Networking Fix

RTSP ALG — The Hidden Reason Your Camera Stream Breaks

RTSP ALG is a router feature designed to help RTSP traverse NAT, but on modern networks it usually breaks camera streams instead. If your RTSP camera connects yet shows no video, disable RTSP ALG first.

Quick Answer

RTSP ALG is a router feature designed to help RTSP work through NAT — but on modern networks it usually BREAKS streams. If your camera connects but shows no video, try disabling RTSP ALG on your router.

What is RTSP ALG?

ALG means Application Layer Gateway. It is a feature in routers and firewalls that inspects specific protocols like SIP, FTP, and RTSP, then rewrites packets so they can cross NAT (Network Address Translation).

RTSP ALG specifically intercepts RTSP SETUP messages and rewrites the IP addresses and ports that define where RTP media should flow. This was intended to help when both the client and camera were behind NAT and needed the router to assist.

That behavior made more sense in older enterprise networks. On modern home networks with a single NAT and local LAN cameras, RTSP ALG usually creates more problems than it solves.

Why RTSP ALG Breaks Modern Streams

RTSP ALG modifies RTSP packets in transit, but it often does so incorrectly. Modern cameras, encrypted links, TCP transport, and vendor-specific RTSP dialects make those packet rewrites fragile.

Connects, then times out

The router drops or mangles the SDP response, so the viewer never learns the real RTP ports.

Video freezes after a few seconds

RTSP ALG garbles RTP port negotiation, so media starts briefly and then stalls.

Works in one app but not another

Different RTSP clients negotiate streams differently, which can confuse the router helper.

Some cameras fail, others work

RTSP ALG behavior depends on the exact RTSP dialect, SDP format, and transport mode used by the camera.

Many consumer router vendors — ASUS, TP-Link, Netgear, and MikroTik among them — still ship with RTSP ALG or related conntrack helpers enabled by default.

How to Disable RTSP ALG on Popular Routers

ASUS Routers

Advanced Settings → WAN → NAT Passthrough → RTSP Passthrough → Off

TP-Link / Archer

Advanced → NAT Forwarding → ALG → RTSP → disable

Netgear

Advanced → WAN Setup → Disable SIP ALG. On many Netgear models RTSP ALG is bundled into the same helper setting.

MikroTik

IP → Firewall → Service Ports → find rtsp → disable

/ip firewall service-port disable rtsp

Ubiquiti EdgeRouter / UniFi

Configure → Firewall/NAT → NAT → disable RTSP ALG

delete system conntrack modules rtsp

pfSense / OPNsense

Firewall → Settings → Advanced → disable RTSP in conntrack helpers.

Generic Router

Look for settings named ALG, SIP/RTSP passthrough, NAT helpers, or conntrack modules, then disable specifically the RTSP or SIP/RTSP helper entry.

RTSP ALG vs RTSP Passthrough — What's the Difference?

RTSP ALG
  • Actively modifies RTSP packets
  • Rewrites ports and IP addresses
  • Often destructive on modern networks
RTSP Passthrough
  • Allows RTSP traffic through the firewall
  • Does not rewrite the protocol payload
  • Usually the better option if you must expose RTSP internally

Best practice: disable RTSP ALG, leave simple passthrough behavior enabled if needed, and use a VPN for remote access instead of public port forwarding.

Still Not Working? Try These Fixes

Switch to TCP mode: use rtspt:// instead of rtsp:// in SmartRTSP to force TCP transport and bypass many UDP negotiation issues.

Check port 554: confirm your firewall is not blocking TCP/UDP port 554 between the viewer and the camera.

Verify subnet placement: make sure your camera and viewer are on the same subnet unless you intentionally routed traffic between VLANs.

Use a VPN for remote access: if you're outside the home, use Tailscale or another VPN instead of exposing RTSP directly to the internet.

Read the VPN remote access guide →

Frequently Asked Questions

What does RTSP ALG stand for?
RTSP ALG stands for Application Layer Gateway — a router feature that inspects and modifies RTSP packets traversing NAT.
Should I enable or disable RTSP ALG?
Disable it. On modern home networks RTSP ALG usually breaks streams instead of helping them.
My RTSP stream connects but no video — is it ALG?
Very likely. This is one of the most common RTSP ALG failure patterns, especially when the app stays on “connecting” or freezes after initial video.
Does SmartRTSP work with RTSP ALG enabled?
SmartRTSP works best with RTSP ALG disabled. If you cannot change the router immediately, try rtspt:// in SmartRTSP to force TCP transport.