SIP Call Setup sees a delay when SNAT is Disabled

Products

• VMware NSX

Issue/Introduction

When operating a Virtual Machine (VM) running a SIP-based application, following issues are observed:

• SIP traffic and call setups delaying to complete over the internet.

• The call setup hangs for a specific duration before it gets completes.

• Verification: When Source Network Address Translation (SNAT) is enabled, the call setup completes successfully. When SNAT is disabled, the connection stalls.

Cause

This issue occurs when the SIP application is configured to perform a DNS lookup request at the initiation of every call.

With SNAT enabled, the VM has outbound internet reachability, allowing the external DNS lookup to succeed and the SIP call to proceed. When SNAT is disabled, internet access is removed. Consequently, the outbound DNS query is dropped, causing the application to time out.

Resolution

 If the VM is expected to resolve external DNS queries, it must maintain outbound internet reachability (e.g., via SNAT).

However, if external DNS resolution is not strictly required for the SIP application to function, you can remove the external DNS server entries from the VM's network configuration. Doing this prevents the SIP application from attempting the lookup entirely, bypassing the timeout period and allowing the call to initiate immediately.