Outbound NAT

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Overview

Outbound NAT determines how traffic leaving a pfSense system will be translated.

Outbound NAT does not control which interface traffic will leave, only how traffic is handled as it exits. To control which interface traffic will exit, use policy routing or Static Routes.

Outbound NAT is configured under Firewall > NAT on the Outbound tab.

Automatic Outbound NAT

The default "Automatic" scenario is to have all traffic that enters from a LAN (or LAN type) interface to have NAT applied so it is translated to the WAN IP address before it leaves.

For static IP configurations, an interface is considered a WAN by the presence of a gateway on the interface's settings, e.g. Interfaces > OPT1. Having a gateway defined under System > Routing is not enough, it must also be selected on the interface configuration or it will not be considered a WAN for NAT or other purposes.

Manual/Advanced Outbound NAT Settings

In order to use Manual/Advanced Outbound NAT rules, navigate to Firewall > NAT on the Outbound tab and select Manual Outbound NAT rule generation (AON - Advanced Outbound NAT), and save. The list should then be populated with the equivalent of the automatic rules, which can then be edited, deleted, or added as needed.

There are a several more possibilities with outbound NAT rules beyond the standard address fields:

Protocol Choice

A protocol may be specified for outbound NAT traffic. This would allow for outbound PPTP to use a separate IP, for instance.

Address Pools

Address pools allow use of a subnet or list of external IP addresses when performing outbound NAT as opposed to the traditional situation which translates traffic to a single external address. Multiple external addresses can help in situations where the resources of a single external IP may not be enough for a large number of internal users.

Choosing Addresses for a Pool

Address pools for outbound NAT translations may be used in several ways:

  • A Proxy ARP VIP subnet (ex: 10.10.10.128/29) shows up in the drop-down for translation target
  • An alias of IP addresses can be chosen from the drop-down list (note: See limitations below)
  • By choosing Other Subnet from the drop-down list, any arbitrary subnet can be used.

Address Pool Options

When an address pool is used, there are several options available that control how NAT translations happen on the pool. These options are:

  • Round Robin
Loops through the translation addresses one at a time in sequence.
  • Random
Selects an address from the pool at random.
  • Source Hash
Uses a hash of the source address to determine the translation address, ensuring that the redirection address is always the same for a given source.
  • Bitmask
Applies the subnet mask and keeps the last portion identical; 10.0.1.50 -> x.x.x.50.
  • Sticky Address variants of Random and Round Robin types
The Sticky Address option can be used with the Random and Round Robin pool types to ensure that a particular source address is always mapped to the same translation address, so long as a state exists for a given internal IP. If all of an internal system's states expire, it may get a different translation IP for the next external connection.

Limitations

Only Round Robin types work with Host Aliases. Any type can be used with a Proxy ARP or "Other Subnet" pool.

Disable NAT

To completely disable NAT to have a routing-only firewall, do the following.

pfSense 2.2 And Later

  1. Navigate to Firewall > NAT on the Outbound tab
  2. Select Disable Outbound NAT rule generation (No Outbound NAT rules)
  3. Click Save
  4. Apply changes

Prior Versions

  1. Navigate to Firewall > NAT on the Outbound tab
  2. Select Manual Outbound NAT rule generation (Advanced Outbound NAT (AON))
  3. Click Save
  4. Delete all rules from the list on the page
  5. Click Apply changes

NAT may be performed on some interfaces and not others by configuring Outbound NAT rules accordingly.

Details may be found in the pfSense book.

Disable NAT and Firewall

To completely disable NAT and all firewall functions from all interfaces, do the following. Note that the previous section ("Disable NAT") is skipped when taking this approach.

  1. Navigate to System > Advanced, Firewall / NAT tab
  2. Check Disable Firewall / Disable all packet filtering
  3. Click Save

See also:

This article is part of the How-To series.