Varnish error for failed backend

[Danila Vershinin]

Hi,

I have configured Varnish backend probes + grace mode. Works great.

One thing I’d like to address with this setup (this is Magento website, but this “issue” applies to other website types) is more relevant error pages.

If a constant PHP error occurs in the backend and there is no cache - we see Backend fetch failed.
In some cases (e.g. when adding a store view to Magento website, and not yet configured CMS page, Magento returns 404 which of course fails the probes) - we see Backend fetch failed as well.
Finally, if Magento sends too long header and we haven’t upped some Varnish settings, etc. (misconfiguration issue) - we see Backend fetch failed.

While I’m working with Varnish setups quite often, it always takes time to explain to the clients that the Varnish is not to blame and ideally there would be a way to deliver different Varnish error page for these 3 cases:

* 500 error-ed backend
* 404-ed backend
* actual problem talking to the backend (HTTP etc.)

Is there any way to achieve this with grace mode on? I assume this requires at very least being able to get information from last probe...

Best Regards,
Danila

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