SIP ALG: Why It Breaks VoIP and How to Disable It
Introduction
If you’ve ever encountered one-way audio, dropped calls, or registration failures in your VoIP deployment, there’s a good chance SIP ALG is the cause. Often enabled by default on routers and firewalls, SIP ALG tries to help — but usually makes things worse.
Let’s unpack what SIP ALG is, why it causes problems, and how to turn it off.
What Is SIP ALG?
SIP ALG (Application Layer Gateway) is a network feature found in many consumer- and enterprise-grade routers. It’s designed to:
- Inspect SIP packets
- Modify SIP headers
- Rewrite IP addresses for NAT traversal
In theory, this sounds helpful — especially for VoIP — but in practice, it messes with packet structure, breaks session integrity, and interferes with NAT traversal techniques like STUN, TURN, and ICE used in WebRTC.
Common Symptoms of SIP ALG Interference
- Phones register but can’t make or receive calls
- One-way audio (especially on WebRTC softphones)
- SIP messages show incorrect internal IPs
- Calls drop after 30 seconds
- Random call setup failures
Why It’s a Nightmare for VoIP and WebRTC
WebRTC and SIP rely on precise session negotiation between devices and servers. When SIP ALG modifies packets midstream, SIP proxies or SBCs can no longer validate headers or negotiate media, leading to failed calls.
Worse still, it’s often hard to diagnose — especially when users are behind remote networks, VPNs, or mobile hotspots.
How to Disable SIP ALG
Step 1: Check If SIP ALG Is Enabled
You can use command-line tools or apps like:
- sipalg-detect
- Wireshark (look for IP mismatches in SIP headers)
Step 2: Disable It on Common Routers
Most enterprise routers allow SIP ALG to be disabled. Here are some resources:
For larger teams or hosted solutions, work with your IT or VoIP provider to enforce router settings company-wide.
Bonus: Use a WebRTC Proxy That Bypasses ALG Chaos
One workaround is to place a WebRTC proxy or gateway (like Siperb) between your client and SIP server. This isolates the browser from direct NAT complications and handles session negotiation reliably — even if ALG is active downstream.
Conclusion
SIP ALG is the invisible gremlin behind many VoIP headaches. If you’re serious about WebRTC reliability — especially across remote networks — disabling it should be a top priority.
We’d love your questions or comments on today’s topic!
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Thought for the day:
“The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.” Bob Marley
