Example: Configuring Source NAT for Multiple Addresses with PAT

This example describes how to configure a source NAT mapping of a private address block to a smaller public address block using port address translation.

• Requirements
• Overview
• Configuration
• Verification

Requirements

Before you begin:

1. Configure network interfaces on the device. See the Junos OS Interfaces Configuration Guide for Security Devices.
2. Create security zones and assign interfaces to them. See Understanding Security Zones.

Overview

This example uses the trust security zone for the private address space and the untrust security zone for the public address space. In Figure 116, the source IP address in packets sent from the trust zone to the untrust zone is mapped to a smaller block of public addresses in the range from 1.1.1.1/32 through 1.1.1.24/32. Because the size of the source NAT address pool is smaller than the number of potential addresses that might need to be translated, port address translation is used.

Note: Port address translation includes a source port number with the source IP address mapping. This allows multiple addresses on a private network to map to a smaller number of public IP addresses. Port address translation is enabled by default for source NAT pools.

This example describes the following configurations:

• Source NAT pool src-nat-pool-1 that contains the IP address range 1.1.1.1/32 through 1.1.1.24/32.
• Source NAT rule set rs1 to match all packets from the trust zone to the untrust zone. For matching packets, the source IP address is translated to an IP address in the src-nat-pool-1 pool.
• Proxy ARP for the addresses 1.1.1.1/32 through 1.1.1.24/32 on interface ge-0/0/0.0. This allows the Juniper Networks security device to respond to ARP requests received on the interface for those addresses.
• Security policies to permit traffic from the trust zone to the untrust zone.

Configuration

CLI Quick Configuration

To quickly configure a source NAT mapping from a private address block to a smaller public address block using PAT, copy the following commands and paste them into the CLI.

Step-by-Step Procedure

The following example requires you to navigate throughout various levels in the configuration hierarchy. For instructions on how to do that, see Using the CLI Editor in Configuration Mode.

To configure a source NAT mapping from a private address block to a smaller public address block using PAT:

1. Create a source NAT pool.
   [edit security nat source]user@host# set pool src-nat-pool-1 address 1.1.1.1 to 1.1.1.24
2. Create a source NAT rule set.
   [edit security nat source]user@host# set rule-set rs1 from zone trustuser@host# set rule-set rs1 to zone untrust
3. Configure a rule that matches packets and translates the source address to an address in the pool.
   [edit security nat source]user@host# set rule-set rs1 rule r1 match source-address [10.1.1.0/24 10.1.2.0/24 192.168.1.0/24]user@host# set rule-set rs1 rule r1 match destination-address 0.0.0.0/0user@host# set rule-set rs1 rule r1 then source-nat pool src-nat-pool-1
4. Configure proxy ARP.
   [edit security nat]user@host# set proxy-arp interface ge-0/0/0.0 address 1.1.1.1 to 1.1.1.24
5. Configure a security policy that allows traffic from the trust zone to the untrust zone.
   [edit security policies from-zone trust to-zone untrust]user@host# set policy internet-access match source-address any destination-address any application anyuser@host# set policy internet-access then permit

Results

From configuration mode, confirm your configuration by entering the show security nat and show security policies commands. If the output does not display the intended configuration, repeat the configuration instructions in this example to correct it.

If you are done configuring the device, enter commit from configuration mode.

Verification

To confirm that the configuration is working properly, perform these tasks:

• Verifying Source NAT Pool Usage
• Verifying Source NAT Rule Usage
• Verifying NAT Application to Traffic

Verifying Source NAT Pool Usage

Purpose

Verify that there is traffic using IP addresses from the source NAT pool.

Action

From operational mode, enter the show security nat source pool all command. View the Translation hits field to check for traffic using IP addresses from the pool.

Verifying Source NAT Rule Usage

Purpose

Verify that there is traffic matching the source NAT rule.

Action

From operational mode, enter the show security nat source rule all command. View the Translation hits field to check for traffic that matches the rule.

Verifying NAT Application to Traffic

Purpose

Verify that NAT is being applied to the specified traffic.

Action

From operational mode, enter the show security flow session command.

Related Topics

• Junos OS Feature Support Reference for SRX Series and J Series Devices
• Understanding Source NAT
• Source NAT Configuration Overview
• Understanding Source NAT Pools with PAT