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VPN, CGNAT, and UDP discovery troubleshooting

This guide explains how VPNs, CGNAT, and UDP broadcast affect iShareRadio (iSR) Client remoting with Flex Radio. It complements the networking overview on the Support page.

Two separate network paths

Remoting uses two paths that behave differently:

Path Purpose Typical interface
Local bind / LAN discovery TCP proxy on your computer (SmartSDR connects here) and UDP discovery broadcast on your operator LAN Wi‑Fi or Ethernet (en0, eth0, …)
WAN to the radio Outbound TCP from your computer to the Flex radio's external (UPnP or port-forward) address Whatever route your OS chooses — may include a VPN tunnel

Choosing Broadcast Network Interface (Wi‑Fi/Ethernet on your LAN) fixes local bind and UDP discovery. It does not turn VPN off or force WAN traffic off the tunnel.

VPN can stay connected

A full- or split-tunnel VPN does not need to be disabled for iShareRadio to work. If connection fails, pick the correct LAN interface for local services first; only consider VPN routing if WAN to the radio still fails afterward.

Full-tunnel vs split-tunnel VPN

VPN mode Default route LAN traffic (e.g. 192.168.x.x) WAN to Flex radio
Split tunnel Internet via VPN; RFC1918/LAN often direct Usually stays on Wi‑Fi/Ethernet May use VPN or direct path depending on VPN rules
Full tunnel All traffic via VPN tunnel Often still local for same-subnet / LAN exceptions Often via VPN; depends on VPN split exclusions

The iSR Client auto-detects when the system's "outbound probe" IP is on a VPN tunnel interface and, when possible, switches local bind/advertise to a physical LAN address. That avoids binding the local proxy or UDP discovery on an address that cannot serve SmartSDR on your LAN.

Full-tunnel VPN does not automatically break remoting:

  • Local SmartSDR connects to the proxy on your LAN address.
  • UDP discovery is broadcast on the LAN segment you select.
  • WAN to the radio may still succeed through the VPN if the path is reachable (unless CGNAT or firewall blocks it).

UDP discovery (LAN broadcast) limitations

Flex remoting uses UDP broadcast so SmartSDR on your operator LAN can discover the iSR Client proxy. UDP broadcast has important limits:

  • Scope: Broadcasts stay on the local L2/L3 segment of the interface you choose. They do not cross the internet, most VPN tunnels, or arbitrary routers.
  • Interface matters: The client must bind and advertise on the same LAN as SmartSDR. Wrong interface → grey/red LAN icon, SmartSDR never finds the session.
  • VPN tunnel addresses: Tunnel interfaces (e.g. utun on macOS, tun0 on Linux) often cannot bind a local TCP listener or originate LAN broadcasts the way Wi‑Fi/Ethernet can. The client may show Local TCP proxy could not bind or LAN discovery broadcast failed.
  • Not a WAN transport: UDP discovery is not how traffic reaches the radio over the internet. WAN uses TCP to the Flex external address (UPnP/port forward).

What to do when LAN discovery fails

  1. Expand the iSR Client window (drag the bottom edge down).
  2. Open Broadcast Network Interface.
  3. Select the Wi‑Fi or Ethernet interface on the same network as SmartSDR (the one with your LAN IP, e.g. 192.168.x.x).
  4. Connect again from the Hub marketplace.

VPN can remain connected while you do this.

CGNAT and WAN path issues

CGNAT (Carrier-Grade NAT) means your ISP does not give you a unique public IPv4 address. This affects WAN connectivity, not local bind:

Symptom Likely layer
Red LAN icon; SmartSDR never sees the client Local bind / UDP discovery / wrong interface
Green LAN, blue cell, SmartSDR cannot reach radio WAN path, port forwarding, UPnP, CGNAT, or firewall
Connection starts then drops on cellular/VPN paths Routing, NAT hairpin, or double-NAT

CGNAT on the operator side or shack side can prevent inbound/outbound Flex WAN paths even when VPN and LAN setup are correct. That is an infrastructure limitation, not something Broadcast Network Interface alone fixes.

Indicators in the iSR Client:

  • Cell / WAN icon red after LAN is green → investigate routing, Flex UPnP/port forward, and CGNAT; see Flex networking diagram.
  • Use the clipboard copy from the info box when posting on Groups.io — include device IDs and the status lines.

Quick reference

Goal Action
Fix local proxy / UDP discovery with VPN on Broadcast Network Interface → Wi‑Fi/Ethernet on operator LAN
Understand status icons iSR Client GUI layout
Gather diagnostics for support iSR Client support
Report a platform outage System status

Still stuck?

Post on the iShareRadio Groups.io forum with:

  • Clipboard copy from the iSR Client (include device ID and hub URL).
  • VPN product and whether you use full- or split-tunnel (if known).
  • Whether SmartSDR is on the same LAN as the iSR Client (required for Flex remoting).
  • Screenshot of Broadcast Network Interface and status icons.

For account or credential issues, email support@ishareradio.com instead of the public forum.