start-iap-tunnel unable to connect to a listening port - ssl

Question

I'm installing OpenVPN Access Server on a Google Cloud instance. Its webUI listens on port 943 using https. It has a self-signed certificate whose name doesn't match the server's hostname (10.150.0.2). I can't start an SSH tunnel. I'm looking for a way to troubleshoot the connection from the IAP service to my server.
The command I'm running is gcloud compute start-iap-tunnel vpn 943 --local-host-port=localhost:943 I receive the normal Testing if tunnel connection works message.
It errs out with ERROR: (gcloud.compute.start-iap-tunnel) While checking if a connection can be made: Error while connecting [4003: 'failed to connect to backend']. (Failed to connect to port 943)
If I add --log-http to the command invocation the relevant information follows (it looks like a normal req/resp cycle with a 200 that I assume is from my client to the IAP service):
Testing if tunnel connection works.
=======================
==== request start ====
uri: https://oauth2.googleapis.com/token
method: POST
== headers start ==
b'content-type': b'application/x-www-form-urlencoded'
b'user-agent': b'google-cloud-sdk gcloud/367.0.0 command/gcloud.compute.start-iap-tunnel invocation-id/db27de82264f47fcb63f6680afaa8327 environment/None environment-version/None interactive/False from-script/False python/3.7.9 term/xterm-256color (Macintosh; Intel Mac OS X 21.2.0)'
== headers end ==
== body start ==
Body redacted: Contains oauth token. Set log_http_redact_token property to false to print the body of this request.
== body end ==
==== request end ====
---- response start ----
status: 200
-- headers start --
Alt-Svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000,h3-Q050=":443"; ma=2592000,h3-Q046=":443"; ma=2592000,h3-Q043=":443"; ma=2592000,quic=":443"; ma=2592000; v="46,43"
Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Content-Encoding: gzip
Content-Type: application/json; charset=utf-8
Date: Fri, 24 Dec 2021 02:11:52 GMT
Expires: Mon, 01 Jan 1990 00:00:00 GMT
Pragma: no-cache
Server: scaffolding on HTTPServer2
Transfer-Encoding: chunked
Vary: Origin, X-Origin, Referer
X-Content-Type-Options: nosniff
X-Frame-Options: SAMEORIGIN
X-XSS-Protection: 0
-- headers end --
-- body start --
Body redacted: Contains oauth token. Set log_http_redact_token property to false to print the body of this response.
-- body end --
total round trip time (request+response): 0.246 secs
---- response end ----
----------------------
ERROR: (gcloud.compute.start-iap-tunnel) While checking if a connection can be made: Error while connecting [4003: 'failed to connect to backend']. (Failed to connect to port 943)
To my knowledge this is the limit of easily accessible troubleshooting for start-tap-tunnel.
Moving on to the local machine we can connect to 10.150.0.2:943 before puking a la certificate.
root#viongier:/usr/local/openvpn_as# wget https://10.150.0.2:943
--2021-12-24 02:01:47-- https://10.150.0.2:943/
Connecting to 10.150.0.2:943... connected.
ERROR: The certificate of ‘10.150.0.2’ is not trusted.
ERROR: The certificate of ‘10.150.0.2’ doesn't have a known issuer.
The certificate's owner does not match hostname ‘10.150.0.2’
It seems to me that my client happily connects to the IAP service which fails to connect to my server. I would expect to see an IAP error if it was erring out because of the cert. The only thing I can think of to test this is by generating a certificate whose issuer google likes. (LetsEncrypt for example.)

Answer 1

This message means that the backend does not have a socket open in the listening state. Common reasons are that no service has been started or a firewall is blocking the port.
To allow the Identity Aware Proxy into your VPC, allow traffic from 35.235.240.0/20.
ERROR: (gcloud.compute.start-iap-tunnel) While checking if a
connection can be made: Error while connecting [4003: 'failed to
connect to backend']. (Failed to connect to port 943)
This error means that the certificate provided does not match the address that the connection is made to:
ERROR: The certificate of ‘10.150.0.2’ is not trusted. ERROR: The
certificate of ‘10.150.0.2’ doesn't have a known issuer. The
certificate's owner does not match hostname ‘10.150.0.2’
Some clients, such as wget support ignoring SSL certificate validation. For wget see the --no-check-certificate flag.
Once you solve that problem you will run into another set of problems:
Under normal circumstances, you can not use HTTPS with tunnels. Tunnels are a form of man in the middle. There are tricks that can be employed, none of them secure.
Commercial SSL certificates do not support IP addresses only public domain names. You would need to create your own self-signed certificate, which would not be trusted or do not validate the certificate.
The last issue is that HTTPS endpoints require encryption negotiation from the client party. The start-iap-tunnel command does not initiate encryption (TLS negotiation). This command also does not do any form of certificate exchange and that is why you do not see an IAP error about certificates. This command only transfers data between the tunnel endpoints.
In summary, you cannot use HTTPS with TCP / SSH tunnels without deploying tricks and/or disabling features which defeats the purpose of HTTPS.

Answer 2

Allow IAP traffic through the firewall allowed my external client to connect to the internal port 943 via an IAP tunnel.
Allowing port 943 from 35.235.240.0/20 solved my problem.
More information is available at the GCP IAP docs