> Source: [sk172805](https://support.checkpoint.com/results/sk/sk172805)

# sk172805 - IPSec VPN uses an incorrect source IP address for NAT-T when initiating encrypted traffic, IKE negotiation uses correct IP address

| Property | Value |
|----------|-------|
| Solution ID | sk172805 |
| Date Created | 2021-03-29 |
| Last Modified | 2026-01-21 |
| Technical Level | General |
| Products | Security Gateway |
| Versions | R82.10, R82, R81.20, R81.10 (EOS), R81 (EOS) |

## Symptoms

- * IPSec VPN uses an external IP address. The user configured a different IP address in the incoming/outgoing link selection when initiating encrypted NAT-T traffic.
* IKE negotiation uses the correct link selection IP address.
* Response traffic to peer-initiated traffic uses the correct link selection IP address.
* Remote Access uses the correct link selection IP address.
* The resolved_link table shows the correct source IP address.

## Cause

IPsec VPN uses the wrong source IP address when it initiates NAT-T encrypted traffic.

## Solution

This problem was fixed (requires manual configuration of the new kernel parameter).  
The fix is included starting from:

* [Check Point R81.10](https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid=sk170416)
* [Jumbo Hotfix Accumulator for R81](https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid=sk170114) starting from Take 51
* [Jumbo Hotfix Accumulator for R80.40](https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid=sk165456) starting from Take 126
* [Jumbo Hotfix Accumulator for R80.30](https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid=sk153152) starting from Take 241

Check Point recommends to always upgrade to the most recent version ([Security Gateway](https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doShowproductpage&productTab=downloads&product=435)).

### Hotfix

If you choose not to upgrade, [Contact Check Point Support](https://www.checkpoint.com/support-services/contact-support/) to get a Hotfix for this issue.   
A Support Engineer will make sure the Hotfix is compatible with your environment before providing the Hotfix.   
For faster resolution and verification please collect [CPinfo](https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid=sk92739) files from the Security Management and Security Gateways involved in the case.

**Hotfix installation instructions:**   
Refer to [sk168597 - How to install a Hotfix](https://supportcenter.checkpoint.com/supportcenter/portal?eventSubmit_doGoviewsolutiondetails=&solutionid=sk168597).

### Configuration Instructions

This fix adds a new kernel parameter "*override_source_ip_with_ls_for_natt*" that controls the IP address for the NAT-T encrypted traffic:

|-----------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| Parameter Value | Security Gateway Behavior                                                                                                                                            |
| 0               | This is the default. The Security Gateway uses the IP address of the external interface for NAT-T traffic.                                                           |
| 1               | The Security Gateway uses the IP address configured for the Link Selection for NAT-T traffic (and updates the kernel table "*orig_route_params*" table accordingly). |

Procedure:

1. Connect to the command line on the Security Gateway / each Cluster Member.
2. Log in to the Expert mode.
3. Configure the value "1" for the kernel parameter "*override_source_ip_with_ls_for_natt* ":  
   `fw ctl set -f int override_source_ip_with_ls_for_natt 1`  
   This command changes the value in the current session and in the configuration file *$FWDIR/boot/modules/fwkern.conf* to survive reboot.

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