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[ScreenOS] SIP, H.323, RTSP ALGs have problems with Interface Based NAT



Article ID: KB7407 KB Last Updated: 14 Dec 2017Version: 8.0
SIP, H.323, RTSP connections are not working and Trust Interface is configured in NAT mode (interface-based NAT)
  • SIP
  • H.323
  • RTSP

Any application that uses SIP, H.323, or RTSP will not work properly if Interface Based NAT is configured.  The ALG will not translate the IP properly in the payload.

These VoIP applications will only properly function when using policy based NAT.  This also affects IPSec VPNs.

To resolve this issue, it is strongly recommended that policy-based NAT (Source Network Address Translation within the policy) is used on the policies that the SIP, H.323 and RTSP traffic passes through.  Typically, when policy-based NAT is used, the Trust or Source Interface is changed to Route mode, and all policies (that are required to be NAT'd) are configured to use policy-based NAT.   However, It is okay that the Trust or Source Interface still remain in NAT mode, as long as the SIP, H.323 pass through a policy with NAT enabled on the policy. 

For information on configuring Policy-based NAT, consult: KB4771-Using the NAT Option in a Policy

Note: Refer to the Concepts and Examples ScreenOS Reference Guide for configuration examples of Source Network Address Translation in a policy too:
Volume 8:  Address Translation
Chatper 2: Source Network Address Translation
ScreenOS 5.4:
ScreenOS 6.0:


Modification History:
2017-12-07: Article reviewed for accuracy. Minor grammatical changes. Rest of the Article is correct and complete.
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