more, a limited number of ports are available for use by local applications, which, in turn can cause security issues such as denial-of-service DoS attacks.
The port numbers used by local applications can easily be identified by DoS attacks, leading to security threats. This restriction is specific to all NAT overload configurations for example, interface overload or pool overload configurations that use a logical, loopback, or physical address for NAT configurations.
Configuring zone-based policy firewall high availability with NAT and NAT high availability with zone-based policy firewalls is not recommended. If the NAT outside local address matches with any logical interface address, interface IP address, or a tunnel-configured address; then packets are software-switched.
NAT uses the following definitions: Inside local address—An IP address that is assigned to a host on the inside network. You can configure inside source address translation of static or dynamic NAT as follows: Static translation establishes a one-to-one mapping between the inside local address and an inside global address. Figure 1. NAT Inside Source Translation The following process describes the inside source address translation, as shown in the preceding figure: The user at host Based on the NAT configuration, the following scenarios are possible: If a static translation entry is configured, the device goes to Step 3.
Overloading of Inside Global Addresses You can conserve addresses in the inside global address pool by allowing a device to use one global address for many local addresses. Figure 2. NAT Overloading Inside Global Addresses The device performs the following process in the overloading of inside global addresses, as shown in the preceding figure.
The user at host Based on your NAT configuration the following scenarios are possible: If no translation entry exists, the device determines that IP address The following figure shows how NAT translates overlapping networks.
Figure 3. If it is, the device translates the address as described in the following steps: Host Host C receives the packet and continues the conversation. The figure below shows a typical NVI configuration. Figure 4. Domain-specific NAT configurations can be eliminated. The following restrictions apply to an NVI configuration: Route maps are not supported. Figure 5. Benefits of using route maps for address translation are as follows: The ability to configure route map statements provides the option of using IPsec with NAT.
The following restrictions apply to the NAT Route Maps Outside-to-Inside Support feature: Access lists with reversible route maps must be configured to match the inside-to-outside traffic. Outside-to-inside support is not available with PAT. Outside sessions must use an access list.
Denial-of-Service Attacks A denial-of-service DoS attack typically involves misuse of standard protocols or connection processes. Viruses and Worms That Target NAT Viruses and worms are malicious programs that are designed to attack computers and networking equipment. Configuring Static Translation of Inside Source Addresses Configuring Dynamic Translation of Inside Source Addresses Configuring Static Translation of Inside Source Addresses Configure static translation of the inside source addresses to allow one-to-one mapping between an inside local address and an inside global address.
Note Configure different IP addresses for an interface on which NAT is configured and for inside addresses that are configured by using the ip nat inside source static command. Step 2 configure terminal Example: Device configure terminal Enters global configuration mode. Step 3 ip nat inside source static local-ip global-ip Example: Device config ip nat inside source static Step 4 interface type number Example: Device config interface ethernet 1 Specifies an interface and enters the interface configuration mode.
Step 5 ip address ip-address mask [ secondary ] Example: Device config-if ip address Step 6 ip nat inside Example: Device config-if ip nat inside Connects the interface to the inside network, which is subject to NAT.
Step 7 exit Example: Device config-if exit Exits interface configuration mode and returns to global configuration mode. Step 9 ip address ip-address mask [ secondary ] Example: Device config-if ip address Step 10 ip nat outside Example: Device config-if ip nat outside Connects the interface to the outside network.
Note Conditional translation is not supported with ip nat outside source route-map configuration. Configuring Dynamic Translation of Inside Source Addresses Dynamic translation establishes a mapping between an inside local address and a pool of global addresses. Note When inside global or outside local addresses belong to a directly connected subnet on a NAT device, the device adds IP aliases for them. However, a situation can arise where the device answers packets that are not destined for it, possibly causing a security issue.
Such a situation can cause minor security risks. Step 4 access-list access-list-number permit source [ source-wildcard ] Example: Device config access-list 1 permit Step 5 ip nat inside source list access-list-number pool name Example: Device config ip nat inside source list 1 pool net Establishes dynamic source translation, specifying the access list defined in Step 4. Step 6 interface type number Example: Device config interface ethernet 1 Specifies an interface and enters an interface configuration mode.
Step 7 ip address ip-address mask Example: Device config-if ip address This framework enables a Linux machine with an appropriate number of network cards interfaces to become a router capable of NAT. We will use the command utility 'iptables' to create complex rules for modification and filtering of packets. The important rules regarding NAT are - not very surprising - found in the 'nat'-table.
So far no routing decision has taken place, therefore it is not yet known whether the packet would be interpreted locally or whether it would be forwarded to another machine located at another network interface.
In case that the local machine is the recipient, the packet will be directed to the corresponding process and we do not have to worry about NAT anymore. In case that the recipient is located in a sub- net located at a different network interface, the packet will be forwarded to that interface, provided that the machine is configured to do so.
Before we start with our packet manipulations we have to enable the required features. On the one hand we know how IP packets look like, on the other hand we are ready to manipulate packets under Linux and other Unix derivates. Therefore we are ready for our first application!
The most popular question regarding NAT seems to be the one about sharing an internet connection for computers within a private subnet. For this reason I want to start with this particular scenario. First we should consider the following accurate analogon which is hopefully much easier to understand: Let us assume the following situation: there is a landlord with several subtenants. The postman has no idea about the subtenants und would reject every letter that is directly addressed to one of the subtenants.
The landlord has several pigeon holes that can be used for addressing. The subtenants have the possibility to place their letters in a postbox at the landlord's office who will then take the letters to the post office. The question now is: How can all the subtenants fully participate at any kind of mail correspondence i. One solution for this given problem is the following: The landlord takes the letters sent by the subtenants, assigns each subtenant a pigeon hole and then replaces the subtenants address which is in some sense invalid since the postman would reject any answers by the landlord's own address including the pigeon hole number.
The receipient of such a letter will then send the reply back to the landlord including the pigeon hole number and then the landlord could easily hand over after he has replaced his address by the subtenant's address so that the subtenant does not recognise this 'cheat' the letter to the matched subtenant.
This solution is optimal in the sense that it is fully transparent for the subtenants, none of them would ever notice the postman not being capable of sending letters directly to the subtenants! NAT just works similar to the subtenant problem mentioned above.
Every subtenant family represents an IP address in the local net, every subtenant family member represents a port number, the landlords represents a router and the recipient acts as an arbitrary computer in the internet.
Consequently a socket can be seen as a combination of address and pigeon hole or subtenant family und a member of that family. Let us recapitulate: The process of communication is as follows:. We will presume that the standard gateway is set properly at each client. All that is left is to configure the router. Fortunately the netfilter framework automatically adds to each rule its inverse rule, therefore we only have to set one explicit rule. Usually the decision for one of these two rules is made by taking the one with the lower level of undetermination.
For example, the rule 'Replace the sender's address for all packets from the local subnet' is much easier than 'if a client has sent something to a server, then replace the receipient in the server's response by something'. As a rule of thumb can be used that the rule that is executed first is the one that is set explicitly in the kernel. All we want to have is the following: packets arriving from the local net with a receipient's IP address somewhere in the internet have to be modified such that the sender's address is equal to the router's address.
For further command examples let us assume that the first interface 'eth0' is connected to the local net and that the router is connected to the internet via the second interface 'eth1'. The command for a shared internet connection then simply is:.
A quick overview of all available network interfaces gives. Local computers can access the internet, but there are still some restrictions left. A computer located in the internet is not able to establish a connection to a local computer, all he can do is address a port of the router and hope for the best.
Usually the addressed port is currently not used and hence the packet will be rejected. Even if the port is currently used by a local machine the packet might be forwarded but will then usually be rejected since the computer is already communicating with a different computer. In the following example, the goal is to define a virtual address, connections to which are distributed among a set of real hosts.
The pool defines addresses of real hosts. The access list defines the virtual address. If a translation does not exist, TCP packets from serial interface 0 the outside interface , whose destination matches the access list, are translated to an address from the pool. The following example shows how to configure a route map A and route map B to allow outside-to-inside translation for a destination-based Network Address Translation NAT :. The following example shows how to enable static IP address support for the device at The following example shows how to limit the maximum number of allowed NAT entries to The following example shows how to limit the host at IP address NAT commands: complete command syntax, command mode command history, defaults, usage guidelines, and examples.
Internet Assigned Numbers Authority. Address Allocation for Private Internets. The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies. To receive security and technical information about your products, you can subscribe to various services. Access to most tools on the Cisco Support website requires a Cisco. For ease of network management, some sites prefer to translate prefixes rather than addresses.
These sites want the translated address to have the same host number as the original address. The two prefixes must be of the same length. The NAT Host Number Preservation feature can be enabled by configuring dynamic translation with the address pool of the type, match-host. The NAT Performance Enhancement—Translation Table Optimization feature provides greater structure for storing translation table entries and an optimized lookup in the table.
The optimized lookup table enables associating table entries to IP connections. In addition to giving users more control over how NAT addresses are used, the Rate Limiting NAT Translation feature can be used to limit the effects of viruses, worms, and denial-of-service attacks.
This feature is enabled by default when NAT is configured. You cannot disable this configuration. No commands were introduced or modified for this feature. Skip to content Skip to search Skip to footer. Book Contents Book Contents. Find Matches in This Book. PDF - Complete Book 5. Updated: November 13, The following requirements help you decide how to configure and use NAT: Define the NAT inside and outside interfaces if: Users exist off multiple interfaces.
Multiple interfaces connect to the internet. Define what you need NAT to accomplish: Allow internal users to access the internet. Allow the internet to access internal devices such as a mail server.
Allow overlapping networks to communicate. Allow networks with different address schemes to communicate. Use NAT during a network transition. Following is a bind entry in the NAT table: In this example, NAT uses the following definitions: Inside local address—An IP address that is assigned to a host on the inside network.
You can configure inside source address translation of static or dynamic NAT as follows: Static translation establishes a one-to-one mapping between the inside local address and an inside global address. Figure 1. NAT Inside Source Translation The following process describes the inside source address translation, as shown in the preceding figure: The user at host Based on the NAT configuration, the following scenarios are possible: If a static translation entry is configured, the device goes to Step 3.
Overloading of Inside Global Addresses You can conserve addresses in the inside global address pool by allowing a device to use one global address for many local addresses. Figure 2. NAT Overloading Inside Global Addresses The device performs the following process in the overloading of inside global addresses, as shown in the preceding figure.
The user at host Based on your NAT configuration the following scenarios are possible: If no translation entry exists, the device determines that IP address The following figure shows how NAT translates overlapping networks. Figure 3. If it is, the device translates the address as described in the following steps: Host Host C receives the packet and continues the conversation.
Figure 4. Denial-of-Service Attacks A denial-of-service DoS attack typically involves misuse of standard protocols or connection processes. Viruses and Worms That Target NAT Viruses and worms are malicious programs that are designed to attack computers and networking equipment.
Configuring Static Translation of Inside Source Addresses Configuring Dynamic Translation of Inside Source Addresses Configuring Static Translation of Inside Source Addresses Configure static translation of the inside source addresses to allow one-to-one mapping between an inside local address and an inside global address.
Note Configure different IP addresses for an interface on which NAT is configured and for inside addresses that are configured by using the ip nat inside source static command. Step 2 configure terminal Example: Device configure terminal Enters global configuration mode. Step 3 ip nat inside source static local-ip global-ip Example: Device config ip nat inside source static Step 4 interface type number Example: Device config interface ethernet 1 Specifies an interface and enters the interface configuration mode.
Step 5 ip address ip-address mask [ secondary ] Example: Device config-if ip address Step 6 ip nat inside Example: Device config-if ip nat inside Connects the interface to the inside network, which is subject to NAT. Step 7 exit Example: Device config-if exit Exits interface configuration mode and returns to global configuration mode.
Step 9 ip address ip-address mask [ secondary ] Example: Device config-if ip address Step 10 ip nat outside Example: Device config-if ip nat outside Connects the interface to the outside network.
Note Conditional translation is not supported with ip nat outside source route-map configuration. Configuring Dynamic Translation of Inside Source Addresses Dynamic translation establishes a mapping between an inside local address and a pool of global addresses. Note When inside global or outside local addresses belong to a directly connected subnet on a NAT device, the device adds IP aliases for them.
However, a situation can arise where the device answers packets that are not destined for it, possibly causing a security issue. Such a situation can cause minor security risks. Step 4 access-list access-list-number permit source [ source-wildcard ] Example: Device config access-list 1 permit Step 5 ip nat inside source list access-list-number pool name Example: Device config ip nat inside source list 1 pool net Establishes dynamic source translation, specifying the access list defined in Step 4.
Step 6 interface type number Example: Device config interface ethernet 1 Specifies an interface and enters an interface configuration mode. Step 7 ip address ip-address mask Example: Device config-if ip address Step 8 ip nat inside Example: Device config-if ip nat inside Connects the interface to the inside network, which is subject to NAT.
Step 9 exit Example: Device config-if exit Exits interface configuration mode and returns to global configuration mode. Step 10 interface type number Example: Device config interface ethernet 0 Specifies an interface and enters an interface configuration mode. Step 11 ip address ip-address mask Example: Device config-if ip address Step 12 ip nat outside Example: Device config-if ip nat outside Connects the interface to the outside network.
Using NAT to Allow Internal Users Access to the Internet Perform this task to allow your internal users access to the Internet and conserve addresses in the inside global address pool using overloading of global addresses. Step 5 ip nat inside source list access-list-number pool name overload Example: Device config ip nat inside source list 1 pool net overload Establishes dynamic source translation with overloading, specifying the access list defined in Step 4.
Step 6 interface type number Example: Device config interface ethernet 1 Specifies an interface and enters the interface configuration mode. Step 10 interface type number Example: Device config interface ethernet 0 Specifies an interface and enters the interface configuration mode. Note On Catalyst Series Switches, when the NAT translation is done in the hardware, timers are reset every seconds or once the set timeout value is reached.
Changing the Timeouts When Overloading Is Configured If you have configured overloading, you can control the translation entry timeout, because each translation entry contains more context about the traffic using it. Step 3 ip nat translation seconds Example: Device config ip nat translation Optional Changes the amount of time after which NAT translations time out. The default timeout is 24 hours, and it applies to the aging time for half-entries.
Step 4 ip nat translation udp-timeout seconds Example: Device config ip nat translation udp-timeout Optional Changes the UDP timeout value. Step 6 ip nat translation tcp-timeout seconds Example: Device config ip nat translation tcp-timeout Optional Changes the TCP timeout value. Step 7 ip nat translation finrst-timeout seconds Example: Device config ip nat translation finrst-timeout 45 Optional Changes the finish and reset timeout value. Step 9 ip nat translation syn-timeout seconds Example: Device config ip nat translation syn-timeout 45 Optional Changes the synchronous SYN timeout value.
If you want to communicate with those hosts or routers by using static translation. Step 5 ip address ip-address mask Example: Device config-if ip address Step 6 ip nat inside Example: Device config-if ip nat inside Marks the interface as connected to the inside. Step 8 interface type number Example: Device config interface ethernet 0 Specifies an interface and enters the interface configuration mode. Step 9 ip address ip-address mask Example: Device config-if ip address Step 10 ip nat outside Example: Device config-if ip nat outside Marks the interface as connected to the outside.
Step 5 ip nat inside destination-list access-list-number pool name Example: Device config ip nat inside destination-list 2 pool real-hosts Establishes dynamic inside destination translation, specifying the access list defined in the prior step. Step 6 interface type number Example: Device config interface ethernet 0 Specifies an interface and enters the interface configuration mode. Step 8 ip nat inside Example: Device config-if ip nat inside Marks the interface as connected to the inside.
Step 10 interface type number Example: Device config interface serial 0 Specifies a different interface and enters the interface configuration mode. Step 12 ip nat outside Example: Device config-if ip nat outside Marks the interface as connected to the outside.
Enabling Route Maps on Inside Interfaces Before you begin All route maps required for use with this task must be configured before you begin the configuration task. Step 2 configure terminal Example: Device config configure terminal Enters global configuration mode. Note When you configure the ip nat outside source static command to add static routes for outside local addresses, there is a delay in the translation of packets and packets are dropped.
Benefits of configuring NAT of external IP addresses only are: Allows an enterprise to use the Internet as its enterprise backbone network. Allows the use of network architecture that requires only the header translation. Unless you are doing some subnetting here, I would suggest keeping those internal addresses at their defaults which are:. I'd like to discuss some configuration instructions that I can't get working. You seem to have a handle on this tech. Either you bought the Always use private adresses inside the NATed network!
I would suggest 0. Then linux users want to know why companies are still running Microsoft. This how to will make me run back.
I m not a linux engineer but i have responsibilty to solve that Hi, I will give an relevant update for users that need it on XenServer 6. Uncomment the following line to enable packet forwarding for IPv4 and other stuff. I have only a LAN network I need all the packets coming from network For all the hosts in network Of course the Ubuntu box needs to translate the incoming packets destination address from I am stack with the outgoing packets source address translation from
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