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Barracuda Phone System

What is a SIP ALG, and how does it affect the Barracuda Phone System?

  • Type: Knowledgebase
  • Date changed: 4 years ago

Solution #00005672

 

Scope:

This solution applies to all Barracuda Phone Systems.

 

Answer:

An Application Layer Gateway is a security filter or service included in many network firewalls and routers. It is intended to handle application-level Routing, Filtering, and NAT-Traversal of various services used in inter-networking today.


What purpose does a SIP ALG serve?

Application Layer Gateways preform a few optional services on a network:

  • NAT Traversal for certain application services that do not support it by default
  • Session control for application traffic which requires specific ports to remain open
  • Policy-based routing and re-direction of traffic for certain applications
  • Granular control of application functions, commands, and features

How do SIP ALGs work?

An Application Layer Gateway is a security filter or service included in many network firewalls and routers. It is intended to handle application-level Routing, Filtering, and NAT-Traversal of various services used in inter-networking today.


ALGs use Stateful Package Inspection or Deep Package Inspection in order to to detect and modify their target application data. ALGs perform in a manner similar to Proxies, but do so without interaction from the application itself,


How can you identify a SIP ALG on a firewall or router?

SIP ALGs are labeled with a variety of naming conventions including:
  • SIP Inspect Service
  • SIP Protocol Fixup
  • SIP Pass-Through
  • SIP Cleanup
  • SIP Helper
  • Voice Filtering Rules
Most firewalls/Routers that have SIP ALG services enable them by default. This often means that, particularly with Cisco Devices, showing the current configuration may not indicate that a SIP ALG is running if the device does not list default values. Reviewing the Firewalls/Routers documentation will be the best method of determining if there is an ALG enabled by default. The SIP ALG services are often included as part of a Stateful Packet Inspection service

What are the symptoms of a SIP ALGs that is not compatible?

An Application Layer Gateway that is not compatible can cause a wide range of issues for inbound and outbound calling. The most common symptoms of an Application Layer Gateway can include, but are not limited to:

  • One way calling issues
  • One direction calls not passing audio
  • Remote phones disconnecting after less than one minute
  • No audio on calls
  • Particular types of SIP packets blocked (update, notification, session progress...)


What are Barracuda's recommendations with regard to SIP ALGs?

The Barracuda Phone System is designed with NAT traversal in mind and, in most instances, will handle the same functions as an ALG most effectively without an external filtering server/service in place. In almost all known cases, it is recommended that you disable any SIP ALGs.


Link to this page:

https://campus.barracuda.com/solution/50160000000IYCtAAO