Support / Voice / Firewall / ASUS

Disable SIP ALG on ASUS routers.

ASUS calls SIP ALG "SIP Passthrough." It's enabled by default on RT-AX, ZenWiFi, and ASUSWRT-based routers. Disable it before placing production traffic.

Steps

  1. Sign in to the router admin UI: http://router.asus.com or http://192.168.50.1.
  2. In the left sidebar, open WAN (under Advanced Settings).
  3. Click the NAT Passthrough tab.
  4. Find SIP Passthrough — change the dropdown to Disable.
  5. Click Apply at the bottom of the page.
  6. Reboot the router (Administration → System → Reboot) to clear any cached SIP sessions.

Merlin firmware

If you're running ASUSWRT-Merlin, the path is identical. Some Merlin builds also expose a nf_conntrack_sip kernel-module toggle under Tools → Other Settings — leave it disabled.

UDP NAT timeout — increase if registration drops

Stock ASUSWRT doesn't expose a UDP conntrack timeout knob in the GUI. If you see SIP registration drops between calls (typical symptom: inbound calls only work right after an outbound call refreshes the mapping), use one of these:

  • Best: set the SIP REGISTER interval on your device to 360 seconds with NAT keepalive at 30 seconds; the keepalive refreshes the NAT mapping between registers.
  • If running ASUSWRT-Merlin: SSH in and apply echo 600 > /proc/sys/net/netfilter/nf_conntrack_udp_timeout_stream; persist via the firmware's nat-start script under Tools → User Scripts.

Verify

  • Place a test call in both directions (inbound and outbound).
  • Confirm two-way audio for at least one minute.
  • Stay on the call past 30 minutes — re-INVITE failures from ALG often surface there.
  • Wait 5–10 minutes between calls without traffic; place an inbound test call. If it goes to voicemail / fails until you place an outbound call first, raise the UDP timeout or shorten REGISTER interval.