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How To Troubleshoot Port Forwarding Issues

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TitleHow To Troubleshoot Port Forwarding Issues
Objective
  • Troubleshoot port forwarding issues
  • Fix port forwarding
Environment
Procedure
Below are possible port forwarding issues listed respectively from most to least common. 
  1. Verify the router's WAN has a public IP address. Note that the connection may be NAT'd, and the address seen from the Web (ie: whatismyip.com) may not be the address on the WAN of the Cradlepoint. This is especially common for LTE / 3G connections.
    1. NCOS: How to determine if you have a publicly routable IP address
    2. Remote connection and port forwarding isn't working because of a carrier NAT
  2. Verify that you have the correct TCP/IP information on the client.
    1. The client will need a valid IP address / subnet mask within the range of a network owned by the Cradlepoint.
    2. The client will need its default gateway set to Cradlepoint's LAN interface.
    3. Port forwarding is not working because the default gateway is not set on endpoint device
  3. Verify that the appropriate ports are open on the client.
    1. Test the connection locally (from the LAN). For instance, if you are trying to Remote Desktop (RDP) to a PC on your LAN, see that you can RDP from another computer on that LAN to the client of interest. If you are trying to open a web page / http connection, try it from another computer on the same LAN as the client of interest.
    2. Test the ports using telnet. It may be necessary to enable/install telnet as a Windows feature on a Windows computer. Once the service is accessible, you can test ports by using the following syntax: telnet <hostnameOrIP> <PortToTest> . If the port is open, the session will connect and you will see a blinking cursor. If the port is closed, you will see a connect failure message. 
    3. Be aware that Windows Firewall and other client-side firewalls could be blocking these connections. In the case the port is showing closed, disable any local firewalls and test again. 
    4. It may be necessary to contact the manufacturer of the client device for assistance with confirming the correct ports are configured to listen for incoming traffic.
  4. Verify that the remote connection is reaching the WAN of the Cradlepoint.
    1. Use Remote Admin to test that the port is not being blocked by the carrier. First, use the default port is working for remote connect (port 8080). Then, test the port you are trying to forward. If some ports go through and some do not, the unsuccessful ports are probably being blocked by the carrier. If remote connect does not work on any port, please refer to step 1.
      1. NCOS: How to Configure Remote Administration on a Cradlepoint Router
    2. Test that traffic is being received by the Cradlepoint, and verify that it is being forwarded out the correct LAN using the TCPdump tool. If the traffic is not being received, there is a problem with the remote client's request (could be incorrectly configured) or the port is being blocked by the carrier. If the traffic is being received on the WAN and not forwarded to the LAN, then the client you're trying to access may not have the correct, refer to steps 2 and 3.
      1. NCOS: CLI - Introduction to TCPdump
  5. Swap known working ports. For example, if remote administration is working on port 8443, but port forwarding is not working on port 29000, switch them - use 29000 for remote administration, and set the internet port on the port forwarding rule (not the local port) to 8443.
    1. Retest whether ports are open again with a site like https://www.yougetsignal.com/tools/open-ports/
    2. If the port forwarding starts working, and the remote administration stops working, most like the Carrier is blocking those ports.
      1. Port forwarding and remote admin not working because the Cradlepoint has a public restricted WAN IP address
      2. Port forwarding not working because ports are blocked by the carrier
      3. Remote connection and port forwarding isn't working because of a Secured SIM card
    3. If remote administration still works while using a port that didn't function for port forwarding previously, but the port forwarding stops working while using the previously-working remote admin port, then the device itself is not properly listening on the forwarded ports. Refer back to step 3 for additional troubleshooting.
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