> Source: [sk35945](https://support.checkpoint.com/results/sk/sk35945)

# sk35945 - Security Gateway drops RTSP traffic due to a connection link collision in SecureXL

| Property | Value |
|----------|-------|
| Solution ID | sk35945 |
| Date Created | 2008-09-24 |
| Last Modified | 2025-06-24 |
| Technical Level | Advanced |
| Products | Security Gateway |
| Versions | R82.10, R82, R81.20, R81.10 (EOS), R81 (EOS) |

## Symptoms

- * Movies transmitted over RTSP through Security Gateway are not played.

* With Static NAT or without NAT, RTSP traffic passes correctly.

* Kernel debug on the Security Gateway (`fw ctl debug -m fw + link drop`) during the issue shows link collision for the RTSP connection:

  *

    ```
    
    ;fwconn_set_link: failed to set the link (-3)
    ;fwconn_set_link: link collision ignored by SXL
    ;FW-1: fwconn_init_links_for_data_conn: fwconn_set_link(cls_i) failed
    ;fwconn_pending_intercept: failed to add symlinks for data connection entry
    ```

  *

    ```
    
    fwconn_key_set_link: failed to set the link (-3);
    fwconn_key_set_link: link collision ignored by SXL1;
    FW-1: fwconn_key_set_links_inbound: fwconn_key_set_link(cls_o) failed
    FW-1: fwconn_key_init_links: Failed to set VM-side links;
    fw_handle_first_packet: fwconn_key_init_links failed. Dropping packet;
    ```

## Cause

* The Security Gateway drops the RTSP traffic due to a connection link collision in the SecureXL mechanism.
* The Client's port in the Server's response to the RTSP "Setup" command is not being de-NATed.
* RTSP works in this way - it creates a TCP connection and then establishes data connections over UDP.  
  Check Point Security Gateway does not match these UDP packets with the RTSP service.  
  As a result, the return traffic from the remote RTSP streaming side is not mapped back to the client and is dropped by the "Clean Up" rule.

## Solution

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