Connecting with https to a server application on GCE VM through ngrok tunnel

Question

I am observing a strange behaviour with ngrok on a GCE VM.

I fire up a 'preemptible' VM instance with static external ip, and start ngrok on it normally. I get the URL that maps incoming https request to http on localhost. I am also able to check the tunnel and everything is working perfectly fine. I create a snapshot of the bootable disk attached to this VM. I turn off the VM.

Then I create another GCE VM that has exact same configuration, network settings (with a different static ip of course) etc except that I disable the preemtibility. The bootable disk is also created through the snapshot above, so it is the exact replica of the disk for the preemtible VM above. I start ngrok on this non-preemtible machine. Ngrok doesn't complain anything, it seems to have started a tunnel.

Snapshot of ngrok after starting tunnel on the non-interruptible VM

However, the tunnel doesn't work. No tunnel found

Can anyone point me to possibe causes / directions to investigate?

I already tried following with no success:

1. Logging into ngrok and using ngrok authtoken <token> before starting the tunnel
2. rewriting the host header: ngrok http -host-header=rewrite localhost:3000

Answer 1

I've tried follow your steps on my test project and found no issues.

Please have a look at my steps below:

1. create VM instance with network tags http-server and https-server:
2. create firewall rules to allow HTTP/HTTPS incoming connections:

gcloud compute firewall-rules create default-allow-http --direction=INGRESS --priority=1000 --network=default --action=ALLOW --rules=tcp:80 --source-ranges=0.0.0.0/0 --target-tags=http-server

gcloud compute firewall-rules create default-allow-https --direction=INGRESS --priority=1000 --network=default --action=ALLOW --rules=tcp:443 --source-ranges=0.0.0.0/0 --target-tags=https-server

3. install ngrok and php:

instance-8:~$ sudo apt update
instance-8:~$ sudo apt install snapd
instance-8:~$ sudo snap install ngrok
instance-8:~$ sudo apt install php7.2-cli

4. connect ngrok to the account:

instance-8:~$ ngrok authtoken xxxxxxxxxxxxxxxxxxxxxxxgmfHM

5. start embedded php web-server with a dummy page:

instance-8:~$ php -S localhost:8000
PHP 7.2.24-0ubuntu0.18.04.6 Development Server started at Mon Jul  6 17:19:31 2020
Listening on http://localhost:8000
Document root is /home/username
Press Ctrl-C to quit.

6. start ngrok:

instance-8:~$ ngrok http -host-header=rewrite localhost:8000
                                                                                                                                                                              
ngrok by @inconshreveable                                                                                                                                     (Ctrl+C to quit)
                                                                                                                                                                              
Session Status                online                                                                                                                                          
Account                       XXX (Plan: Free)                                                                                                                       
Version                       2.3.35                                                                                                                                          
Region                        United States (us)                                                                                                                              
Web Interface                 http://127.0.0.1:4040                                                                                                                           
Forwarding                    http://1537504102c2.ngrok.io -> http://localhost:8000                                                                                           
Forwarding                    https://1537504102c2.ngrok.io -> http://localhost:8000                                                                                          
                                                                                                                                                                              
Connections                   ttl     opn     rt1     rt5     p50     p90                                                                                                     
                              2       0       0.03    0.01    0.00    0.00                                                                                                    
                                                                                                                                                                              
HTTP Requests                                                                                                                                                                 
-------------                                                                                                                                                                 
                                                                                                                                                                              
GET /favicon.ico               404 Not Found                                                                                                                                  
GET /                          200 OK       

7. check connection and it works as expected.
8. create new VM instance based on snapshot of the existed VM instance:

gcloud compute disks snapshot instance-8 --snapshot-names=snapshot-1 --zone=europe-west3-a --storage-location=europe-west3
gcloud compute disks create instance-10 --size=10 --zone=europe-west3-a --source-snapshot=snapshot-1 --type=pd-standard
gcloud compute instances create instance-10 --zone=europe-west3-a --machine-type=e2-medium --tags=http-server,https-server --disk=name=instance-10,device-name=instance-10,mode=rw,boot=yes,auto-delete=yes --reservation-affinity=any

9. start embedded php web-server with a dummy page:

instance-10:~$ php -S localhost:8000
PHP 7.2.24-0ubuntu0.18.04.6 Development Server started at Mon Jul  6 17:42:59 2020
Listening on http://localhost:8000
Document root is /home/username
Press Ctrl-C to quit.

10. start ngrok:

instance-10:~$ ngrok http -host-header=rewrite localhost:8000
Your account 'XXX' is limited to 1 simultaneous ngrok client session.
Active ngrok client sessions in region 'us':
  - ts_1eK49pNAq8zIKDN2ikKdCvVhwHz (35.XXX.153.XXX)

ERR_NGROK_108

11. stop ngrok at instance-8 and start it at instance-10:

instance-10:~$ ngrok http -host-header=rewrite localhost:8000
ngrok by @inconshreveable                                                                          (Ctrl+C to quit)
                                                                                                                   
Session Status                online                                                                               
Account                       XXX (Plan: Free)                                                            
Version                       2.3.35                                                                               
Region                        United States (us)                                                                   
Web Interface                 http://127.0.0.1:4040                                                                
Forwarding                    http://38c2f5d9f673.ngrok.io -> http://localhost:8000                                
Forwarding                    https://38c2f5d9f673.ngrok.io -> http://localhost:8000                               
                                                                                                                   
Connections                   ttl     opn     rt1     rt5     p50     p90                                          
                              4       0       0.34    0.39    0.00    0.00                                         
                                                                                                                   
HTTP Requests                                                                                                      
-------------                                                                                                      
                                                                                                                   
GET /favicon.ico               404 Not Found                                                                       
GET /                          200 OK                                                                              

12. check connection and it works as expected.

I was able to reproduce your issue when I checked connection to the shutted down ngrok at instance-8 using old links while ngrok was running at instance-10:

Tunnel 1537504102c2.ngrok.io not found

I've got the same result was when I checked connection to the shutted down ngrok at instance-10 using old links while ngrok was running at instance-8.

I think in your case it could be some issue at the ngrok side and you should contact them via email contact@ngrok.com.