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When you need to deploy ngrok behind a corporate firewall, there may be additional steps that you will need to take to make sure ngrok is working properly. As background, this is usually not an issue. Firewalls usually allow outbound connections, which is what an ngrok Agent makes in order to establish a session with the ngrok service and subsequently your tunnel. However, certain corporate firewalls have more restrictions around outbound connections. For example, we’ve seen that ngrok may be blocked on Fortinet firewalls. If you’re having trouble using the ngrok agent to start a tunnel, the first step is to run ngrok diagnose which will produce a report that will help you identify connection issues. 
$ ngrok diagnose
  Testing ngrok connectivity...

  Internet Connectivity
    Name Resolution                           [ OK ]
    TCP                                       [ OK ]
    TLS                                       [ OK ]
  Ngrok Connectivity
    Name Resolution                           [ OK ]
    TCP                                       [ OK ]
    TLS                                       [ OK ]
  Tunnel Protocol                           [ OK ]

Successfully established ngrok connection! (region: 'us', latency: 54.895145ms)
To resolve these issues, you have a couple options:
  • Work with the network team in charge of the corporate firewall to provide exceptions that allow ngrok to make the necessary outbound connections.
  • Set up a custom Connect URL.
When working with the network team, you’ll want to set up the following exceptions and allow:
  • Connect URLs (our default is connect.ngrok-agent.com, but there is one for each region), so that the ngrok agent can connect with our servers
  • Our update URL (the default is update.equinox.io), so that the ngrok agent can quickly update itself
Setting up a custom Connect URL can be useful because it ensures that no one can bring their own ngrok account. In this case, the network admins could continue to block traffic to our default Connect URLs and only allow your custom branded Connect URL. For this, you’ll need to:

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Certificate Revocation List

One of the steps in agent connection is checking the certificate revocation list. This requires an outbound connection on port 80 to the CRL URL (crl.ngrok.com for agent versions 3.9.0 and before, crl.ngrok-agent.com for agent version 3.10.0 and after).If you are unable to connect to this URL, it is possible to skip the CRL check by setting crl_noverify: true in your configuration file. However, disabling the CRL check does expose you to the possibility of using a certificate that has been revoked which could mean that a third party could intercept and view your traffic.

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Testing in a Kubernetes Cluster

If you are using ngrok from within a Kubernetes Cluster, you may need to diagnose the network connectivity from the cluster to the ngrok cloud. To do this, you can run the previously mentioned ngrok diagnose command using the pre-built docker images for the agent as a Job in Kubernetes. Create a manifest file (for example ngrok-manifest.yaml): 
apiVersion: batch/v1
kind: Job
metadata:
  name: ngrok-diagnose
spec:
  template:
    spec:
      containers:
        - name: ngrok-diagnose
          image: ngrok/ngrok:latest
          command: ["/bin/sh", "-c"]
          args: ["ngrok diagnose"]
      restartPolicy: Never
Apply this manifest to your cluster: 
kubectl apply -f ngrok-manifest.yaml
Wait a few seconds and check its logs to see the diagnose command’s output: 
kubectl logs -l "job-name=ngrok-diagnose"

  TLS                                       [ OK ]
Localhost Connectivity
  Name Resolution                           [ OK ]
Ngrok Connectivity - Region: Auto (lowest latency)
  Name Resolution                           [ OK ]
  TCP                                       [ OK ]
  TLS                                       [ OK ]
  Tunnel Protocol                           [ OK ]
Successfully established ngrok connection! (region: 'auto', latency: 54.895145ms)
To generate the more verbose diagnostic report, update your job’s args with the -w flag and file location: 
apiVersion: batch/v1
kind: Job
metadata:
  name: ngrok-diagnose
spec:
  template:
    spec:
      containers:
        - name: ngrok-diagnose
          image: ngrok/ngrok:latest
          command: ["/bin/sh", "-c"]
          args:
            [
              "ngrok diagnose -w /tmp/diagnose_output.txt && cat /tmp/diagnose_output.txt",
            ]
      restartPolicy: Never