Solved Cloudron v6.2.4 - Nginx Access-Control-Allow-Origin Policy blocks Grafana to access Prometheus

[BrutalBirdie]

Tested on Cloudron v6.2.4:

Install Grafana, install Prometheus.
Add Prometheus in Grafana as Datasource.
Get:

Unknown error during query transaction. Please check JS console logs.

> Access to fetch at 'https://prometheus.domain.tld./login?redirect=/api/v1/query?query=1%2B1&time=1615638775.765' (redirected from 'https://grafana.domain.tld/api/datasources/proxy/5/api/v1/query?query=1%2B1&time=1615638775.765') from origin 'https://grafana.domain.tld' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.
> tti-polyfill.js:4 GET https://prometheus.domain.tld/login?redirect=/api/v1/query?query=1%2B1&time=1615638775.765 net::ERR_FAILED

Testing this on Cloudron v.6.1.2 with no problems.

[BrutalBirdie]

I tried looking into the diff of Box v6.1.2 and v6.2.4

https://git.cloudron.io/cloudron/box/-/compare/v6.1.2...v6.2.4

Found this line

https://git.cloudron.io/cloudron/box/-/compare/v6.1.2...v6.2.4#65b4484c1d799a34df752a8f7b9be90aca54aef6_98_98

- add_header Referrer-Policy "same-origin";
+ proxy_hide_header Referrer-Policy;

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Referrer-Policy

Then I tried to edit /etc/nginx/applications/default.conf and the both apps from Grafana and Prometheus to add cors:

https://enable-cors.org/server_nginx.html

Nothing worked, I am clueless.

[girish]

@brutalbirdie CORS headers are set by the app, it shouldn't be platform related. So, if Grafana cannot access prometheus, it's because prometheus needs some CORS setting.

[girish]

It seems one can set --web.cors.origin as per https://manpages.debian.org/unstable/prometheus/prometheus.1.en.html . I guess we have to fix the Cloudron package to have CLI args.

[BrutalBirdie]

@girish
My problem with that is, both Grafana and Prometheus did not get updated for 2 weeks on the v6.2.4 instance.
Grafana 7.4.3 and Prometheus Server 2.25.0
They did not change but the version of Cloudron got upgraded from v6.1.2 => v6.2.4 that's the only thing that changed.

[girish]

@brutalbirdie Nothing has changed in the reverse proxy configs other than referrer-policy. So, I am surprised that it worked before. I can only think of some app update causing a problem.

[BrutalBirdie]

@girish Yes I'm also very confused about this issue.
And also that no one else reported this so far.

Did you test said issue and could replicate it @girish?

[girish]

@brutalbirdie I haven't tried this yet. Do you know if this setup requires basic auth to work in the apps?

[BrutalBirdie]

@girish Good question.
Since prometheus does use proxyAuth and grafana tries to auth via basic auth.
Could this be a problem? I think I've read something on the forum about a problem with proxyAuth?

[girish]

@brutalbirdie yes, that is most likely the issue. Let me try to push a fix.

[girish]

@brutalbirdie I have enabled it in package v1.4.1 . Can you please try?

[BrutalBirdie]

@girish On it.

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Neat! This fixed it. Big thanks for the uddate.